Studies of water damage in polyester glass laminates
Results are presented of an investigation into the combined effects of hot water and static uniaxial tensile stress on chopped strand mat reinforced polyester resins. Transport of water has been studied by measuring permeation rates, and the damage has been categorised by optical microscopy. Changes in flexural properties are reported. The resistance to crack propagation of polyester resins in the presence of water has also been studied. Isophthalic-acid based polyester resin/chopped strand mat laminates were prepared by the “contact moulding process”. Rates of permeation of water at a series of temperatures (35-80C) were measured by a gravimetric method for up to 2000 hours. The flow was Fickian in most cases, but there were circumstances where the transport became non-Fickian. The non-Fickian behaviour was observed at higher temperatures on prolonged exposures. A specially designed apparatus was used to stress laminates uniaxially in tension and to measure water permeation rates through the stressed samples. The effect of introducing external stress ([not greater than] 25% of the ultimate tensile strength) was to increase the rate of permeation. The progressive changes in the structures, as observed by microscopy took the form of (i) flat plate like circular disc cracks in the resins and resin rich areas of laminates, (ii) surface cracks in the resin casts and gel coats of laminates and (iv) partial failure of glass monofilaments by buckling. In most cases the damage was confined to regions very near the exposed surface. However, straining to 0.4% elongation led to the onset of debonding in transverse fibre bundles throughout the cross-section. Water damage was accelerated and modified by the presence of external stress. The laminates retained their flexural strength and modulus well. Laminates stressed to 0.3% strain at 80C in water for nearly three months retained 80% of their original strength. No catastrophic failure was observed. Single edge notched polyester resin specimens were exposed to water at three different temperatures for up to three months. The resistance to crack propagation as measured by the critical stress intensity factor was lowered at all exposure temperatures. An examination of the fractured surfaces revealed that no stable crack growth had occurred.