Effect of Constraint on Creep Crack Propagation of Mod. 9Cr-1Mo Steel Weld Joint

In order to clarify the effect of constraint on creep crack propagation of modified 9Cr-1Mo steel, 1) 25.4mm thickness C(T) specimen with side grooves (thick specimen) and 2) 10mm thickness C(T) specimen without side grooves (thin specimen) were machined from base metal (BM) and weld joint (WJ), and subjected to creep crack propagation experiments under 650 °C. The crack in WJ propagated along the fine grain heat affected zone (HAZ), where the multi-axial constraint was high due to the difference in deformation properties among HAZ, WJ, and BM. While the thick specimens hardly allow deforming to the thickness direction and showed less crack tunneling, the thin BM specimen showed remarkable reduction of thickness and crack tunneling. The thin WJ specimen showed some thickness reduction and crack tunneling, but also showed larger crack extension than that of BM. In terms of the relationship between the crack propagation rate (da/dt) and the creep J integral (C* parameter), the data shows a wide scatter band. The specimens with higher geometrical constraint (thick specimen, WJ) locate above the lower constraint (thin specimen, BM) ones. All the specimen showed the reduction in thickness and suggests the change in constraint condition during the experiments. By picking up the data without significant thickness reduction, the C*- da/dt relationship has been gathered in a narrow and straight band regardless of the specimen geometry or difference in material. The picked up data locates at the upper bound of the current and literature data.

Three-Dimensional Crack Growth in Ductile Materials: Effect of Stress Constraint on Crack Tunneling

Crack tunneling is a crack growth feature often seen in stable tearing crack growth tests on specimens made of ductile materials and containing through-thickness cracks with initially straight crack fronts. As a specimen is loaded monotonically, the midsection of the crack front will advance first, which will be followed by crack growth along the rest of the crack front, leading to the formation of a thumbnail shaped crack-front profile. From the viewpoint of fracture mechanics, crack tunneling will occur if the operating fracture criterion is met first in the midsection of the crack front, which may be due to a higher fracture driving force and∕or a lower fracture toughness in the midsection. A proper understanding of this fracture behavior is important to the development of a three-dimensional fracture criterion for general stable tearing crack growth in ductile materials. In this paper, the phenomenon of crack tunneling during stable tearing crack growth in a single-edge crack specimen is investigated by considering the effect of stress constraint on the fracture toughness. Crack growth in the specimen under nominally Mode I loading conditions is considered. In this case, crack tunneling occurs while the initially flat crack surface (which is normal to the specimen’s lateral surfaces) evolves into a final slanted fracture surface. A mixed-mode crack tip opening displacement (CTOD) fracture criterion and a custom three-dimensional (3D) fracture simulation code, CRACK3D, are used to analyze the crack tunneling event (but not crack slanting) in the specimen. Results of this investigation suggest that the critical CTOD value (which is the fracture toughness) has a clear dependence on the crack-front stress constraint Am (the constraint measure in this work is the stress triaxiality, which is the ratio of the mean normal stress to the von Mises effective stress). For simplicity, this dependence can be approximated by a straight line within the range of stress constraint values found, with the toughness decreasing as the constraint increases. It is found that crack tunneling in this case is mainly the result of a higher stress constraint (hence a lower fracture toughness) in the midsection of the crack front. Details of the crack growth simulation and other findings of this study will also be presented.

Experiments, Analysis and Simulation of Mixed Mode Ductile Fracture

This paper provides a review of findings of a comprehensive research effort by the authors and collaborators in the area of experiments, analysis and simulation of mixed-mode ductile fracture. Topics include mixed-mode Arcan stable tearing tests, the mixed-mode CTOD fracture criterion and its basis, normalization of ductile crack tip fields, ductile failure envelope, crack tunneling and slanting, effects of stress constraint, custom 2D and 3D mixed-mode crack growth simulation codes, and simulations of mixed-mode stable tearing crack growth tests.