Problem: Catastrophic failure of a motor-driven shaft in service. Full shearing of the shaft has occurred.
Discussion: It was noted from the cross-section of the shaft that the failure propagated from two distinct diametrically opposite fatigue cracks, leading to the ingression of the beach marks that can be observed, occurring each running cycle.
A degree of "blueing" was also evident, indication the steel had been heated >350°C during the course of failure. SEM-EDS analysis determined that the elemental composition of the shaft was consistent with the required grade, whilst further analysis by electron microscopy found no evidence of material or manufacturing faults.
Solution: Failure of the shaft was found to have been caused by flexural stress, otherwise known as "reverse bending", whereby a shaft is subjected to diametrically opposed stresses until the point of failure, much in the same way as repeatedly flexing a paperclip until it breaks. The information was by the client used to facilitate additional in-service checks to identify similar occurrences before they fail in future, leading to improved uptime, and more importantly, a safer working environment.