GENERATION OF THERMAL STRAINS IN CARBON FIBER-REINFORCED BISMALEIMIDE (PMR-15) COMPOSITES .1. THE DETERMINATION OF RESIDUAL THERMAL STRAINS IN CROSS-PLY LAMINATES

The level of residual thermal strain which develops in PMR-15 cross-ply laminates has been determined from the curvature of unbalanced 0-degrees/90-degrees beams. The hysteresis in the temperature dependence of the beam deflection has been attributed to the plasticizing effect of entrapped curing volatiles (mainly water and methanol). Vacuum drying has been shown to remove the hysteresis but increase the magnitude of the residual thermal strain. The latter appears to result from a complex interaction between the enhancement in matrix expansion coefficient, a reduction in transverse modulus and the strain-free temperature (T1). Since T1 is in the region of 300-degrees-C it has not proved possible to demonstrate its reduction directly. These values of thermal strain have been compared with predictions obtained from measured transverse and matrix expansion coefficients (alpha-t, alpha-m). The presence of residual volatiles also appears to be responsible for some variability in the temperature dependence of the matrix-dominated expansion coefficients. Post-curing at higher temperatures gives rise to better reproducibility in the values of alpha-m, alpha-t and epsilon-t 1th. The role of these volatiles is discussed further in Parts 2 and 3.