# Energy absorption of composite materials under high velocity impact

## Abstract

Many studies were directed toward understanding damage patterns in compositelaminates and determining the damage development sequence upon high velocity impact. Damage accumulation depends on projectile velocity and on a number of otherparameters, so that it is not possible to set strict limits between the different regimes.However, experiments show that, for a given set of experimental conditions where theimpact speed is the only variable, there is a certain threshold velocity below which nodetectable damage occurs. Above the threshold velocity, no surface damage is observed except for a small indentation at the contact point, but significant internal damageconsisting of delaminating and matrix cracks is introduced. As the impact velocityincreases further, surface damage due mainly to fiber breakage is introduced. For veryhigh speeds, the target does not have time to deform, and perforation occurs, leaving aclean hole in the sample.The objective of this study is to develop a mathematical model that corresponds to the deformed geometry under high velocity impact applications for composite laminates. A total of 100 tests were conducted on composite laminates, struck by cylindrohemispherical projectiles at normal incidents with velocities up to about 100 mls. The types of materials, used this study, are AS4/3051, IM7/5250 CarbonlEpoxy and TI003Glass/Epoxy. The strain energy was obtained by derivation of the proposed deflectionfunction. The strain energy was plotted with respect to the deflection of the mid-planeand, then correlated through dynamic correlation factors to actual kinetic energyduring the impact. The dynamic correlation factors were determined using a geneticalgorithm regression analysis. Two types of materials were tested, namely plain graphitecomposites and hybrid composites. The growth of the delamination and also the effect of varying the stacking sequence were investigated for the different type of materials and various orientations.The mathematical model appears to provide a reasonable representation of the deformation of composite laminates during the penetration by a cylindro-hemispherical projectile. Furthermore, hybrid composites appear to provide more resistance to the impact, whereas plain composites have less resistance with respect to the higher velocities. It was concluded that, the change of the material in a hybrid composite affects the growth of the damaged area and also reduces the impact penetration resistance. Hence, IM7/E-Glass hybrid has a higher resistance to the penetration. Measurements ofthe energy levels of the hybrid composites indicated that they offer the highest resistanceto ballistic perforation. The hybrid composites perforated at velocities between 77 mlsand 83 (mls), whereas the graphite composites perforated at velocities between 48 m/sand 59 (mls). The higher perforation resistance is attributed to the reduced level ofdelamination generated during the impact, and also the addition of the E-Glass, whichwas capable of absorbing more energy during the impact.In studying the graphite composites, the best orientation in terms of the stacking sequence was found to be [(45, -45, 0, 90) 2 ] S , which indicates that this stacking sequence withstand higher velocity and hence absorbs more energy during the impact. Therefore, the quasi-isotropi corientation [(45, -45, 0, 90) 2 ] S is best for impact resistance if a laminate is not combined with E-Glass. The ballistic-limit velocity prior to perforation for the Quasi-isotropic laminate was measured as 58.9 m/s. This is a significant increase compared to the other plain graphite samples. The energy required for the complete perforation is approximately 48% higher in this stacking sequence as compared to other plain Graphite specimens. It was also found that the energy absorption capability is reduced significantly in the cross-ply laminates. The penetration resistance of the [(0,90,0,90) 2 ] S laminate and the energy required for perforation are approximately 50% less than the other plain graphite specimens.