Fatigue crack initiation and propagation as a consequence of thermal striping

An analysis is given of the initiation and subsequent propagation of surface cracks in a stainless steel section subjected to high frequency thermal shocks (thermal striping) at metal temperatures up to 6500C. Estimates of the maximum sustainable metal temperatures range allowable to avoid crack nucleation in the surface layers have been made using fatigue endurance data corrected for the thermally induced equibiaxial strain field. Using these methods, it is apparent that a metal temperature difference of 630C may be allowed. This represents the application of a safety factor of two, in the conventional manner, to the uniaxial fatigue endurance data in order to account for scatter in fatigue data arising from thermal ageing, surface finish and sodium corrosion effects. A discussion of the assessment is given. The extent of crack propagation under thermal striping has been analysed using a linear elastic fracture mechanics approach and it is shown that, in certain circumstances, crack arrest may occur. Methods are outlined which allow the inclusion of multiple surface cracking (and its effect of reducing the peak stress intensity factor) into the analysis. Consideration is also given to the possibility of arrested thermal fatigue cracks being further propagated by creep crack cracking under conditions of high mean stress. (author)