Elastic stiffness analysis of a thermo-formed plain-weave fabric composite Part II: analytical models

Two models are developed for stiffness analysis of plain-weave-fabric-reinforced thermoplastic laminates after thermo-forming. A repeating element of every fabric-reinforced lamina is modelled such that yarn reorientations in the fabric plane and changes in yarn crimp are reflected. These deformations were schematised in Part I of this paper for two in-plane thermo-forming deformation modes, simple shear and fabric stretching. To account for crimp, effective yarn stiffnesses are calculated with either an isostress or an isostrain assumption. Subsequently, the yarns and neat resin material are replaced by substitute layers, and overall laminate stiffnesses are calculated by using classical laminate theory. For weaves with moderate crimp, closed form solutions are obtained for both isostress and isostrain models. Parametric studies are carried out to establish the effect of very large thermo-forming deformations on laminate stiffnesses. It is found that fabric stretching and simple shear significantly affect predictions.