Part 1: Plastic Pipe: Burst And Fatigue Testing Of PVC And HDPE Pipe: Bending Fatigue Tests On Polyvinylchloride (PVC) Pipe Joints

In a two-year program, seventy-eight specimens of PVC pipe with and without adhesive socket joints were tested in a four-point bending apparatus. Internal pressures were varied from atmospheric pressure (0 psig) to the rated internal pressure of 280 psig. Results of the plain pipe are compared to pipe with joints. All testing was done using a special apparatus developed to accommodate the large deformations required for fatigue testing of PVC pipes. This apparatus is described as well as the test results. Both strains and stresses are plotted against cycles-to-failure. Finite element elastic models of the socket joined specimens were analyzed to establish the areas of stress concentration and to explain the observed failures modes.

Phase I testing indicated that the pipe internal pressure might have a significant effect on fatigue life. Joined pipe specimens with no internal pressure were weaker in bending fatigue than pressurized pipe. As a result, the internal pressure was systematically varied between 0 psig and the rated pressure 280 psig. Results of Phase 2 testing showed that there is a dependence on internal pressure. Also, the effects of an adhesive primer and roughening the pipe surface on joint fatigue strength were also studied.

Strain rate is known to have a large effect on the fatigue of thermoplastic polymers. This effect has not been systematically studied and remains an unknown.

This research was supported by two PVRC grants: 96-15 (Phase 1) and 97-13 (Phase 2). A final report on Phase 1 (dated January 21, 1977) was approved as completion of that portion of the project. This report combines the results of Phase 1 and Phase 2 testing of PVC pipe.

Part 2: Plastic Pipe: Burst And Fatigue Testing Of PVC And HDPE Pipe: On the Determination of Long-Term Hydrostatic Strength of Plastic Pipe

An accurate determination of the long-term hydrostatic strength (LTHS) of plastic pipe is of interest to industry not only due to the continuous innovation of production processes and quality control of products but also due to the complication (and thus time- and cost-consumption) associated with current procedures.

In the first part of this report, a summary is presented of the literature dealing with the long-term hydrostatic strength of plastic pipe for both internal and external pressure loading. From the survey, it was determined that (1) a simple procedure for performing an internal hydrostatic pressure test is needed, and (2) a procedure for an external hydrostatic pressure test should be established.

To examine the feasibility of the survey results, a hydrostatic pressure test apparatus, the Double-Chamber Test Center, was developed. This system is designed for the testing of four-inch diameter plastic pipe, which may be subjected to either internal or external hydrostatic pressure at temperature varying from ambient temperature to 100°C (210°F).

A three-coefficient equation has for some time been employed to predict the life of plastic pipes. In this report, a new procedure that is based on the currently employed internal hydrostatic pressure tests is proposed. The procedure contains a different way to determine the test sample size and yields more reliable values of the three coefficients.

A similar three-coefficient equation for the critical, long-term, external hydrostatic pressure was established, with the three coefficients determined experimentally. A procedure for the experimental determination of the three coefficients is proposed.

Part 3: Plastic Pipe: Burst And Fatigue Testing Of PVC And HDPE Pipe: Ratcheting and Fatigue Characteristics of High Density Polyethylene (HDPE) Pipe

Joining of thermoplastic pipe is done using a number of fabrication techniques. The most popular method for HDPE pipe is to "butt weld" pipe sections by applying heat to melt the ends and supporting the joint until solidification can fuse the material. At times, a straight socket joints or an elbow socket joint is used to connect pipe sections. If the thermoplastic pipe cannot be effectively bonded using an adhesive, the pipe is fused in these sockets or "butt welded" to form a joint. In general, joints can be fused using a variety of fabrication techniques, such as:

1) Spin welding

2) Hot plate welding

3) Electric resistance heating of socket couplings

4) Induction heating of socket couplings

In some applications, thermoplastic pipe is joined to metal pipe by crimping with some type of metal band. However, the difference between the thermal coefficient of expansion of metal and plastics is an order of magnitude. Thus, the joint can suffer from fatigue when there are variations in temperature such as would typically occur in hot water pipes. This thermal cycling could culminate in joint fatigue failure.