Effect of fatigue loading on stiffness degradation, energy dissipation, and matrix cracking damage of CFRP [±45]3S composite laminate

In this study, a concept of using experimental matrix crack density, residual stiffness, and energy dissipation as a comparative life monitoring tool for any arbitrary stacking sequence is proposed. First, IMA/M21 [+/- 45](3S) carbon fibre-reinforced polymer (CFRP) multidirectional (MD) laminates were fabricated by autoclaving technique. The static tension and compression tests were conducted to determine the fatigue stress levels. Then, constant amplitude tension-tension fatigue tests were carried out at two stress ratios (R = sigma(min)/sigma(max)) of 0.1 and 0.5. Three stress levels on the basis of cycles to failure (N-f) were chosen, ie, lower stress run-out (LSR), lower stress fractured (LSF), and higher stress fractured (HSF). The LSF and LSR specimens primarily degraded due to matrix cracking that caused higher stiffness degradation and lower energy parameter, whereas HSF specimens having the least possibility of matrix crack growth showed lower stiffness degradation and higher energy parameter in a fibre-dominated failure.