Normal and Oblique Penetration Studies in CFRP Laminates
The penetration of a 12 mm thick CFRP laminate by a 12 mm steel projectile; impact velocity = 346 m/s
SPONSOR: EPSRC/MoD/Cranfield University
SUMMARY: In this research, woven CFRP laminates have been subjected to impact by a steel sphere. Impact and penetration of targets at normal and oblique incidence were studied using high-speed video. Several hybrid CFRP laminates of different material and geometrical configurations have been subjected to impact by a high velocity steel sphere with an impact energy of c.a. 440 J. It was found that 12 layers of ballistic-grade Kevlar loosely bound to the rear of the CFRP laminate proved to be the most weight-efficient method of dissipating the kinetic energy of the projectile. Furthermore, the impact response of a non-woven symmetrical CFRP laminate has been compared to that of a woven laminate over an impact-energy regime of 92–459 J. At lower impact-energies there were strong indications that the non-woven laminate out-performed the woven laminate whereas at the higher impact-energies the ballistic performance was seen to be approximately the same.
Further studies have examined the impact of a woven 6 mm thick CFRP laminate by an annealed steel sphere up to velocities of 1875 m/s. It was observed that above a threshold impact energy, the percentage of kinetic energy dissipated by the laminate was constant (see below). Further, the level of damage, as measured by C-Scan and through-thickness microscopy remained roughly constant as the impact energy was increased. However, the size of the hole formed increased. This suggested that the energy transferred to the target in the velocity range of interest became independent of the delamination. Consequently, the main energy transfer mechanism at the high velocities of impact is thought to be due to the cavity expansion and more importantly, the kinetic energy of the particulates.
Other research has focussed on two thicknesses of a woven CFRP laminate that have been subjected to impact by a steel sphere in a velocity regime ranging from 170 to 374 m/s. It was shown that for the normal incidence targets at the higher velocities of impact, a conical mass of laminate was ejected ahead of the projectile. Furthermore, despite the energy transferred to the plate increasing with impact energy, the degree of delamination in the thicker targets decreased indicating a change in projectile penetration mechanism. Eventually, the degree of delamination in the thicker targets appeared to approach an asymptotic level whereas for the thinner targets the degree of delamination appeared constant regardless of impact energy. For oblique targets, more of the kinetic energy was transferred from the projectile when compared to the same thickness of target that had been subjected to a normal incidence impact. However, this was merely due to a geometrical effect. Further, thicker panels appeared to behave more efficiently by absorbing more kinetic energy per effective linear thickness at the lower impact energies where petalling is a dominant factor in the penetration. This advantage appeared to disappear as the impact energy was increased.
MORE INFORMATION:
Hazell PJ, Appleby-Thomas GJ. A study on the energy dissipation of several different CFRP-based targets completely penetrated by a high velocity projectile. Composite Structures. Vol. 91. (1) pp 103-109 (2009)
Hazell PJ, Cowie A, Kister G, Stennett C, Cooper GA. Penetration of a woven CFRP laminate by a high velocity steel sphere impacting at velocities of up to 1875 m/s. International Journal of Impact Engineering. Vol. 136 (9) pp 1136-1142 (2009)
Dr Paul J Hazell
T: +44 (0)1793 784195
E: p.j.hazell@cranfield.ac.uk
