Effects of low-velocity impact on the quasi-static and fatigue performance of 3D woven composites

This work investigates the static and fatigue degradation after low-velocity impact (LVI) and the corresponding mechanisms of 3D woven carbon/epoxy composites with a fiber volume fraction of 53 %. Drop weight tests employing impact energies ranging from 6 J to 20 J are performed to introduce damage to the specimens. Quasi-static tensile tests after impact (TAI) and tension–tension fatigue tests after impact (FAI) are then carried out to study the post-impact behaviors. The results of TAI tests show that the residual strength decreases linearly by up to 62 % as impact energy increases to 20 J. Similarly, residual stiffness shows a linear decline with increasing impact energy until 15 J. However, as the impact energy increases to 20 J, the residual stiffness decreases significantly and deviates from linear decreasing trend due to the impact causing fiber breakage. The FAI test results show that LVI can reduce the fatigue life. But such effect is limited for small loading levels. It is worth noticing that even slight impact damage can reduce the static and fatigue performance significantly. To understand the mechanisms behind it, the damage evolution in TAI and FAI tests is analyzed through the CT technique. The results show that even though low energy LVI does not cause fiber fracture, the stress concentrations induced by matrix cracking and delamination still reduce the uniformity of the internal stresses, thus causing fiber fracture in sequence and reducing the mechanical and fatigue performance of 3D woven composites.