Y.X.Zhang,Zhi Zhu,Richardson Joseph,Isfakul Jamal Shihan

School of Engineering,Western Sydney University,Kingswood,NSW,2751,Australia

Keywords: Aircraft Composite structures Damage mechanisms Direct energy system Laser system Experiment Numerical studies

ABSTRACT This paper presents a comprehensive review of the research studies on direct energy system effect on aircraft composite structures to develop a good understanding of state-of-the-art research and development in this area.The review begins with the application of composite materials in the aircraft structures and highlights their particular areas of application and limitations.An overview of directed energy system is given.Some of the commonly used systems in this category are discussed and the working principles of laser energy systems are described.The experimental and numerical studies reported regarding the aircraft composite structures subject to the effect of directed energy systems,especially the laser systems are reviewed in detail.In particularly,the general effects of laser systems and the relevant damage mechanisms against the composite structures are reported.The review draws attention to the recent research and findings in this field and is expected to guide engineers/researchers in future theoretical,numerical,and experimental studies.

1.Introduction

Composite materials have been widely used in aircraft industries due to their excellent strength/stiffness to weight ratio,leading to signi ficant weight reduction and strength improvement for the aircraft structures.In addition,composite materials offer excellent fatigue and corrosion resistance[1].Composite materials caught attention in military applications before their commercial use[2].They have been widely used in military aircrafts,and the most desirable aspect of weight reduction would render the aircraft to carry more weapons and increase its range.

Directed energy systems are relatively new weapon systems that have been developed.These systems have seen applications in battle field,and have been used to attack unmanned air vehicles(UAVs)and helicopters.With further development,these systems will be used even more widely.In general,all weapons in one way or another are devices that deposit energy in targets to achieve a certain type of damage.Directed energy systems are defined as the systems in which a beam of concentrated electromagnetic energy or atomic or subatomic particles are used to degrade,damage,or completely destroy the targets.Directed energy systems are broadly classi fied into three main categories:1)lasers systems,which use an intensely focused beam of energy to destroy objects or to dazzle or to disorient peoples;2)systems which use electromagnetic waves of other wavelengths such as microwaves;and 3)systems which use particle beams to damage the target.It is important to provide adequate guidance and reference for engineers/researchers in analysis and design of composite aircrafts against the attack from direct energy systems.

To better understand the effect of the direct energy systems on the aircrafts and the relevant damage mechanisms on the composite structures,a large number of research studies have been conducted.This paper presents a comprehensive review of the research studies on direct energy system effect on aircraft composite structures to develop a good understanding of state-of-theart research and development in this area.This paper focuses on the experimental and numerical studies of aircraft composite structures subject to the effect of laser energy systems.The experimental studies reported include the studies and experimental techniques investigating the thermos-mechanical response,the ablation behaviour caused by laser energy dissipating in the plasma layer near the target surface,the interlaminar damage mechanism and other damage mechanism of various composites(CFRP,GFRP,etc).The effects from both single laser loading and combined laser and mechanical loading are included.Numerical studies reviewed include the thermal-mechanical response,ablation behaviour,interlaminar effects and dynamic response of composite materials under laser irradiations.This review is expected to guide engineers/researchers for future theoretical,numerical,and experimental studies.

The paper begins with the introduction of the application of composite materials in the aircraft structures and their particular application and limitations as presented in Section 2.An overview of directed energy systems is given in Section 3,with the commonly used systems discussed and the working principles of laser energy systems described especially.The experimental studies relating to the laser system effects on the aircraft composite structures are reviewed in Section 4.The numerical studies using finite element method to simulate the direct energy system effect on composite structures are presented in Section 5.Finally,the review is summarized in Section 6 with recommendations of future research.

2.Composite application in aircraft structures

In general,three groups of composite materials have been developed for aircraft industries[2],including Fibre-Reinforced Plastics(FRPs),Fibre-Reinforced Metal Laminates(FRMLs),and Metal Matrix Composites(MMCs).

2.1.Fibre-Reinforced Plastics(FRPs)

Due to the requirements of high mechanical,chemical and thermal material properties,the demand of fibre-reinforced composites to replace the metal alloy materials started since the end of the 1960s.The fibres used are usually carbon,glass,Kevlar or the combination of these,while thermosetting epoxy is generally used as the matrix material in fibre-reinforced composites.Fibrereinforced composites have been used increasingly in civilian aircraft applications in recent decades.Extensive applications of these types of composites are usually seen in Boeing 757,767,777,787 and Airbus A310,A320,A330,A340 and A350.For example,the fibre-reinforced composites have been used in the fuselage of Boeing 787 Dreamliner,and the outer and centre wing box,fuselage and the empennage from the Airbus A350 XWB airframe[3].

Carbon Fibres Reinforced Plastics(CFRPs)and Glass Fibres Reinforced Plastics(GFRPs)are used in modern fighter aircrafts such as the Lockheed Martin F-35 Lightning II.Their particular areas of application are in the load-bearing structures such as vertical stabilizer,tailplane,flaps and wings skin.Future military cargo Airbus A400 M,C17(USA),JSF or F35(USA)and EFA(Europe)contains up to 40%of composites in the structural mass,covering around 70%of the surface area of the aircraft[4].Another class of fibre used in the FRPs is Kevlar,which is known for its extremely high strength,low speci fic weight,outstanding thermal properties and dimensional stability.Kevlar has been used in the rotor blades of the American helicopter Boeing AH-64 Apache,however,its use in aircraft industries is rather limited due to the high cost associated with it.

Although FRPs offer advantages in terms of weight reductions,corrosion resistance,improved fatigue life and use of fewer number of components(lesser no.of fasteners in particular)in the aircrafts,there are some inherent disadvantages,such as environmental degradation,material and processing costs,impact damage and damage tolerance[5-7].

2.2.Fibre-Reinforced Metal Laminates(FRMLs)

Although the application of advanced aluminium alloys and fibre reinforced composites has potential to reduce the cost of aircrafts,they have their corresponding advantages and disadvantages.For instance,aluminium alloys offer poor fatigue strength while fibre-reinforced composites have lower fracture toughness.In order to overcome the disadvantages of these classes of materials,the idea to develop hybrid composite structural material was initiated[8].As a result,FRMLs are developed using the advantages of both alloys and fibre reinforced composites,in which the composite laminates are adhesively bonded to thin metal sheets[9].

FRMLs are broadly classi fied into two main categories,i.e.1)FRMLs with aluminium alloys and 2)FRMLs with alternate metal alloys.Some of the commonly used FRMLs with aluminium alloys are Aramid Reinforced Aluminium Laminate(ARAL),Glass Reinforced Aluminium Laminate(GLARE)and Carbon Reinforced Aluminium Laminate(CARALL).On the other hand,FRMLs with alternate metal alloy include titanium based FRMLs and magnesium based FRMLs[10].The most commonly used type of FRML in the aircraft industry is GLARE,which was employed in the bulk cargo floors on Boeing 777,United airlines Boeing 737 and 757 aircraft,and all Boeing aircraft at QANTAS.It was also employed in Midwest Express DC9,explosion hardens LD3 containers,Laser Jet 45 forward bulkhead,AT&T aircraft electronics cabinets,and the fuselage on the A380 Airbus[11].In the context of the military application,in which higher fracture toughness is desirable,GLARE was found to exhibit outstanding fracture toughness during bullet hole calibre penetration[12,13].FRMLs offer low moisture absorption,however,the main disadvantage associated with FRLMs is the weak adhesive and cohesive strength between laminas.Economically,the cost of FRMLs is five to ten times of that of a traditional aluminium alloy used in the aircraft industry,but they can reduce the overall structure weight by 20%[1].

2.3.Metal Matrix Composites(MMCs)

MMCs,which are metals or metals alloys that incorporate particles,whiskers or fibres of different materials,offer unique properties suitable to speci fic design needs[14].In the context of the aircraft industry,the materials with lower speci fic weights are used as matrix materials such as aluminium,magnesium,copper,silver,tin,lead,titanium,intermetals(NiAl,Ni3Al,Ti3Al,TiAl,MoSi2),and superalloys.MMCs were used in the 5th generation aircraft Lockheed Martin F-22 Raptor and the heavy transport plane Boeing C-17 Globemaster III.MMC composite based on alloy TiAl6V4 and continuous fibre made of silicon carbide was used in the turbine aircraft engine.In Ref.[2],the composites were made from the metal matrix and reinforced with a very fine ceramic or metallic particle with a diameter of 0.01-0.1μm to about 15%of the composite volume.MMC composite B-Al(Boron-Aluminium)type of laminates was used on the leading edges of main and tail rotor blades of helicopters:Sikorsky S-76D,Sikorsky S-92 or Sikorsky S-70[15].

MMCs are ideal materials where high strength and excellent thermal properties are required.They are extremely reliable for components which are subjected to higher thermal and mechanical loadings.However,the use of MMCs is restricted due to the extremely higher cost associated with them.Some of the commonly used MMCs con figurations are presented in Table 1.

Table 1 Different fibre-matrix con figurations for MMCs[2,14,16].

Table 2b A summary of experimental studies.Experimental studies on ablation behaviour.

Table 2a A summary of experimental studies.Experimental studies on thermo-mechanical responses.

Table 2c A summary of experimental studies.Experimental studies on interlaminar effects.

3.Directed energy system(DES)

The three types of directed energy system and their working principle are introduced in this section with a focus on the laser system.

3.1.Working principles and types of DES

3.1.1.Lasers

The acronymlaserstands for “l(fā)ight ampli fication through stimulating emission of radiation” ,and the laser system is considered as a device that produces a highly energetic and intense beam of electromagnetic light.Laser light has the following qualities[16]:

?the light released is monochromatic,i.e.with one speci fic wavelength,which is dependent on the material of the laser and the method of stimulation;

?the waves of the emitted electromagnetic radiations are in phase in both space and time,i.e.the light is coherent;

?the emitted light is highly concentrated and strong;and

?it does not disperse over a long distance due to the coherent nature.

Laser light is thus a special case of electromagnetic radiation and is defined by the wavelength,frequency,and speed of light in vacuum or some other medium[17].In the context of military applications of laser system,the type of system is determined based on the intended target and the operating environment.In particular,the vulnerability of the target and the range that it must be engaged are two of the main factors in determining the type of laser to be selected[18].

3.1.2.Microwaves

Microwaves are another type of electromagnetic radiation with a comparatively much longer wavelength and much lower frequency than light.Microwaves have been used in various devices for military applications such as radars,communication links and missile seekers[17].However,the use of intense radio frequency waves(100 MHz-3 GHz)is directed towards military targets to accomplish certain desirable military operations.Such systems can be used to disable electronic systems by inducing a voltage to the hardware to destroy or disrupt electronic circuit boards,their components and software controls.The four levels of effects are roughly categorised as[18]:

?noise,in which the operating signals generated by the source is not extracted by the receiver;

?false information,which is generated via the receiving end;

?induced voltage from the source which causes transient upset,i.e.upset the logical operation of the targeted electronic equipment;and

?induced voltage,which could permanently destroy the target.

In general,depending upon the frequency and waveform,two basic types of high-power microwave systems are available,i.e.the narrow band and the wide band system.The selection of different combinations of the technologies along with the available knowledge of the intended target is used to develop a high microwave source to generate an induced voltage.

