Micromechanical Bilinear Behavior of Composite Lamina Subjected to Combined Thermal and Mechanical Loadings

Abstract

The bilinear elastic degradation called the "knee" phenomenon, observed well in the transverse tensile stress–strain curves of some metal-based composites, is modeled through both a simplified three-phase cylindrical model and a hexagonal-arrayed unidirectional composite. The interphase is modeled by spring layers which account for continuity of tractions, but allow radial and circumferential displacement jumps across the interphase that are linearly related to the normal and tangential tractions. Even though constituent materials are in the elastic range all the way through, the possible low stiffness of the interphase and the residual stresses induced by uniform cooling yield bilinear elastic behavior in the stress–effective strain curves. However, perfect bonding or low stiffness in the interphase with no residual stresses creates a linear curve. The effects of interphase stiffness, fiber volume fraction, temperature change, and transverse tensile load on both the micro- and macro-thermomechanical behaviors of unidirectionally fiber-reinforced composites are analyzed numerically using the boundary-element method. These results are then compared to the elastic solutions of the three-phase model in a qualitative manner.