An experimental study of low velocity impact damage in woven fiber composites

A study is made on the low velocity impact response and post-impact mechanical capacity of woven fiber [0/90, - 45/45,0/90], carbon epoxy composite plates. This complements the work done by numerous other researchers who have also examined low velocity impact of composites, but have focused on uni-directional, cross-ply or quasi-isotropic laminates. In the present study, post-impact static uniaxial tension, compression, as well as tension-compression fatigue tests are performed. The damage mechanisms for woven laminates are found to be predominantly delamination and fiber breakage, with the area of impact-induced delamination increasing linearly with impact energy for the range of energies examined. Damage extent and type are also dependent on the curvature of the impactor tip and deformation generated by a sharp impactor is more localized. The existence of a threshold energy level below which no delamination discernible by C-scan occurs is noted. In contrast with previous findings, it is observed that the peak in the impactor deceleration-time response is not associated with the onset of fiber failure, which can occur earlier. The amount of energy absorbed when this peak occurs therefore does not indicate the energy required to initiate fiber breakage. It is observed that residual tensile strength is a function of delamination area and impactor tip radius. For static compression and tension-compression fatigue, the residual load-bearing capacity is only dependent on delamination area. Under compression, delamination promotes the micro-buckling of fibers, whereas in tension, failure is predominantly via fiber breakage. For a common impact energy and impactor, a damaged specimen is weaker in compression than in tension.