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A theoretical development based on a simple physical model is proposed to help the designer predict high strain multi-axial fatigue behaviour. This approach hypothesises that the maximum shear strain y*, on planes driving the crack through the thickness, controls the fatigue crack propagation rate and hence the life. The direct strain E.* acting normal to the plane of y* can exert a secondary modifying influence. Experimental results from several research laboratories have been analysed in this manner with some success. NomenclatureSurface principal strain ratio ; smallerflarger Principal direct strains Principal direct stresses Poisson's ratio Equivalent Poisson's ratios Shear strain Octahedral shear strain Maximum shear strain Direct strain normal to plane of ymax Max. shear strain on plane 45" to surface; larger of y,, and yo, Direct strain normal to plane of y* Ratio applied shear strain/applied tensile strain

InstitutionThe applicability of the C* parameter for the prediction of creep crack propagation rates is considered. A new method for estimating C* is presented, the results from which show good agreement with those from an existing technique. Experimental results from creep crack growth tests, conducted on a 1 CrMoV steel using both compact tension and single edge notch bend specimens, indicate that good correlation with C* is obtained once the effects of stress redistribution become negligible. Finally, comparisons are drawn between C* and other possible correlating parameters, and the limitations of each approach are discussed.

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A 1 Cr Mo V steel has been studied at 565 °C under a cyclic strain axial mode with various combinations of tensile and compressive dwell periods. Compared with the continuously cycled data, the presence of a tension only dwell caused a considerable decrease in the number of cycles to failure. The addition of a compressive dwell to the cycle already containing a tensile dwell tended to reduce or eliminate the harmful effects of the latter. Metallographic evidence suggests that the compressive dwell reduced the incidence of internal intergranular cracking. A ratchetting grain boundary damage model to account for this is proposed.