Zekan HE , Haijun XUAN ,*, Conger BAI, Manli SONG ,Zhuoshen ZHU

a College of Energy Engineering, Zhejiang University, Hangzhou 310027, China

b Collaborative Innovation Center for Advanced Aero-engine, Beijing 100083, China

KEYWORDS Containment equation;Fan blade;Inner metal ring;Kevlar fabric;Soft wall casing

Abstract Aramid fabrics have been commonly used in the civil turbofan engine fan blade containment system for its excellent performance. To investigate the behavior and capability of soft wall containment casing,a series of fan blade released tests were conducted on the high-speed spin tester.The soft wall casing was fabricated by wrapping multiple layers of Kevlar49 plain woven fabric around a thin steel ring.Casings with different inner metal ring and outer fabric layers number were compared.The method of using the explicit dynamic software LS-DYNA to establish the finite element analysis model for the quantitative analysis of the containment process was developed and conducted. The simulation results are in good agreement with the test results. It is shown that the containment process of the soft wall casing can be divided into three impact stages. The casing with low-stiffness inner metal ring will get severe overall deformation and lose the structural integrity when it suffers the blade impact.Kevlar fabric layers will appear large bulge on outside surface and absorb the most impact dynamic energy of the high speed released fan blade. By summing up the results of the test and simulation,an empirical critical equation was derived to describe the relationship between the released blade dynamic energy and the Kevlar fabric thickness.

1. Introduction

Aramid fabrics have been commonly used in the civil turbofan engine fan blade containment system. An alumi-num containment case protected by a composite fiber fabric wrap is a suitable alternative to the traditional heavy steel or titanium containment case.1In this fabric wrap structure, the high strength fabric plays a major barrier role. The released fan blade is arrested by the outer fabric after penetrating the inner thin metallic wall.The damage area is localized and the casing can maintain structural integrity.2This is called ‘‘soft wall”containment.3Benef iting from the excellent performance of the composite fiber and the innovative structural concepts,the ‘‘soft wall” containment system provides a lighter weight and a lower manufacture cost and has been used in lots of fan engines, such as Trent700, PW4084 and GE90.4

It is specif ied in the US Federal Aviation Regulation(FAR 33)5that a full engine Fan Blade Out(FBO)test must be conducted to demonstrate that the engine has the ability to contain a fan blade released at full operating speed. Such a FBO certif ication test is very time consuming and costly6and thus will not be carried out unless it is fully prepared. Ballistic impact tests which can be conducted on a ballistics facility and component containment tests which can be conducted on a spin rig are two effective test approaches to investigate the containment capability of the engine casing before the final full scale test. However, the data for the impact and containment test research of the fabric wraps structure is relatively rare in the public literature, especially in the recent three decades.

Bristow et al.7introduced the research of Boeing Company in 1970s on engine burst fragment impacts and the development of an engine burst containment system using Kevlar material.They tested over 20 different shield design conf iguration by using a ballistic impact facility,and accumulated much basic data for design criteria of such a system. Weaver8(Pratt& Whitney Company) presented the evaluation and development of using Kevlar fabric as a lightweight containment material to contain blades released from gas turbine rotors.They conducted ballistic impact evaluations, laboratory tests,spin pit tests and engine tests. A variety of application techniques was established for Kevlar containment applications.Stotler and Coppa9,10(GE Company) conducted a series of subscale ballistic-type tests and large scale tests in a rotating test rig to develop a light weight Kevlar containment system for turbofan engine fan blade. The relationship between the fabric thickness required and the energy of released blade were established.Salvino et al.11presented the tests conducted in an air-drive spin rig to determine design guidelines for turbine rotor fragment containment rings. Shockey et al.12performed a series of ballistic tests to develop lightweight barrier systems for turbine engine fragments. They conducted small-scale and full-scale tests to compare the effectiveness of different material type and fabric corner failure. Pereira and Revilock13carried out large numbers of ballistic impact tests by using a flat rectangle projectile to impact on an inclined fabric wraps ring.The purpose of their program was to provide validation data for the development of numerical models and compare the properties of two different fabrics. Liu et al.14carried out ballistic impact tests on wrapped multi-layer Kevlar49 woven fabric systems with a flat blade projectile to investigate the impact response of wrapped fabric during a fan blade out event. The influences of the numbers of Kevlar layers and pre-tension were analyzed and discussed.

