Thermal effects on polymer matrix composites: Part 1. Thermal cycling

Polymer matrix fibre reinforced composites are usually cured at an elevated temperature and then cooled to ambient conditions. Because of their heterogeneous nature and the very dissimilar expansion/contraction behaviour and mechanical properties of the two components thermal stresses are easily generated and can prove troublesome. The effects are exacerbated by thermal cycling, which can occur in a working environment. Furthermore, composites are usually employed at a temperature within approximately 150 °C of the glass transition temperature, Tg, of the matrix and in extreme conditions within approximately 20 °C or less of Tg. This tends to be a harsher thermal environment than is experienced by other materials and can lead to degradation of the composite and a reduction in properties. Both effects, thermal cycling and thermal degradation, are surveyed in this two-part paper with, mainly, reference to work reported during the last decade. For thermal cycling most work has involved carbon-fibres, principally with a modulus in the range 250-300 GPa and a CTE of -0.3-1×10-6 °C, along the fibre axis, compared with properties for the polyimide and epoxy matrices of approximately 3 GPa and approximately 50×10-6 °C-1, respectively. With a 100 °C temperature change and a unidirectional laminate stresses of ±15 MPa can be generated, with somewhat higher values for a typical 0/90 laminate (the type most often studied experimentally). Under thermal cycling damage, mainly matrix cracking resulting in reduced flexural and transverse properties, was widely reported and the role of moisture and volatiles in reducing cracking, noted. Unfortunately the simple theoretical treatment developed some time ago was rarely used in assessing the results.