Micromechanical bilinear behavior of composite lamina subjected to combined thermal and mechanical loadings

The bilinear elastic degradation called the "knee" phenomenon, observed wel l in the transverse tensile stress-strain curves of some metal-based compos ites, is modeled through both a simplified three-phase cylindrical model an d a hexagonal-arrayed unidirectional composite. The interphase is modeled b y spring layers which account for continuity of tractions, but allow radial and circumferential displacement jumps across the interphase that are line arly related to the normal and tangential tractions. Even though constituen t materials are in the elastic range all the way through, the possible low stiffness of the interphase and the residual stresses induced by uniform co oling yield bilinear elastic behavior in the stress-effective strain curves . However, perfect bonding or low stiffness in the interphase with no resid ual stresses creates a linear curve. The effects of interphase stiffness, f iber volume fraction, temperature change, and transverse tensile load on bo th the micro- and macro-thermomechanical behaviors of unidirectionally fibe r-reinforced composites are analyzed numerically using the boundary-element method. These results are then compared to the elastic solutions of the th ree-phase model in a qualitative manner.