In a unidirectional polymer-matrix composite under axial tensile loading, the failure process is governed by the dynamic localization and clustering of multiple fiber breaks. The composite fails catastrophically once a critical cluster size (10-30 fiber breaks) is reached. The brittle failure of a fiber is a locally dynamic process which leads to stress concentrations in the interface, matrix and the neighboring fibers that can propagate at high speed over long distances.
To study this local dynamic event, a fiber-level Finite Element (FE) model of a 3-dimensional array of S2-glass fibers embedded in an epoxy matrix along with an experimentally-determined ‘zero-thickness’ interfacial cohesive traction law has been developed. The effects of thermal residual stresses and frictional sliding of the debonded fibers along the interface have also been incorporated in the 3D model. The modeling results indicate the importance of designing the fiber, matrix and interphase as a system in order to maximize energy-absorption during dynamic tensile failure of these composites. To determine the optimal combination of matrix and interphase properties, we need to experimentally map the size and spatial distribution of critical defects in the S2-glass fibers.
A novel continuous fiber bending experiment has been developed where all cross-sections along the length of multiple 20mm-long fiber segments are subjected to progressively reducing radii of curvature. The 7 different radii of curvature used in this experiment are obtained by embedding steel wires of radii ranging from 350um down to 25um onto the polished edges of razor blades. The spatial distribution of critical fiber defects obtained through this experiment will be presented along with a description of the methodology that will be used to integrate these inputs into the 3D micromechanical FE models. SEM images of the representative fracture surfaces obtained at the different radii of curvature will also be presented.