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.

Glass Expert Performs Failure Analysis of Glass to Metal Seals

California failure analysis expert and manufacturing engineer was engaged to perform a root cause failure analysis of shallow cracking in the seal glass. These glass to metal seals were made by one manufacturer and then sent out to another processor to be plated. The photograph on the left shows two sight glasses. The body of the seal has male pipe thread and there is a screw driver slot on the top. The intent is for these seals to be screwed into a container wall. The glass window allows one to view the container interior (i.e. see the fluid level). The photomicrograph on the right shows a typical crack. These cracks occured during plating; they are always shallow and always associated with the one of the screw driver slots. The "rainbow" that is seen is due to the air wedge created by the shallow crack. The stress pattern of these seals is symetrical; therefore, sealing stresses are not implicated. Because glass only fails in tension and because these cracks are associated with a screw driver slot, the glass expert believes that they occur when the seals come out of the hot plating bath (195°F) and cool rapidly either in the air or when spray rinsed.