A stochastic lifing method for materials operating under long service conditions: with application to the creep life of a commercial titanium alloy

Abstract

This paper extends the theta prediction methodology so that life predictions for materials operating under long service conditions can be made that also have a degree of confidence associated with them. Ways in which this model can be applied to the fatigue as well as the creep of all materials is also discussed. For comparison purposes two failure criteria are built into the stochastic model and the determinants of failure derived. This stochastic theta model is then used to investigate the nature of the creep failure time distribution for the Ti 6.2.4.6 alloy under constant uniaxial conditions. The distributions for each θ _j were found to be very different—with only some of them being normally distributed. The others had very pronounced skews to both the left and right. The empirical distributions for predicted failure times were also found to have long tails reaching out to higher failure times—although the failure time distributions were more symmetric when using the Monkman—Grant failure criteria. For Titanium 6.2.4.6 operating at 773 K and 580 MPa the chances of failure before 410 hours is 1%. At 480 MPa and 773 K the chances of failure before 780 hours is 1%.