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 f iber spacing was systematically decreased and interphase properties we re varied. Both the computational and experimental results demonstrate d that interphase properties and fiber spacing alter the location of t he maximum equivalent stress and the initiation of microcracks. Microc racks were predicted and observed to initiate first (at the lowest the rmal load) in the case of a higher modulus interphase. The cracks init iated at the fiber/interphase interface at a lower thermal load than i f no interphase were present. In contrast, computations for a low modu lus interphase predicted the maximum equivalent stress to be lower tha n the case of no interphase and to occur in the matrix. Experimentally , microcracks were observed to initiate in the matrix at the interphas e/matrix interface with higher thermal loads. The presence of the low modulus coating prevented cracks from reaching the fiber surface. Over all, the investigation demonstrated the ability of the interphase to e nhance or hinder microcracking in a cluster of fibers. The interphase can be tailored to reduce the local stress state and reduce the initia tion of microcracks in the composite.