Transition modelling of surface flaws to through cracks

Abstract

Cracks starting at surfaces will grow under fatigue loading conditions both along the surface and in the thickness directions of the component geometry.In those cases where the crack grows through the thickness, the fatigue crack may transition to a corresponding through crack geometry. While the fatigue crack growth behaviour of both the surface flaws and complete through cracks are well understood, the method for modelling the process by which they transition from one to the other is not. This paper seeks to bring greater clarity and understanding to the transition process by implementing a transition method and developing the associated codes and equations to do so based on careful consideration of boundary conditions, experimental data, and finite element simulations. K E Y W O R D S fatigue crack growth, flaw transition, marker banding, part-through cracks, surface flaws

Time‐ and cycle‐dependent growth of surface cracks in alloy 718

“…26 It is thought based on evidence from Figure 11, and the bifurcated/lighter surface indications on some specimens that shear lips may have a major role in controlling whether the TAZ region reaches the surface of the material. The FG material is more susceptible to shear lip formation based on both corner crack specimens tested at RT 51 and surface crack specimens at 650 C (Figure 11).…”

“…The plane strain nature of the internal part of the crack could cause it to grow TAZ at an enhanced rate, changing the normal aspect ratio evolution of the crack and causing it to breakthrough and transition more quickly. 51 This idea will be explored in the current work through TMF testing on the surface crack configuration to highlight differences in the plane stress and plane strain regions of the crack front. Using the insights gained from the new experiments, the MPYZ-TMF fracture mechanics-based model is modified to include the influence of the grain size on TAZ formation and its subsequent influence on fatigue crack growth rate.…”

“…Due to the differences in the behavior of the TAZ from the level of constraint, where plane strain and plane stress are the extremes, it is not currently clear how this may affect the growth of these cracks. The plane strain nature of the internal part of the crack could cause it to grow TAZ at an enhanced rate, changing the normal aspect ratio evolution of the crack and causing it to breakthrough and transition more quickly 51 . This idea will be explored in the current work through TMF testing on the surface crack configuration to highlight differences in the plane stress and plane strain regions of the crack front.…”

Multiaxial creep–fatigue failure mechanism and life prediction of a turbine blade based on a unified numerical solution approach

“…The actual loading condition generates the combination effect of bending and torsional loading, which has a significant effect on the creep-fatigue properties of Inconel 718 superalloy. [13][14][15][16] It is essential to address the potential relationships and distinctions between multiaxial loading conditions in practical applications and uniaxial loading conditions in experiments. Prior studies concentrated on the impact of structural discontinuity locations, such as notches and holes, on the creep-fatigue damage of Inconel 718 under multiaxial stress states.…”

“…More specifically, the turbine blades are identified as the key structure that is highly susceptible to failure due to the multiaxial loading effect caused by the centrifugal and pneumatic forces under actual loading conditions. The actual loading condition generates the combination effect of bending and torsional loading, which has a significant effect on the creep–fatigue properties of Inconel 718 superalloy 13–16 . It is essential to address the potential relationships and distinctions between multiaxial loading conditions in practical applications and uniaxial loading conditions in experiments.…”