Failure investigation on high velocity impact deformation of boron carbide(B4C)reinforced fiber metal laminates of titanium/glass fiber reinforced polymer

S.Suresh Kumar,P.A.Shankar,K.Lalith Kumar

Department of Mechanical Engineering,Sri Sivasubramaniya Nadar College of Engineering,Kalavakkam,Chennai,603 110,India

Keywords:Ti/GFRP FML Ballistic impact Armour piercing projectile Boron carbide Depth of penetration Plugging

ABSTRACT High velocity ballistic impact deformation behaviour of Titanium/GFRP Fiber Metal Laminates(FML) has been explored.Both single and multiple projectiles impact conditions were considered.Ti/GFRP FML targets were fabricated with addition of 5% and 10% weight percentage of boron carbide (B4C) particles.Mechanical properties of Ti/GFRP FML targets were determined as per ASTM standards.High velocity ballistic experiments were conducted using Armour Piercing Projectile (APP) of diameter 7.62 mm and velocity ranging between 350 and 450 m/s.Depth of penetration of the projectile into the target was measured.The deformation behaviour of Ti/GFRP targets with and without the presence of ceramic powder (B4C) was investigated.“Ductile hole growth”failure mode was observed for pure GFRP target when subjected to single projectile impact whereas“plugging”failure mode was noted for Ti/GFRP targets.The presence of B4C(5%by weight)particles has significantly improved the ballistic resistance of the Ti/GFRP FML target by offering frictional resistance to the projectile penetration.Further addition(10% by weight) of B4C has reduced the ballistic performance due to agglomeration.None of the targets showed ‘brittle cracking’ or ‘fragmentation’ failures.When compared to the published results of Aluminium (Al 1100/GFRP and Al 6061/GFRP)FMLs,Ti/GFRP FML showed lesser DoP which increases its potential application to aerospace industry.

1.Introduction

Fiber metal laminates (FML) comprises alternate layers of metals and fiber reinforced composites (Fig.1) and fitted to the group of hybrid composites.FMLs have many advantages such as,sustainability,durability when subjected to fatigue load,better corrosion and impact resistance compared to fiber reinforced composites [1].Commercially available FMLs are ARALL (Aramid Reinforced Aluminium Laminate),GLARE (Glass Reinforced Aluminium Laminate),CARALL (Carbon Reinforced Aluminium Laminate) which are based on Aramid fibers,high strength glass fibers and carbon fibers.Generally FMLs are manufactured with metal layers of Aluminium,Magnesium,and Titanium with fiber layers of glass,carbon,Aramid or Kevlar fibers based on the applications.These FMLs are widely used in aircraft skins,wings and tails.ARALL FMLs are used in cargo doors of the C-17 military aircraft and GLARE s used in the upper fuselage of the Airbus 380[2].Titanium based FMLs are generally used at regions of aircraft where high temperature and high velocity impact load needs to be tolerated.Fig.2 shows the applications of FMLs at different regions of an aircraft [3].

Amid several types of damages in an aircraft such as fatigue,stress corrosion and bird impact,major proportion of failures were stated to impact damage.Thus it is very important to understand the behaviour of FMLs when subjected to low-velocity and high velocity impact loading.The strengths of the FMLs depend on several parameters,such as layup configurations,the properties of the constituent materials and the fiber orientation [4].

Fig.1.Fiber metal laminates.

For both domestic and military aircrafts,impact damage is frequently occurs at regions such as,nearby the doors,on the nose of the aircraft,at the cargo compartments and at the tail.In addition,it may also happens by runway debris,hail,maintenance GLARE,ARALL and CARALL FMLs [9].Chen et al.[10] investigated the inter laminar failure behaviour of FMLs using short beam,cantilever beam and fiber bundle composite beam methods.Wu et al.[11]developed the analytical model and numerical simulation to predict the nonlinear deformation behaviour of GLARE 4 and GLARE 5 FMLs.Maximum strain criterion was used to determine the failure of GFRP layers.The authors noted a deviation between analytical model and experimental results at high strain rates.Vlot[12]analyzed the failure behaviour of monolithic Al 2024-T3,7075-T6,ARALL and CARE FMLs using low and high velocity impact tests.Smaller damage region was observed for FML targets compared to fiber reinforced composite materials.damage,and dropped tool,collisions with service cars or cargo,bird strike,ice from propellers striking the fuselage,engine debris,tyre rupture and ballistic impact[5].Vogelsang and Vlot[6]mentioned that,among the repair work of 71 Boeing 747 fuselages,57.6% of them required repairs due to fatigue cracks,29.4%for corrosion and 13.0% for impact damage.

