To travel hopefully… in search of the instant creep test

ABSTRACT

Proverbially ‘To travel hopefully is better than to arrive.’ Many roads have been travelled in search of a short-term mechanical test that would reliably predict long-term component creep life. The motive is clear; whatever progress is made in inspection and monitoring, direct measurement of the relevant properties of the subject material – archival or ex-service – will always provide the most reliable indicator of its state and performance capability. The difficulties are equally clear: mechanism changes over the stress, temperature and time ranges involved; issues of specimen size and geometry – and their relation to stress state; adequate emulation of in-service variation in operating condition; and, for post-exposure tests, translating information between the sample position and the critical location.

This paper focuses on:

• issues of mechanism change, both in simple extrapolation and across boundaries of stress state and test type

• identification of parameters that allow true comparison between forward creep, stress relaxation and constant strain rate testing

Data obtained on low-alloy ferritic steels are examined and analysed, largely using the framework of continuum damage mechanics. Particular attention is given to the nature of the equilibria represented by tensile instability and the minimum creep rate.

It is concluded that the ‘instant test’ is an unreachable ideal. However, the knowledge and understanding gained through the search have benefitted the larger objective and – as in every legendary quest – have made the journey worthwhile.

KEYWORDS:

• Creep
• rupture
• stress-relaxation
• constant strain rate
• post-exposure testing
• life fraction rule
• continuum damage mechanics
• remaining life

Acknowledgments

Thanks are due to the many colleagues with whom valuable discussions have taken place.

ETD are to be congratulated for staging the online MIMA Conference.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Notes

1. That is, the evolution of damage, ω, can be described by an equation in the form $\dot{\omega }=f\left(\omega \right).g\left(\sigma \right)$ such that it can be re-expressed with all terms in ω and all terms in σ on opposite sides of the equality. The equation can thus be solved by the technique of separation of variables.