Failure probability prediction of fast-fractured NT451 SiAlON diesel exhaust valves

The fast-fracture failure probability of a prototype NT451 SiAlON diesel exhaust valve was predicted using volume-censored flexure and tensile strength data. A tandem set of life prediction codes developed by AlliedSignal Engines was used to make these predictions in this exercise; these codes also yielded 95% confidence estimates on determined two-parameter Weibull distributions and failure probabilities. The prediction of fast-fracture failure probability was compared to actual NT451 SiAlON valve fast-fracture strengths. All the valves failed from volume-flaws, so volume-censored input strength data were required for the fast-fracture failure probability predictions. In addition to the failure probability predictions, the effects of several independent parameters on strength, and ultimately on the failure probability, were also examined in this exercise: the specimen geometry tested; specimen machining parameters consisting of grinding direction and grinding wheel grit size; and the utilization of a post-machining anneal. Within 95% confidence estimates, the effect of specimen geometry, machining parameters, and a post-machining anneal had no effect on the calculated Weibull Moduli and scaling parameters and the consequential predicted reliability.