Fracture phase field modeling of 3D stitched composite with optimized suture design

Abstract

Stitch, a flexible method of enhancing interlayer toughness, is widely applied in engineering. The strength of sutures with various stitching patterns dominates the interlaminar fracture toughness. Understanding the breakage process of sutures is vital to determine the interlayer failure of laminated composites. The failure modes of the stitches, however, have proven challenging to predict due to a series of potential complex responses such as debonding, interfacial crack, edge delamination, and breakage. In this paper, we propose a new phase field scheme to explore the progressive failure mechanism of single and multiple stitches, leveraging the advantages of the phase field method for complex fractures without introducing additional failure criteria. We reconstruct the phase field driving force by considering both tensile and compressive components of strain energy in the phase field evolution to deal with multiple forms of damage simultaneously. The fracture energy is modified to distinguish mode-I and mode-II fracture toughness. The zero-thickness smeared interface model is derived from the original allocation with a thin interphase layer by asymptotic analysis, effectively approximating the coherent imperfect interface between sutures and polymer matrix. The feasibility of the developed phase field scheme is verified through benchmark examples and experiments. The stepped fracture process and strength of 3D stitched composites with various stitching patterns are examined and compared with each other to reveal the fracture laws and damage mechanism. This study implies that the failure of the stitching system is strongly associated with the structural symmetry, closure, and distribution of the hole positions. On this basis, a new stitching strategy is proposed with a 13% enhancement in tensile strength with identical stitching density, which can be further extended to optimize the bearing capacity of multilayer structures with more complex stitching patterns.