Fatigue life determination based on infrared thermographic data for MultiDirectional (MD) CFRP composite laminates

Abstract

Infrared thermography is a useful tool for the rapid determination of fatigue properties. A previously proposed fatigue life model, which combines the stiffness degradation k(N) with thermographic data ΔTstab, has been successfully applied to UD and ±45° CFRP laminates. In view of more complex damage mechanisms in multidirectional laminates, the model is modified in this work to overcome the conservative predictions. The S-N curves determined by the modified model are compared with the experimental results of traditional fatigue tests on Quasi-Isotropic (QI) and cross-ply CFRP laminates. Very good consistency was observed. In particular, the S-N curve corresponding to fatigue life of Nfl=106 cycles is conservative compared to that from the traditional fatigue test results, but the maximum safety factors are no more than 1.15 and 1.08 for QI and cross-ply laminates, respectively. Both S-N curves corresponding to Nfl=107 cycles for two tested laminates lie within the 95% confidence interval of traditional experimental results. Therefore, based on infrared thermographic data, the modified model proposed in this study is more general. It allows to determine the S-N curves of QI and cross-ply CFRP laminates just in about ten hours of testing machine time instead of one month and a half using traditional fatigue tests, even though more experimental data (other types of MD stacking sequences, composite materials, under other fatigue modes, etc.) are necessary to demonstrate the applicability of modified model to all types of MD laminates.