This article originally appeared in The Journal of Practical Failure Analysis – April 2001
A failure analysis was conducted on a flow-sensing device that had cracked while in service. The polysulfone sensor body cracked radially, adjacent to a molded-in steel insert. This article describes the investigative methods used to conduct the failure analysis. The techniques utilized included scanning electron microscopy, Fourier transform infrared spectroscopy, differential scanning calorimetry, thermomechanical analysis, and melt flow rate determination. It was the conclusion of the investigation that the part failed via brittle fracture, with evidence also indicating low cycle fatigue associated with cyclic temperature changes from normal service. The design of the part and the material selection were significant contributing factors because of stresses induced during molding, physical aging of the amorphous polysulfone resin, and the substantial differential in coefficients of thermal expansion between the polysulfone and the mating steel insert.
A flow sensor was submitted for analysis because the plastic body section had failed while in service in an industrial application. The part was representative of approximately 10 other sensors that had been returned from service with generally comparable features. It was also anticipated that similar failures went otherwise unreported, and that the premature failures posed a significant risk to other parts still in service.
Engineering drawings specified that the body section be made from an unfilled polysulfone resin. Polysulfone is a transparent amorphous engineering thermoplastic, which, by comparison with other resins, is generally considered to be a ductile and tough material. Like many other plastics, however, it is somewhat notch sensitive. As a class of materials, polysulfone is known for good electrical insulation properties, excellent thermal stability, and exceptional hydrolytic stability. Because of its unique combination of thermal and hydrolytic properties, polysulfone is commonly used in several demanding applications including food processing, medical components, and fluid handling.
The polysulfone body of the failed sensor had been injection molded around a tubular stainless steel insert to form the final component. Background information obtained on the application showed that the part was routinely exposed to alternating hot and cold cycles. The fluid passing through the flow sensor had ranged from 40 to 200 °F (4 to 93 °C). Cleaning agents, including chlorine-based and acid-based chemicals had also been put through the sensor. In addition to the failed part, a typical sample of molding resin was also received for comparison with the sensor material.
The failed flow sensor shown as-received, exhibiting cracks
radially within the plastic collar around the metal insert.