3.1.3.Particle beam

In principle,the particle beam systems are close to the conventional kinetic energy systems in the sense that they rely on kinetic energy.However,instead of the projectile used in the conventional kinetic energy systems,the particle beams are composed of small particles of high density moving at the speed of light.The density is usually of the order of 1011particles per cubic centimetre[17].The aim of employing this type of system is to destroy or disrupt the molecular or the atomic structure of the target.The two main types of particle beam systems are dependent on the types of the particles being used either particle possess electrical charge(electrons or protons)or the electrically neutral particles.Usually,the electrically charged particles are suitable for application within the earth’s atmosphere,while the neutral particles are suitable for space operations.However,the huge cost associated with the power supply and large fixed installations makes them susceptible to attack and render them of limited military use.

3.2.Laser energy system

As previously stated,laser is an intense beam of electromagnetic radiation that is usually defined by the wavelength,frequency,speed of light and the coef ficient of refraction.Some of the most common applications of lasers are range finders,target designators,beam riding guidance,laser radar,laser communications and laser energy systems[18].

3.2.1.Types of laser energy systems

The type of a particular laser system and its properties such as beam energy,wavelength,mode,peak power,and useful engagement,for a particular application,is dependent on the intended target and the operating conditions.In terms of energy levels,the lasers are characterised as low-energy,medium-energy,and highenergy laser.The associated power for the low,medium,and high energy laser is in the range of<1 kW,10 kW-100 kWand>100 kW respectively,while the energy range is<1 mJ/cm2,>J/cm2and>1 kJ/cm2respectively.The damage linked with the low energy and medium laser energy system is usually the destruction of a small circuit or an electronic device.On the other hand,a high energy laser system is used for aiming the destruction of a structure.In this context,the high-energy laser systems are the first choice from a military point of view.Lower energy laser systems may produce a targeted flash or continuous beam that temporarily blind human being[16].

Lasers are also defined by means of the lasing media,i.e.solid state,liquid or gas.In particular,the high energy laser systems are powered by a chemical fuel,electric power,or a generated stream of an electron.The chemical laser setups may achieve high energy level lasers,however their use for the military applications is limited due to the challenging requirements of volume,weight and fuel logistics,which require large platforms and stationary installations.On the other hand,the solid-state lasers are more stable and easily transported but are very low in ef ficiency as most of the energy is lost as heat.Moreover,the free-electron lasers use a stream of electrons that passes through alternating magnetic fields to generate high energy laser beams,but their use is restricted due to the huge size.

3.2.2.Laser-target interaction

The laser-target interaction is primarily dependent on the coef ficient of refraction-nand an attenuation coef ficient-K,which describes the extent to which the beam flux is reduced as it passes through a speci fic material.The beam’s intensity is decreased as it travels from the source to the target.The beam parameters which may be adjusted to compensate the decremented effects are energy,pulse width,beam diameter,and the wavelength.From a broader perspective,the target effects are either thermal or mechanical.The effects are also found to be dependent on the presence of plasma,which is defined as the state of matter in which the ionised gaseous molecules become electrically conductive to deliver long range electrical and magnetic fields,dominating the matter’s behaviour.

3.2.2.1.Heating and melting.The most basic laser-target interaction effect is heating.When laser light is incident upon the target surface,some fraction of the energy is absorbed,which is presented as heat.The heat by itself is not enough to damage the target unless the target is very soft[17].Hence,from a military point of view,heating alone would not be a desirable effect to damage the target material and structure.However,when the surface of the target is heated,the energy which deposits on the surface will start to propagate into the material.The propagation depends on the thermal diffusivity of the material and the time,and as a result the surface temperature rises.The target surface may begin to melt if the temperature of the heated region reaches the melting point of the material.At the same time,when the intensity on target is weakened(which reduces the heating as well)other energy loss mechanisms,such as convection and the re-radiation of energy will come into play.Similar to the heating,melting alone would not be damage the target in military applications.To cause signi ficant damage,it is necessary for the laser beam to make a hole into the target surface,and the rate at which the hole increases in depth is called the material’s erosion rate.If the molten material is removed from the hole,the new material would be exposed to the laser intensity and hence more damage will occur.However,if the molten material remains in the hole,more energy would be needed for further damage.

3.2.2.2.Vaporization.If the molten material remains in the hole,it must be vaporized before the laser causes more damage.Therefore,the incident laser intensity on the target surface must be high enough to accommodate the energy required for the vaporization.For faster vaporization of the molten materials and hence further damage,the heat vaporization must be faster than the erosion.In general,propagation losses will require the laser fire with much greater intensities,in order to hit the target with the intensity necessary for damage.The vaporization,in turn,would transfer the momentum into the depth thus causing the damage by mechanical effects.For example,the vapour at the surface would act as a small jet and further exerts reaction forces back to the target which causes deformation without physically vaporizing the bulk material.

3.2.2.3.Mechanical effects.Due to the vaporization of the target material,the momentum transfer into the material’s depth is termed as the mechanical effect of laser system onto the target.As mentioned above,the reaction forces exerted by the vapour at the surface serves to deform the target even without physically removing bulk material.Thus,the energy required for the mechanical damage of the target could be lesser than that for the thermal damage,though a higher intensity beam is required for the mechanical effects.From the military point of view,the pressure and impulse required to cause the mechanical effect on the structure and its material are dependent on the degree of damage required.

Overall,it is well understood that the required intensity for the target erosion(melting or vaporization)damage is comparatively less than that for the mechanical damage.On the other hand,the mechanical damage may require higher laser intensity but less energy to carry on.In several laser-target interaction phenomena,the plasmas are more likely to occur and in fluence the interaction at higher intensities[17].

4.Experimental studies

In this section,the experimental studies of the laser system effects on aircraft composite structures are reviewed.

4.1.Laser loading only

4.1.1.Thermal-mechanical response

Herr et al.[19]investigated the effect of high energy laser system(HEL)on CFRP panels and the thermal-mechanical response of the CFRP panels.A 1.07-μm,2-kWcontinuous wave ytterbium laser was used to irradiate the CFRP panels of varying thicknesses(1.7,2.4,3.2 mm)with 4,6 and 8 plies of 6 K 2×2 twill weave carbon fibre fabric based on Bisphenol A diglycidyl ether(DGEBA)epoxy resin blended polymer matrix.The temperatures at the front and back surface of the irradiated panels were recorded by mid-infrared camera,and the spatial and temporal laser beam irradiance variation was recorded by near-infrared(NIR)camera.A 30-Hz visible camera was also used to monitor each test which was conducted on an open optical table with ceiling mounted ventilation hood.All test panels were shaped to 10.38 cm×10.38 cm.Laser with 5,10,36,and 64 W/cm2output was used to irradiate the 3.2 mm thickness panels,and laser with 10 and 36 W/cm2was used to irradiate the 1.7 mm and 2.4 mm thickness panels respectively.The laser spot diameter was 2.3 cm for the 64 W/cm2output and 6 cm for all others.All tests were run for 2 min or until surface ignition occurred.A FLIR SC6000 MWIR camera was used to record thermal imageries.The recorded data were processed via a thermal model with a single set of temperature-dependent thermal,optical and kinetic parameters based on heat diffusion equation,coupled with a sequence of visible images,to estimate the CFRP thermal properties and kinetic parameters during matrix decomposition.Ignition was found to occur at T=1198±50°C under any circumstances,which was found to accord with a laser threshold of 21 W/cm2.It was found that high energy system(HEL)posed detrimental effects to the CFRPs,albeit being incapable of completely removing its materials.

Thermal shock strength of the laminated carbon-carbon(C/C)composite subjected to laser heating up was investigated by a few researchers.The manufacturing process and fibre texture are highly associated with the mechanical properties of the C/C composite,and the failure mechanisms of the C/C composite was found to be very sensitive to the failure between the two layers with low shear strength,thus entailed the developing of a new method of evaluation for thermal shock shear strength of laminated C/C composite[20-23].Li et al.[24]evaluated the thermal shock strength of the laminated C/C composite subjected to laser heating up,and the acoustic emission(AE)was used to detect fracture which corresponded to the critical power density of the laser to obtain the thermal shock strength.The fracture was assumed to be initiated by the induced stress over the shear strength of the material,and the critical fracture curve was derived as a function of power density and beam diameter.The laminate of eight-harness stain-weave cloth with 40%overall volume fraction of carbon fibres with 7.8 MPa shear strength,137.3 MPa compressive strength,and 606 MPa tensile strength,was used in the experiments.CO2laser with the maximum power of 1 kW was used to irradiate the rectangular specimen(50 mm×50 mm×20 mm).Experimental results concluded that the shear strength criterion was appropriate for the evaluation of the thermal shock strength of the C/C composites.Additionally,the maximum shear stress was found to occur at the periphery of the laser beam and beneath the surface and increase with the growth of the laser beam diameter and power density.

Uhlmann et al.[25]studied the thermal damage on the unidirectional(UD)carbon/epoxy laminate caused by laser grooving.The laser beam was supplied by two CO2lasers of 10000 Wand 1500 W,respectively with 0.17 mm and 0.25 mm focus spot.Grooving experiments were carried out at beam power ranging from 350 to 1500 W in continuous mode and at scanning velocities of 2.5-110 mm/s,and an N2gas jet flow coaxially to the laser beam was applied to protect the focusing lens from debris and to provide an inert environment for beam-material interaction.Thermal conductivity of the carbon/epoxy was measured at 23°C by the Laser Flashing Method using a Holometrix Micro flash instrument,conforming to the ASTM E146-92.Experimental results showed that the heat affected zone was approximately proportional to the speci fic laser energy density.Also,it was found that less thermal damage would be produced under higher laser traverse velocity,evidenced by the fact that the heat affected zone was reduced as laser traverse velocity increases.

Leplat et al.[26]investigated the thermal response and damage evolution of composite laminates subjected to laser heating in the test chamber of the BLADE(Banc Laser de cAracte’risation et de Degradation)facility developed at ONERA to analyse the anisotropic and heterogeneous behaviour of decomposing composite laminates.The square(80 mm×80 mm)test coupon(T700GC/M21 composite laminate)was heated up by continuous laser(1080 nm wavelength,maximum power of 50W)inside an air-filled pressureand temperature-regulated chamber.Sixteen 260μm thick M21/35%/268/T700GC unidirectional 268 g/m2prepreg plies were stacked into the composite laminate.Especially,the M21 resin demonstrated high tolerance to damage under high energy impacts.The orientations of the plies in the quasi-isotropic layup were[0°/45°/90°-45°/0°/45°/90°/-45°]s.The average diameter of fibres was 7μm and the volume fraction of fibres was 0.57.The final thickness was 4.16 mm.At the unheated side of the test coupon,the transient temperature was measured by quantitative infrared thermography at 10 Hz acquisition frequency.Additionally,the test chamber was vacuumed to reach 3 mbar(300 pa)to avoid any convective heat transfer and volatiles flaming.The quantitative infrared thermography technique used a FLIR SC7650 infrared camera equipped with a mid-wave[3-5μm]high-sensitivity InSb detector.Four different integration time was used with speci fic calibrations for each temperature range to precisely cover the response of the test coupon with a dependable temperature resolution.The orientation of each test coupon in the experiment was accurately performed upon aligning the fibre direction of the first 0°-ply along the horizontal plane.The laser-generated a constant non-uniform heat flux of maximum 220 kW/m2which exerted 40-Wthermal loading on the material front surface.The heat flux was assessed prior to the decomposition experiments using a nonlinear inverse heat conduction method.Following each test,a post-decomposition micrographic analysis was conducted from longitudinal cross sections of the coupons.The highest temperature magnitude was observed at the centre of the coupons where the thermal loading at the front side was the highest.No in-plane deformation was detected from the IR measurements.However,temperature-activated chemical reactions occurred which affected the resin through a pyrolysis charring process.From 150 s exposure duration,the composite coupons experienced critical temperature drops which can reach 60 K as observed for the third text coupon.Such sudden decreases in temperature could only result from breaks within the continuous medium,which caused local thermal contact resistances.When the maximum temperature on the cold surface reached 550 K,delamination was seen onset in all tests.Nevertheless,identical experimental conditions and thermal loading resulted in different delamination damage and postdamage behaviours in terms of onset time,suggesting that delamination may depend on many parameters with temperature and exposure time strongly correlated.The deepest delamination cracks were identi fied as the consequence of the mechanical damage induced by high thermal gradients and a non-symmetrical stacking sequence.Moreover,experimental results con firmed that the large resin-rich regions at ply interfaces offered preferential paths for the cracks initiation and propagation of T700GC/M21 laminates.

