Sunggi Baik

Mechanisms of creep-fatigue interaction

Experiments on a model material have shown that creep-fatigue interaction is possible by one of several mechanisms. At low temperatures failure occurs by initiation and propagation of a crack, and is transgranular. At higher temperatures failure is caused by cumulative damage of intergranular cavities dispersed throughout the specimen. At the transition a regime is found where failure occurs by initiation of an intergranular crack near the surface which propagates as a Stage II transgranular crack. In the cumulative damage mode of failure a further distinction is possible in terms of ‘r’ type and ‘w’ type cavities. Which one of them is dominant depends on the cycle shape. The behavior of the present aluminum alloy should be representative of those materials in which nucleation of cavities is difficult, such as austenitic stainless steels and precipitate strengthened nickel base alloys, but not of materials which cavitate rather easily,e.g., copper and oxide dispersion strengthened alloys.

Wedge type creep damage in low cycle fatigue

A model is proposed to quantify the accumulation of wedge type creep damage in low cycle fatigue. It is proposed that such damage is produced primarily during the ramp periods of the cycle. Equations are developed for estimating incremental accumulation of damage per cycle in fully reversed, multiaxial loading. The rate of accumulation of damage depends on the strain-rate, the temperature, and the microstructure. The analysis is kept simple by making physically reasonable assumptions. Cycles to failure are predicted by invoking a fracture criterion. The model is applied to two sets of data; one set is a well characterized life test data on an aluminum alloy, and the other is phenomenological data on austenitic stainless steels. In both cases the predictions are good enough to prompt further experimental evaluation of the model. This paper deals with only one mechanism of creep-fatigue interaction. Other mechanisms of failure,e.g., ‘r’ type cavitation, or fatigue crack initiation and propagation, are also viable. The model described here may be expected to apply only under those conditions when wedge damage is the dominant failure mechanism.

Creep-fatigue interaction in OFHC-copper

Abstract OFHC-copper which is more susceptible to interaranular cavitation than the aluminum-five percent magnesium alloy in monotonic loading is also more susceptible to intergranular fracture under fatigue loading. The difference is most remarkable for the equal-ramp cycle shape. Attempts to predict fatigue life of copper specimens using stress rupture data and assuming linear time-dependent integration of damage, consistently overestimated the fatigue life. Less constraint to the growth of cavities in the fatigue case, because of a change in the grain morphology, and because of a lowered creep resistance of the grain matrix, are suggesteed as possible reasons for the discrepancy.