Effect of gap and overlap on mode Ⅱ interlaminar fracture toughness of automated fiber placement prepreg laminates

Automated Fiber Placement(AFP) technology facilitates the manufacturing process of composite structures with complex geometry owing to its high efficiency and accuracy.However,the unavoidable imperfections induced by the automated layup method bring challenges to the stability of the final mechanical properties of composites.The influence of AFP-induced gaps and overlaps on the mode Ⅱ interlaminar fracture process of oven-cured laminates is experimentally investigated and explicitly revealed.End-Notched Flexure(ENF) tests were performed to measure the interlaminar fracture toughness of unidirectional laminates under three different defect configurations,namely "Gap" "Overlap" and "Gap & Overlap".A marginal decline(about-1.2% in non-precracked tests,-5.1% in precracked tests) and an obvious increase(about 15.2% in nonprecracked tests,5.2% in precracked tests) in fracture toughness were observed for the "Gap" group and the "Overlap" group,respectively.Fractographic studies which involve the metallographic observation and Scanning Electron Microscopy(SEM) observation revealed that the increases in fracture area and crack length helped improve the fracture toughness of the "Overlap" group.However,the resin-rich area with high porosity induced by gaps was detrimental to the delamination resistance.To capture the interlaminar stress distribution local to imperfections,a three-dimensional numerical model was established.Gap defects facilitate the local delamination process,and the overlap defect postpones the damage onset and the propagation.