Prediction of long-term fatigue life of quasi-isotropic CFRP laminates for aircraft use

We have proposed the accelerated testing method to predict the fatigue life of FRP under an arbitrary combination of frequency, temperature, and stress ratio based on the time–temperature superposition principle and discussed experimentally about the validity and applicability of our proposed method. It has been clarified from these studies that our proposed method is applicable to predict the tensile and flexural fatigue strengths for unidirectional CFRP using PAN-based carbon fiber and epoxy resin. In this paper, the long-term flexural fatigue strength of quasi-isotropic CFRP laminates using PAN-based carbon fiber and epoxy resin was predicted based on our proposed method. Two kinds of quasi-isotropic CFRP laminates for aircraft use were employed. One is for nose structure UT500/135 which consists of twill-woven UT500 carbon fiber and 135 epoxy resin. The other is for wing structure T800S/3900-2B which consists of unidirectional T800 carbon fiber and 3900 epoxy resin with toughened interlayer. As results, the validity of our proposed method was confirmed for these CFRP laminates. Furthermore, it is cleared from predicted results that the flexural fatigue strength of UT500/135 decreases with increasing time as well as the number of cycles to failure. That of T800S/3900-2B decreases more strongly with increasing time comparing with that of UT500/135, however that decreases scarcely with increasing load cycles.