Search

Thermoplastic pipes are often manufactured with profiled walls to maximize the flexural stiffness of the pipe for a given amount of polymer. Thin elements in the profile can buckle under the influence of large earth pressures associated with deep burial or other extreme loading conditions. Earth load tests have been conducted on high density polyethylene pipes with a number of different wall profiles. Two high-pressure pipe test cells have been used to conduct these tests. Observations of local buckling in the internal liners of these products have been examined and compared to stability assessments based on the conventional equation for buckling in stiffened plate structures (following modification of that equation to an equation that defines critical strain instead of critical stress). The strain levels that develop in the liner are, however, dependent on three-dimensional bending within the pipe profile. Provided the effects of three-dimensional bending in the pipe profile are considered, the modified Bryan equation appears to be a useful tool for quantifying liner stability and should be considered for inclusion in limit-state design procedures for these structures.

Two uniaxial constitutive models, a nonlinear viscoelastic (NVE) model and a viscoplastic (VP) model, have recently been developed for time dependent inelastic analysis of ductile polymers. This study extends these models to provide a multiaxial formulation. The new multiaxial models are implemented in a general purpose finite element program, together with a conventional treatment of geometrical nonlinearity. The solution procedure is based on a simple explicit integration method that is suitable for the new material models and is easy to implement. Two example problems are studied to evaluate the performance of the new finite element procedure. Finite element simulations of a perforated high density polyethylene (HDPE) plate under remote uniaxial loading are performed, and finite element model simulations of an HDPE pipe section under parallel plate loading are also performed. The finite element modelling results are then compared with the measured experimental results. It is found that the finite element model provides good predictions of the inelastic deformation of HDPE under various loading histories, including cases that involve a highly nonlinear material and geometrical response.