Uniform fields, yielding, and thermal hardening in fibrous composite laminates

Abstract A uniform strain field is found in fibrous composite laminates with isotropic matrix and transversely isotropic fiber under uniform phase thermal strains and overall uniform stresses which are functions of thermoelastic properties of these phases. The only restriction on the structure of the laminate is that the plies be identical except for the fiber orientation. Thermoelastic properties of the phases are functions of temperature. The resulting uniform strain field in the laminate is isotropic. The corresponding stress field is uniform and isotropic in the transverse plane of each lamina and piecewise uniform in the longitudinal direction. In any case, the matrix stress is isotropic which causes no plastic deformation in plastically incompressible materials. The solution leads to a correspondence between thermal and mechanical loads in laminates which converts, in an exact way, any thermomechanical loading path to an equicalent mechanical path. Application of the thermomechanical uniform fields to initial yielding of composites identifies thermal hardening of the overall yield surface with translation along a stress vector that is a function of the phase thermal strains. Examples of thermal hardening in intermetallic matrix composite laminates are shown.