Type I size effect and failure behavior of woven composites under biaxial flexure

This study presents an experimental investigation into the failure and size effect exhibited by woven composites under biaxial flexure, induced via the ring-on-ring plate bending test. Square-shaped, epoxy/carbon twill woven composite plates of various sizes, scaled in three dimensions are considered. The biaxial flexural response of all specimens is found to be ductile and nonlinear with failure initiation at the top surface edges under combined in-plane compression and out of plane shear. These combined compression/shear cracks propagate towards the plate center, exhibiting a four-fold symmetry, and causing the plate to deform into a cone. Elastic stress analysis is presented to interpret and support the observations. The nominal stress at the onset of non-linearity is found to obey a type I size effect law, exhibiting a marked decrease with increased plate size but approaching a constant value. This size effect must be accounted for when designing woven composite parts to be subjected to biaxial flexure.