Simple models for triaxially braided composites

Abstract

Comparison of measurements and theory shows that the elastic properties of some triaxially braided glass/urethane composites are predicted as well as they can be by a simple laminate model modified to account for measured tow waviness. Any remaining discrepancies are the result of inevitable irregularity in the positioning of tows and are not grounds for more sophisticated modelling. The same modified laminate model provides estimates of the partitioning of in-plane stresses among tows of different orientation. Estimates of these local stresses correlate well with measured compressive and tensile strengths in the primary load-bearing direction. Absolute values for compressive strength are predicted quite accurately by kink band models, using independently measured values for tow misalignment angles and the critical shear stress for resin flow. Some summary design principles are presented based on the model of elasticity and observations and modelling of tensile and compressive strength.