Computational investigation of stress, contact conditions, and buckling of thin-walled pipe liners

Thin walled polymeric liners are often used to rehabilitate deteriorated pipe lines. The host pipes into which these liners are installed are typically assumed to be structurally sound, and the liner is only expected to carry the external pressure exerted by the groundwater. This external pressure will induce creep deformation and radial deflections that may eventually result in collapse of the liner within the host pipe. The aim of this work is use computational modeling to better understand the evolution of conditions leading up to collapse so that improved liner design models can be developed. Emphasis is placed on a close examination of the contact forces, contact areas, displacements and stresses for short-term and long-term liner buckling. The contact force is seen to enhance the buckling resistance of the liners by inducing a reverse moment which decreases the deflections and stresses at the critical point in the liner. For pressure levels less than 30% of the critical pressure, the stresses at the critical point in the liner are dominated by compression, indicating that compressive material properties are most appropriate for liner design. The formation of inverse curvature at the liner buckling lobes indicates that failure is imminent, since the rate of stress relaxation can no longer keep pace with the rate of stress increase due to increasing curvature and deflections at the critical point. The liner tends to perform more like a beam rather than an arch after inverse curvature has occurred. The value of the applied pressure and the creep properties of the material are seen to have a tremendous effect on the expected lifetime of liner systems. An improved short-term liner buckling model is developed that accounts for all of the couplings between the liner to host pipe gap, the diameter to thickness ratio, host-pipe ovality, and local intrusion imperfections. Three-dimensional finite element models are used to show that the critical length to diameter ratio for specimens used in liner buckling experiments around five. Finally, the effect of multiple local imperfections on the deformation history, short-term buckling pressures, and long-term buckling times are explored using an improved two-dimensional finite element model in which asymmetric deformation modes are permitted, allowing the liners to buckle in a natural way. These results indicate that any variations in material or geometrical parameters that induce scatter in short-term liner buckling tests are expected to induce much more scatter in long-term tests