# 1995 Edinburgh
In this contribution, some aspects of the huckling hehaviour of polymer pipes are treated; particular emphasis is placed on the hehaviour of pipes under lateral pressure. This work contains a classification of the failure cases in the plastic pipes due to instahility, a summary of some of the existing stahility criteria for polymer pipes, results of some long-term experiments on the pipe sections, results of a number of huckling experiments on the PE-HD pipes, and also results of a number of theoreticallnumerical simulations. The material tests, which were performed in this investigation, consisted of short-time and longtime diagonal compression of the PE-HD pipe (Q280 mm) ring sections. Furthermore, shortterm and long-term huckling tests were performed on 3 m long PE-HD pipes (PN 3.2) made of the same material. For huckling tests, an equipment was built which was capable of inducing vacuum inside the hanging pipes; the lateral deformation of the pipes was measured hy 8 gauges which were installed at the midheight of the pipes. The huckling tests showed that the pipes would experience lateral deformation up to ahout % I 0 of their diameter under negative pressure; this was found to he below the calculated theoretical huckling pressure. Part of the theoretical study included an analytical modelling of the pipe ring experiments. In this part, for determination of the long-term material properties, a fourparameter viscoelastic model was used and, through its correlation with the pipe ring data, short-term and long-term (creep) properties of the PE-HD pipe material were ohtained. The buckling calculations consisted of analytical and Finite Element numerical simulations of the critical pressure of hydrostatically as well as partially loaded pipes. From the comparison of the experimental results, the theoreticallnumerical simulations. and the existing standards it has heen concluded that the various huckling calculations and also the cited stahility criteria give different results. which are not the same as the experimental results. Hence it is proposed that, for establishment of valid stahility criterion, coordination of various methodologies and determination of correlation factors among various methods on one side and the experimental results, on the other side should he made. Based on the present investigation and upon reference to the existing stahility criteria, and through comparisons of theoretical and experimental data a procedure for the stability design of plastic pipes is proposed.