Herr[27]measured the emissivity-adjusted surface temperatures from laser irradiated CFRP using a mid-wave infrared thermal camera.In the study,the CFRP targets were irradiated with a 1.07μmytterbium doped continuous wave fibre laser at irradiances ranging from 5 to 525 W/cm2and 780-3000 W/cm2,respectively.Surface temperatures of the specimens were measured.The CFRP testing panels were of 10.38×10.38 cm2and thicknesses of 1.7,2.4,3.1 mm,irradiated by a 2-kW continuous wave IPG Photonics ytterbium doped fibre laser at 1.07μm.The test set up is shown in Fig.1.The panels were manufactured by layering multiple plies of 6 K 2×2 twill weave carbon fibre fabric in a mould and injecting epoxy resin under vacuum.The samples contain 4,6 and 8 plies.A DGEBA based epoxy resin was used.The 3.1 mm thickness panels were irradiated at 5,10,36,and 64 W/cm2by the 2.3 cm diameter laser beam.A beam splitter was used to illuminate a stationary scatter plate and the spatial and temporal laser beam variation was recorded by a calibrated near-infrared(NIR)camera.Tests were set up on an open optical table with ceiling mounted ventilation hood,and thermal imagery was recorded by a FLIR SC6000 MWIR camera operated with a bandpass filter from 3.8 to 4.0μm and neutral density filter of O.D.1.0.The spectral emissivity of samples was observed from 2 to 25μm with an SOC-100 HDR(Hemispherical Directional Re flectometer).

Decomposition produced billowing clouds of volatile products with soot beginning at surface temperature of around 430°C.Enough volatiles were produced on the backside of the panel which was ignited by the front side flames at around 600°C.The radius of visible change ranged from around 2-3.5 cm for 5-35.7 W/cm2using the 3 cm laser beam radius.No signi ficant mass loss was observed for total incident energies of less than 7.7 kJ and 5.2 kJ for non-ignition and ignition cases,respectively,at 6-cm laser spot diameter.However,no mass loss was observed for total incident energies of less than 2.8 kJ at 2.3-cm laser spot radius.The appearance of combustion flames increased the mass loss rate by 45%.Resin matrix removal resulted from HEL heating was found to be incapable of completely compromising the material,however,it did in flict several detrimental effects including fouling optics and electronics,providing a fuel source for combustion,which reduced the repressive strength of the CFRP.

Fig.1.Experimental setup[27].

Berlin et al.[28]carried out an investigation to ascertain the effects of heat radiation on several polymer composites,including composites reinforced with carbon fibres and E-glass.Laminates of carbon/polyetheretherketone(PEEK),carbon/epoxy and glass/epoxy namely Ciba Geigy 914C and 914G were used in the experimental study.Ciba Geigy 914C and 914G are thermosetting matrix composites made of epoxy matrix reinforced with T300 carbon fibres and E-glass fibres,respectively.The specimens were exposed to radiation on the entrance of an oven set at 800°C with minor variation between the specimen and the heat source of approximately≤1%at intensities≤30 kW?m2.On the backside,laminates were subjected to a heat flux of 8-60 kW·m2.The temperature of the specimens was measured using infra-red(IR)pyrometer.A microbalance was used to determine the loss in mass after exposure to radiation.In this study,the deterioration of the mechanical properties of carbon-reinforced composite was found to be attributed to the delamination.The authors further argued the decay of composites was preceded by visual defects formed as a result of exposure to heat radiation.The specimen’s(914C and 914G)damage was found to occur at temperature of approximately 300°C.

Predicting the general laser-matter interaction demands the accurate thermal analysis(as the fundamental element)in predicting the overall composite structural reliability when subjected to rapid high intensity heating.One of the earliest work to study the transient thermal response of fibre-reinforced composite plates was conducted by Grif fis et al.[29].In this work,a one-dimensional heat transfer model was developed to predict heat conduction in the thickness direction.The area exposed to the laser irradiation was considered to be signi ficantly smaller than the other characteristics dimensions of the structures.The numerical results of the thermal analysis(temperature distribution and ablative characteristics)were compared with the experimental data and employed to evolve thermomechanical stress analysis and failure criteria.The experiments were conducted on the twenty-ply AS/3501-6 graphite epoxy laminated composite panels of in-plane dimensions 5.6 cm×10 cm.The thickness of the panels was set as 2.54 mm and the panels were subjected to rapid heating using the 15 kW,continuous wave,CO2laser.In order to measure the thermal response during irradiation,one thermocouple was embedded in the middle while the second one was placed at the rear surface.In addition,an optical pyrometer was installed to measure the temperature of the front surface.Testing was conducted at several laser intensity levels,using a fixed 25 mm beam diameter with variable power output.Furthermore,to accommodate the aerodynamic cooling effects,Mach 0.3 air flow was applied parallel to the irradiated surface during each test.This research served as the benchmark for several later reported studies.

Furthermore,Lacroix et al.[30]studied the thermo-mechanical behaviour of carbon epoxy composite under laser irradiation.The laser matter interaction was assumed to be linear for the heat transfer process.The Fourier thermal conduction model was used to predict the heat conduction,and the mechanical degradation during laser exposure was obtained through an “Equivalent Ablated thickness” criteria.This criterion was defined to assess the temporal progression of the thermal flow inside the material despite the resistance of the carbon fibres to the laser illumination by means of a cantilever bending test con figuration.The experimental setup consisted of G939/M18-1 carbon/epoxy laminate samples(based on 8 wraps of G939 50/50 bi-dimensional prepregs with M18-1 epoxy resin),irradiated with 10 kWIPG fibre laser with a 1.07μm wavelength.The illumination area diameter was 2 cm and the power density were supposed to be homogenous.Experimentally,the carbon fibres were found to withstand approximately 1 kW/cm2of power density with the surface temperature reaching 3250°C.

4.1.2.Ablation behaviour

It was indicated that a highly concentrated laser energy may cause energy waste due to laser energy dissipating in the plasma layer near the target surface as high-power density induced optical breakdown[31-33].Ablation effect was strongly dependent on the deposited energy in the target material,emphasizing the importance of determining the threshold of laser power density to trigger optical breakdown which caused energy dissipation.Wu et al.[34]investigated this phenomenon in their study of the laser ablation of mechanism of CFRP composite.In their study,CFRP specimens(T800 plain woven carbon fibre cloth reinforced polymer laminate of 50 mm×50 mm×4.1 mm)were tested under three groups of laser irradiation to obtain ablation behaviours and morphologies of the CFRP.The power density for continuous wave laser,long pulsed laser(200 ns pulse duration),and short pulsed laser(10 ns pulse duration)was 3.54×106W/m2,1.50×1012W/m2,and 3.00×1013W/m2respectively.All lasers were designed to irradiate the square CFRP specimens for 10 s with random polarizations.The surface morphologies were photographed by optical microscope and then compared to the CAD drawing.Under continuous laser irradiation,the CFRP specimen was ablated by several layers followed by voids appearing throughout the epoxy layer after 10 s.The ablation effects on the under-epoxy laminate closely attached to the top carbon fabric were reduced because laser-induced plasma absorbed a large part of the incident laser energy as air breakdown occurred after the surface carbon fibre fabric locally evaporated.However,long pulsed wave laser drilled a conical hole through the laminate with largest radius around 1 mm which was smaller than the laser beam radius,as compared to that the short-pulsed laser only ablated the thin surface carbon fabric of CFRP.

Zhu et al.[35]studied the ablation behaviour T300/AG80 carbon fibre reinforced laminate following layup sequence[±45°/0°/90°/0°]Sof a shape of 200 mm×200 mm×1.5 mm,under Nd:YAG generated laser beam with 1064 nm wavelength for 5 s irradiation.The laser power density was 50 W/cm2,and the laser beam radius was 10 mm,20 mm,and 30 mm respectively.They used a CRONOSPL2-DIO dynamic strain meter to measure the specimen response.At 0°,45°,and 90°directions at front and back surfaces of the specimens,strain foils were attached.Experimental results showed that the larger the laser beam radius,the greater the thermal shock exerted upon the specimens.The maximum shocking tensile strain recorded in 20 mm and 30 mm laser beam radius cases are 5.7 and 8.4 times the maximum shocking tensile strain in the 10 mm case,respectively.Generally,the strain value at different locations within the laser spot decreased as it was farther from the centre of the spot.

Stratoudaki et al.[36]investigated the effect of laser generated ultrasound on epoxy resins using three different lasers,i.e.,TEA CO2(Coherent,Hull,Laserbrand150),Q-swtiched fundamental Nd:YAG(Spectron Laser Systems),and XeCl excimer(Lamda Physik).A Michelson interferometer was used to directly measure the ultrasonic waveforms and record the absolute epicentral displacements on the opposite side of the sample,and the samples were inspected under optical microscope.The generation beam spot size was kept between 0.02 and 0.03 cm2throughout the experiment.The carbon fibre reinforced composite(CFRC)had a thin super ficial layer of resin with a mean thickness of~12μm.Pure epoxy resin samples were prepared using resins commonly used for the manufacture of composites,of which the first was a cold-curing epoxy resin,and the second a warm-curing epoxy resin.The Nd:YAG operated in TEM00mode with pulse duration at FWHM was 10 ns and its spot size was~0.02 cm2.The FWHM of the TEA CO2was 50 ns,and the spot size was~0.03 cm2.The excimer laser had a‘top hat’beam pro file,and its FWHM was 40 ns and its spot size was~0.02 cm2.Results showed that in the case of the Nd:YAG laser,most of the energy was absorbed in the first layer of carbon fibres.In the case of the excimer laser,the ablation threshold was found to be very low but the damage was localized at the super ficial resin layer.No exposure of fibres was observed in all three cases.

Pan et al.[37]investigated the ablation mechanism and the effects of laser parameters in laser ablation of carbon fibre reinforced silicon composites.The researchers used six different laser power densities and six levels of pulse numbers.Results revealed that damage to surface morphology included three areas:the board region,the transition region,and the centre region.As the laser power density increased,the ablation at the centre region increased signi ficantly with the surface cracking with spherical composite particles observed in the transition region.The surface morphology of the composite showed that the degradation caused by laser irradiation at different power densities ranged from 4.77×102W/cm2to 12.1×102W/cm2.An increase in the laser energy contributed to ablation of the surface coating because of the high temperature at the centre of the lase spot.In addition,the composite matrix was decomposed when exposed to strong laser resulting in the exposure of the non-ablated carbon fibre layers.The damage to the composite was resulted from the high laser temperature which reached the decomposition temperature of the composite material,although it did not reach the melting point of the composites.As a result,the composite was ablated while carbon fibres were retained but exposed on the surface of the sample.The ablation centre showed large spherical particles at the ablation centre which was an oxidation product of the composite material.A high number of large spherical materials were also found to be distributed at the ablation edge of the composite coating.As the laser power on the composite material increased,the boundary between non-irradiated region and the irradiated region became clearly visible.Both the carbon fibre and composite matrix surface were ablated.In addition to complete sublimation of the first layers of the composite and the matrix,the carbon fibres were also sublimated.The sublimation temperatures of the silicon carbide and carbon composite were 2700°C and 3550°C,respectively,and the laser temperature at the centre of the composite material was above 3550°C.

