简介概要

Fracture mechanism of a laminated aluminum alloy plate during ballistic impact

来源期刊：Rare Metals2017年第9期

论文作者：Ming-Yuan Li Bai-Qing Xiong Guo-Jun Wang You-Zhi Tong Xi-Wu Li Shu-Hui Huang Zhi-Hui Li Yong-An Zhang

文章页码：737 - 745

摘要：The multilayered 7XXX series aluminum alloy was impacted by 7.62 mm ogival projectiles at velocities ranging from 787 to 851m·s^-1. The deformed microstructure under various impacting velocities and fracture surfaces of different sections were investigated at different physical scales to determine the process of failure.Optical microscopy（OM）,electron back-scattered diffraction（EBSD） and scanning electron microscopy（SEM） were used in the investigation. The results show that crater is constrained in the 7B52 front layer and two types of adiabatic shear bands which are transformed bands and deformed bands and different types of cracks are observed.Spall fracture is the significant failure mode of 7B52 front layer, and the resulting delamination leads to the presence of bending tensile fracture instead of the shear plugging.The ductile 7A01 layer blunts and deflects the spall crack tips, preventing the targets from full spall, and induces a constraint of 7A52 rear layer. The level of the constraint determines different fracture modes of 7A52 layer,accounting for the asymmetry of damage.

稀有金属(英文版) 2017,36(09),737-745

Fracture mechanism of a laminated aluminum alloy plate during ballistic impact

Ming-Yuan Li Bai-Qing Xiong Guo-Jun Wang You-Zhi Tong Xi-Wu Li Shu-Hui Huang Zhi-Hui Li Yong-An Zhang

State Key Laboratory of Nonferrous Metals and Processes,General Research Institute for Nonferrous Metals

Northeast Light Alloy Co., Ltd.

收稿日期：17 December 2014

基金：financially supported by the National Basic Research Program of China (No.2012CB619504);the National Natural Science Foundation of China (No.51274046);

Fracture mechanism of a laminated aluminum alloy plate during ballistic impact

Ming-Yuan Li Bai-Qing Xiong Guo-Jun Wang You-Zhi Tong Xi-Wu Li Shu-Hui Huang Zhi-Hui Li Yong-An Zhang

State Key Laboratory of Nonferrous Metals and Processes,General Research Institute for Nonferrous Metals

Northeast Light Alloy Co., Ltd.

Abstract：

The multilayered 7XXX series aluminum alloy was impacted by 7.62 mm ogival projectiles at velocities ranging from 787 to 851m·s^-1. The deformed microstructure under various impacting velocities and fracture surfaces of different sections were investigated at different physical scales to determine the process of failure.Optical microscopy(OM),electron back-scattered diffraction(EBSD) and scanning electron microscopy(SEM) were used in the investigation. The results show that crater is constrained in the 7B52 front layer and two types of adiabatic shear bands which are transformed bands and deformed bands and different types of cracks are observed.Spall fracture is the significant failure mode of 7B52 front layer, and the resulting delamination leads to the presence of bending tensile fracture instead of the shear plugging.The ductile 7A01 layer blunts and deflects the spall crack tips, preventing the targets from full spall, and induces a constraint of 7A52 rear layer. The level of the constraint determines different fracture modes of 7A52 layer,accounting for the asymmetry of damage.

Keyword：

Laminate; Aluminum; Ballistic impact; Spall fracture; Adiabatic shear bands;

Author： Bai-Qing Xiong,e-mail:xiongbq@grinm.com;

Received： 17 December 2014

1 Introduction

Laminated metallic plates have many advantages over monolithic plates,such as the improvements of damage tolerance,fatigue behavior and ballistic performance [ 1, 2] .The idea of using metallic laminate targets laminate plates in place of monolithic targets has been proposed for a long time for the potential to enhance bulletproof ability of the ballistic systems within restrictive weight requirements [ 3] .

