Tensile fatigue damage and its mechanism of stitched composite laminates

Stitched laminate is a new kind of composite structure toimprove the interface strength of composite. But the in-plane properties ofthe laminate may reduce because there are local fiber bending in thelaminate plane and in the thickness direction, and the damage induced by thestitching threads. A lot of researches have been carried out on theproperties of laminates to push this material into engineering applications,such as in-plane tension and compression, compression after impact,hygrothermal properties, etc. But few studies have been performed onstitched composite fatigue properties. The aim of this paper is tocomprehend the fatigue damage development and mechanism of stitchedlaminates under tensile loads. The material of the laminates was uni-weaveT300/QY9512 stitched in 0^\circ direction. Un-stitched laminates were studiedfor comparison with the stitched laminates.Experimental study was carried out on the fatigue performance of stitchedand non-stitched composite laminate. The finite element models wereestablished to obtain the stress or strain distributions in the laminates.The mechanism of tensile fatigue damage origination and propagation ofstitched laminates was analyzed. The results show that the tensile fatiguedamage origination and propagation mechanisms of laminates have been changedby stitching. The stress concentration of in-plane stress \sigma_x andinterlaminar shear stress near the stitching node plays an important role indamage origination and propagation. Stress concentration of interlaminarstress at the free sides has effect on fatigue performance of stitchedlaminates. The stress concentration of interlaminar stress at the free sidesis the main reason that causes fatigue damage and its propagation ofnon-stitched laminates.