Fig.2.Applications of FMLs in aircraft structures (Ferreira et al.[3]).

In the field of defense sector,FMLs were developed due to increased requirement of fracture toughness to withstand low and high velocity impacts.Generally FMLs have better impact resistance compared to composite&monolithic targets when subjected to ballistic impact.The recent development of FMLs show 90% of airframe structures are fabricated by FMLs and caused major weight reduction.It is expected that lesser weight of military aircrafts can carry more weapons and increases its range at the combat environment [7].When FMLs are subjected to impact loading,Compston et al.[8] observed specific damage modes such as,fiber breakage,matrix cracking,delamination of the composite layers,plastic deformation,buckling in the metal layers and debonding between the different layers.

Even though many researchers have studied the static and dynamic behaviour of targets subjected to low and high velocity impacts,primarily they have reviewed the mechanical properties of Wang et al.[13] stated that,impact loading of FML can cause degradation and damages such as,tensile fracture,shear failure,delamination and rupture of the fibers.Sohn and Hu[14]measured the dynamic delamination toughness of targets under impact and high strain rate conditions.Heimbs et al.[15]indicated that,matrix cracking and delamination are the prime damage modes or patterns for low-velocity impacts,whereas“fiber rupture”happens during high-velocity impacts.Findik and Camci[16]suggested that ballistic experiments can be divided into three categories such as low velocity ballistic (V＜50 m/s;V-inlet velocity of the projectile) medium velocity ballistic (50 ＜V＜1300 m/s) and high velocity ballistics (V＞1300 m/s).Vlot et al.[17] determined the minimum energy required to cause initial failures on Aluminium 2024-T3,GLARE-3 and GLARE-4 targets using drop weight tester and gas gun set up.It is noted that,GLARE showed better impact resistance compared to Aluminium 2024-T3 target due to strengthening of glass fiber at higher strain rates.Kaboglu et al.[18]investigated the influence of yield strength,ductility of Aluminium layer,surface treatments and number of layers on flexural and impact resistance.It is noted that,increasing the strength of Aluminium alloy improves the adhesion between fiber and metal.Zhang et al.[19]investigated the damage mechanism of Aluminium alloy 2024 FML subjected to oblique impact conditions using nonlinear finite element model.Deflection of the projectile was observed during the oblique impact and the deflection angle increases with increasing the impact velocity.

Published [12-14,17-19] ballistic performance of FML targets are limited to Aluminium alloys based FMLs and they are limited to low to medium temperature applications.In order to withstand high temperature and high velocity ballistic impacts,Titanium based FMLs are important.Table 1 shows the application of titanium material in aircraft structure design.The aim of this work is to investigate the deformation behaviour of Ti/GFRP FMLs which is used for high temperature aerospace applications.

Titanium FMLs are mainly used in supersonic aircrafts in order to withstand severe environmental conditions and operating temperatures as high as 177C.Nakatani et al.[2] studied the damage caused by low velocity impact of Ti/GFRP FMLs and reported that the inter-laminar delamination occurs mainly due to crack initiation in the titanium layer of non-impacted side.Many researchers [21-24] have determined the influencing parameters such as,sample thickness,surfacing techniques,effect of fiber type and metal volume fraction of ballistic limit of Ti/CFRP FMLs.Chai et al.[21] determined the structural response of titanium fiber metal laminates(TFML)subjected to high velocity ballistic impact.Significant increase of ballistic limit was observed with increase of target thickness.Failure modes such as“Plugging”and“Petaling”were observed at the entry and exit regions of the projectile.The influence of surfacing techniques such as annealing,sand blasting and anodizing on ballistic performance of Ti/CFRP/Ti laminates was revealed by Li et al.[22].Both,sand blasting and anodizing treatments showed enhanced the metal contact interface.

