Khushi Rm ,Krtikey Krtikey ,Puneet Mhjn ,Nresh Bhtngr ,*

a Department of Mechanical Engineering, IIT Delhi, New Delhi,110016, India

b Department of Applied Mechanics, IIT Delhi, New Delhi,110016, India

Keywords: Shock tube Peak-over pressure Shock wave Blast mitigation Plastic deformation

ABSTRACT Shock tube experiments were carried out to investigate dynamic behavior of Ultra-high hardness(UHH)steel and Aluminium (Al) sheets of 0.8 mm thickness at 0.55,0.9 and 1.18 MPa peak-over pressure.Experimental results showed that center point deflection increases with an increase in peak-over pressure for Al sheets.However,UHH steel sheets showed negligible deformation when loaded at low peak-over pressures and showed sudden brittle failure at high peak-over pressures.Similar results were obtained by quasi-static testing,UHH steel failed abruptly while Al showed ductile behavior.Results from literature indicate that to protect structures against shock loading it is necessary that they dissipate energy via plastic deformation.The Al sheets were shown to deform plastically both in quasi-static and shock loading.Thus,hardness along with ductility is required to dissipate supersonic shock waves.

1.Introduction

The use of improvised explosive devices by anti-national elements nowadays has created a need for blast-resistant structures.It is essential to understand behavior of materials under shock and blast loading to fabricate effective blast-resistant structures.A wide variety of metals such as steels,Al,magnesium,and titanium sheets are being investigated to test their blast mitigation properties [1].Armox?steels that are used to protect from ballistic threats are also being used to provide protection against explosions [2].Also,metals of different thicknesses are widely used for the fabrication of fiber metal laminates and sandwich composites to protect structures from impact loading [3].

A blast mitigating material deform plastically in order to absorb blast energy and attenuate transmitted impulse[4].There are four different ways in which a blast mitigating material behaves to protect against blast waves [5].First is impedance mismatching,second is increasing the time duration of blast wave,third is arrangements of different layers of materials so that blast waves are deflected,and last is introducing disruption in the blast wave path.Armox? steels absorb more energy as compared to mild steel because they have superior mechanical properties i.e.,high yield and tensile strength [6].When a supersonic shock wave impacts mild and Armox? steel they fail,tensile mode of failure occurs in mild steel while brittle failure occurs for armox steel [7].

Thimmesh et al.[8]investigated the effect of geometry of door panel under blast loading using finite element modelling.Authors observed that irrespective of gemotery and cross-section thickness the maximum displacement observed at center of the panel.They also observed that rate of blast loading has no effect on maximum displacement.Wang and Shukla[9]expressed the energy of a shock wave generated via a shock tube in analytical form.They showed that a part of incident wave energy is used to deform specimen,less amount of energy is reflected back and rest converted into sound,heat and other forms.Vo et al.[10]examined the behavior of different grades of Al and fiber metal laminates against lowvelocity (450 m/s) blast waves.Results showed the behavior of different grades of Al against blast loading is dependent on the yield strength of the material with higher the yield strength resulting into lower center point deflection.Barik et al.[11]investigated the properties of AA 5052-H32 sheets before and after shock loading and they studied the forming behavior of pre-strained sheets using shock tube.Results showed that 1.5 mm sheets deformed less than 1 mm sheet due to higher rigidity of the sheet.The pre-strained sheets deformed more as compared to flat sheet due to indentation deformation superimpose before changing into flexural mode of deformation.Ray et al.[12]showed that there is no change in grain size and gemotery of AA 5086 grains before and after shock loading.However,the hardness of shock loaded sheets increased probably due to strain hardening.Kumar et al.[13]showed that there is an optimum curvature value for the different materials so they can sustain shock loading without catastrophically failure.As the curvature decreased the flexural deformation also decreased but indentation deformation increased.