4.1.3.Interlaminar effects

Wu et al.[38]conducted a comprehensive study on the mechanical and thermal properties of glass fibre reinforced epoxy composites.The diglycidyl ether of bisphenol F epoxy resin with the epoxy equivalent weight of 164-172 g/mol was used.The curing agent was diethyl toluene diamine(DETD,HY5200,Huntsman Advanced Materials)with an amine weight equivalence of 44.5 g/mol.The boron free glass fibre cloth was treated by silane coupling agent.The fabric was 0.2±0.022 mm thick with count of 18±1 threads/cm in the warp and 14±1 threads/cm in the fill.The prepared composite panel was cut into specimens for the short beam shear test with the dimensions of 24 mm×8 mm×4 mm.60Coγ-ray with dose rate of 300 Gy/min at ambient temperature was applied upon the specimens.The total doses of 1 MGy,5 MGy and 10 MGy were applied respectively.Through the short-beam shear(SBS)test according to the ASTM D2344,the apparent interlaminar shear strength(ILSS)was determined.The specimens were dipped inside a cryostat filled with liquid nitrogen to achieve cryogenic condition.After the SBS tests,a Hitachi S-4300 SEM was adopted to observe the fracture surfaces of the specimens.The UV-Vis spectra of the specimens were measured by a Cary 5000 spectrometer.Fourier transform infrared spectroscopy was performed on an Excalibur 3100 spectrometer.Distinct failure due to the interlaminar shear was found in all cases.No clear effect of the gamma ray irradiation on the interlaminar shear strength was found when the total doses was less than 5 MGy.However,the ILSS sharply decreased after exposed to the total 10 MGy dose.Two radiation-induced processes,i.e.,the molecular chain scission and crosslinking,were identi fied to associate with ILSS reduction mechanisms.Speci fically,the chain scission generally reduced the cryogenic strength and stiffness of the matrix which resulted in the degradation of the ILSS.After 10 MGy irradiation,the ILSS was found being decreased by around 58%.Thermogravimetric analysis showed that the initial degradation temperature(IDT)of the specimens decreased drastically after the irradiation with a decrease of 18%and 25%in IDT respectively from 5 MGy to 10 MGy compared to that of the non-irradiated specimen.Overall,the composite laminate could resist the dose of 5.0 MGy.

Following the work done by Wu et al.[34],Liu et al.[39,40]further investigated the CFRP composite laminate interacting with infrared wave laser,and interlaminar damage of CFRP laminate under continuous laser irradiation.The 2 mm thick CFRP lamina synthesized from CCF-700 carbon fibre and BA9916-II resin matrix was autoclave treated and then being irradiated by infrared laser of 1064 nm wavelength and 200 ns pulse duration repeating at 10 Hz.The specimen was compacted by 16 laminates of a thickness of 0.125 mm by[45°,0°,-45°,90°]2ssequence,and then shaped into 50 mm×50 mm×50 mm cube with 145±g/m3.The specimen was irradiated by the infrared laser of 1064 nm wavelength by pulse duration of 200 ns repeating at 10 Hz.The laser spot radius was 1.1 mm and the output power were around 1.1 J.Optical microscope was used to observe cut cross sections of the CFRP specimens.It was observed that the longer irradiation would cause wider hole at the back surface of the specimens after it was penetrated and around 10 s were required to drill the specimen through.In addition,Finite Element model was utilized to investigate the temperature and phase change of the specimens.A wider hole than that from the experiment as well as a coarse aperture wall were predicted from the numerical simulations.

When being exposed to high temperature,CFRP laminates became very susceptible to deterioration in mechanical properties,and composite gradients are prone to flaming[41,42],and interlaminar thermal damages induced by ablation could lead to disastrous consequence on CFRP laminate.In theory,it was the propagation of the interlaminar cracks that governs the interlaminar separation of the CFRP and was greatly dependent on the fibre lay angles.However,the multi-interlaminar shear failure mode was basically attributing to the weak fibre-matrix interface in the CFRP.In the work done by Liu et al.[40],the interlaminar damage morphologies of the CFRP laminate under continuous laser irradiation was recorded.Acquired interlaminar damage pattern and its spatial distribution were further analysed.An optical microscope and a scanning electron microscope(SEM)were used in the study.The same specimen in the previous study[39]was used again but was irradiated vertically by continuous wave laser in a cabinet filled of Nitrogen.The continuous wave laser of 1070 nm wavelength was used to irradiate the specimens for 3 s by output of 500 W,800 W and 1000 W respectively.The optical microscope and SEM were used to observe the interface damages at cross section of the specimens.Morphologies at cross section were recorded,illustrating pyrolysis occurring in up to eight laminate from the surface layer where fibre fracturing under direct irradiation.Similar to the phenomenon observed in carbon/carbon composite exposed to elevated temperature[43],the interlaminar cracks were found to increase in width and length as laser power output rise,and at around the backward surface large interlaminar cracks were seen.

4.1.4.Damage analysis

Gay et al.[44]studied the local tensile stress caused by laserinduced shock within CFRP composite laminates involved in aeronautic and defence industry.They selected the carbon fibres G40-800-24 K reinforced epoxy Cytec?5276-1 with 4 and 8 ply laminates by layup sequence[0°/90°]Sand[0°/-45°/90°/45°]Srespectively as test specimen which were shaped to 15 mm×15 mm with a thickness of 600μm and 1200μm respectively.The average diameter of the carbon fibres is 5μm and their volume fraction is 70%.A linear-elastic law[45,46]was used to describe the dynamic behaviour of the composites.In their adhesion test,the load was generated by a Nd:YAG laser which delivered a calibrated pulse with a duration of 9.3 ns at Full Width at Half Maximum(FWHM)and an energy of 1.5 J with 532 nm wavelength.The high intensity pulse drove a compression wave within the specimen to test and was eventually released to relax the material to its initial state.The pulse propagating through the sample thickness to the opposite surface where it bounced back in tension could induce damage.Firstly,the experiments were performed on the 4-ply lamina and a delamination threshold for an incident intensity of[0.9-1.03]GW/cm2was detected.Secondly,the 8-ply lamina was tested under irradiation of 450 ns pulse duration laser,and the 1.49 GW/cm2laser intensity was found to be strong enough to induce delamination.The microtomography X-Tek HMXST 225 was used to examine recovered samples.The incident intensity to induce delamination in the 8-ply case was found higher than that in the 4-ply case,and it was observed that laser-induced stress waves could produce on-axis tension which caused delamination of the specimens.The tensile strength of both the 4 and 8 ply lamina was found to be around 292 MPa,which was in the same range as the strength evaluated by Yu and Gupta[47],and Riedel et al.[48].

Laser shock wave techniques have also been used to experimentally examine the damage of directed energy on composites which are widely used in the aerospace industry.Ecault et al.[49-51]first examined how laser shock wave impacted on carbon fibre reinforced polymer(material T800/M21).The T800/M21 is a common composite material in the aeronautical industry,made from mon-conventional matrix that is mixed from thermoplastic nodules and thermoset epoxy resin whose mechanical characteristics are created to improve shock resistance.Besides their adequate mechanical properties,T800/M21 composite materials are also used in the aeronautics industry because of their light weight.Characterization of T800/M21 materials for various defects has been achieved in the past using Interferometric Confocal Microscopy,X-ray Radiography,and Optical Microscopy.Ecault et al.used Carbon Fibre Reinforced Polymer(CFRP)samples for the experiment to assess how laser shock waves impacted on composite materials.The researchers first conditioned the samples by shocking them with various laser energy levels to generate diverse levels of internal damage.The samples were then recovered from the set-up for analysis using several diagnostic setups.The first shocks were performed on thin T800/M21 samples measuring 1.5 mm that were extracted by cutting out from thicker materials.Optical micrography was used to analyse the samples which were exposed to laser shock waves.The analysis was used to assess the correlation between laser intensity and damage characteristics on the composite material.The second shock waves were done on thicker materials of 6 mm as shown.Resulting damage was analysed using Interferometric Confocal Microscopy(ICM)and X-ray radiography on the back face of the samples.The obtained data was used to assess the damage threshold done on the T800/M21 CFRP because of laser shock dynamic loading.Results from the 1.5 mm T800/M21 composite samples revealed that the damage induced by laser shock was cone shaped through the sample thickness.The cone basis was situated at the back of the face.

For the different laser shocks,the sample exhibited a similar kind of damage despite the laser strength used.Ecault et al.[49-51]also performed laser shocks on 66 mm thick T800/M21 CFRP samples.Like the thin samples,four different laser pulse intensities were used.Results showed that since the samples were thick,the laser did not spall them.The laser shock wave amplitude was decayed through the material’s thickness as there was a longer distance to close before emerging from the other side of the face.The resulting damage was evident in the form of small blisters on the material back face based on the ICM and X-ray radiography measurements.In summation,it was observed that the damage resulting from the laser shock wave propagation on composite materials was resulted from laser intensity.A potential damage scenario for the T800/M21 composite due to high laser irradiation was shown revealing the damage tolerance of aircraft composite materials when exposed to high energy laser wave shocks.