Li et al.[23]investigated the effect of fiber type(CFRP and hotpressed ultra-high-molecular-weight polyethylene fiber) on the failure of titanium based FMLs.Target with Dyneema fiber showed higher ballistic limit compared to CFRP and the rolling direction of titanium plays an important role in impact energy absorption.Low velocity impact experiments conducted by Jakubczak et al.[24]indicates that,the effect of metal volume fraction on maximum force,total contact time and damage range was observed to be marginal.The authors considered target thickness of 2.5 mm.Sundaram et al.[25] investigated the effect of target velocity on ballistic performance of Al6061/GFRP FMLs.As the target velocity increases,deflection of the projectile was observed.In addition,“Plugging”failure mode was observed at the front face of the target.

Significant number of literatures have been reported to determine the low and high velocity impact behaviour of FMLs(Aluminium alloy 2024-T3,GLARE-3,GLARE-4 and Ti/CFRP) using numerical and experimental techniques.GFRP laminate was considered in the present work due to its strengthening behaviour during high velocity impact[17].The following gaps were observed from the summary of literature review.

i.There are no reports in open literature on the ballistic behaviour of GLARE or Aluminium based FMLs subjected to multiple projectile impact.

Table 1 Application of titanium alloys in aircraft structures(Inagaki et al.[20]).

ii.There is ample scope to investigate the ballistic impact failure caused by single and multiple projectile impact on Ti/GFRP FML targets.

iii.Not much work has been reported on the effect of boron carbide (BC) addition on high velocity ballistic impact failure of Ti/GFRP targets.

iv.There no reports in the open literature to investigate the inter-laminar failure behaviour of Ti/GFRP FMLs subjected to high velocity projectile impact.

In the field of defense aviation,airframe maintenance cost can be reduced to a greater extent by selecting materials with good fatigue strength,resistance to crack propagation,high fracture toughness,and corrosion resistance.Ti/GFRP FMLs are predominantly used in regions where high temperature and high fracture toughness are required.Hence it is mandatory to study the interlaminar failure behaviour of titanium based FML targets subjected to high velocity projectile impact.The prime objectives of the present paper are.

1.To investigate the ballistic failure mechanism of Ti/GFRP FML targets subjected to high velocity single projectile impact(7.62 mm Armour piercing projectile)

2.Extension of objective-1 to Ti/GFRP FML targets subjected to multiple projectiles impact and to explore the nature of failure mechanism at the front and rear side of the target.

3.To understand the influence of ceramic powder (BC) addition(5% and 10% by weight) on ballistic impact failure of Ti/GFRP FML targets.

4.To compare the inter-laminar failure behaviour of Ti/GFRP FML caused by projectile impact with published results[25,26]of Al 1100/GFRP and Al 6061/GFRP FMLs.

In the present work,in order to investigate the high velocity ballistic impact deformation behaviour of Titanium/GFRP Fiber Metal Laminates samples were fabricated using hand lay-up technique.Mechanical properties were determined by conducting experiments as per ASTM standard.High velocity ballistic experiments were conducted using single and multiple projectile impacts.

2.Preparation of Titanium fiber metal laminates (Ti/GFRP+B4C FMLs)

Titanium based FMLs were fabricated by using ‘hand lay-up’technique.Samples were prepared with 8 mm thickness comprising three layers of metal sheets(dimension:120×120×8)of 1 mm thickness and 2 layers of glass fiber (dimension:120×120×8).A release gel was sprayed on the mold surface to avoid sticking of polymer to the surface.Epoxy resin (Araldite LY556)and hardener(HY917)in the ratio of 10:1 was prepared and applied in between the metal and composite layers using a brush(Fig.3).The layers were stacked by applying the resin one over the other and excess resin was removed from the samples.A separate fixture(Fig.4)was designed to hold and apply uniform pressure to the metal and composite layers together under resin coated conditions.The specimens were allowed to cure under uniform pressure so that better adhesion between the metal and composite layers can be ensured.

