The purpose of this paper is to study the hygrothermal effect on fatigue behavior of quasi-isotropic carbon/epoxy plain weave aerospace laminates containing artificial flaw under axial tension–tension loading. Dry and wet specimens were tested at tensile load-controlled cyclic loading with a stress ratio R = 0.1 and a load frequency of 7 Hz at room temperature (RT) and at 82℃ under different stress levels. Allowable stiffness change as a failure criterion was used to determine the delamination propagation onset threshold under cyclic tensile loading at each environmental condition. The delamination propagation onset was verified using the ultrasonic imaging (C-Scan) technique. The experimental results show that (1) fatigue life of CFRP specimens was more individually affected by moisture than by temperature and (2) combined moisture and temperature cause a drastic decrease in fatigue life. Finally, an investigation of the effect of hygrothermal conditions on stiffness degradation and damage of composite laminates subjected to tensile fatigue loading has been also carried. On the basis of the residual stiffness degradation, a damage variable was presented and phenomenological damage models were proposed by employing fatigue modulus and secant modulus concepts as measure of material damage.