In addition to the experimental investigation, many researchers made efforts to develop the modeling techniques and analytical methods of fabric. Scida et al.15developed an analytical model to predict 3D elastic and failure properties of several woven structures and fiber composite materials based on the classical thin laminate theory. Model prediction properties were compared with test data. Jiang et al.16developed a unit cell model with the assumption of that the stress and strain were uniform in a representative volume cell of the plain woven fabric. The global strain distribution was solved by using numerical method. The finite-element computation results were consistent with the experimental data.Duan et al.17modeled a single-ply plain-woven fabric under ballistic impact in the yarn level. The orthotropic material model was used. The friction between yarns, the friction between projectile and fabric and the fabric boundary conditions were discussed. Iannucci and Willows18developed an energy based damage model for woven carbon composites which is under high strain loading.Plain stress shell unit was used to represent the composite material and the evolution of damage is controlled via a series of damage-strain equations. Stahlecker et al.19developed a strain-rate dependent and anisotropic material model for dry fabrics. The model was implemented as a user-defined material subroutine in LS-DYNA. Then it was used to simulate a suite of ballistic tests. Zhao et al.20developed a numerical simulation method to estimate the ballistic resistance of Kevlar 49 fabric for gas turbine engine containment system. Material model based on representative unit cell and three-element viscoelastic constitutive equation was used in their method to describe the dynamic response of single-layer and multi-layer fabrics.

In this paper,in order to investigate the containment capability and behavior of the soft wall casing structure,a series of fan blade containment tests were conducted.The soft wall casing was fabricated by wrapping multiple layers of Kevlar49 plain woven fabric around a thin steel ring. The fan blades used in the tests were from a real engine. Some effective test techniques were developed,including the fabric wrapping technique, the heating blade release method, the trigger control and measurement,etc. Five tests with different structural conf igurations were conducted and recorded by a high-speed digital camera system. The impact response behavior of casings with different inner metal ring rigidity and outer fabric layers number were studied. Then the containment system was modeled using finite element method. The simulation results matched the test results well. The containment process and failure mechanism were analyzed and discussed. By summing up the results of the test and simulation in this paper and from the previous research,21a design equation for the soft wall containment criteria was established.

Fig. 1 Two types of inner steel ring.

Fig. 2 Plain woven Kevlar49 fabric.

Table 1 Main properties of test components.

2. Blade containment tests using high-speed spin tester

2.1. Soft wall containment casing

The soft wall containment casings have the equivalent inner diameter with a real engine casing and are simplif ied in structure. It consists of two parts with the inner thin steel ring in two different kinds of stiffness and the outer Kevlar fabric in different layers. One steel ring has a thickness of 0.8 mm and many holes are evenly distributed on it, as shown in Fig. 1(a).It is very weak in stiffness and strength with minimal shape support for outside Kevlar fabric. Another steel ring has a thickness of 2.5 mm and no hole is on it, as shown in Fig. 1(b). It has more stiffness and pro-vides a small amount of resistance to blade impact.Both rings have an inner diameter of 960 mm, a total height of 296 mm with f lange on two sides and material of Q235 steel in Chinese (Gr.D steel in US standard. The outer fabric is plain woven by Kevlar49 fiber with 7 yarns per centimeter both in warp and fill direction,as shown in Fig.2.Among the main properties of the test components,as shown in Table 1, the mass of inner metal ring is measured without two f lange edges,the axial and radial stiffness of inner metal ring is calculated by finite element method.A continuous 280 mm wide plain woven Kevlar49 fabric strip was wrapped around the metal ring utilizing a self-designed wrapping machine.21The fabric wrapping process and the soft wall casing after wrapping are shown in Fig.3.In the fabric wrapping process,the tension in fabric was kept at about 80 N to make sure that the fabric can be wrapped onto the casing tightly. Both sides in wide-direction of the Kevlar fabric were impregnated by the epoxy resin during wrapping. In the outmost layer,about a quarter circle on each side of the fabric cutting end was impregnated with resin.