Ecault et al.[49-51]then examined composite material damage resulting from laser-induced shock waves on 10 mm long and 5 mm thick transparent epoxy composites using optical shadowgraphy.The shock waves from high laser intensity were focused on composite material for 3 ns pulse duration(1.2-3.4 TW/cm)generating a pressure of between 44 GPa and 98.9 GPa.Results revealed that the shock wave and release wave generated by the laser reverberation at the back face was followed by a dark zone.The results indicated that the shock wave and release wave resulting in dark zone at the back face corresponded with the creation of tensile zone due to crossing on the loading axis of the release waves that came fromthe edge of the impact region.The shock wave setting up at t=0.5μs and propagation t=1μs and 1.5μs revealed two main shapes.At t=1μs there were thin curved black lines which were the shock wave.At this stage,the composite was compressed because of the pressure state that contributed to the observed darkness.Considering that the shock was short,the pressure was released once the loading stopped.The shock wave was followed by a grey area which was a colour that was evident in unloaded areas.A sizeable black blur behind the first two waves was identi fied which traduced a tensile loading due to the geometry of the laser impact.The phenomenon was called the edge effect which relied on the principle of spherically propagating release waves inside the composite from the edge of the impacted area.Upon reaching the free surface,the shock waves were re flected releasing a curved grey line,which propagated backwards fromright to left and crossed the black blur which was the incident tensile stressed region.The back face was progressively loaded with tensile stress in line with the spallation phenomena and shock wave propagation.Ecault et al.[49-51]also investigated the impact of laser shocks on ecocomposites to assess how the materials degraded under laser impact loading.The researchers compared the impact of laser shock induced damage based on back observations of composite samples for various types of eco-composites.Inside delamination,residual blister and spallation resulting from laser impact on fibre length were also tested using the Terahertz technique.The laser was focused on the surface of the specimen.Aluminium coating caused the matter/laser interaction to be generated on the sample surface,which caused the high-pressure plasma created from the process to expand rapidly.Reactions inside the material contributed to a generation of shock waves.The directed shock propagated through the composite material depending on the geometry and characteristics of the samples.Due to impedance mismatch,the incident shock wave was re flected into the release wave upon reaching the sample blackface creating a release backward wave propagation.The release wave crossed the incident release wave which came from initiation at the end of the loading(returns to the initial state).The crossing of two release waves contributed to high tensile stress which could damage the composite material if the threshold was exceeded.In composites with short fibres such as TWA and WA,damage resulted from ejection and separation of fragments from the surface,which was referred to as the spallation phenomenon.For the WA and TWA composites,the spallation phenomenon was noted to have variations in its damaging ability where the area of damage for the TWA was 6.6 mm compared to 4.7 mm damage recorded in WA composite under the same laser shock.These observations demonstrated that thermal treatment of spruce fibres caused high damage due to brittleness of the composite material when exposed to high energy laser.When considering the composites with flat mat reinforcement,no spallation was recorded.In this case,the inside damage resulting from laser shock generated residual relief which caused small blisters on the back-face surface of the FBB and FPLA composites.The measurement of diameters of blisters resulting from laser directed shocks on the composites showed that there were also differences in behaviours.The FPP sample had a blister diameter of 8.5 mm compared to the blister diameter of 10 mm on the FPLA sample.Thus,laser shock created more damage to FPLA samples than to the FPP samples.Finally,the HE composites showed partial blister spallation with a diameter of 9.4 mm,while that of the GE composite had a whitened zone of 7.7 mm.It was found that composites which were used in the aerospace industry exhibited different behaviours when being exposed to laser impact loading and directed energy sources,largely depending on distribution,length,type of fibres,and the matrix used for composite materials.When the same laser impact intensity was used,the composite materials recorded three types of damage at the back-face samples.These damages included inside delamination,residual blister,and spallation.Short fibres were damaged from the spallation phenomenon which indicated their low strength.Non-woven fibres indicated residual blisters which was a composite damage step preceded spallation.In contrast,woven fibres showed combined spallation and blister damage,but woven glass composites showed whitening on back face surface.

Fig.2.Experimental setup from top view[52].

Garcia et al.[52]investigated the possibility of lasers defeating targets without penetration but with heat re-radiating to internal components by using a thin carbon fibre-reinforced polymer(CFRP)and a steel disc to represent target skin and internal component,respectively.A FLIR?A325sc infrared camera was used to measure steel disc temperature.Thin carbon fibre composite laminate consisting of 2-,4-,and 6-ply sheets and having 50%carbon fibre-resin mass ratio were shaped into 5.08 cm×25.4 cm strip specimens.The composites were reinforced with a 3K plain-weave,5.7 oz/yd2carbon fibre cloth.After being vacuum bagged for 1 h at 1 atm pressure,the composites were further heated to 121°C and cured using a 2-ton press for 1.5 h.A YLR-100-AC fibre laser served to output laser beamwith a M2value of 1.07,maximum 100 W,and 5-mm diameter.The actual range of on-target irradiances was between 101.9 W/cm2and 509.3 W/cm2due to lab limitations.A 1 cm diameter metal disc made from 1008 carbon steel with a nominal thickness of 0.15 mm was used to simulate the internal component.Its temperature was measured by an attached Type K thermocouple.In the experimental setup,an IR camera was offset 53°from the plane perpendicular to the sample plate to avoid any damage as laser could potentially penetrate the composite.The setup of the experiment is shown in Fig.2.High energy laser was placed perpendicular to the sample plate at other side.The specimens were subjected to five different irradiances of between 152.8 W/cm2and 356.5 W/cm2.The resulting temperature was measured at the back of the composite.Results con firmed that without complete penetration,the intense local heating caused by laser radiation on a CFRP laminate can damage the internal components.Radiation from the heated internal component in turn contributed to the development of flames at the back surface of the CFRP.The released heat elevated the temperature of the 6-ply CFRP back surface by 237 K during the 305.6 W/cm2test and by 240 K during the 356.6 W/cm2test.In a typical unmanned aerial vehicle(UAV)fuselage,the heat gave rise to internal flames that could damage flight control systems and ignite the fuel mechanism.

In the study conducted by Voisey et al.[53],SEM and Raman spectroscopy were used to investigate the effects of the fibre type on the extent of laser-induced fibre swelling in carbon fibre composites being laser-drilled by pulsed Nd:YAG laser(λ=1.06μm).T300,high modulus(HM)and P100 fibres were heat treated for 12 h at 2000°C prior to the experiment.An average output power of 135 W,M2value of~25 with single pulsed of energy 1.0 J and duration 1.0 ms Nd:YAG laser was adopted.An average power density of around 30 kW/mm2was yielded by the 100μm focused spot diameters.Prior heat treatment was found to have substantially reduced the swelling exhibited by the T300 fibres but had little effect on other two fibres.The obtained spectra showed laser drilling had rendered the T300 fibres more graphitic and less disordered.It was proposed that the rapid heating and high thermal gradients generated by laser drilling tended to volatilise noncarbon impurities and create high gas pressures.These high pressures could act as a driving force for the new structure to be more open which was apparently retained after the impurities were driven off.This mechanism explained why fibres containing larger quantities of residual(volatile)impurities were more prone to laser-induced swelling.

Instead of the total failure/destruction of the targeted structure,the recent military interest has been shifted to tactical threats,such as cruise missiles and unmanned aerial vehicles(UAVs),which can be countered with comparatively lower laser irradiance.The idea is to bring the incident laser wavelength to the near Infrared region,which can penetrate the matrix resin of most of the composite materials.One such study was conducted by Wu et al.[54]in which it was argued that the weakly absorbed,more deeply penetrating near-IR radiation softened the resin matrix resulting in structural failure under load at lower laser intensity than what was required for mid-IR lasers.The study used two materials for illustration,i.e.translucent fibreglass composite and opaque carbon composite.Since the study was related to the deep penetration of the laser,the first step in the experimental program was to determine the absorption properties in material samples,so that the comparison of the results with the theoretical values could be made.Following this,the measure of the compressive load strength under near-IR laser irradiation was made.The composite plate samples were held vertically in a hydraulic load machine designed to apply a constant compressive force.A uniform illumination on the samples was achieved by passing light from a 1.5 kWdiode laser array and the wavelength of 0.8μm through a lens duct and imaging lenses.Digital and IR cameras were used for recording purposes.The exhausts were provided for the fumes to be exhausted.The research discovered that the volume heating by direct in-depth absorption of laser energy in case of translucent fibreglass composite material and rapid conduction of surface-deposited energy as in the case of opaque carbon composite may cause the failure at one to three orders of magnitude lower than the intensity required to produce a lethal effect by target penetration.

The intense heating produced by the laser system can adversely affect the mechanical properties of the composite structures such as the tensile/shear strength and elastic modulus.Meanwhile,severe thermal irradiation may also induce signi ficant geometrical changes in the structure’s surface ablation or localized burn through.It was well understood that if the surface temperature reached the ablation temperature of the composite,the material would undergo a sublimation reaction.The burn-out material reduced the structural stiffness of the remaining laminate and increased the mechanical stresses,thus degrading the structure’s load-carrying capability.Moreover,due to the presence of highly non-uniform temperatures over the thickness of the composite laminate,thermal in-plane forces and bending moments might arise.Obviously,these thermal loads play an important role in the change of constitutive relations of a laminate,initially subjected to mechanical loads.Hence,methods for predicting composite damage under coupled laser irradiation and mechanical loading have been developed by the researchers[55,56,61].

Candan et al.[57]investigated laser threats on fibre-based ballistic-resistant composites,and the Spectra Shield SR-3136 composite material was used in the investigation.Spectra Shield SR-3136 was a thermoplastic composite made of ultra-high molecular weight polyethylene(UHMW-PE)fibres reinforced with lowdensity(LD-PE)fibres.The composite was exposed to a continuous wave laser beam at 915 nm of a fibre-coupled diode laser module.The specimen exposed to the laser beam had a puncture with signi ficant swelling around the entry hole subjected to temperature of 450°C or more.A full penetration was observed at a laser energy of 20 kJ and greater.Furthermore,as a result of the slight tapering induced by asymmetric behaviour close to the entrance,the entry hole had a diameter of 16 mmwhich was larger than the diameter of the exit hole of 2.4 mm.

4.2.Combined loading

Allheily et al.[58]investigated the thermo-mechanical behaviour of aeronautic materials(CFRP)subjected to powerful electromagnetic fluxes to assess the likely impact of directed energy systems on aircraft composite structures.The method proposed was meant to find a way to neutralise unmanned aerial vehicles that posed threat to defence installations.In the preliminary study the authors carried out three tests namely thermal surface characterization,thermal in-depth characterization,and thermomechanical characterization on G939/M18-1 specimens(Hexcel Composites)made of plain-woven carbon fabrics.In all the tests the specimens were initially exposed to a homogenous irradiation with a 10 kW fibre laser from IPG.Approximately 3%of the laser energy was produced with a beam splitter.Concurrently,the front side of the specimen was exposed to gas flow which was applied parallel to the irradiated surface before,during and after the irradiation.The purpose of gas flow was to assess the carbon fibres combustion impact on temperature readings by comparing experiments with air or nitrogen.After each experiment the specimens were weighed to determine mass loss based on laser intensity and the irradiation time.

Results show that with growing laser power the temperature of the irradiated zone increased and reached a maximum value of 3300°C for power densities that were greater than 1000 W/cm2.The maximum power recorded aligned with the values recorded by other researchers[49-51],and it referred to the sublimation temperature of the carbon composite fibres.When observing post irradiation samples,the same property was also observed while irradiations with power density of less than 1000 W/cm2only impact on epoxy matrix because the sublimation temperature was lower than that of the carbon material where drilling became visible when the composite sample were irradiated using laser power density that was greater than the threshold value.The evolution of the temperature gradient was evident inside the composites where the results revealed that CFRP was a good thermal insulator since it could take some seconds before the backface side temperature became signi ficant.The observed thermal resistance was attributed to strong carbon fibres which could absorb almost all laser energy in a single fabric layer and sustaining high temperatures without resulting in ablation.Once the carbon ablation conditions were attained,the amount of laser energy needed to damage the composite material was high because of the substantial carbon ablative enthalpy.All these observations explained for the 10 s continuous laser irradiation resulting in very high temperatures at the composites’front face(above 2000°C for 200 W/cm2),while the rear side that was 4 mm in thickness reached only 250°C.In contrast,the pyrolyzes process on the matrix started after irradiation.The side post-irradiation and infrared observation revealed an exothermic pyrolysis at between 300°C and 350°C that preceded the delamination front.The chemical degradation temperature ranged in line with the thermogravimetric analysis resulting in pure resin component with an inert ambient medium.The composite recorded thickness reduction during irradiation and post-irradiation since the bending displacement increased even when the irradiation stopped.The study indicated a good thermal insulation of the composite material,which was largely because of the potential of carbon fibres to absorb and support heat fluxes.Furthermore,the delamination of the composite reduced energy deposition on the specimen.

