The investigation of the stress cracking behavior of PBT by acoustic emission

Abstract

AbstractDespite being a major cause of failure of polymer products under service, environmental stress cracking (ESC) is still a phenomenon poorly investigated and, as consequence, not fully understood. This article aims to contribute to this field in a study of the stress cracking behavior of polybutylene terephthalate (PBT) exposed to ethanol (a common fluid, used in several daily situations), by means of two types of mechanical tests (tensile and stress relaxation). During mechanical testing the technique of acoustic emission (AE) was applied to identify certain mechanisms of internal failure. As a non‐destructive technique, AE is widely used with metals to monitor the integrity of components but it is very scarcely applied to polymers. The article, therefore, innovates by using AE as a tool to better understand the ESC behavior of an engineering polymer produced under different conditions. The PBT samples were prepared by injection molding, using different mold temperatures (20°C, 40°C, and 80°C), resulting in different crystallinities. The results showed that the tensile properties were highly dependent on the crosshead speed and that ethanol affected the mechanical performance of the materials, anticipating the necking, but its evaporation during the experiment reduced the actual temperature on the sample bar. This latter effect caused an unexpected behavior, with an increase in tensile strength and reduction of stress relaxation rate, the opposite trends that would be expected from a stress cracking agent. The crystallinity of PBT had some influence on the results, with the more crystalline samples being more resistant to ESC, as observed by both stress relaxation and AE analyses. The technique of AE showed to be a very useful tool to understand the mechanical behavior and to differentiate the various testing conditions, with the detection and quantification of hits and released energy, related to the several stages of deformation and failure, including the formation of shear bands, nucleation and growth of crazes and cracks, and so forth. The fracture surface analyses by scanning electron microscopy were very consistent with the AE experiments, in which rough topography were related to high acoustic activities.