Plastic Failure Analysis: Automobile Part

Read Consulting was asked to perform plastic failure analysis on a broken pickup truck tailgate latch. Attached are a series of photographs demonstrating the latch and the cause of failure. The latch is mostly held to the back gate with two machine screws that are screwed into PEM inserts pushed into bosses when the plastic is still hot. Over time the flexing of the steel PEM insert in the plastic boss generated fatigue cracks at the sharp edge of the bottom of the insert. This is a typical plastic failure created by the stiffer steel insert applying forces to the weaker plastic receptacle. Often the result of such an interaction is plastic fatigue.. 

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 Failed Plastic Chair

Plastic Failure Analysis Expert performs a failure analysis on a plastic "lawn chair" that collapsed when someone sat in it. The right arm separated from the seat. Upper left is a photograph of the subject chair with the separated leg. Fractography, top center photograph, indicates that the failure was progressive (i.e. a fatigue failure). Examination of exemplar chairs at the site showed identical small cracks where the leg joins the side of the seat. All the chairs examined had this manufacturing defect, and these will eventually show fatigue faikure. This is a product defect (design defect) that must be corrected to improve the performance of the chair. Otherwise, this is a product liability that exposes the manufacturer.

Product Liability Expert Asses Changing Part Suppliers

Product liability is strongly dependent on part quality. This is a big issue when changing part suppliers, and there are times a product liability expert must be hired to examine parts from the new supplier.

In this case the supplier of a molded nylon part was changed and plastic failure analysis tests showed that the thread strength of the replacement part was inadequate. The possibilities considered were a change in the poloymer, improper glass filling and incomplete thread geometry. The new supplier actually failed on all three counts. He substituted a different polymer, he used shorter glass fibers, and he did not properly mold the part. A failure analysis expert performed a root cause failure analysis which indicated that incomplete molding of the threads was the main cause of the decrease in strength. A failure analysis expert cross sectioned representative parts and found the female threads were incomplete, and these provided inadequate strength. The cross section photomicrograph on the upper left shows the representative threads of a part from the original supplier. Upper right is that of the alternate supplier. One can see that the alternate supplier had formed incomplete threads.

CPVC Plastic Failure Analysis of Hot Water Pipe Fitting

Plastic failure expert performs a failure analysis on a failed CPVC 90° 3/4 inch elbow. This elbow was in a 150°F hot water recirculation system in a large facility. Plastic fatigue was the alledged cause of failure. However, a plastic failure analysis found no evidence of fatigue. Instead, the detailed root cause failure analysis indicated that this elbow had molding defects and had deteriorated in service; in the top left photograph one can see that the CPVC inner and outer surfaces have turned brown. The failure originated at a stress concentration point caused by the interior 90° angle. It initiated as a brittle fracture on the outside. It then became an overload failure in mid thickness. Finally, there were layer separations toward the interior of the elbow. The photo on the right is of the vent hole. This shows that the CPVC came out in layers. Thus, during plastic molding there were layers of CPVC that did not "recombine". Given that this elbow is formulated to operate continuosly at 180°F, this elbow was a defective product due to both plastic molding defects and plastic formulation problems.

Plastic Failure Analysis

Plastics failure expert performs a root cause failure analysis on a plastic automobile door handle. The handle is from a van's sliding side door. The product failure occured when the user was trying to close the van's sliding door. This is a products liability issue because the handle failed in a brittle manner and a personal injury occured. The lower photograph is of the handle assembly after it was removed from the vehicle. The two arrows point to the two fracture surfaces on the broken handle. The upper photograph is of the fracture surface on the handle piece that came free. The failure started as a fatigue crack (see arrow). It initiated at a molding defect similar to the one indicated by the second arrow on the left of the photograph. Molding defects are manufacturing defects caused by improper molding conditions. The molding defect created a stress riser allowing crack initiation. The crack progressed due to fatigue. The final failure mode was overload, and it was mostly a brittle failure.

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.