ELASTICITY AND FRACTURE IN PARTICULATE COMPOSITES WITH STRONG AND DEGRADED INTERFACES

Silane-coated glass microspheres randomly embedded in an epoxy polymer matrix have been employed as a model system to investigate the degrad ation of disordered composite materials by water, and to test various models of deformation and fracture. Numerous composites containing sod alime (A) glass in the range 0 to 25% by volume were tested dry and im mersed in saturated NaCl at 40 degrees C for periods up to 70 days bef ore testing. Enhanced osmotic water uptake due to percolating interfac e damage was observed for composites containing more than 15% glass. T he electrical resistance of similar composites filled with conducting spheres confirmed the existence of a percolation transition, though wi th high resistance values implying no direct contact of the spheres. T ensile measurements conducted on dry material at a nominal strain rate of about 10(-3) s(-1) showed an increase in elastic modulus and a dec rease in the fracture strength with increasing glass content. New deta il was apparent in these curves and confirmed by statistical analyses. For wet specimens, in addition to a general embrittlement effect of w ater absorption, there was a distinct plateau or small peak in fractur e strength in the range 9 to 12% glass, and an abrupt drop between 12 and 15%. The plateau can be related to favorable crack interaction eff ects between disconnected clusters of interfaces just below the percol ation threshold, The steep increase in elastic modulus with glass cont ent seen in the dry material vanished entirely in wet material, which behaved like a porous polymer above 6% glass, owing to osmotic interfa ce damage within particle clusters.