Thermally Induced Interfacial Microcracking in Polymer Matrix Composites

Novel experiments on a cluster of fibers combined with finite element analysis was utilized to investigate the influence of both inter-fiber spacing and interphase properties on thermally induced microcracking. Local thermal stresses were predicted and microcracking observed as fiber spacing was systematically decreased and interphase properties were varied. Both the computational and experimental results demonstrated that interphase properties and fiber spacing alter the location of the maximum equivalent stress and the initiation of microcracks. Microcracks were predicted and observed to initiate first (at the lowest thermal load) in the case of a higher modulus interphase. The cracks initiated at the fiber/interphase interface at a lower thermal load than if no interphase were present. In contrast, computations for a low modulus interphase predicted the maximum equivalent stress to be lower than the case of no interphase and to occur in the matrix. Experimentally, microcracks were observed to initiate in the matrix at the interphase/matrix interface with higher thermal loads. The presence of the low modulus coating prevented cracks from reaching the fiber surface. Overall, the investigation demonstrated the ability of the interphase to enhance or hinder microcracking in a cluster of fibers. The interphase can be tailored to reduce the local stress state and reduce the initiation of microcracks in the composite.