2.2. Containment test plan

The fan blade containment tests are conducted on a high-speed spin tester. Fig. 4 shows a schematic diagram of the basic test structure and measurement system.The fan blade and disk are driven by a spin shaft which was powered eventually by a motor.A set of bearings are mounted around the upper section of the spin shaft to limit excessive vibration of rotor after the blade released.This limit bearing set protects the fan disk and shaft from a large unbalance load and only causes minor damage. The fan disk and shaft can be reused in the next test,which signif icantly improved test efficiency and reduced cost.

Fig. 3 Fabricating process of soft wall casing.

Fig. 4 Sketch of blade containment test.

The casing is installed under the tester cavity cover through a mount base. Trigger wire is glued on inside surface of casing.As the released blade cut off the trigger wire,a pulse signal will be sent to the control system to shut down the driver motor.Meanwhile the high-speed camera is triggered to record data.The test plan is listed in Table 2. The total mass of soft wall casing in Table 2 is calculated by the mass of inner metal ring plus the mass of outer fabric layers.

In order to control the blade release speed more precisely,an innovative method by locally heating the dovetail of blade is applied. A heating rod is inserted into the hole which is drilled along the dovetail of blade, as illustrated in Fig. 5(a).Utilizing the inverse relationship between the tensile strength of Ti-6Al-4V and temperature,the blade can be released under centrifugal pull load by being heated up locally.This method is described in detail in the patent.22The inner radius of the metal ring Riis 480 mm, the thickness of metal ring δiis 0.8 or 2.5 mm. The number of Kevlar fabric layers wrapped outside varies from 20 to 45, corresponding thickness of Kevlar fabric δois from 6.0 to 13.5 mm.

2.3. Test results

In each test,two blades were mounted on the disk with 180°of symmetry and released off successively within 3 ms.The blade released speed was controlled in 9200±10 r/min in the five tests,as listed in Table 3.Under the impact of the released blade,the inner steel ring was perforated and the outer fabric is greatly deformed in the impact local. The‘‘Maximum deformation of fabric”in Table 3 refers to the radial direction height of bulge appeared on the fabric outside surface. The ‘‘Residual kinetic energy of blade” in Table 3 was calculated by the ‘‘Residual velocity of blade” after it perforated the casing. The fabric deformation and blade residual velocity were measured by the motion image analysis software of the high-speed camera.

Fig. 5 Local magnif ication sketch and photograph of test structure before test.

Table 3 Test results summary.

From Test1 to Test3, the thickness of inner metal ring was 0.8 mm and the number of fabric layers increased from 30 to 45.In Test1,the blade perforated the thin steel ring and Kevlar fabric. The fiber in the impact area damaged severely with obvious fiber fracture, sliding, and pulling out, as shown in Fig. 6. In Test2 and Test3, the Kevlar fabric was not perforated and fiber damage only occurred in the inner fabric layers. However, in all these three tests, the inner metal ring and outer fabric ring deformed dramatically as a whole, as shown in Fig. 6. The casing did not maintain the structural integrity in the impact process as the inner steel ring was too thin to provide suff icient structural rigidity. The fabric wraps ring even translated in position or dropped from the badly damaged casing in these three tests. Thus, it is not reasonable to conclude that the casings in Test2 and Test3 are qualif ied containment structure, even though the fabric stopped the blade from flying out successfully.

In Test4, the thickness of inner metal ring was adjusted to 2.5 mm and 20 layers of Kevlar fabric were wrapped outside. The steel and fabric ring deformed and damaged in the blade impact area locally as two blades both perforated the casing and flew out. The thicker steel ring showed a much better structural rigidity. Two perforation holes occurred on the fabric after the fabric’s large bulge deformation and obvious fiber damage can be observed, as shown in Fig. 7. According to the results of motion image analysis, the casing absorbed about 85% of the initial impact kinetic energy of blade. The maximum size of the bugle deformation of fabric is about 1/6 of the casing diameter.