Zhao et al.[59]conducted an experimental study on the failure behaviour of CFRP composites subjected to tensile loadings under thermal environments and laser irradiation.T700/BA9916 epoxy prepreg was cured at 180°C,and the ply pattern was[45°/0°/-45°/90°]2s.The epoxy and fibres accounted for 0.467 and 0.533 respectively in the laminated composites which had a thickness of 2.40 mm and width of 20 mm.Electric resistance wire was wound into a heating band,being set around the specimen to conduct the tensile tests at thermal environments.Thermal couples of NiCr-NiSi type were installed to monitor the specimen temperature and heating rate,and tensile strain was acquired by a DIC system.The CFRP specimens were irradiated by continuous wave laser by power densities ranging from 1.5 MW/m2to 12.7 MW/m2in a nitrogen protection environment for the sake of avoiding possible combustion.The laser source was a YLS 1070 nm fibre laser of 2 kW provided by IPG Ltd.The defective specimen was further applied with tensile loading at 2 mm/min rate in MTS until failure.Results showed that complex thermal damages could happen once CFRP composites were exposed to thermal loadings which could lead to decrease of the Young’s modulus and failure strength.Residual strength was found to decline as target temperature or laser power density increased.Besides,residual strength was found falling drastically before laser power density reached 6.3 MW/m2after which it decreased at a slower rate.However,a temperature plateau zone was identi fied before 200°C,which was the thermal pyrolysis activation temperature for epoxy resin.Sublimation(the phase transition of substance directly from solid to gas)of carbon fibres occurred once temperature reached 3000°C.The turning point at which delamination cracks occurred was found to be at a power density of 2.5 MW/m2.

The failure behaviour of the T700/603 carbon fibre/epoxy composite laminates under pre-load compression or tension and laser irradiation was tested to study the effect of main parameters including pre-stress ratio,laser power density as well as the thicknesses of the specimens on failure time [60] of 140 mm×10 mm specimens with various thickness consisting of 0.15 mm thick ply.A Zwick/Roell-Z100 universal testing machine was employed to apply pre-load of 35%or 50%of the average ultimate compressive collapse stress and 50%,65%or 80%of the average ultimate tensile fracture stress to the composite laminate specimens.The laser spot was 15 mm in diameter perpendicularly focusing on the centre of the specimen with 0.202,0.492,0.953 and 1.935 kW/cm2power intensity.Specimens with the same thickness were tested under the same pre-load and same irradiation laser intensity.Laser irradiation was kept on until material completely failed.CO2CWlaser systemwas used to generate laser with 10.6μm wavelength.The failure time was defined as the time from the start of irradiation to the complete fracture of the specimens.Results showed that the laser power intensity at 0.202 kW/cm2,pre-stress ratio at 0.5 and thickness of specimen at 1.5 mm would cause complete fracture after 9.90 s irradiation.On the contrary,the failure time in the compression test is much shorter than that in the tension test.Additionally,the failure mechanism in tension test mainly included fibre drag break,while delamination and buckling were what caused failures in compression test.The failure time of the specimens with the same thickness will be shortened exponentially as laser power intensity increased.The relationship between the specimen failure time and the main parameters under tension load combined with laser irradiation and under compressive load combined with laser irradiation are expressed in Eq.(1)and Eq.(2)respectively.

wheretT,tcare the specimen failure time under tensile load and compressive load,respectively,his the thickness of composite laminated specimen,rσis the pre-stress ratio,Iis the laser power density.

Kibler et al.[61]conducted a series of experimental and analytical studies on the response of graphite-epoxy composites to continuous-wave CO2laser radiation.Tensile coupons of one graphite-epoxy system(Narmco 5208/T300)and one aluminium alloy(2024(T81))were exposed to laser irradiation.Three composite laminates,namely(0°/±45°)C,(/±45°/90°)2S,and(90°//±45°)2Swith 12-ply of nominal thickness of 0.073 mm were used.In the low power laser experiments,the laser irradiations were performed with a model 41 Coheren Radiation Laboratory laser with incident beam power of 200 W.A Mach 0.2 air flow was maintained parallel to the specimen surface.An iron-constantan thermocouple at various positions on the back surface of the specimen was used to monitor the temperature.In the high-power laser experiments,a GTE Sylvania model 971 with incident beam power of 750 W was used for the first series of exposures,and a Mach 0.1 air flow was maintained across the width of the specimen surface.The 10 Kilowatt‘flat-top’laser was used for the second series of exposures.It was determined that the aluminium was more susceptible than composites to damage at a high laser intensity,especially,both penetration time and strength retention were less for aluminium than for the composites.A strength retention in terms of fracture mechanics-based predictions was proposed for the partially penetrated and laser-damaged composites.Moreover,specimens loaded in tension and irradiated to failure were found to fracture at a slightly lower preload than the strength retention of specimens irradiated but unloaded.

Laser-induced damage on mechanically loaded laminates in unmanned aircraft was explored with a view to predict the thermal-mechanical response of the heated panel irradiated by laser[62].The laser damage was assessed on specimens under load.This was achieved by applying bending stress of 100 MPa on the specimens,which was produced by a loading device consisting of a load cell and target fixture that was used to fix the specimen.An evaluation was then undertaken to ascertain the thermalmechanical parameters of laser irradiated specimens.The investigation was carried out on glass fibre reinforced polymer(GFRP)and Carbon Fibre Reinforced Polymer(CFRP)specimens.The specimens were cut into thin rectangular sheets of 120 mm×45 mm.The test specimen was subjected to a radiation density of 1 kW/cm2.A carbon dioxide laser with an operating wavelength of 10.6μm was used.The damage of the specimen was studied by LYNX Dynoscope.At an irradiation of 1 kW/cm2,the CFRP samples had a reduction of 72%in bending strength recorded at a bending stress of 20 MPa.Besides,the tensile strength was reduced by 36%for the loaded samples subjected to the bending stress of 100 MPa whilst for GFRP specimens a 31%reduction was obtained.In comparison,the tensile strength reduction for unloaded samples were 22%and 14%respectively.The tensile strength of specimens was measured before and after laser irradiation by universal testing machine(UTM).Signi ficant damage occurred under mechanically loaded conditions resulting in resin sublimation.The extent of the damage was analysed by assessing the ablated mass of the specimen.It was noted that the ablated mass was more for all the specimens when the test samples were irradiated by laser under loaded conditions.High intense radiation induced signi ficant mass loss as a result of resin sublimation.

The experimental studies reviewed are summarized in Table 2,with the experimental studies on thermo-mechanical responses listed in Table 2a,experimental studies on ablation behaviour in Table 2b,experimental studies on interlaminar effects in Table 2c,and experimental studies on damage analysis in Table 2d.A summary of damage mode identi fied in the experimental studied is listed in Table 3 with those under single loading only listed in Table 3a and those under combined loading listed in Table 3b.

Table 2d A summary of experimental studies.Experimental studies on damage analysis.

Table 3a A summary of damages identi fied in the experimental studies.A summary of damage mode identi fied in the experimental studied under single loading.

Table 3b A summary of damages identi fied in the experimental studies.A summary of damage mode identi fied in the experimental studied under combined loading.

5.Numerical studies

5.1.Thermal mechanical response

The finite element analysis(FEA)was conducted for analysing transient heat conduction and thermal stress of laminated carboncarbon(C/C)composite in Ref.[24].A cylindrical model of 50 mm diameter and 12 mm height in MARC programwas used to simulate the experimental specimen which was irradiated at its upper surface by a laser pulse in the FEA.The material used is laminated C/C composite with 7.8 MPa shear strength,137.3 MPa compressive strength,and 606 MPa tensile strength.The isotropic and homogeneous material is modi fied for the laminated C/C composite materials assuming that the material is regarded to be equivalently homogeneous.Totally 975 elements with an arbitrary quadrilateral axis symmetric ring model was applied for the finite element analysis.The laser pulse was modelled with beam diameter of 20 mm,power density of 3.0 W/mm2,with constant distribution and 1-s duration.Simulation results demonstrated that the temperature rapidly decreased in the surface layer and did not change much below 4 mm depth.The critical power density Pcwas con firmed to be a reliable measure of thermal shock strength for the laminated C/C composite and was corresponding to the critical temperature difference in the traditional quenching test.The asymptotical value of Pcwas found to be around 2 W/mm2on the maximum negative shear stress and 2.8 W/mm2.

The ultrasonic wave generated by short pulse laser could be optically absorbed by composite material to cause thermal expansion,and FEA is very handy in dealing with thermoelastic problems owing to its adaptivity in modelling complex geometry and obtaining full field numerical solutions.Wang et al.[63]developed a finite element model of laser ultrasonic generation in thin transversely isotropic fibre-reinforced laminate composite to study this phenomenon,coupled with thermal conductive equation and thermo-elastic mechanism generation equation.Speci fically,they studied the structural response of a thin fibre-reinforced composite plate to normal-direction laser line irradiation.The transient temperature and temperature gradient field were calculated,and the laser-generated transient Lamb waves were obtained.The thermoelastically generated waves were calculated in a thin fibrereinforced composite plate(40-400μm thickness)which was meshed by elements by 20μm in length.The laser energy was 13.5 mJ and the pulse rise time was 10 ns,and the radius of the pulsed laser spot on the sample surface was 300μm.The material properties used in the calculation including thermal conductive coef ficient,density,thermal capacity,Young’s modulus,and thermal expansion coef ficient were pre-speci fied.The results obtained from numerical simulation clearly showed that the evolution of the dispersive waveform was a function of the target distance and the plate thickness.In addition,the velocity of Lamb wave propagating along the fibre direction was found faster than that normal to the fibre direction.The dispersive nature of Lamb waveform in two directions was found to be different.

In the numerical study conducted by Leplat et al.[26],a 2D axilsymmetrical approach which neglected edge effects was used.The test coupon(80 mm×80 mm)was discretised into a Cartesian regular mesh composed of 40×16 cells,and the exact same boundary conditions as in the experiment were introduced into the computation which took account the laser heating distribution and all surface exchanged with the test chamber.All physical properties of the material including thermal conductivity tensor,speci fic heat,density,porosity,permeability tensor,spectral emissivity/absorptivity,and model parameters(Arrhenius coef ficient for chemical reactions kinetics,heat of reactions)were assessed experimentally using ONERA facilities.Likewise,the space distribution of the heat flux density generated by the laser was assessed prior to the experiments.The thermal response obtained from the numerical simulation showed that heat transfer from the exposed face caused the temperature of the condensed phase grow to cause pyrolysis of the matrix into a considerable depth through the laminate.M21 matrix was found to decompose faster than the resin as temperature grew inside the laminate and offered preferential paths for cracks propagation within resin-rich regions between fibre tows.The numerical simulation involved heat and mass(decomposition gas)transfer model within homogenised porous media and was proved to agree well on thermal response of the laminate subjected to stead laser flux from experiments,although the delamination cracks interacting with heat diffusion and gas transport processes were neglected.The transformations of the material were found to induce the gas-phase creation at high temperature and high pressure because of temperature raised fast enough to active the thermo-chemical reactions on the exposed surface.

Tresansky et al.[64]presented a numerical heat transfer model to capture heat flow and material damage to polymer/carbon fibrereinforced composites subjected to laser beam irradiation.COMSOL Multiphysics? numerical modelling program which included packages for heat transfer,fluid dynamics,electro-optics,solid mechanics,electricity and magnetism was used to establish the computational heat transfer model.A 2.5 mm thick,20 ply carbon fibre/epoxy laminate was tested using Cytec 5215 prepreg with a 1K tow in plain-weave architecture.A 16 inch×16 inch panel was made by first debulking three sets of six plies under vacuum for 30 min before curing at 200°C for 3 h under vacuum.A Gaussianpro file,surface heat flux to simulate the laser,forced convection,and surface to ambient radiation was applied to the top surface of the selected axisymmetric model.A 1/4-inch-thick sheet of Acrylite GP was modelled as a surface absorber for a 1070 nm,6.12 mm 1/e2diameter Gaussian beam with 107 Wof total power.Material data related to the thermochemical characteristics experimentally determined was used by the model.A mesh was created with 40 elements in the radial direction and 180 in the through-thickness direction.A numerical technique of varying the material properties was employed to simulate material removal.The model was validated by the experiment.A 100-W-nomial power,Yb:YAG fibrecoupled laser produced by IPG Photonics that lases at 1070 nm was measured using a 5°silica wedge-beam splitter and a Spiricon SP620U pro filer with BeamGage software.Additionally,the CFRP laminate was tested for comparison with the theoretical model.It was found that the model was unable to predict the experimentally observed laser-drilled hole evolution.Also,the model predicted a higher recession rate than experimentally observed result.A recession rate of 3.06 mm/s in the Acryilite,which was 7.6%greater than the experimental results was obtained.

