Effect of inter-laminar toughened layers on process-induced strain and deformation of L-shaped composites

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 increases the manufacturing costs, attracting significant attention to the mechanisms of spring-in deformation. Meanwhile recent aerospace-grade composites include inter-laminar toughened layers composed of thermosetting resin and thermoplastic particles to enhance the inter-laminar fracture toughness and to improve the impact resistance. Even though the inter-laminar resin layers can affect the process-induced strain and deformation, their effects have not yet been studied in detail. Therefore, this current study performed fiber-optic-based internal strain measurement and evaluated the residual deformation of L-shaped parts with inter-laminar resin layers. 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 viscoelasticity of the thermoplastic particles is important. Viscoelastic finite element analysis supported this finding and indicated that the effect of inter-laminar toughened layers on the residual deformation should be considered to optimize curing processes.