Abstract
The physical techniques of polarising microscopy, including the measurement of small (<0.1λ) optical retardations, have been used to investigate elastic fields adjacent to short (1-3 mm length) graphite fibers in epoxy resin composites. In as-cured specimens, i.e., in specimens self-stressed on account of resin cure shrinkage, the elastic field in the neighbourhood of fiber ends is, for surface treated fibers, characteristically different from that for untreated fibers and reflects the more efficient load transfer expected of the former. The regions of highest shear stress, defining the limits along the fiber length between which load transfer is effective, are seen to move inwards, towards each other, during hot (80°C) water uptake. This is interpreted as the propagation of "end cracks" and the con stant rate at which it occurs is shown to be dimensionally compatable with a diffusion con trolled mechanism. Also during water uptake, isolated pockets of interfacial pressure develop and these are attributed to water of crystallization at sodium carbonate impurity inclusions in the fiber surface. The magnitude of this pressure has been estimated from its temperature coefficient to be of the order of 200 bar (20 MPa) at ambient tem peratures.
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