Boer et al.[65]modelled the thermo-mechanical responses of the CFRP laminates to continuous laser irradiation using finite element method in which a technique of coupling and decoupling the Degree of Freedom(DOF)was employed for interlaminar cracking simulation.A quarter CFRP laminate was modelled under laser irradiation.The laser beamwas modelled to be of radius 5 mm and perpendicularly irradiating the CFRP laminate,while the surface heat flux 6.37×106W/m2,10.19×106W/m2and 12.74×106W/m2were applied on areas where under direct irradiation in the three cases.Numerical results indicated that the increase in laser power would only increase the temperature magnitudes,and that the high displacement gradient occurred near the edge of the laser spot inducing large shear stress therein.Also,the maximum normal stress acted on the interface between adjacent laminae was higher than 20 MPa and the maximum shear stress higher than 25 MPa,which was around half the interlaminar strength of the CFRP laminates under room temperature[66].The progressive cracking was observed at the backward surface of the specimen under 800 Wlaser irradiation during the unloading stage and the interface cracks propagate as specimen cooled down.

In addition,a 3D thermal model(coded in MATLAB?)was used to interpret the laser-CFRP interaction resulting in heating and decomposition[27].The model was meshed by 30×30 uniform elements in the plane perpendicular to the incoming laser,with an element for each carbon fibre ply in the parallel direction.The changing material properties were assumed first to be a function of evolving CFRP decomposition phase,in proportion to the relative amounts of each decomposition phase present in each cell.The incident laser intensity distribution was measured throughout each experiment,combined into an average irradiance map and inputted into the thermal model.At the laser spot the temperature peaked to 634°C after 30 s at a laser irradiation of 9.9 W/cm2.Working outward the edges of the panel maintained an average temperature of 100°C.The overall specimen temperature distribution was radially symmetrical.Modelling CFRP without the endothermic reaction enthalpies or volatile heat capacity increased the final backside temperatures by 100°C and 90°C at 5 W/cm2and 10 W/cm2laser intensity,respectively.In all combustion cases,model temperatures during cool down were underestimated due to combustion being neglected in the heat transfer model.

5.2.Ablation behaviour

Temperature patterns of the CFRP being irradiated by continuous wave laser,long duration pulsed wave laser and short duration pulsed wave laser were analysed using finite element method in the study of Wu et al.[34]in which the specimens were modelled axis-symmetrically.An axis-symmetrical finite element model was set up to analyse the thermo-physical responses of the specimens(50 mm×50 mm×4.1 mm).The flow of the sublimation product was neglected.The power density for continuous wave laser,long pulsed laser(200 ns pulse duration),and short pulsed laser(10 ns pulse duration)was 3.54×106W/m2,1.50×1012W/m2,and 3.00×1013W/m2respectively.All lasers were designed to irradiate the square CFRP specimens for 10 s.The numerical computation was carried out with the minimum time step of 0.25 ns to allow one round of heating and cooling complete in each time step.It was found that the peak temperature remained stable under continuous laser irradiation,while pulsed wave laser irradiation would result in stable temperature after ultra-high temperature(over 25000 K)caused by first laser pulse evaporating the surface carbon fibre fabric.The numerical results showed that the laser power density threshold was around 3.0×1013W/m2to trigger optical breakdown hence making the irradiating ineffective.

A three-dimensional model taking the difference of ablation performance along different fibre orientations and the laminated structure of composites into consideration was established to study the ablation characteristics,temperature distribution,heat affected zone(HAZ),and the ablation morphologies of composite laminates impacted by laser[67].Governing equations for energy conservation,mass conservation and momentum conservation were presented in details.An air flow at 0.029 Mach was introduced in this study,erosion was neglected to simplify the problem so that surface ablation could be assumed to be governed mainly by the oxidation of material.As stated by Deng et al.[68],the oxidation was comparable with erosion of fibre only when flow velocity was higher than 0.5 Mach.The heat flux from the laser beam was modelled by a circular 2D Gaussian function,and the laser beam was modelled to normally hit the carbon fibre composites by 38 W power output with the laser radius of 1.25 mm.A quarter of the CFRP laminate of 20 mm×20 mm×0.56 mm was modelled considering the symmetry.The models were solved by the FEA and the deformed geometry of COMSOL was adopted to simulate the interface migration.The T300 carbon fibre and E-51 epoxy with initial porosity of 0.06 was chosen,and the volume fraction of fibre and resin was 0.64 and 0.3 respectively.Computational results indicated the direction of heat transfer within material changed by time owing to different fibre orientations in adjacent layers,which created a non-monotonic line of material surface temperature,and the shape of HAZ changed over time.The deviation of temperature along different axis was distinct within the laser spot and gradually decreased as ablation time elapsed.

A numerical study was further conducted[35]in which composite laminate(200 mm×200 mm×1.5 mm)was modelled using eight node bilinear secondary hot shell element in ABAQUS.Boundary conditions were exactly the same as those in the experimental study,and Simpson integration method with 3 integration points,coupled with external laser loading program coded by Fortan,was used to simulate the laser loading(power density of 50 W/cm2,laser beam radius of 10 mm,20 mm,and 30 mm).Elastic modulus and other thermophysical properties were according to literatures[69,70].Numerical results revealed the strain value decreased as further away from the centre of the laser spot,which aligned with the experimental findings.In addition,the maximum strain occurred at the 90°degree at the concentric circle of the laminate,which was at least 1.5 times of the value at 0°degree.Additionally,the increasing of laser spot radius was found to be capable of exerting a sudden shock upon the laminate,causing the strain at the same location to increase as well.The maximum deviation of 18.4%was identi fied in the 10 mm beam radius test,and the temperature rise effect in the experiments was 16.9%at most,which suggested the numerical model was reasonably reliable.

Chang et al.[71]further investigated the ablation temperature filed and recession rate of the carbon/epoxy composite laminate subject to laser irradiation with the creating of the finite element ablation model with variation in boundary conditions.They selected T300/epoxy resin laminate with 20 plies and 4 mm thickness.CO2continuous laser with an intensity of 1.528 kW/cm2and 3.82 kW/cm2was employed to irradiate specimens at a 10 mm diameter spot.The specimen was shaped in 5 cm×5 cm,and the SEMwas used to monitor the ablation.The ablation was assumed to be one-dimensional(only long the thickness of laminate)in the cases of high energy lasers irradiating composite laminate for short time.ABAQUS software was used to model the two-dimensional parts which was further used to calculate the ablation over thickness of the laminate.0.1 mm×4.0 mm rectangular shape was created in‘part’mode,and elements were discretised into 10×400 with 10μm DC2D4 elements.Using integrating sphere method and Lambda900 spectrograph,the laser energy absorbing coef ficient was measured to be 0.86.The thermal radiation coef ficient and thermal convection coef ficient was set to be 0.92 and 187 W/(m2·°C),respectively[72].A python program was designed to realize that thermal load affecting zone varying with time,and control the whole finite element analysis.Results showed the temperature gradient was relatively steep at locations near the laser spot,and the ablation has reached balance very soon.When laser irradiation intensity reaches 1.528 kW/cm2,the irradiated surface was ablated at a recession rate of 0.15 mm/s and pyrolysis thickness was around 0.08 mm.At laser irradiation intensity of 3.82 kW/cm2,the irradiated surface was ablated at a recession rate of 0.77 m/s and around 0.05 mm pyrolysis thickness.

5.3.Dynamic response

The degradation of material property,burn-out and induced thermal loading due to rapid laser heating will greatly in fluence the dynamic responses of the composite laminate.The thermal interlaminar forces and bending moments would be induced when composite laminate was heated nonuniformly and combined with other thermal effects it would pose great change in dynamic behaviour such as vibration amplitude of the composite laminate.Chen et al.[73]studied the transient response of laminated plates to combined mechanical load and laser irradiation.In their study,a moving organic-matrix composite plate suddenly subjected to uniform and highly intense laser irradiation was examined.Initially,a modi fied Crank-Nicholson finite difference scheme which incorporated the effects of surface ablation,degradation of thermophysical properties and heat loss through radiation and convection was employed to analyse temperature distribution and sublimation reaction.Then mechanical load was combined with thermal load which was evaluated by the temperature distribution.The burn-out geometry con figuration from previous step was used to conduct forced vibration analysis wherein both small and large de flection composite plate theories were adopted.The specimen was modelled as a 20-ply graphite/epoxy laminate with［±，90，03，±，90，03］2sequence and 30.48 cm×2.54 cm×0.25 cm plate.A nine-node isoparametric plate finite element was employed.Laser was irradiating at the top or bottom surface of the specimen,coupled with transverse loads.Both laser irradiation and transverse loads were step functions of time.0.25 kw/cm2,1.0 kw/cm2,and 2.5 kw/cm2were applied on the entire top surface of the specimen,which was simply supported at its shorter edges having in-plane motion suppressed.AMach 0.3 air flow was applied to pass over the plate surface.Computational results showed that the amplitude of the oscillation was governed by the magnitude and rise time of the induced thermal bending moments and plate bending rigidity.The most signi ficant oscillation occurred in the case of 1.0 kw/cm2laser irradiation.Also,with the irradiated surface different from the pressure surface,vibration was elevated.The change in the stiffness and mass density of composite laminate was also indicated,and the laser irradiation was found to increase the peak amplitudes of vibration in all cases.However,the dynamic response of composite laminates was in fluenced by laser intensity with the location of the plate surface being irradiated.Higher laser power ensured neither greater escalation nor reduction of the vibration.

Chen et al.[72]studied the effects of laser irradiation on strength reduction and on the dynamic and buckling characteristics of organic-matrix composite laminates.The heat-transfer process was modelled by a modi fied Crank-Nicholson finite difference scheme.In this study,a moving composite plate(20 ply 30.48 cm×2.54 cm×0.254 cm graphite/epoxy laminated composite)was assumed to be suddenly exposed to highly intensive laser irradiation(1.33 kW/cm2and 2.79 kW/cm2).A Mach 0.3 air flow was assumed to pass over the lamina.A nine-node isoparametric plate finite element was employed.Computational results showed that the higher the laser intensity,the faster the degradation and the ablation of the material property.It also showed that the worsening of material degradation and material removal did not result in monotonical decreasing of the natural frequency.The buckling load was found to have signi ficantly reduced as the laser exposure time increased.