In order to study the effect of ceramic powder(BC)addition on ballistic performance,boron carbide powder (BC) was intruded into the resin and uniform dispersion was assured using sonicator facility.Ceramic powder intruded resin was then applied to the interface of metal and composite layers.Fig.5 shows the step by step procedures involved in the fabrication of BC embedded Ti/GFRP FMLs.

Fig.3.Preparation of Titanium based FML.

Fig.4.Arrangement of Ti and GFRP layers.

2.1.Determination of mechanical properties of titanium and Al6061 FMLs

The composition of Titanium grade-2 alloy is listed in Table 2.Prior to high velocity ballistic impact experiments,the mechanical properties of Titanium grade 2,GFRP laminate and Ti/GFRP FMLs were determined by conducting tensile tests as per ASTM standards such as ASTM-E08,ASTM-D638 and ASTM-D368.In addition to Ti/GFRP FMLs,mechanical properties of Al 6061/GFRP was also determined and compared.This is mainly to study the effect of material plasticity on ballistic deformation behaviour.Table 3 shows the properties of E-glass fiber used in the present work.The tested samples are shown in Fig.6 and the values are tabulated in Table.4.

Fig.7 shows the comparison of ultimate tensile strength of Aluminium 6061 and Titanium based FMLs.It is noted that tensile strength of Ti/GFRP based FML is higher than Al 6061/GFRP FML.It is also noted that,Ti/GFRP FML has 61.7% of its base metal which indicates that Ti/GFRP FMLs are suitable for light weight aircraft applications.

3.Description of high velocity ballistic performance of Ti/GFRP FML

Ballistic performance of Ti/GFRP FMLs was determined by conducting high velocity impact experiments.Fig.8 shows the schematic arrangement of experimental test set up.The experiments were conducted using 9 mm Armour Piercing Projectile(APP).The projectile velocity was controlled by varying the amount of gunpowder in the cartridge prior to each shot.Titanium FML targets of dimension 120×120×8 was considered and fixed along with the backup plate.APPs with hard steel core having a mass of 7.85 g were fired at 0angle of attack from a distance of 10 m from the target plate.The striking velocity of the projectile and range between the gun barrel and target were selected as per the military standard MIL-DTL-32333 (MR).An infrared light-emitting diodes were placed in between the gun barrel and the target area to measure the striking velocity of the projectile for each shot.The depth of bulge caused by the projectile impact into the backup plate was measured.

In order to investigate the deformation behaviour of Ti/GFRP FML target subjected to high velocity projectile impact,the following experimental test matrix (Table 5) was considered.Published results [25] of Al 6061/GFRP FML targets and Al 1100/GFRP FMLs were also considered in order to understand the effect of material plasticity on ballistic deformation.

4.Ballistic deformation behaviour of GFRP laminates

The deformation behaviour of GFRP laminates subjected to high velocity (372 m/s) projectile impact is shown in Fig.9.Similar to FML targets,the GFRP laminates were also fabricated by using‘hand lay-up’ technique.‘Ductile hole growth’ failure mechanism was observed without causing any plastic deformation.The enlarged view of the impacted specimen clearly indicates the brittle cracking of fibers and matrix material.No delamination and radial cracks were observed both at front face(Fig.9a)and rear side(Fig.9b) of the target.It is evident that,at higher velocities the projectile can easily penetrate the GFRP laminate with marginal reduction of its initial velocity.Thus one can expect marginal ballistic resistance of pure GFRP composite targets.

4.1.Ballistic deformation behaviour of Ti/GFRP FML:Single projectile impact condition

The deformation behaviour of Ti/GFRP FML subjected to high velocity projectile impact is shown in Fig.10.The striking velocity of the projectile was 432 m/s at a range of 10 m from the target.“Plugging”failure mode was observed at the front face(Fig.10a)of the target and significant plastic deformation was observed at the rear side (Fig.10b) of the target.It is also noted that,the striking projectile has caused delamination of the fiber and metal sheet as shown in Fig.10c.In order to understand the inter-laminar failure caused by projectile impact,the FML layers were separated manually.The exploded view of the delaminated target (Fig.11)clearly indicates that,the projectile could able to penetrate up to the first layer of Titanium sheet and this deformation has progressed towards the successive layers.

