Nonlinear deformation and elastoplasic collapse of a thermoplastic liner in a steel (or composite) pipe is investigated by a combined theoretical and numerical method. Results of the study on the effect of tightness on thermoplastic liner collapse resistance are reported in this paper. The predictive method is formulated by the use of large-deformation theory of
plastic instability and nonlinear computational mechanics. The collapse (buckling and postbuckling) processes of a liner in a steel pipe have been modeled quantitatively for the cases with various tightness conditions. Failure modes, critical pressure, and associated liner collapse resistance for each case are determined. Comparing with previously reported elastic collapse of the liner problem, one finds that liner tightness significantly increases its collapse resistance and that liner plasticity reduces substantially the critical collapse pressure.