Fig. 6 Post-test photographs of casing in Test1.

Fig. 7 Post-test photographs of casing and blade in Test4.

Fig. 8 Post-test photographs of casing and blade in Test5.

In Test5, the thickness of inner metal ring was 2.5 mm and the number of fabric layers increased to 40. The first release blade perforated the casing with only a little kinetic energy left(1.6% of initial value), and the second release blade was arrested by the fabric with some parts embedded into fabric layers. It is considered to be a threshold containment state.The post-test photographs of casing and blade are shown in Fig.8.It can be seen that in the first release blade impact area,fiber damage also occurred in the outside of fabric, but the breach was much smaller than the one in Test4. In the second release blade impact area, the fabric bulged outward but the fiber in the outer-most layer remained intact. The two blades deformed similarly. Both of them fragmented into two parts at the shrouds.

3. Numerical simulation

Numerical simulation was carried out to obtain quantitative analysis results using the explicit dynamic software.The results of numerical simulation are in good consistency with the test results.

3.1. Material model

In order to simplify calculation, the plain woven fabric is represented by an equivalent orthotropic continuum,as shown in Fig. 9. Gasser’s biaxial tests and three-dimensional finite element simulations show that the Poisson’s ratio ν12, ν13and ν23of dry fabric are negligibly small.23Hence, the constitutive behavior can be defined as Eq. (1) with a decoupled stressstrain relationship. In this equation, direction 11 is defined as the fabric warp direction, direction 22 is defined as the fabric fill direction,and direction 33 refers to the direction perpendicular to both warp and fill directions.

‘‘Dry Fabric” (MAT_214) material model in LS-DYNA was selected. This material model was firstly established and proposed by Simmons et al.24for using in a numerical simulation model of Kevlar wraps ring ballistic test.13Then Rajan et al.25conducted a lot of material mechanical properties tests about Kevlar and Zylon fabric. Based on the material tests data, they improved the material model and proposed a more developed model for the high strength woven fabric used in propulsion engine containment system, body armor and personal protections. In order to obtain the material properties,some tension tests and picture frame shear tests were carried out,as shown in Figs.10 and 11.The test methods are referred from Naik26and Peng et al.27.Based on the test results showed in Figs. 10(b) and 11(b), the behavior of fabric tension and shear in this material model is divided into some regions by using piecewise-linear approximation. The fabric tension behavior is divided into four regions: the crimp region, linear pre-peak region, linear post-peak region and nonlinear postpeak region. In the initial crimp region, the yarns in fabric are straightened with a small increase in tension load. Then in the linear pre-peak region,the tension load rapidly increases linearly to the peak value.After the peak point,the fiber begins to damage and the tension load decreases quickly to the initiation of the nonlinear post-peak region. The relationship of the stress and strain in the nonlinear region is defined by an equation which contains an exponential function.19The stress and strain of initial point of nonlinear region is defined as(σ*11,ε*11)in Fig.10(b).The fabric shear behavior in the picture frame shear test can be divided into three linear regions.

Fig. 9 Modeling fabric as a continuum.

Fig. 10 Tension test photo and results.

Fig. 11 Picture frame shear test photo and results.

The shear modulus increases as the shear strain increases. In the initial fabric shear deformation region,the shear resistance is very low as it is mainly caused by the friction between the yarns when the yarns rotate.Then as the shear strain increases,the resistance continually increases by the reorientation and packing of yarns. Finally, when the shear angle becomes very large, the edge of fabric begins to wrinkle and the shear modulus becomes much larger.

The unloading/reloading behavior, compression behavior and the strain-rate effects of the fabric were referred from Stahlecker et al.19and Cowper and Symonds.28A modified form of Cowper-Symonds (CS) model was used to account for the strain rate effect of the fabric as follows:

where σmaxis the static peak stress, σmax(adj)is the adjusted peak stress due to strain-rate effects, ˙εis the strain rate, C and P are control parameters.It means that there is an increase in the peak stress with an increase in strain rate. The main parameters of Kevlar fabric for MAT 214 are listed in Table 4.

