Experimental and mathematical determination of initiation and propagation of fatigue cracks in pressure vessels

In cyclic loading tests carried out on eight 34 CrMo 4 pressure vessels the formation of fatigue cracks originating from starter notches could best be determined by the DC potential drop method and the measurement of crack opening displacement. As the cycles to fatigue crack initiation could be calculated only roughly when applying the constants for a similar material known from the literature, more appropriate constants were derived from the tests. A comparatively plain crack propagation program based on fracture mechanics relationships was found to give too low an estimate of the number of load cycles required for the propagation of semi-elliptical surface flaws in vessels if the stress limits during cyclic loading were kept constant or the minimum stress was raised respectively. In case the maximum stress was reduced, the underestimate was greater by far as the reduction in crack growth rate produced by the compressive residual stress of the high stress was not taken into account. (orig.)