Effect of annealing on the thermal expansion and residual stresses of bidirectional thermoplastic composite laminates

Thermal expansion measurements have been conducted on various [(±30)N]S laminates fabricated by combining different types of matrix and fibers. The main objective was to link the coefficient of thermal expansion (CTE) of these laminates to the thermophysical characteristics of the matrix. The results show how these laminates can be used to analyze the influence of annealing and glass transition temperatures on the CTE and the release of residual stresses. It is shown that to ensure a reproductive expansion during thermal cycling, the composite must be annealed at a temperature high enough over Tg of the matrix (below the melt temperature) in order to completely release the residual stresses induced during the molding phase and uniformize the crystalline structure of the matrix. Once this is done, the response of the material under thermal cycling gives reproducible coefficient of thermal expansion (CTE) with almost no distortion after each cycle.