Johnson-Cook (J-C) model is selected to be the material model of the inner steel ring and TC4 fan blade. The Johnson-Cook constitutive relation29can be expressed as follows:

where A,B,C,n and m are material constants;σeis the equivalent von Mises stress;is the equivalent plastic strain; ε˙*is a dimensionless strain rate; T*is the homologous temperature.The Johnson-Cook fracture criterion30is based on damage evolution and can be described as:

where Δεpeis the increment of equivalent plastic strain that occurs during an integration cycle and εfis the fracture strain.Failure is assumed to occur by element erosion when D equals unity.

The material model parameters of the inner Q235 steel ring are referred from Guo31and Lin et al.32The material model parameters of the TC4 fan blade are referred from Lesuer33and Kay.34The parameters used in J-C model are listed in Table 5.

3.2. Finite element model

Considering that the 0.8 mm inner metal ring can only provide suff icient structural stiffness in static condition. In the blade impact process, the 0.8 mm inner metal ring broke seriously quickly. Its stiffness and strength is negligible in the impact dynamic condition. Thus the inner metal ring is ignored in the numerical model of Test1-Test3 due to its extremely thin thickness. However, all parts are included in the numerical model for Test4 and Test5, and the boundary condition isset on the metal ring and fabric parts.The finite element models of the fan blade,inner metal ring and outer fabric wraps are shown in Fig.12.The fan blade is modeled by hexahedral solid elements,with a basic mesh size of 1 mm.The inner metal ring is also modeled by hexahedral solid elements, with a basic mesh size of 4 mm. The fabric is modeled by quadrangle shell elements, with a basic mesh size of 4 mm. The mesh size conf iguration is decided by considering both the simulation accuracy and efficiency. All the fabric layers are modeled by one layer of shell elements. The element type is Belytschko-Lin-Tsay shell element, which has 5 integration point in each element through its thickness. The thickness of shell element is set as half of the thickness of fabric wraps, as the voids in the fiber woven structure should be removed when the woven structure is modeled as continuum. This one layer shell element conf iguration improves the computation efficiency significantly,while the simulation accuracy can also be satisfied.The nodes in the top and bottom surface of metal ring are constrained in displacement in all directions. The nodes at both ends of the fabric in width direction are constrained in displacement in the radial and circumferential directions. The constraint conditions in the model are determined based on the actual installation conditions and the adhesive fixed conditions.The initial velocity of blade is set the same as the bladeoff speed. The contact cards between different parts in the model all use the ‘‘Eroding Surface to Surface” contact type in LS-DYNA.The stiffness of the fabric part is quite different from the metal part,so the contact stiffness algorithm which is based on the soft constraint equation is selected.

Table 4 Material parameters of Kevlar fabric for MAT 214

Table 5 J-C constitutive relation and fracture criterion constant of TC4 and Q235.

3.3. Comparison between test and simulation results

Fig. 12 Finite element model of casing and blade.

Fig. 13 High-speed camera recordings and numerical simulation predictions of Test1.

Fig. 14 High-speed camera recordings and numerical simulation predictions of Test5.

The impact process recorded by the high-speed camera shows a good consistency with the numerical simulation predictions,as shown in Figs.13 and 14.Fig.13 shows the impact process in Test1, the blade eventually perforated the casing with small residual kinetic energy both in the camera recordings and simulation predictions. The inner metal ring and outer fabric wraps ring both deformed badly and lost the structural integrity.Fig.14 shows the impact process in Test5,the interaction process between the blade and casing was similar to the one in Test1.But the deformation of casing was not so serious as the one in Test1.The damage area was localized.Fig.15 shows the deformation and damage of the casing and blade in Test5 and corresponding simulation results. The inner metal ring was penetrated easily and a rectangular breach was formed.A large bulge deformation appeared on the outside surface of fabric and eventually the blade perforated out with very small residual kinetic energy.The blade fragmented into two major parts at the shrouds. It can be seen that the numerical simulation matches the test results very well.