It is also evident that the projectile could penetrate the composite layer more (reduced deformation) compared to titanium sheets and caused large plastic deformation on last metal layer.This observation shows good correlation with published results of Vlot[12]who has analyzed the failure behaviour of FMLs and observed smaller damage compared to metal sheets.It is transparent that,the impact energy of the projectile has been utilized to cause delamination of the layers and large plastic deformation on the metal sheets.

Fig.5.Steps involved in fabrication of Ti/GFRP FMLs+B4C targets.

Table 2 Alloy composition of Titanium-grade2.

4.2.Ballistic deformation behaviour of Ti/GFRP FML:Multiple projectile impact condition

The ballistic delamination behaviour of Ti/GFRP FML targetto 4th layer of the FML target.Only the third projectile could able to perforate the target and caused final fracture.The high velocity projectile impact has caused severe plastic deformation on titanium sheet and affected the inter-laminar shear stress prevailed between titanium and GFRP laminates and caused delamination of the FML targets.In addition,bridging of plastic deformation caused by projectiles 2 and 3 was also observed in layer 3.

Table 3 Mechanical properties of glass fiber (Sathish Kumar et al.[26]).

Fig.6.Tested samples of Al 6061/GFRP and Ti/GFRP FMLs.

Fig.7.Comparison of tensile test properties.

4.3.Ballistic deformation behaviour of Ti/GFRP FML+5%B4C:Single projectile impact

Fig.8.Schematic arrangement of high velocity ballistic experimentation set-up.

Table 4 Tensile test properties of Al 6061/GFRP and Ti/GFRP FML.

The ballistic deformation behaviour of boron carbide (5%)intruded Ti/GFRP FML target is shown in Fig.14.Similar to the observations of without boron carbide condition,‘plugging’failure mode was absorbed.It was understood that,the presence of boron carbide particles on the matrix has significantly reduced the penetration of the projectile by offering frictional resistance.The frictional resistance has reduced the projectile speed and caused more plastic deformation.Thus enhanced ballistic resistance of FML targets can be expected by adding ceramic particles.Thesubjected to multiple projectile impact is shown in Fig.12.Larger plastic deformation of titanium sheets and delamination of composite laminates were observed compared to single impact condition.It is apparent that,the Ti/GFRP FML target could able to prevent maximum of two projectile impacts of same velocity without causing perforation.The third projectile caused perforation of the target leaving behind ‘petaling’ mode of deformation.From the exploded view of the FML target,it is interesting to note that,the first projectile could able to penetrate up to first layer of Ti/GFRP FML(Fig.13)and the second projectile could able to pierce up exploded view(Fig.15)of the projectile impacted target shows that the projectile could able to penetrate up to the second layer whereas without adding ceramic powders,the projectile could able to penetrate up to third layer.The inclusion of ceramic particles has improved the toughness of the composite laminates.

Table 5 Test matrix for high velocity ballistic impact on Ti/GFRP FML.

4.4.Ballistic deformation behaviour of Ti/GFRP FML+10% B4C:Single projectile impact

Fig.9.Ballistic failure behaviour of GFRP Laminate.

Fig.10.Ballistic failure of Ti/GFRP:Single projectile impact.

Fig.11.Inter-laminar deformation behaviour of Ti/GFRP FML:single projectile impact.

