Fatigue strength properties of interlaminar toughened CFRP laminates under cyclic loading in the out-of-plane direction

Abstract

In this study, fatigue strength properties of interlaminar toughened CFRP laminates in the out-of-plane direction, or through thickness, were investigated. Thick laminates whose stacking sequence was unidirectional were formed with 88 plies of T800S/3900-2B prepreg. The T800S/3900-2B prepreg is constituted of fiber layer and interlaminar toughened layer in which polyamide particles are dispersed. The material properties of the thick laminates were measured by compression test and 4-points shear test. Spool specimens machined from the thick laminates were loaded in the out-of-plane direction. Stress distributions of spool specimens were evaluated by FE analysis. Fiber layer and interlaminar toughened layer in the each ply were modeled separately in the analysis. In comparison to the out-of-plane direction, the properties of in-plane transverse direction were investigated with 90° thin laminates. To evaluate the fatigue strength properties quantitatively, an analytical equation was introduced for the results of fatigue test. The fracture surfaces of the specimens after static and fatigue tests were observed by SEM. From the observation of the fracture surfaces after fatigue test, it was found that the interfacial debonding between fiber and matrix was occurred due to cyclic loading in both of specimens. In addition, it was observed that crack generated from debonding grew in the fiber layer until ultimate fracture. Moreover, the experimental and analytical results showed that, in comparison to the in-plane transverse direction, the fatigue life in the out-of-plane direction is shorter.