Process-induced residual stress in non-crimp fabric composites: Experimental and numerical evaluation considering viscoelasticity
Sera Koo, Yamato Hoshikawa, Yoshiaki Kawagoe, Kazuki Ryuzono, Tomonaga Okabe
Abstract
Non-crimp fabric (NCF) composites exhibit unique defects owing to their heterogeneous internal structure. These defects include manufacturing-induced fiber waviness and resin gaps caused by fiber tows stitched together with yarns. This study introduces viscoelastic bi-scale modeling to predict process-induced residual stress in NCF composites with epoxy (thermosetting) and PA6 (thermoplastic) resins. The prediction is performed using the time–temperature-dependent properties obtained from dynamic mechanical and thermomechanical analysis tests of the target resins. In the micro-scale, the finite element analysis (FEA) incorporating viscoelastic constitutive law was applied to a periodic unit cell model at the fiber/matrix level to obtain homogenized unidirectional lamina properties. The extracted lamina properties were passed to the macro-scale FEA model to simulate the processing of a cross-ply laminate. The proposed modeling reflected the resin gaps in the NCF to capture in-plane stress localization. The curing (chemical) and thermal responses during processing were analyzed, with viscoelasticity-induced stress relaxation integrated during the cooling process. The proposed method was validated by fabricating a cross-ply laminate of the NCF composite with both resin types and visualizing their respective warpage curvatures, which correlated with the residual stress. The FEA reproduced the measured curvatures well and demonstrated localized residual stress distribution. The findings thus underscore the development of residual stress and its viscoelasticity-induced relaxation in predicting the residual stress of NCF composites with appropriate consideration of heterogeneities.