Fig.16 shows the deformation behaviour of Ti/GFRP FMLs contains 10% of BC particles.Contrast to the observations of 5%BC addition,ballistic performance of FMLs with 10% BC significantly reduced the ballistic performance.It is well-known that,increase in weight percentage of BC particles has improved the bonding strength between the titanium sheets and GFRP interface.Increase in bond strength may improve the structural properties of the FML whereas the ballistic resistance has greatly reduced and the projectile could penetrate up to the final layer of the FML target(Fig.16 b &c).This is due the fact that higher weight fraction might have caused ‘agglomeration’ of the BC particles in matrix material.Agglomerated B4C particles might have offered reduced frictional resistance to the projectile penetration.Similar effect was observed by Jiheon Jun et al.[27] when trying to determine the tensile strength of epoxy resin based and BC intruded composites prepared with and without ultrasonic dispersion (UD) technique.Composites (epoxy resin and B4C) prepared with UD technique showed improved tensile strength and further addition of BC particles caused reduction of tensile strength which was due to the agglomeration of BC particles in epoxy resin.The authors also observed that,weak interfacial adhesion occurs between agglomerated particles with epoxy resin.

Fig.12.Ballistic failure of Ti/GFRP:Multiple projectile impact.

Fig.13.Inter-laminar deformation behaviour of Ti/GFRP FML:Multiple projectile impact.

Fig.14.Ballistic failure of Ti/GFRP+5% B4C:Single projectile impact.

Similarly Nambiraj et al.[28] tried to improve the mechanical properties of polymer composite materials by varying the weight percentage (0%,1%,3%,5% and 10%) of silicon filler material.Maximum tensile strength of 284 MPa was achieved for the particle size of less than 10 μm with a weight percentage of 5.When the weight percentage was increased to 10,the authors observed a significant reduction in mechanical properties due to agglomeration of filler material in matrix material.They reported that,agglomeration significantly increases the void size and reduces the mechanical properties.

Fig.15.Inter-laminar deformation behaviour of Ti/GFRP FML+5% B4C:Single projectile impact.

Fig.16.Ballistic failure of Ti/GFRP+10% B4C:Single projectile impact.

The exploded view (Fig.17) of the projectile penetrated FML target showed higher damage region for GFRP laminates compared to earlier condition.In addition,strong bond between the first layer of Ti sheet and GFRP laminate was observed.

Comparison of depth of penetration(DoP)of the projectile into the Ti/GFRP+0% BC,Ti/GFRP+5% BC,Ti/GFRP+10% BC FML targets is shown in Fig.18.The DoP of Ti/GFRP FML prepared without adding boron carbide was around 39.6 mm whereas the value reduced to 25 mm and 19.5 mm when the percentage of BC added was 10% and 5% respectively.It is evident that,target contains 5% of BC particle shows lower DoP (45-50%) and higher ballistic limit compared to other two conditions.Further addition of ceramic powder reduces the ballistic limit as it increases the brittleness of the GFRP laminates due to the formation of agglomeration of BC particles.

Fig.17.Inter-laminar deformation behaviour of Ti/GFRP FML+10% B4C:Single projectile impact.

Fig.18.Comparison of depth of bulge of targets.

Comparison of deformation behaviour of individual layers of Ti/GFRP FML prepared by adding various percentage of BC particles is shown in Table 6.It is understood that ‘perforation’ failure mode was observed for GFRP laminates and ‘penetration’ failure mode was observed for titanium metal sheets.Smaller damage region was noted for GFRP laminates compared to titanium sheets and irrespective of the BC addition,layer 1 was subjected to‘plugging’failure.Similarly layer-5 shows higher plastic deformation without fracture except condition of target with 10% BC addition.The middle layer of the FML is layer-3 and it shows significant deformation plasticity.The deformation behaviour of layer-4 (GFRP)clearly indicates,fiber/matrix breaking failures.None the layer shows ‘fragmentation’ and ‘brittle cracking’ mode of failure which supports the suitability of Ti/GFRP FMLs to parts of the domestic/fighter aircrafts which are prone to high velocity impact loads.

4.5.Effect of target plasticity on ballistic failure mechanism

In order to investigate the effect of material plasticity on ballistic deformation behaviour of FML targets,failure behaviour of Ti/GFRP and Al 1100/GFRP FMLs were compared with published results of Sundaram et al.[25] who have determined the effect of multiple projectile impacts on Al6061/GFRP FMLs.The material properties of various types of Aluminium alloys considered in the present work are listed in Table 7.

