Mechanism of surface microcracking of matrix in glass-reinforced polyester by artificial weathering

The first stage in the deterioration of glass-fibre reinforced polyester (GRP) composites, fibre prominence, has been reported. The mechanism of the second stage, surface microcracking, is now described. Under controlled conditions GRP sheets were subjected to cyclic variation of moisture and temperature and to radiation. It is proposed that surface microcracking takes place under the combined action of radiation-induced tensile stresses in the surface region and physically-induced stress-fatigue. Tensile stresses in the surface region are caused by shrinkage of the matrix that results from cross-linking induced by the ultra-violet portion of radiation. Stress fatigue is imposed on the composite system by physically-induced alternating stresses produced by cyclic variation of temperature and, probably, moisture resulting from thermal and moisture gradients and inhomogeneities. Stress-fatigue probably plays a dominant role in microcracking induced by artificial weathering, whereas radiation-induced stresses in the surface region are more important in microcracking occurring in outdoor weathering.