According to results of ballistic performance of a range of in contact laminated and monolithic targets [ 4, 5, 6, 7, 8, 9, 10, 11, 12, 13] ,the presence of adiabatic shear bands is a distinguished damage mode when the thickness of the target reaches a specific value [ 4, 5, 6, 7] .Adiabatic shear bands in monolithic targets reach the rear surface,inducing shear plugging easily,whereas,in laminated targets with the same total thickness,adiabatic shear plugging is limited on the entry side of the impacting and the dominate target failure mode changes from shear plugging to stretching and bending on the exit side.Shear localization within a narrow region observed in monolithic target is of quite limited consequence in energy absorption.Stretching of the exit-side layers of laminated targets becomes a major source of energy absorption during ballistic impacting because the stretching work term is much greater than the plugging work term [ 9] .

Aluminum targets are widely used in the ballistic protection systems,and numerous studies were carried out to reveal the ballistic behavior of aluminum targets [ 14, 15, 16, 17] .A roll-bonded multilayer laminate of 7A52,7B52 and7A01 is designed to be an important candidate for armors.And the emerging of the target fracture is an important phenomenon which should be taken into account when assessing the performances of the laminated targets against ballistic impact.As for roll-boned laminate target,the interactions of layers are more complex than the unbonded in contact laminates.However,research on such kind of targets is not thoroughly investigated.In this paper,an attempt was made to explore the characterization during ballistic penetration of the latest three-layered aluminum laminate composite.

2 Experimental

7A52/7B52 aluminum laminate composite was prepared,containing three layers of different alloys which were7A52,7B52 and 7A01.The multilayer laminate was processed by hot roll bonding and peak aging.The total thickness of the laminate composite is 19 mm.The laminated structure and properties of the component alloys(tested in the rolling direction) are shown in Fig.1 and Table 1,respectively.

The ballistic tests were carried out at normal impact angle using 53 type 7.62 mm WO-109C armor-piercing incendiary according to GJB 59.18-88.The high strength7B52 aluminum alloy layer was put on the projectile entry side of the targets,and the softer 7A52 aluminum alloy was used as rear layer.The velocities of impacting were ranging from 787 to 851 m·s^-1.Figure 2 illustrates the rear side of the plate after ballistic tests.The penetration channel was cut along the penetration axes and polished and etched to reveal the microstructure across the crater area.

Optical microscopy (OM,Zeiss Axiovert 200 MAT and Leica DM400) was used to investigate the micros tructure of the crater areas.The surface was ground by SiC emery paper up to 5000 mesh and then polished by 1μm diamond paste.The etchant was a modified Keller’s reagent consisting of 2 ml HF,3 ml HCl,20 ml HNO₃,and175 ml H₂O.Scanning electron microscopy (SEM,JSM-7001F) equipped with energy dispersive spectroscopy(EDS) was used for observing the characterization of fracture surfaces of the deformed areas and the microstructural characterization of the undeformed areas.Electron back scatter diffraction (EBSD) orientation mapping in the SEM was used to observe the characteristic of initial and deformed grains in order to judge the role of the grain structure on the failure behavior.Samples were mechanically grinded to 5000 mesh and then electropolished in a 10 vol%perchloric acid and 90 vol%ethyl alcohol solution for 15 s and an applied voltage of 30 V at ambient temperature.The distributions of different elements in the bond interface were detected by electron probe microanalysis (EPMA,PEGASUS-XM2).Shear tests of either interface were carried out according to the Chinese standard GB/T6396-2008 to evaluate shear strength.

Fig.1 Structure schematic diagram of multilayered 7XXX series aluminum alloy

3 Results and discussion

3.1 Initial microstructure

The component alloy plates were bonded together by hot rolling.Both the thicknesses of the diffusion layers of7A01/7B52 and 7A01/7A52 are about 70μm,as revealed by the concentration profiles of Al and Zn elements across the interfaces (Fig.3).Metallurgical bond between the layers is created.Shear tests of interfaces of 7B52/7A01and 7A52/7A01 were performed to characterize the shear strength and obtain information on its quality and resistance to delamination.Their interlaminar shear strengths are 73 and 69 MPa,respectively,indicating that the interfaces are well bonded.

