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.. 

Similarities of Fatigue Failures in Various Materials

A California materials expert has performed hundreds of materials failure analysis on a wide variety of materials. Surprisingly fatigue failures have the same appearance regardless of type of material. Above are three examples of fatigue. Upper left is a sample of a rubber fatigue failure. This is very similar in appearance to the CPVC pipe plastic fatigue failure (center) and the metal fatigue failure (right). All these fatigue failures start at one point and "progressively" spread out leaving redily distinguishable fatigue "beach marks" indicating the direction of travel.

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.