The effect of low velocity impact damage on the performance of a woven CFRP.

A wide ranging study of the effects of low velocity impact on the performance of a
quasi-isotropic, woven CFRP laminate has been conducted. The study considered
the response of the laminate to constant velocity impact up to an incident energy
of approximately 9J. The resulting damage, a complex network of delaminations,
matrix cracking and fibre failure, was related to the incident energy and also to
the residual static tensile and compressive strength of the material.
The growth of matrix cracking and delamination and also the reductions in
tensile stiffness, measured locally over the impact damage site, were followed under
constant amplitude zero-tension fatigue on specimens impacted at approximately
3J and 7J, representative of two characteristic damage states. The growth of
matrix micro-cracking was found to be very rapid with numbers of cycles and was
related to increases in tensile strength of plain, notched and impacted specimens,
but substantial decreases in tensile stiffness. This type of fatigue related damage
was observed to act as a 'pseudo-plastic' zone providing stress relieving around
stress concentrations. No growth of this type of damage was noted in impacted
specimens when the ratio of maximum fatigue stress to residual static strength
was reduced to approximately 20%.
Growth of delamination was found to be related to the original impact damage
and was only rapid towards the end of specimen life. The propagation of this type
of damage under zero-tension fatigue was also apparently related to reduction in
tensile stiffness.
The applicability of available 'equivalent flaw' models to the residual tensile
and compressive strengths was investigated. In order to widen the applicability
of the equivalent flaw approaches, a model has been suggested which predicts
the fatigue strength of CFRP subjected to low velocity impact and subsequent
zero-tension fatigue loading.