Many studies were directed toward understanding damage patterns in composite
laminates and determining the damage development sequence upon high velocity impact. Damage accumulation depends on projectile velocity and on a number of other
parameters, 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 the
impact speed is the only variable, there is a certain threshold velocity below which no
detectable damage occurs. Above the threshold velocity, no surface damage is observed except for a small indentation at the contact point, but significant internal damage
consisting of delaminating and matrix cracks is introduced. As the impact velocity
increases further, surface damage due mainly to fiber breakage is introduced. For very
high speeds, the target does not have time to deform, and perforation occurs, leaving a
clean 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 TI003
Glass/Epoxy. The strain energy was obtained by derivation of the proposed deflection
function. The strain energy was plotted with respect to the deflection of the mid-plane
and, then correlated through dynamic correlation factors to actual kinetic energy
during the impact. The dynamic correlation factors were determined using a genetic
algorithm regression analysis. Two types of materials were tested, namely plain graphite
composites 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 of
the energy levels of the hybrid composites indicated that they offer the highest resistance
to ballistic perforation. The hybrid composites perforated at velocities between 77 mls
and 83 (mls), whereas the graphite composites perforated at velocities between 48 m/s
and 59 (mls). The higher perforation resistance is attributed to the reduced level of
delamination generated during the impact, and also the addition of the E-Glass, which
was 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.
Impact Strength of Glass/Kevlar Fiber Hybrid Composites with Various Stacking Sequences and Impact Energies