Fig.19 shows the multiple projectiles penetrated Al 1100 FML(Fig.19a and b),Al 6061 FML (Fig.19c and d) and Titanium FML targets (Fig.19e and f).Regardless of the target material,the projectile impact has caused ‘plugging’ (Ti/GFRP and Al 6061/GFRP FMLs) and ‘perforation’ (Al1100/GFRP) fracture mode at the front face of the targets.At the rear side,complete“Petaling”failure mode was observed for Al 1100/GFRP FML targets (Fig.19b) and partial“Petaling”failure mode was observed for Al 6061/GFRP and Ti/GFRP FML targets(Fig.19d and f).Similar failure mechanism was observed by Chai et al.[21]while investigating the failure analysis titanium FMLs subjected to high velocity ballistic impact.Compared to Al1100 and Al6061 FMLs,titanium based FML showed higher ballistic limit and lesser depth of bulge.This is due higher yield strength and elongation percentage of titanium alloy compared to other Aluminium alloys.The observation shows good correlation with findings of Kaboglu et al.[18] who has reported that,increase in yield strength improves ballistic limit of FMLs due to improved adhesion between metal layer and GFRP laminates.From the experiments,it is transparent that titanium based FMLs are suitable for aircraft structures which are subjected to high temperature and high velocity loading.

5.Conclusions

FMLs are generally designed for protective structures of the aircraft as they often subjected to high velocity impact during service condition.Investigations of the inter-laminar failure behaviour of Ti/GFRP FMLs subjected to high velocity projectile impact is essential for practical application.In this paper,various failure mechanisms produced by projectile impact (single and multiple) and addition of ceramic particles on Ti/GFRP FMLs have been explored.

· When the Ti/GFRP FML was subjected to high velocity impact(432 m/s)“plugging”failure was observed at the front face of the target and no perforation was observed.At the rear side of the target,higher plastic deformation was observed.This is in contrast with ballistic behaviour of pure GFRP composite laminate where complete ‘perforation’ failure mode was observed.Similar to the observations of Vlot [13],failure analysis of individual layers indicate higher damage region for titanium sheets compared to GFRP layers.In addition,the projectile's impact has caused inter-laminar shear fracture between the titanium sheets and GFRP laminates.

·The deformation behaviour of multiple projectiles (three)impacted Ti/GFRP FML target showed that it can resist first two projectiles.Whereas the third projectile has produced perforation and caused“petaling”mode of failure at the rear side.Comparison of deformation behaviour of individual layers clearly indicates that,multiple projectile impact has triggered joining of deformation in metal layers and caused severe delamination.No such behaviour was observed in GFRP laminates.

·It was noted that,addition of boron carbide powder(5%weight)has significantly improved (45-50%) the ballistic limit of Ti/GFRP FML targets.The depth of bulge for the Ti/GFRP FML prepared with 5% addition of boron carbide was less (19.5 mm)compared to conditions of 0% (39.6 mm) and 10% addition(25 mm).This is mainly due to frictional resistance offered by the ceramic powders which has retarded the projectile speed and improved the ballistic resistance of the FML target.Further increase of ceramic powder(10%)has reduced the ballistic limit which may be due to‘agglomeration’of B4C particles in matrix material.

·For the same projectile velocities,Ti/GFRP FML showed lesser deformation compared to Aluminium 6061/GFRP and Al1100/GFRP based FML targets.Published results of Kaboglu et al.[18]also reveals that target with higher yield strength will have improved ballistic limit.No ‘radial cracks’ and ‘fragmentation’failures were observed during high velocity projectile impact of Ti/GFRP FMLs which recommends their suitability for aircraft applications.

Table 6 Layer wise comparison of ballistic failure mechanism of Ti/GFRP FMLs.

Table 7 Material properties of Ti and Aluminium alloys [25,29,30].

Fig.19.Effect of target plasticity on ballistic behaviour.

The authors declare that there are no conflicts of interest regarding the publication of this paper.

The authors would like to acknowledge the financial support received from the management of SSN.Thanks to ‘Tata Advanced Materials Limited,Bangalore for conducting high velocity ballistic experiments.