Finite Element - Based Strength Prediction for Notched and Mechanically Fastened Woven Fabric Composites

A literature review has been carried out relating to the damage and fracture behaviour of composite laminates based on woven fabric reinforcement containing a stress raiser in the form of either a circular hole or of a mechanically fastened joint – the tensile failure modes in these two types of problem are very similar. Closed form and finite element based approaches that enable the stress distributions in these two classes of problem are presented prior to a review of some of the existing failure models. The failure models considered are based on strength and fracture mechanics approaches, which are applied in some cases at the laminate level and in others on a ply-by-ply basis. A number of these approaches invoke the use of a characteristic distance, which is a material property, but may also be specimen geometry dependent. The aim of the present work is to develop a more unified model for damage and fracture at tensile stress concentrations within a finite element analysis. To this end a traction-separation law (based on physically meaningful material parameters) is implemented within ABAQUS CAE and used to predict the strength of woven composite plates containing open holes and mechanically fastened joints. This approach is applied first to the open hole problem within a two-dimensional framework, with reference to several data sets from the literature. Agreement is good, both with prior experimental data and other modelling approaches. The bolted joint problem is then considered and a two dimensional approach is applied to model net-tension failure data for woven GFRP bolted joints from the literature, using the same traction-separation law as applied to the open hole problem. Agreement is reasonable, but a need for a model that incorporates bolt clamp-up is apparent. Subsequently an extensive experimental data set is obtained for the failure strength of a range of woven CFRP bolted joints, both double-lap and single-lap construction with various lay-ups, plate geometries, hole sizes and bolt clamp-up. These test configurations are then modelled using a 3-D finite element framework. Good agreement between the predicted and measured bearing stress at failure was obtained for double-lap joints that failed in the net tension failure mode. Less good agreement was obtained for the single-lap joints, where the tensile fracture mechanisms were more complex and not captured fully in the model.