Yahya et al.[14]manufactured carbon-fiber reinforced epoxy composites and observed buckling,fracture and delimitation of composites after blast loading.Theobald et al.[15]investigated airblast test (6-30 g explosive charge of PE4) on sandwich panels composed of steel face sheets (0.6 and 1 mm) with unbounded Al foam and hexagonal honeycomb cores.Core thickness of Al foam was 25 mm and honeycomb was 13 and 29 mm.Experimental results indicated that 1 mm face sheet showed increased in relative performance compared to monolithic plates.Hexagonal honeycomb showed better performance than Al foam and 29 mm honeycomb panels shows the highest relative performance of all the core materials.In foam brittle fracture is evident while in honeycomb cell separation and tearing are evident.Menkes and Opat[16]studied the deformation behavior of fully clamped thin sheets under uniform blast loading.Three types of failure response were observed i.e.,large inelastic deformation,tensile tearing and transverse shear failure.In most experiments,the metallic plates failed by large inelastic deformation,and few sheets show the tearing mode of failure.

For the fabrication of fiber metal laminates and sandwich laminates,it is required that face sheets possess out-of-plane stiffness and strength.However,studies on the deformation behavior of UHH steel against super-sonic shock waves are scarce.In the present work,the blast mitigation behavior of UHH steel and Al sheet is studied using in-house design and developed shock tube.The deformation behavior of thin sheets at various peak-over pressure is compared and analyzed for the fabrication of blast-resistant fiber metal laminates.

2.Method and materials

2.1. Materials

The UHH steel and the 5005-H14 Al sheets of dimensions 300 mm × 300 mm × 0.8 mm were procured locally.The UHH is special grade of steel fabricated by Star Wire(India)Limited for our research purpose and its equivalent to their commercial grade BP600 steel.

2.2. Hardness measurements

Hardness tests of UHH steel and Al sheets were carried out using a vickers hardness testing machine at 50 kgf and 5 kgf load,respectively.Small 20 mm ×20 mm specimens were cut out from the sheets using abrasive water jet machining for hardness test.The hardness test specimens were polished and cleaned with acetone.

2.3. Quasi-static tensile test

Tensile test specimens according to ASTM E8[17]were prepared using abrasive water jet cutting machine.Tension test was performed using Zwick Roll Z 50 kN for Al sheets and Zwick Roll 250 kN for UHH steel sheets at a crosshead speed of 1 mm/min.

2.4. In-house developed shock tube

Shock tube is a laboratory instrument which is used to generate super-sonic shock waves.Shock tube is a long cylindrical tube consisting of two sections,one is a high-pressure section known as the driver section and other is a low-pressure section known as the driven section as shown in Figs.1 and 2 also shows a photograph of the shock tube used for the study.The two sections of a shock tube are separated by a diaphragm.The pressure difference between the two sections results in rupturing of the diaphragm and the generation of a high-intensity shock wave.Fig.3 shows the punctured diaphragm sheets after the test.This high-intensity shock wave travels through the driven section and impacts the specimen mounted at the end of the tube.The shock tube used in the present study has an overall length of 10 m and an internal diameter of 100 mm.Driver section has 2 m length and driven section has 8 m length divided into three lengths using flanges.At the end of driven section a flange of stainless steel(SS304)having 50 mm thickness is welded on which specimen is mounted with the help of 10 mm mild steel ring and four C-clamps.The specimen is held between stainless steel flange and mild steel ring and clamped with C-clamp.A leak proof seal also used between stainless steel flange and specimen to eliminate the loss of energy and to make a perfect clamping.After test,deformation of specimen is measured using height gauge,dial gauge and surface plate.

Fig.2.In-house designed and developed shock tube.

Fig.3.Punctured diaphragm.

Compressed air is used as driver gas to develop the shock waves.A compressor having maximum capacity to compress air up to 3 MPa is used and this compressed air is stored in a cylinder.Compressed air from cylinder to driver section is filled by high pressure tubes and their flow is controlled using solenoid values.If the diaphragm not ruptured at a preset value,the evacuation of driver section is done with the help of an automatic system to avoid accidents.The experiments were performed at three driver section pressures of 0.8 ± 0.02 MPa,1.6 ± 0.02 MPa and 2.4 ± 0.02 MPa.Different driver pressures were achieved by varying thickness of diaphragm between driver and driven sections.The rupturing of diaphragm resulted in slight variations in the driver pressures and subsequently peak-over pressure.A piezo-electric pressure sensor mounted at the driver section is used to measure rupture pressure of the diaphragm.Three similar piezo-electric pressure sensors were mounted on the driven section.Two pressure sensors,2 m apart on driven section,are used to measure the shock strength.One sensor,at a distance of 100 mm from the specimen,is used to measure the peak-over pressure.Fig.4 shown the peak-over pressure shock wave loading curve at 0.55,0.9,and 1.18 MPa on specimen.