The grain structures of 7A52 alloy and 7B52 alloy are depicted in Fig.4.Both the layers have deformed fibrous grain structure formed for the reason that a large reduction in thickness is necessary to obtain reasonable bond strength between the layers.It is notable that,in the 7B52 layer,the recrystallized grains with random crystallographic texture can be seen in-between the fibrous grains.The recrystallized grains flat in the rolling direction,and the two layers have high-angle boundaries.Intergranular fracture is propone to occur along the high-angle grain boundaries,and the fracture toughness is expected to be lower along loading directions where the projected area of the highangle grain boundaries on the main fracture plane is high [ 18] .

下载原图

Table 1 Composition,mechanical properties (yield strength (YS),ultimate tensile strength (UTS) and elongation) and thickness of multilayers

Fig.2 Macrograph of rear side of target after ballistic test

The coarse constituent particles formed during casting can be seen in both layers,as shown in Fig.5.These particles mainly consist of iron-rich and silicon-based intermetallics and are more or less elongated in the rolling direction for the severe deformation during hot rolling.The relationship between the coarse constituent particles and toughness has been well documented [ 18, 19] .On the microscale,the coarse constituent particles are prone to contribute to an inhomogeneous strain field and preferential fracture initiation [ 18] .

Fig.4 EBSD images of characteristic of grains in laminates:a 7A52and b 7B52 (L longitudinal,LT long transverse,ST short transverse)

Fig.5 SEM images of particle distribution of laminates:a 7A52 and b 7B52

3.2 Microstructural characterization of craters

From the macroscopic perspective (Fig.6),the whole crater is limited in the 7B52 layer.The front part of projectile was surrounded by the partly punched plug during the ballistic impacting.

Shear localization is observed in the crater walls.The kinked grains shown in Fig.7a indicate the presence of localized shear deformation during impact processes.As the shear strain increases,the shear localization zones narrow and adiabatic shear bands arise.There is a competition between work hardening and thermal softening during the process of shear deformation caused by penetration.When thermal softening is the dominate factor,instability of plastic flow appears,resulting in the formation of adiabatic shear bands [ 20] .Two types of localized shear bands can be seen:the one without distinct boundary is called deformed shear band (Fig.7b),and the other with a well-defined border is called transformed shear band(Fig.7c) [ 21, 22] .The latter type is the further development of the former type,and critical values of shear strain are confirmed to exist for the occurrence of both types [ 22] .As shown in Fig.8,even border of the tip of transformed shear band is not that well-defined,indicating that the strain of localized shear deformation decreases in the propagating orientation of shear bands.Special patterns such as bifurcation and crossing of shear bands are observed (Fig.9).The crack nucleation,growth and coalescence in these shear bands are speculated to be much easier,indicated by Fig.10.

Fig.3 EDS results of across interface of component alloy plates:a 7A01/7B52 and b 7A01/7A52

Fig.6 Macrograph of polished cross section,indicating failure modes of a penetrated area in 7B52 layer

There are different types of shear cracks in the deformed areas of the 7B52 layer.The type of shear cracks estimating from the crater wall,as shown in Fig.11,is often named primary cracks,inclined about 45°with respect to the penetration direction.With the projectile entering into the target,material was pushed to the side by the front part of the projectile generating localized shear strain.The localized deformation is attributed to the formation of the primary cracks,which can be indicated by a shear crack running along a shear band (Fig.11b).It can be seen that the fracture surface of primary crack is smooth with many parallel lines appearing on (Fig.12a).When viewed in high magnification,a lot of edges and corners<1μm in size can be observed in some regions (Fig.12b).The presence of intergranular fracture mode in these regions illustrates dynamic recrystallization that took part in the deformation.

