Numerical investigation of creep crack growth in cross-weld CT specimens. Part II: influence of specimen size

Abstract

A numerical investigation of the influence of specimen size on creep crack growth in cross-weld CT specimens with material properties of 2.25 Cr1Mo at 550 °C is performed. A three-dimensional large strain and large displacement finite element study is carried out, where the material properties and specimen size are varied under constant load for a total of eight different configurations. The load level is chosen such that the stress intensity factor becomes 20 MPa√m regardless of specimen size. The creep crack growth rate is calculated using a creep ductility-based damage model, in which the creep strain rate ahead of the crack tip perpendicular to the crack plane is integrated - taking the degree of constraint into account. Although the constraint ahead of the crack tip is higher for the larger specimens, the results show that the creep crack growth (CCG) rate is higher for the smaller specimens than for the larger ones. This is due to much higher creep strain rates ahead of the crack tip for the smaller specimens. If, on the other hand, the CCG rate is evaluated under a constant C * condition, the creep crack growth rate is found to be higher for the larger specimens, except when the crack is located in a HAZ embedded in a material with a lower minimum creep strain rate; then, the creep crack growth rate is predicted to be higher for the smaller specimen. In view of these results, it is obvious that the size effect needs to be considered in assessments of defected welded components using results from CCG testing of cross-weld CT specimens.