A state-of-the-art review is presented of the field of thermal fatigue. Following a brief historical review, the concept is developed that thermal fatigue can be viewed as processes of unbalanced deformation and cracking. The unbalances refer to dissimilar mechanisms occurring in opposing halves of thermal fatigue loading and unloading cycles. Extensive data summaries are presented and results are interpreted in terms of the unbalanced processes involved. Both crack initiation and crack propagation results are summarized. Testing techniques are reviewed, and considerable discussion is given to a technique for thermal fatigue simulation, known as the bithermal fatigue test. Attention is given to the use of isothermal life prediction methods for the prediction of thermal fatigue lives. Shortcomings of isothermally-based life prediction methods are pointed out. Several examples of analyses and thermal fatigue life predictions of high technology structural components are presented. Finally, numerous dos and don'ts relative to design against thermal fatigue are presented.

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Low-cycle thermal fatigue

Finite element simulation of creep-fatigue crack growth behavior for P91 steel at 625 °C considering creep-fatigue interaction

During the operation of a compressed air energy storage (CAES) salt cavern, the surrounding rock experiences creep damage during the stages of constant internal pressure and undergoes fatigue damage due to the periodical injection-production. To describe the damage evolution of salt rock under creep-fatigue loading, a novel damage accumulation model based on the ductility exhaustion concept is proposed by applying a nonlinear summation method to represent the synergistic effect of creep and fatigue damage. Low-cycle fatigue (LCF) and creep-fatigue tests of rock salt were conducted under stress-control mode for various cycle stress amplitudes and hold times. Results show that the deformation of rock salt under creep-fatigue loading consists of initial, steady and accelerated phases. The proposed model matches well with the test data and can accurately describe the damage evolution as the applied stress amplitudes and dwell times change. The introduction of the hold times at the upper limit stress causes a strain increment and life reduction, which become more evident as the duration periods prolong and can be understood by the dislocation theory of crystals.

Damage Evolution and Deformation of Rock Salt Under Creep-Fatigue Loading

Mechanical properties of rock salt under combined creep and fatigue

Plane strain finite deformation finite element calculations of mode I crack growth under small scale creep conditions are carried out. Attention is confined to isothermal conditions and two time histories of the applied stress intensity factor: (i) a monononic increase to a plateau value subsequently held fixed; and (ii) a cyclic time variation. The crack growth calculations are based on a micromechanics constitutive relation that couples creep deformation and damage due to grain boundary cavitation. Grain boundary cavitation, with cavity growth due to both creep and diffusion, is taken as the sole failure mechanism contributing to crack growth. The influence on the crack growth rate of loading history parameters, such as the magnitude of the applied stress intensity factor, the ratio of the applied minimum to maximum stress intensity factors, the loading rate, the hold time and the cyclic loading frequency, are explored. The crack growth rate under cyclic loading conditions is found to be greater than under monotonic creep loading with the plateau applied stress intensity factor equal to its maximum value under cyclic loading conditions. Several features of the crack growth behavior observed in creep-fatigue tests naturally emerge, for example, a Paris law type relation is obtained for cyclic loading.

Creep crack growth by grain boundary cavitation under monotonic and cyclic loading

Experiments regarding the effect of simultaneous fatigue and creep on life have been performed for a 20 pct Cr-35 pct Ni stainless steel at 700°C. It was found that a life fraction rule, comprising an interaction term besides the terms of pure fatigue and creep damage, adequately accounts for the observed lives. The interaction term contains the product of the fatigue and creep damage, and may physically be interpreted as the damage generation due to the interaction of the fatigue and creep processes. There is evidence that the surface crack density represents a measure of the total damage. The cracks created during the combined fatigue and creep test were both transgranular and intergranular. They were probably nucleated intergranularly and later, during their growth, were transformed to transgranular cracks. As the creep damage of the test was increased gradually, more cracks remained intergranular throughout the test.

R. Attermo

Papers

The effect of combined low-cycle fatigue and creep on the life of austenitic stainless steels