Fractal scaling and crack-size effects on creep crack growth

Scaling effects on the creep crack growth behaviour are investigated by analyzing the results of compact tension (CT) tests on different-sized notched steel specimens appearing in the literature. Creep crack growth rate data are correlated to the elastic stress-intensity factor in terms of a Paris-type law, da∕dt = C0Kq, where C0 turns out to be a crack-size dependent coefficient of proportionality. Considering specimens with the same loading configuration (CT) and the same thickness, the observed crack-size effect on the creep crack growth rate is discussed on the basis of self-similarity considerations, and geometrically interpreted in terms of fractal tortuosity of the crack profile. A size-independent formulation of the creep crack growth law correlating renormalized quantities is finally deduced and confirmed by the experimental results.

Mismatch Constraint Effect of Creep Crack With Modified Boundary Layer Model

Mismatch effect plays a crucial role in weldments, and an independent mismatch constraint parameter M* is proposed to characterize the material mismatch constraint effect in this paper. A mismatched modified boundary layer (MBL) model for creeping solids is developed to simulate the stress field of creep cracks in mismatched weldments. It can be found that there still exists the similarity between creep crack tip stress fields under different mismatch factors. Numerical results show that M* obtains the minimum value on the under match condition and the maximum value on the over match condition. Comparisons between M* and other geometric constraint parameters (A2(t) and Q22) are carried out and the applicability of M* is verified. A modified assessment formula for creep crack growth rate ratio is proposed based on the parameter M*. It is found that M* is a reasonable and remarkable parameter to characterize the mismatch constraint effect of creeping cracks.

Numerical Prediction of Creep Crack Growth Rate in Cr-Mo-V Steel for Specimens with Different Constraints

The creep crack growth rate in Cr-Mo-V steel has been numerically predicted for specimens with different constraints for a wide range of C* by using stress dependent creep model and ductility, and the simulated da/dt-C* curves were compared and analyzed with experimental data. The results show that the simulated da/dt-C* curves agree well with experimental data. At low and transition C* regions, the crack-tip constraint has obvious effect on CCG rates, while at high C* region it almost has no effect. With increasing constraint, the CCG rates and transition region size on da/dt-C* curves increase due to higher stress traxiality ahead of crack tip and stress-regime dependent creep ductility. If the extrapolation CCG rate data of standard high constraint CT specimen from high C* region (above the turning point 2) or from transition C* region are used in life assessments of the components with various constraints at low C* region (below the turning point 1), the non-conservative or excessive conservative results may be produced. Therefore, the CCG rate data for considering constraint effect should be obtained for a wide range of C* by long-term laboratory tests or numerical predictions using the stress dependent creep model and ductility.