Fig.7 OM images of shear localization in 7B52 layer:a shear localization zone,b deformed shear band,and c transformed shear band

Fig.8 OM image of a typical transformed shear band a and EBSD images of b middle region of transformed shear band and c tip of transformed shear band in 7B52 layer

Fig.9 OM images of bifurcation and crossing of shear bands in 7B52layer

Fig.10 OM images of nucleation,growth and coalescence of microcracks in shear band in 7B52 layer

This type of shear cracks is often intersected by another type of shear cracks which is normal to the penetration axes.In fact,the second type of shear cracks can be seen throughout the deformed areas.Intense shear between the grains results in such type of shear cracks which has a great relationship with the fibrous rolled grain structure.It can be indicated by the intersection of a shear crack and a shear band (Fig.13).

3.3 Failure modes of 7B52 layer and effect of 7A01layer

Spall fracture is a significant failure mode of the 7B52layer,as shown in Fig.14.In the cross-sectional view,the diameter of spall zones can be six times as large as that of the craters.Such type of fracture is a kind of tensile failures induced by reflected waves from the free end of the target [ 23] .

Fig.11 OM images of primary crack:a overview and b high magnification in 7B52 layer

Fig.12 SEM images of fracture surface of spall fracture in 7B52layer:a overview and b high magnification

Fig.13 OM image of shear band intersecting with shear crack in7B52 layer

Figure 15a shows that the spall cracks nucleate and grow along the recrystallized grain boundaries.It also can be seen that coarse constituent particles are mostly situated inside the recrystallized regions (Fig.15b).Because the pancake-shaped recrystallized grains are flattened along the rolling direction,the projected area of the recrystallized grain boundaries on the rolling plane is high.The coarse constituent particles are initiation sites for cracks,and the recrystallized grain boundaries are weak interfaces [ 18] .Therefore,when the tensile stress loads are perpendicular to the rolling plane,the area fraction of intergranular fracture is quite large and the broken constituent particles can be seen in the intergranular fracture surfaces (Fig.16).

Fig.14 OM images of cross section of 7B52 layer after impacting

Fig.15 Spall cracks of 7B52 layer in 7B52 layer:a EBSD image and b OM image

Fig.16 SEM images of fracture surface of spall fracture in 7B52layer:a overview and b high magnification

The spall cracks in different planes are prone to coalesce.The coalesced spall cracks slice the 7B52 layer into relatively thin plates (Fig.14).When the primary cracks encounter the gaps of these thin plates,crack blunting can be resulted.Under the load of the projectiles,bending fracture appears.The in-plane stresses induced by bending are off-axis from the spall fracture plane.In Fig.17,the cross-sectional view and the appearance of fracture surface of bending fracture region can be seen.The main fracture mode is partly intergranular and partly trans-granular(Fig.18).High density of dimples with<1μm in diameter can be seen in the trans-granular fracture surfaces(Fig.18b).Because the stress triaxiality is high,secondary cracks which seem to have the character of delamination along the recrystallized boundaries can be seen (Fig.17).

As for kinetic energy dissipation,despite the amount of energy absorbs due to the presence of new interfaces as delamination occurs,bending and stretching are the main sources.The dissipation during the propagation of localized shear cracks is of little consequence because the volume of material participating in impact process is quite limited [ 8, 9] .

Fig.17 OM image of bending fracture in 7B52 layer

Fig.18 SEM images of fracture surface of spall fracture in 7B52layer:a overview and b high magnification

Former studies illustrated the influence of laminate structures on enhancing the fracture mechanical behavior [ 9, 10, 11, 12, 13] .As for 7A52/7B52 aluminum laminate composite,one of the significant benefits arising from such three-layer structure is the positive effect on the resistance of full spall of the whole laminate composite.Crack blunting and crack deflection can be resulted when the propagating spall cracks encounter the ductile 7A01 layer (Fig.19).The loaded spall crack produces regions of intense effective plastic strain that are off-axis to the plane of crack tip opening [ 1] and the off-axis deformation can result in large-scale yielding of the ductile 7A01 layer.The largescale yielding of materials around the front of the crack tips blunts the crack and activates the nucleation and growth of plastic voids in the crack front.Stress localization is induced in the ligaments between plastic voids and the crack tips under the load of projectile.As the ligaments become thick enough,they break ahead the crack tips,leading to the deflection of cracks.Further crack growth in the 7A52 layer requires re-nucleation process which results in an enormously increase in the amount of energy required for the full spall of the target.

