Plastic Failure Analysis: Fusion Welded HDPE

Poor Welding of a Carrier
Pipe Joint There was no
 Melting on a part of the joint

Read Consulting, a California failure analysis lab, has performed numerous plastic failure analysis on various types of plastic pipe. This includes PVC, CPVC, Delrin and Polypropylene. In this case a plastic failure analysis was performed on a failed containment waste transition line made with fusion welded high density polyethylene pipe. The line was several miles long. The interior transmission pipe had an OD of 8 3/8" and a 1/2" wall thickness.  The containment pipe had  12 3/8" OD with a wall thickness of 1". There were numerous leaks in the line,  and all were found to result from poor fusion welding procedures. The upper right photograph shows a weld on the interior carrier pipe that leaked. In this case the ends of the pipe had not been properly welded before the pipes were fused. This resulted because the pipe end never contacted the heater plate. Obviously, the fusion weld procedures were not properly followed.  It is important to follow exactly the manufacturer's recommended procedures when fusion welding high density polyethylene pipe. In addition, all joints should be leak checked before being buried in the ground. Poor fusion welding creates leaks that are hard to find and are very expensive to remedy. 

Failure Analysis of CPVC Pipe Fitting

Failure Expert Examines Plastic Pipe Failure

A 90° CPVC Elbow connected to a hot water line failed after three years of service in a system operating at 130°F. The photograph at the right shows the failed elbow after it was removed. The crack is on the inside of the 90° angle.

This photo is a closeup that shows the crack in the elbow. Although it only goes partially around the elbow, it leaks.

The elbow was intentionally broken to expose the initial fracture surface. The failure shown at the right is a fatigue failure initiating from the outside and working inward. The arrow points to the fatigue striations. Also, one can see staining where the water was leaking out through the crack. Apparently cyclic stresses were applied to the elbow. These could have come from either vibrations or stresses created by thermal expansion of the attached pipes when they are heated up by the 130°F water.