A simple continuum damage model is developed based on an understanding of the effect of the damage mechanisms on effective material response parameters. A detailed micromechanical analysis, using the mathematical theory of homogenization is conducted, to understand the coupled influence of damage mechanisms and volume fraction on the degradation of the primary stiffness coefficients. The study further explains the generation of additional residual strains due to damage, through the degradation of the coefficient of thermal expansion. Using these studies, a macrolevel free energy expression is developed, and based on this expression, constitutive equations for macrolevel stresses and thermodynamic forces, corresponding to damage parameters, are derived. Further, a thermodynamically consistent, simplified model for evolution of damage is developed. The simplified model developed is effective in capturing the observed diffuse damage response of a coupon of any unidirectional composite, in the absence of laminate level delamination. © 2012 Elsevier Ltd.