Tension-tension fatigue performance of unidirectional out-of-autoclave composite manufactured by automated fiber placement

In manufacturing of Out-of-Autoclave (OOA) composites based on Automated Fiber Placement (AFP), it is inevitable to have defects such as gaps, which will affect the mechanical and fatigue properties of the final parts. In this work, out-of-autoclave prepregs IM7/CYCOM5320-1 are selected, Non Defective Samples (NDS) and Defective Samples (DS) are manufactured by automated fiber placement process. The influence of defects on the tension-tension fatigue behavior of 0° unidirectional out-of-autoclave composites is investigated. The fatigue life model and the residual strength degradation model are developed. The results reveal that due to laying defects, the static tensile strength of the composite decreased by 5.90%, and the tensile modulus decreased by 6.54%. As the fatigue load increases, the fatigue life of both non defective and defective composites shows a downward trend, while the fatigue life of the defective composite is always less than that of the non defective one. The longer the fatigue life is, the more differences are there between non defective and defective composites. Meanwhile, the influence of defects also increases with the fatigue life. In the investigation of the residual strength, both non defective and defective composites exhibit a constant behavior at the beginning of tension-tension fatigue tests, followed by a dramatical drop at specific fatigue cycles. Considering the influence of defects on residual strength of composites, the higher the stress level is, the more obvious the effect becomes. However, defects exhibit a similar influence on residual strength when the stress ratio is different.