Fig.4.The peak -over pressure shock wave loading curve.

3.Results and discussions

3.1. Hardness testing results

Five indentations were performed to measure the hardness of each material.Average hardness value for UHH steel is 600 HV and for Al is 70 HV.

3.2. Tension test results

Blast mitigating materials require high strength along with high elongation to failure to sustain sudden impact loading[18].Tension test results showed that UHH steel has higher strength as compared to Al.The engineering stress-strain curve for Al and UHH steel sheets are shown in Figs.5 and 6.The ultimate tensile strength(UTS) of UHH steel is 2200 ± 5 MPa while its value for Al is 185 ± 5 MPa.The elongation observed in Al is 3.6% while in UHH steel its 3%.Fig.5 shows that Al showed ductile mode of failure and UHH steel sheet showed brittle mode of failure.Another important result observed from the tensile results that Al showed small amount of strain hardening phenomenon and this is not seen in UHH steel.The strength of UHH steel is 12 times more than Al.

3.3. Shock loading results

In the present study,UHH steel and Al sheets were fully clamped,and loading of the one-dimensional shock wave.After the shock loading,as observed by various researchers,the center point deflection for Al sheets increased with an increase in peak-over pressure due to their ability to absorb shock energy via plastic deformation.Table 1 shows the center point deflection observed against the peak-over pressure for the Al sheet.

Table 1 Shock loading results for Al Sheet with a thickness of 0.8-mm.

Fig.7 shows the cross-section of deformed Al sheets at 0.55,0.9,and 1.18 MPa peak-over pressure.Fig.8 shows the deformed Al specimen at 1.18 MPa peak-over pressure.As seen from the results,Al sheets deform in the form of dome of diameter 100 mm and steel sheet shows cracks and fractured into pieces.It is observed from the deformed sheets that one-dimensional shock wave uniformly spread over the 100 mm cross-section and due to plastic nature of Al it stretches out to form a dome shape.At the outer periphery of a circular thinning of sheet is observed.It can be speculated that if peak-over pressure further increased it will tear out from this zone.

Fig.7.Cross-section of deformed Al sheets at 0.55,0.9 and 1.18 MPa peak-over pressure.

Fig.8.Deformed Al sheet at 1.18 MPa peak-over pressure.

When the UHH steel is subjected to shock loading,the observed deformation behavior is different from previous studies on mild steel and other grades of steel [7].Initially,under low peak-over pressure,i.e.,0.55 MPa (The 0.55 MPa is peak-over pressure on specimen when driver pressure was 0.8±0.22 MPa),shock loading UHH steel resulted only vibration without any significant deformation as shown in Fig.9(a).The region where shock loading is done is marked by a circle in Fig.9(a).When the shock wave loading is at 0.9 MPa peak-over pressure,the UHH steel sheets showed small deformation and some minute cracks were also observed over the surface of the sheet as shown in Fig.9(b).At high-intensity shock loading of 1.18 MPa,the UHH steel exhibited brittle fracture as shown in Fig.9(c).Thus,the failure of UHH is steel under shock loading is due to propagation of cracks and resulting fracture as observed with other hard materials like ceramics.

Similar to the results observed by tensile tests,Al showed plastic deformation under shock loading while UHH steel showed brittle fracture.The specimen shown in Fig.9(c) broke into a number of pieces.If UHH steel is used for blast mitigation,during explosion the UHH steel sheet may break into a number of pieces.These pieces may attain high velocity and act as secondary splinters to cause further damage.Thus,during shock loading it is required that a material deforms and absorb shock energy but does not fracture to create splinters.

4.Conclusions

The following conclusions can be drawn from this study:

(1) To mitigate blast energy high hardness along with ductility is required so it can deform and absorb shock energy.

(2) To sustain blast loading without failure material is required to shows plastic deformation.

(3) No material should fails in such a manner that it can work as a splinter during explosion and acts as a secondary elements to create fatility.

(4) For the fabrication of fiber metal laminates UHH steel alongwith Al sheet can be used to mitigate shock energy in such type of laminates the advantage of high hardness material and ductilie material is incorporated.

(5) The UHH steel mitigates blast energy by impedence mismatching and low hardness Al mitigates blast energy by plastic deformation.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.