18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS NOVEL CHARACTERISATION TECHNIQUE FOR ASSESSMENT OF DAMAGE TO PLATES BY BALLISTIC IMPACT K. Kandan 1 , B.P. Russell 1* , V.S. Deshpande 1 and N.A. Fleck 1 1 Department of Engineering, Cambridge University, U.K * Corresponding author (bpr23@cam.ac.uk) Keywords : composite, damage, ballistic loading, multi-impact (Fig. 1b). These points can be charted in 1 Introduction space, and the resultant trajectory shows the A novel characterisation technique is proposed for susceptibility of a particular plate to ballistic the assessment of ballistic damage of plates and the damage. residual capacity of such plates to sustain further 2.1 The form of the limit-boundary ballistic loading. Damage mechanisms in composites are complex and numerous [1,2] and is generally Of this limit-boundary two coordinates are known a assessed by either visual techniques such as priori , the location where the ballistic limit is microscopy [3] or 3D X-ray tomography or residual reached immediately ( and ) and property tests where for example, the knock-down in lying on the other axis, the point assuming a zero compressive strength of an impacted specimen is velocity initial impact ( and ). compared with an undamaged one [4,5]. Of course, the equation governing all locations The technique presented here considers the knock- between these points, ( ) , will be some a down in penetration velocity of a projectile on a function of the material properties and plate plate that has been previously hit by an identical geometry, and is likely to exhibit a non-linear projectile at the same position. Multi-hit scenarios form as mechanistic changes might well be are of interests in both military and civilian assume to occur at certain transition velocities. applications, for example, bird strike on the fuselage A couple of „model‟ limit - boundarie s can be of commercial composite airliners, containment of plotted on the chart describing armour plates aircraft engine blade fragments, munitions rounds and blast entrained debris. Here it is of interest to showing idealised interaction behaviours. The know the residual resistance to a subsequent impact simplest limit-line shows a non-interaction that might arise from collision with a flock of birds. effect. In this case, the equation of the line is independent of and for all values of 2 A novel charaterisation technique , up to (Fig. 1c). In a material which is The degradation characteristic of a plate subjected to elastic up to failure, this may be expected to be a non-penetrating ballistic event can be mapped out the characteristic for such a plate. Another limit- in the following way. A projectile is impacted on to boundary might be defined from the sum of the a plate target at some velocity such that kinetic energies of the two impacts (Fig. 1c), i.e. , where is the ballistic limit of the plate. A second projectile is launched at a velocity and ) ( impacted at the same location on the plate , and the survival or failure status of the plate is noted (Fig. 1a). In this way, for each , a series of tests at This characteristic would be typical of a material different values of are conducted such that the that showed large ductility to failure, where the boundary between survival and failure is established
kinetic energy of the projectile was absorbed both the carbon fibre composite and stainless steel primarily through plastic deformation of the plate. plate specimens (Fig. 3). The characteristics for these two plates are quite distinct. 2.2 Definition of a scalar parameter Whilst this method shows clearly the damage 4.1.1 Stainless steel specimens characteristics of a plate, it does not lend itself to an These plates show a gradual deterioration in the easy comparison between many such plates, due to second limit velocity as the velocity of the initial the potentially non-linear boundary between the impact is increased. This reduction appears to be linear up to about 85% of the . In this region survival and failure regions. Many instances in which a knowledge of the multi-impact behavior of the degradation is observed to fall in accordance a plate is desirable concern events where the first with the equation, and second impact are of similar mass and velocity. For such instances, it can be helpful to define a scalar quantity that will give a measure of the multi- impact response. A line of slope one in For a above 85% of the , the deterioration space, i.e. where , will intersect with the is more rapid. If we compare the limit-boundary survival-failure limit-boundary at a velocity where with that of an ideally plastic material (cf. Fig. the second projectile is on the cusp of failing the 1c), we observe that the initial region ( plate (Fig.1d). This double equal velocity ) falls below this curve, while the latter penetration limit not becomes the critical design region ( ) matches quite closely. value, rather than the undamaged ballistic limit, for Plastic deformation is seen at all initial impact design scenarios where multi-impact performance is critical. velocities, and the consequential thinning of the plate has a significant effect on the residual 3 Experimental capacity to resist an impact. Carbon fibre/epoxy composite plates with a lay-up [ ] and 304 stainless steel plates both of the 4.1.2 Carbon fibre specimens same areal mass were impacted with spherical steel A markedly different limit-boundary is seen for the projectiles fired from a gas gun, Fig. 2. Square plates carbon fibre composite specimen. Here, virtually no measuring 150 mm along the length were through- deterioration in the limit velocity is seen in the holed and clamped with an annular ring such that the region where . Beyond this region unsupported diameter was 100 mm. The area density there is a sharp drop-off in the strength of the was kept constant at 5.7 kg m -2 by using plates of composite with further increases in . The thickness 3.5 and 0.71 mm for the composite and following expressions define the boundary- stainless steel respectively. The projectile used was a limit, spherical steel ball bearing measuring 12.7 mm in diameter and weighing 8.6 g. The velocity range used for the experiments was 25 to 200 ms -1 . High speed photography was used to capture the deformation at both the front and back faces of the Comparing the composite with the idealised elastic plates. A Phantom v12 camera (Vision Research, material limit-boundary shows the composite to be NJ, USA) was used with a inter-frame time of 10 µs close to this ideal before the onset of damage. When and an exposure time of 1 µs. damage does occur in the composite, this is significantly more detrimental to the ballistic 4 Results + Discussion performance than that seen in the stainless steel. 4.1 Boundary-limit characteristics Damage mechanisms such as delamination and fibre breakage impair the structural integrity significantly By making use of the methodology detailed in compared with the more benign plastic dissipation Section 2, maps of space were plotted for mechanism seem in the stainless steel.
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