Fig.19 OM images a-c of crack blunting and crack deflection in 7A01 layer at different regions

3.4 Failure modes of rear layer

Bulging is the typical failure feature of the rear side in this study,accompanied by cracks occurring on one side along the periphery of bulging in some cases,as shown in Fig.2.In cross-sectional view (Fig.6),it can be clearly seen that bending of materials ahead the projectiles results in bulging.It should be noted that the cracks along the periphery of bulging are mainly parallel to the rolling direction.From the LT-ST plane perspective,damage is asymmetric.Shear crack,even partly separation,induced by bending is observed on one side (Fig.20),while less damage is seen on the other side (Fig.21).

When viewed in high magnification,adiabatic shear band can be seen in the heavier damage side (Fig.20b).The adiabatic shear band can propagate rapidly to reach the rear plane and slip along it occurs easily [ 4] ,illustrating that intense shear deformation plays an important role in the failure of the heavier damage side.The total deflection of the 7A52 rear layer on this side is a combination of a bending deflection and an intense shear deflection.And the intense shear deflection is the dominate factor leading to fracture.

As a comparison,the same deflection is reached by a relatively uniform deformation on the other side inducing less damage.Figure 21 shows the typical feature of the less damage side.Tearing fracture along the interface is found.It should be highlighted that there is a constraint of 7A52layer induced by 7A01 layer.The delamination of the interface of 7A01 layer and 7B52 layer means a relaxation of the constraint,which allows more materials to participate in deformation.Once the kinetic energy of projectile is large enough,tensile failure rather than sheared deformation emerges on this side.Slices were cut every 2 mm to investigate this process,including nucleation,growth and coalescence of microfractures,as shown in Fig.22.High density of parabolic tear dimples can be seen in the fracture surface of the tearing fracture region between 7A01 layer and 7B52 layer,as shown in Fig.23.The appearance of pores on the fracture surface illustrates that defects inheriting from rolling processing play an important role in the fracture properties.The ideal situation is that the extent of delamination caused by tearing is larger to enhance the laminate toughness.

Fig.20 OM images of bending fracture of 7A01 layer in heavier damage side:a overview and b high magnification

Fig.21 OM images of tearing fracture in 7B52 layer

Fig.22 OM images a-d of evolution of tensile failure on less damage side at different regions in 7B52 layer

Fig.23 SEM images of fracture surface of tearing fracture in 7B52layer:a overview and b high magnification

Fig.24 SEM images of fracture surface of 7A52 layer:a overview and b high magnification

The asymmetry of fracture in 7A52 layer accounts for the presence of different features of the fracture surfaces.The main feature is partly along the grain boundaries and partly through the grains,as shown in Fig.24.The flat and elongated appearance of grains and the large distance between the grain nodes in the rolling plane make cracks grow easily in or near the grain boundary.However,the fraction of intergranular fracture surfaces is lower than that in 7B52 layer for the absence of recrystallized grains.Large amount of coarse intermetallic particles are found in the intergranular fracture surfaces,indicating that intermetallic constituent particles act as sites for the formation of cracks.

4 Conclusion

An investigation of fracture mechanical of the 7A52/7B52laminates during ballistic impact was conducted.Shear localization causes the formation of two types of adiabatic shear bands (transformed bands and deformed bands) and the primary shear cracks which estimate from the crater.Intense shear between grains induces another type of shear cracks.Spall fracture in the 7B52 layer slices the target into thin plates,resulting in bending and stretching tensile failure.The 7A01 layers prevent the targets from full spall by blunting and deflecting the spall cracks.Bending shear fracture and bending tensile fracture are typical failure modes of the rear 7A01 layer.The transition is determined by the propagation of cracks along the 7A01 layer.