Investigation of the influence of a multi-axial state of stress on the behaviour of intercrystalline creep micro-cracks

In a polycrystalline metal, the damage process in tertiary creep stage is highly influenced by the initiation, growth and linkage of intercrystalline creep micro-cracks. This procedure is dependent on the intensity and the distribution of the applied stress and strain. The investigation of a multi-axial stress state on the behaviour of micro-cracks was carried out on the basis of creep tests on different circumferentially notched tension bars with a ratio of nominal diameter to root diameter of (1.125 less than or equal to D/d less than or equal to 2.5) and a diameter radius ratio of (2.7 < D/R less than or equal to 62.5) with different applied stresses (100 MPa less than or equal to sigma less than or equal to 200 MPa). Metallographic investigation and finite-element simulation give further information about the criterions of creep crack growth rate. The metallographic evaluation includes the length of the micro-cracks in grain boundary facets, the number of micro-cracks in each length class, the relative number of micro-cracks per unit area and the orientation of the micro-cracks referring to the load direction. Using these results and investigations of Lindborg [1] and El-Magd [2], a creep crack growth rate can be calculated with a modified model [3] with two elementary assumptions: (1) every micro-crack grows within a short time from one grain boundary wedge to another, where it stops for a longer time and then goes on growing to the next boundary wedge; i.e. the length of a crack is always an integral multiple of a grain boundary facet. (2) Initially, each crack is of length 1 and grows step by step to next higher length classes. The finite-element computations are used to determine the distribution of the stresses and the strains in the narrow of the notch root in order to have a better understanding of the influence of a multi-axial stress state on micro-crack initiation and micro-crack growth. The experimental results as well as the finite-element simulation show that the creep behaviour and the micro-crack initiation and growth are highly dependent on the notch geometry.