5.4.Damage analysis

Complex chemical and physical processes including pyrolysis of matrix,formation and growth of pores and char,oxidation of residual char and carbon fibres,thermal expansion and contraction,matrix cracking,and delamination may occur when CFRP composite with epoxy matrix was exposed to thermal environment[39].Fig.3 shows section morphologies and surface topography of a 2 mm thick autoclave cured CFRP laminate from CCF-700 carbon fibre and BA9916-II resin matrix under infrared laser of wavelength 1064 nm,pulse duration 200ns and repetitive frequency 10 Hz for 6 s,8 s,and 10 s irradiation time by common optical microscope and 3D microscope.An Arrhenius-Equation based model with multi-step decomposition was employed to describe the mass-loss behaviour of CFRP composite under elevated temperature by Zhao et al.[56].Additionally,a mesoscopic to macroscopic model including thermal degradation of both matrix and fibres for obtaining the thermo-mechanical properties of the CFRP composite under heating,along with a progressive destruction program was employed for theoretical predictions.Specimens were predicted to fail in a progressive damage way at elevated temperatures,with the failure sequence of 90°plies,±45°plies and 0°plies.Young’s modulus and residual strength decays predicted by the analytical model was in well agreement with experiment observation.

Fig.3.Cross section morphologies and the surface topography of CFRP laminates bring drilled under pulsed infrared laser[39].

Zhang et al.[74]developed a parametric program using APDL language from ANSYS,based on three-dimensional Hashin principle to study the accumulative damage of laminated composites under laser erosion.Speci fically,the initiation,propagation and structural damage of the macroscopic failure after laser erosion on the composite laminates containing holes were simulated,yielding cloud charts of damages of different layers under inner static strain.T300/KH304[75]composite laminates containing hole with different diameter(6.21,7.97,6.19,6.15,6.22,and 6.21 mm)and ply angle were selected.The SOLID46 laminate element in ANSYS was chosen for discretization,and re finements were applied near the hole where stress concentration was likely to occur.For unpenetrated laser erosion,the part being irradiated drastically decreased in strength.Speci fically,at erosion depth of 1.24 mm and 0.62 mm,the failure load for composite laminate was calculated to be 38.39 kNand 50.36 kN,respectively,as compared to the failure load of 63.57 kN when no laser erosion exerted upon composite laminate.By analysing the damage propagation,it was found that the damage initiated from the ply with the largest angle and propagated to the ply with the smallest angle.The damages at the plies with larger angles were found to be mostly cracks,as opposed to the fractures at the plies with smaller angles.

Chang et al.[76]presented a numerical study investigating the progressive damage regulation of graphite fibre/epoxy laminate under combined effects of laser and mechanical loading.An improved bridging model based on the one-dimensional ablation model was adopted to calculate the tensile strength of the laminate under different laser power density and irradiation time.Representative volume element(RVE)was introduced into their study to calculate stiffness attenuation and laminate thermal stress.The AS/3501-6 composite laminate with[45°/-45°/90°/0°/0°/0°/0°/-45°/45°/0°/0°]Slayup sequence and 0.62 fibre volume fraction was selected.The specimen was modelled into 2.54 cm3cube.The progressive damage under laser irradiation(1.0 kW/cm2and 2.5 kW/cm2)for 0.5,1.0,and 5 s was calculated.Calculation showed that the maximum tensile load the laminate can withstand decreased as irradiation time prolonged,and the decreasing rate was associated with the laser irradiation intensity.At 1.0 kW/cm2intensity,the carbon fibre was not fractured thus remained to carry most of the loadings.However,at 2.5 kW/cm2,both fibres and resin matrices were burned out very quickly,followed by the failure of the main load-carrying plies(0°plies)after which the tensile strength plummeted.

Liu et al.[77]developed a multiscale based bridge model to predict the progressive damage of carbon epoxy composite under the combined laser and mechanical loading.Firstly,the thermal response of the composite plate was evaluated by means of an ablation model.The ablation process was divided into three forms,i.e.heat exchange,matrix decomposition,and carbon fibre sublimation,based on the different responses of the composite plate under laser irradiation.The model was employed to obtain the ablation temperature field and recession rate of the carbon/epoxy composite laminate.The numerical results were compared with the laser irradiation experiments of the composite material laminates,subjected to a high-power CO2laser with a power density of 1.528 kW/cm2and 3.82 kW/cm2respectively.The numerical and experimental results were found to be comparable.Moreover,the bridge model(multiscale bridge model)was revised based on the Ramberg Osgood model,using the polynomial strengthening model for reducing the input parameters of the matrix.Thus,the improved bridge model was found to be capable of dealing with existing thermal stresses in the thickness direction while predicting the progressive damage and the ultimate strength of composite plate.

The high intensity laser beam may incur damage to the composite structure by heat re-radiation even without penetration or material degradation at the target’s surface.From the military point of view,this is extremely useful as far as the laser lethality of UAVs and helicopters is concerned.In order to explore this possibility,an explicit uni-dimensional heat transfer model was developed at the Naval Research Laboratory to study the heat re-radiation from a carbon-fibre composite skin to the internal components[78].Although this model was a unidimensional model,i.e.it did not account for the changing material properties of the carbon fibre composite,it did provide useful insights to the researchers interested in exploring the heat re-radiation laser effects on composite materials.Mechanical analysis on composite failure revealed that the tensile strength occurred at the 9th lamina failure.At the 9th lamina,the fibre strength was 21000 MPa which revealed the composite’s ultimate tensile strength did not occur at the 0°lamina breakage moment.The temperature did not change the modulus of carbon fibre,but the extent of ablation was associated with composite strength.Under coupled laser,tensile load and composite strength were in fluenced by the fibre strength variation of 0°laminas.Fibre strength of lamina coordination larger than 1.8 mm was maintained at a lower stage(532 MPa).The fibre strength of laminas 1.0 mm-1.8 mm showed a gradual decline.At this point,the composite strength reduced by 50%although the fibre did not record any ablation activities.The phenomenon can be explained by the fact that under high temperature,the matrix decomposition was induced by laser irradiation.

Ablation was only reported to occur at the 1st lamina.In this case,the composite strength decreased because of the matrix decomposition.Liu et al.[77]further noted that the 1-4th laminas were ablation with a residual thickness of irradiation being observed in the region of 2.0 mm.After laser irradiation,only 25%of the composite strength remained.Two reasons can explain the observations:1)fibre ablation induced thickness loss of the composite,and 2)the temperature that increased through the composite thickness caused the degradation of the fibre strength.In summation,it was found that the tensile strength of the composite plate was controlled by the high strength fibres at the 0°lamina,while fibre ablation laminas and matrix decomposition hardly in fluenced the strength of the composite plate.The findings agreed with those from the previous literatures that carbon fibre ablation and epoxy matrix thermal decomposition were the primary damage modes for degrading carbon/epoxy composites under directed energy such as laser irradiation[72,73,75].Carbon fibre ablation and epoxy matrix thermal decomposition were the main damage modes on composite strength decrease.

Kujawinska et al.[79]conducted experimental study to investigate the interaction of a high power near-IR laser beam and composite material samples composed of sandwich structures of fibre-reinforced polymer,and to determine the symptoms and effects against UAV’s airframe shell by a laser beam.Specimens of carbon and glass fabric woven resin shells and carbon reinforced/epoxy laminate were irradiated by impulse laser beam Nd:YAG laser with 1.06μmwavelength.The specimen was illuminated with 10 impulses of 5J each.The laser had a maximum power of 20 kW.3D digital imaging projected the interaction of the specimens and laser beam as displacement maps and temperature maps.An infrared camera(FLIR SC 7500)near the back surface of the specimen took the maps.Two-100 W LED re flectors were used to enhance illumination on the specimen.DIC software(VIC 3D)was used to process the recorded images.The specimens were made of sandwich structures for typical UAVs which composed of two carbon and glass fibre woven fabrics face-sheets reinforced by a foamcore.The results obtained indicated that glass-reinforced sandwich required a smaller number of laser impulses to start degradation of the composite structure compared to the carbon-reinforced sandwich of the same thickness(5 mm).On the other hand,carbonreinforced sandwich needed approximately twice the number of laser impulses to disintegrate compared to the carbon reinforced laminate.Specimens reinforced with a foam-core had a better strength and resistance to the laser beam pulses.A decrease in the thickness of the foam-core,however,resulted in the reduction in strength of the composite structure.A specimen of a 1.5 mm thick core was destructed more severely compared to a sample with a 5 mm foam-core.

5.5.Other studies

Boley et al.[80]developed models of laser interactions with composite materials consisting of fibres embedded within a matrix.A detailed ray trace model that can calculate the absorptivity and energy enhancement of composite materials,starting with the optical parameters of the constituents and the material structure was established to serve as components of a comprehensive model of composite behaviour under the effects of laser radiation.The ray trace model was found to be sufficient for the description of carbon-based composites.The calculation of the absorptivity of the composite material for different frequencies of incident radiation and the energy enhancement was presented.It was concluded that the ray trace model would be more time consuming when applied to glass fibre-based composites.Additionally,their approach was able to calculate the energy deposition in the material which could be part of a predictive thermomechanical description of material behaviour under the effects of high-power laser radiation.

A summary of the numerical studies is given in Table 4 with the loading situation,materials and structured modelled,software used for the modelling and the typical failure mode for each research focus.

Table 4 A summary of numerical studies.

6.Summary

In this paper we presented a review of literature on damage to aircraft composite structures caused by directed energy systems.We firstly discussed the application of composite materials in the aircraft structures and the common types of directed energy systems and the mechanisms of their effect.We then presented a detailed literature review on the reported experimental and numerical studies of aircraft composite structures subject to the effect of energy systems.The general damage mechanisms of laser systems hitting aircraft composite structures,and the failure behaviour of composite materials under combined laser and mechanical loadings are particularly reported.

The experimental studies reported include the investigation on the thermos-mechanical response,the ablation behaviour caused due to laser energy dissipating in the plasma layer near the target surface,the interlaminar damage mechanism and other damage mechanism of various composites(CFRP,GFRP,etc).The effects from both sole laser loading and combined laser and mechanical loading are included.It was found that most of the reported experimental studies were mainly focused on CFRP laminate composites-.Laser systems are found generally effective against target of composites;however,complete destruction of the target will require extremely powerful output and for a considerably long duration.

Numerical studies including thermal-mechanical response,ablation behaviour,interlaminar effects and dynamic response of composite materials under laser irradiations are reviewed.These numerical studies further provide an insight into the damage mechanisms of laminate composites under laser irradiation.A variety of numerical methods have been employed to simulate and analyse the sort of problem,which paves the way of establishing generic models of composite interaction with energy systems.

Despite the fruitfulness of the current research,a few gaps for further research to fill are identi fied in this paper.Hitherto most of the experimental studies have been focused on CFRP.There is still a large room for further research on other types of composites including carbon-carbon composite,Kevlar-epoxy composite,GFRP and window glass composite responding to energy hitting.Especially,studies that compare the performance of different types of composite laminate to withstand consistent laser irradiation would be highly constructive.In addition,studies on the distinction of different types of energy system with identical power output impact on the same targets are highly recommended for future research to understand the effects of the different energy systems.For numerical analysis,there are also room to further investigate the numerical algorithm for better simulation of the contact and impact between the direct energy and the target.More accurate material models for the composites considering the high energy impact effects will need to be further developed.In addition,studies focusing on parallel contrast between the accuracy and ef ficiency of different numerical models on simulating laser effects on composites are also yet to be suf ficed.Finally a recommendation for further research is to conduct experimental and computational investigation on prediction of the residual strength of composite structures subject to laser damage for the purpose of structure integrity assessment.

Declaration of competing interest

The authors declare that there is no con flict of interest.

Acknowledgement

Funding support from Department of Defence,Australia is acknowledged.The authors acknowledge the advice and valuable suggestions on this research and paper writing from Dr.John Wang from Defence Science and Technology Group,Australia.