Thermal Deformations and Residual Stresses in Fiber Composites

Abstract

The basic composite lamina is highly anisotropic thermally. Thermal expansion is lowest in the fiber direction and highest in the matrix-dominated transverse direction. The low coefficient of thermal expansion (a) and high modulus in the fiber direction, especially in graphite composites, allow fabrication of angle-ply laminates with near-zero thermal expansion. This is of great importance in structures requiring exceptional dimensional stability. The coefficient a for a single ply can be calculated knowing the thermal and mechanical properties of the constituents and their geometric distribution. Thermal expansion in angle-ply laminates can also be predicted reasonably well by means of lamination theory. An important result of thermal anisotropy is the introduction of lamination residual stresses in angle-ply laminates during curing. They can reach values comparable to the transverse strength of the ply and thus induce cracking of that ply within the laminate. They are equilibrated with interlaminar shear stresses transmitted from adjacent plies and can thus cause delamination. Residual stresses have been investigated analytically and experimentally. It was found that the significant strains recorded during the cooling stage of curing correspond to thermal expansion of the laminate. Residual or restraint strains are computed from measured restrained and unrestrained thermal expansions. Residual stresses are computed using appropriate orthotropic constitutive relations. Results have been obtained for a variety of materials including boron, graphite, Kevlar, S-glass and hybrids with epoxy or polyimide matrices, for a variety of lamination angles. It was found that residual stresses do not relax appreciably with time. Results show that, for graphite and Kevlar laminates, residual stresses at room temperature are high enough to have caused damage in the transverse to the fiber direction.