In this communication, the inelastic behavior of a unidirectional graphite/polyimide composite, whose constituent phases are both elastic and brittle, is characterized based on the hypothesis of shear-dominated fiber/matrix interfacial degradation as the primary cause of the observed nonlinearity. To accommodate a combined thermo-mechanical multiaxial loading in the off-axis specimens, the finite-volume direct averaging micromechanics (FVDAM) with damage evolution capability is established within a unified framework that uses discontinuity functions in conjunction with the bilinear cohesive-zone model. Meanwhile, the stand-alone FVDAM homogenization approach is combined with the particle swarm optimization algorithm to identify consistent in-situ fiber and matrix properties. The accuracy and efficiency of the present model with deduced properties are validated by comparing the simulated stress-strain responses against the experimental data in the literature with various fiber orientations and good agreements are obtained for all cases. Concomitant local stress distributions at different load steps are examined to demonstrate the stress transfer mechanism from the region of the damage interface to the surrounding matrix. For the first time, this study reveals that the off-axis dependent nonlinearity in this material system comprised of elastic fibers and brittle, linearly elastic matrix may be accurately captured using a damage evolution model rather than plasticity, viscoelasticity or viscoplasticity approaches typically employed for the matrix phase.