4. Containment analysis

4.1. Containment process

Fig.16 shows the time history of the energy and the interaction force between the blade and casing in Test1 and Test5 as two typical tests with different inner steel supporter and different results.

Fig. 15 Deformation and damage of casing and blade in Test5 (experimental versus numerical).

Fig. 16 Time history curve of blade kinetic energy and interaction force between blade and casing in Test1 and Test5.

In Test1, the blade perforated the casing and flew out finally.Combining Figs.13 and 16(a),the containment process in Test1 can be divided into three stages. In the first stage, if the moment of blade releasing was assumed as the zero time,the blade tip began to impact casing at the time about 0.3 ms.In this stage,the blade bent slightly in the tip part and the interaction force between the blade and casing increased to the first peak value of about 56 k N at about 0.5 ms. Then in the second stage, the fan blade bent at the shrouds and the main part of blade continued to impacted outwards. At the time about 1.5 ms, the interaction force came to the second peak value of about 218 k N.The inner metal ring almost made no contribution to resist the blade impact and was damaged very quickly and easily. A bulge deformation occurred on the outside surface of Kevlar fabric.The bulge becomes larger while the blade keeps on moving and deforming. When the bulge reached the largest size, some yarns in the impact area could not withstand the load (mainly tensile load) and began to break. Then in the third stage, the deformation and breakage of fabric were more severe.The blade penetrated the casing and flew out. The interaction force came to the third value of about 75 k N.

In Test5, the result was considered to be a threshold containment state.Combining Figs.14 and 16(b),the containment process in Test5 can be analyzed and also can be divided into three stages. The deformation and damage process of blade and fabric were the same as the one in Test1. An obvious difference is that the interaction force peak value in each stage was higher than the one in Test1.The force peak value reached 90 kN, 275 k N, 122 kN in the three stages successively. It means that the casing in Test5 has a much stiffer structure.

Both in Test1 and Test5,the third stage took up most of the time as the fabric could withstand large deformation.The second stage occupied the second long time.The decrease of blade kinetic energy in these two tests is similar. Most of the blade kinetic energy was absorbed in the second stage (about 75%of total decrease). Very little energy was absorbed in the first and third stage (about 10% of total decrease in each stage).The blade impact energy was mainly absorbed by the deformation and failure of fabric, about 85% in Test1 and 67% in Test5. In Test1, the energy absorbed by the inner metal ring was ignored. The energy absorbed by the blade accounts for about 15% of total blade kinetic energy decrease, as shown in Fig. 16(a). In Test5, the energy absorbed by the blade and metal ring accounts for about 20%and 13%of the total blade kinetic energy decrease respectively, as shown in Fig. 16(b).

4.2. Effect of inner metal ring

The inner metal ring is an indispensable part as the shaper supporter for the soft wall casing.In Test1 to Test3,the metal ring has extremely poor stiffness with thickness of 0.8 mm and many holes evenly distributed on it. In Test4 and Test5, the inner metal ring is much stiffer than the one in Test1 to Test3 with a thickness of 2.5 mm. In the term of reducing weight of the structure, the inner metal ring should be as light as possible, which means lightweight mate-rial and small wall thickness should be used and designed. However, the soft wall casing with too thin inner metal ring thickness cannot meet the containment requirements, even though the fabric wraps has a suff icient thickness to resist the blade impact.For example, in Test2 and Test3, the blade was stopped in the casing and there is no fiber damage in the outermost layer of fabric wraps.But the casing was badly overall deformed and lost the structure integrity,as shown in Fig.17.In this f igure,the outer contour line of casing (fabric wraps ring) before and after deformation in Test1 and Test4 are delineated based on the test photos. It can be seen that the casing which has a thicker inner metal ring in Test4 deformed locally in the blade impact area. In contrast, the casing in Test1 deformed integrally very seriously.When it was impacted by blade,Cracks appeared on the inner metal ring and propagated circumferentially quickly.The inner metal ring even broke into two parts finally, which caused the fabric wraps ring to lose its supporter and dropped down in the tester cavity in the later stage of containment process. Even though the blade did not penetrate out, the casing structure made no sense in term of blade containment.

