An analytical/experimental investigation was performed to study the effect of material nonlinearities on the response of composite tubes subjected to combined axial and torsional loading. An elasticity based analytical model was developed to predict stresses and deformations in composite tubes subjected to combined thermomechanical loading. Material nonlinearities were modeled using the Endochronic Theory. The effect of residual stresses on subsequent mechanical response was included in the investigation. Subsequently, experiments were performed on P75/934 graphite/epoxy tubes with a stacking sequence of [15/1/ ± 10/0/-15], using pure torsion and combined axial/torsional loading. The in-plane material properties needed for incorporation into the analytical model were determined using tests on flat coupons made from P75/934. In the presence of residual stresses. the analytical model predicted a reduction in the Initial shear modulus of a tube subjected to torsional loading. Experimentally. a difference in the nonlinearity of the stress-strain response was found between pure torsion loading and combined proportional loading. This difference is due to coupling between axial loading and shear strain. These phenomena were predicted by the nonlinear analytical model where a linear model did not. The experimentally observed linear limit of the global shear response was found to correspond to the analytically predicted first ply failure. The observed nonlinear response thus appears to be due to a combination of material response at the ply level and gradual damage accumulation. Further, based on cyclic torsion tests, the failure of the tubes was found to be path dependent above a certain critical load level. / Master of Science