The creep of metals under interrupted stressing

Abstract

The creep behaviour of polycrystalline lead subjected to interrupted stress, or ‘stress pulses’, has been examined with an apparatus which enables the creep curve to be recorded as a magnified photographic trace. When the metal is reloaded after an interruption, a new creep transient is exhibited which can be adequately expressed by assuming a fraction n of the material to have recovered its original Andrade β -flow properties, whilst the remaining fraction (1 — n ) continues to deform as if unloading had not occurred. It is shown that this analysis is valid whether the material exhibits a finite Andrade K -flow or not. The variation of n with the time of creep ( T ) previous to the interruption, for a constant recovery time ( R ), and the variation of n with R for a constant T , are each investigated. The full component analysis is worked out for four stages, but the exact calculation rapidly becomes too unwieldy, and approximations are developed to deal with the case of regular repetitive stress pulses. It is shown that the creep under these conditions may be very closely formulated from measurements of uninterrupted creep coupled with knowledge of the value of n for the first interruption. The problem of a constant stress cycle, i. e. ɤ + R = constant (= λ ), is specially treated, and the significant effect of the ɤ/λ ratio is demonstrated; it is, for example, possible, with a given duration of test, for interrupted stress to bring about a greater creep strain than uninterrupted creep at the same stress. Because of the character of the β -againststress curves, nearly square β -pulses can occur in structures subjected to sinusoidal stress cycles, and the treatment has, therefore, a very wide application.