4.3. Effect of outer Kevlar fabric

Fig. 18(a) shows the total absorption of blade kinetic energy and the energy absorbed by Kevlar fabric in each test. The Kevlar fabric played a major role in absorbing the blade impact energy through its large deformation.The blade kinetic energy is absorbed about 80% in Test1 to Test3 and about 65% in Test4 and Test5 by the outer Kevlar fabric through the stretching deformation and failure of yarns and fibers.The bulge on the outside of the Kevlar fabric will grow larger as the blade continues impacting and sliding.If the bulge is not rupture when it grows to maximum, the released blade is contained. Otherwise the blade will perforate the casing and f ly out. Fig. 18(b) shows the outer diameter of casing before deformation and the maximum bulge height size in the outside surface of fabric when the bulge appeared and grew on the fabric out surface during the impact process in each test.The maximum bulge height is defined as the radial distance from the highest point of bulge to the original outer surface of fabric wraps and measured by utilizing the motion image analysis software of the high-speed camera.It can be seen that the fabric wraps in the five tests had a similar amount in bulge deformation. The maximum size of the bugle deformation is about 1/6 of the fabric ring original diameter.

Fig. 17 Outer contour line of casing before and after test.

4.4. Empirical equation for containment criteria

In order to understand the critical Kevlar fabric thickness required to resist the released blade with different kinetic energy, a design curve is plotted as shown in Fig. 19. In this f igure, when the fabric thickness and the released blade initial kinetic energy fall into the top left corner area, the casing can contain the blade.On the contrary,when the conditions are in the bottom right area, the casing will not contain the blade.The test results data of the small-scale containment tests in the previous research phase19 and the large-scale containment tests in current research phase are all displayed in Fig.19.The test data conforms to the division of the contained area and uncontained area. Based on the simulation and test data, the relationship between the required Kevlar fabric thickness and blade impact energy can be expressed as:

Fig. 19 Relationship between required Kevlar fabric thickness and blade impact energy.

Fig. 18 Comparison of blade energy absorption and bulge deformation of fabric in five tests.

where δ(mm)is Kevlar fabric thickness required to absorb the specific impact kinetic energy; Ek(J) is blade impact kinetic energy.

5. Conclusions

The containment capability of soft wall casing wrapped with Kevlar fabric was investigated by means of experimental and numerical simulation comparison. The effect of stiffness of the inner metal support case and the relationship between the fabric thickness and the blade impact energy were discussed. Some conclusions were drawn as follows:

(1) The containment process of soft wall casing can be divided into three stages. In the first stage, the blade tip begins to impact casing and the interaction force between the blade and casing increases to the first peak value. In the second stage, the fan blade bends at the shrouds and the main part of blade continue to impact outwards. A bulge deformation occurs on the outside surface of Kevlar fabric under the blade impact. The interaction force comes to the second peak value. In the third stage,if the yarns in the outmost layer of fabric can withstand the impact load (mainly tensile load) and no fiber fracture occurs,the blade can be contained successfully.On the contrary,if the fiber in outmost layer of fabric starts to break,the blade will perforate the casing and f lies out. The interaction force comes to the third peak value and shows some fluctuation.

(2) The inner metal ring is an indispensable stiff supporter of the soft wall casing. If the inner metal ring is too weak, the casing will cause structure failure even if the fabric wraps has a suff icient thickness to stop the released blade inside. The Kevlar fabric played a major role in absorbing the blade impact energy through its large deformation. A bulge deformation will appear on the outside of the fabric wraps when the blade impacts on casing. In the fan blade containment of large scale casing which is used in the current article,the maximum size of the bugle deformation is about 1/6 of the fabric ring original diameter. Such a large deformation of fabric requires that enough space should be reserved on the outside of soft wall casing to prevent the fabric deformation inf luencing other structure.

(3) An empirical equation and critical design curve is derived to describe the relationship between the blade impact kinetic energy and the required Kevlar fabric thickness.It can be used to help in the design of soft wall casings for aero-engine designer.

Acknowledgement

This project was supported by the Chinese Aviation Propulsion Technology Development Program(No.APTD-1103-07).