Thermomechanics of Composites under High Temperatures

The thermomechanical properties of composites on polymer matrix at high tem peratures are essentially different from those at normal temperatures. The main distinctions briefly consist in the following: • at high temperatures there occurs an irreversible variation (degradation) of all mechanical and thermal properties of a material that usually has a complex non-linear character depending on time exposure under high temperature; • there are complicated internal physico-chemical processes in a matrix and fibres under high temperatures called by the general notion of ablation; the internal physico-chemical and mechanical processes run differently in the matrix and fibres, and this leads to the appearance of considerable internal thermal stresses. Generally speaking, a composite under high temperatures can be considered as a multiphase system consisting of solid, gaseous and fluid phases interact.ing mechanically and chemically with each ot.her. There are t.hree levels of temperature: normal, elevated and high. Normal, or room temperatures are 10 - 30°C; elevated temperatures are 30 200°C; hzgh temperatures are those above 200°C. However, the dividing line between elevated and high temperat.ures depends on the material involved; a temperature is called high for a particular composite material if, at this temperature, irreversible internal physico-chemical transformations occur in the matrix and/ or fibres of the material.