Effect of Thermal Shock on Flexural Modulus of Thermally and Cryogenically Conditioned Kelvar/epoxy Composites

Abstract

Utilization of polymeric matrix composites materials in safety critical structures necessiates their full characterisation especially where changing temperature environment is a concern. An attempt has been made here to assess the effects of thermal shocks on flexural modulus of thermally and cryogenically conditioned kevlar/epoxy composites. Thermal and cryogenic conditions were concurrently followed by downthermal shock(positive to negative temperature excursion) and upthermal shock(negative to positive temperature excursion) treatments respectively on the composite laminates. Residiual thermal stress developed by temperature gradient should be given critical attention in many application areas. The 3-point short beam shear(SBS) test was conducted on the conditioned specimens to evaluate modulus. The test results may indicate the relative level of bond strength in a composite system where only the bonding level is a variable. Post curing strengtening effect of thermal conditioning and mechanical keying factor of cryogenic conditioning are investigated here by electron micrographs. The high radial expansion coefficent of Kevlar fiber causes weakening of interfacial adhesion under the influence of temperature gradient. However, the weak interface may readily allow crack deflection along the interface and improves energy-absorbing capacity.