Effect of Inter-laminar Toughened Layers on Process-induced Strain and Deformation of L-shaped Composites

Abstract

L-shaped composites are fundamental parts of complex-shaped structures and it is well-known that shape distortion (i.e., spring-in deformation) arises after curing due to the orthotropic nature of composites. This residual deformation significantly increases the manufacturing costs, attracting significant attention to the mechanisms of spring-in deformation. Recently developed aerospace-grade composites include inter-laminar toughened layers composed of thermosetting resin and thermoplastic particles to enhance inter-laminar fracture toughness and improve impact resistance. Even though inter-laminar toughened layers can affect process-induced strain and deformation, their effects have not yet been studied in detail. Therefore, fiber-optic-based internal strain measurements were performed and residual deformation of L-shaped parts with inter-laminar toughened layers was investigated in this study. The through-thickness cure shrinkage strain at the corner part was relaxed and the spring-in angle decreased by holding the parts at the cure temperature, indicating that the viscoelasticity of the thermoplastic particles is important. Viscoelastic finite element analysis supported this finding and indicated that the effect of the toughened layers on the residual deformation should be considered to optimize curing processes.