Strain and strain rate behaviour of a low carbon 18Cr-12Ni stainless steel under conditions of creep-low cycle fatigue interaction

Results of an investigation of the application of a low frequency ″rectangular″ cyclic stress, during both the initial and advanced stages of primary creep, on the time dependent strain and strain rate behaviour of a low carbon 18Cr-12Ni (304L) stainless steel are presented. The strain rates immediately before, as well as immediately after, any stress decrement or increment were measured accurately. It is shown that at any given relative amplitude of applied stress Δσ/σ and superimposed applied stress period Δtc, these strain rates depend linearly on time. The relations between these strain rates and the relative amplitude of stress cycling, Δσ/σ, are also linear at any constant cyclic loading stress period Δtc. This finding makes it possible to estimate the contribution of the net strain owing to superimposed cyclic loading to the total plastic strain. Further, it is shown that the creep strain rate after the cyclic loading superposition lasting ∑Δtc = 36 ks, is only slightly affected by this superimposed loading. An interpretation of strain and strain rate behaviour under conditions of cyclic loading superposition is based on the processes of creep strain strengthening and creep recovery, though the backward straining after any stress reduction must be taken into account as well. All the above- mentioned processes depend not only on temperature and the mean applied stress, but also on the cyclic loading history.