The Effect of Crack Depth (a) and Crack Depth to Width Ratio (a/W) on the Fracture Toughness of A533-B Steel

Constraint, as related to specimen crack depth (a) or crack depth to specimen width ratio (a/W), can have a significant effect on fracture toughness. In laboratory specimens, both crack depth and the a/W ratio can be varied. However, it is not always possible to model the constraint of a structurally relevant geometry in the laboratory. Nonetheless, an understanding of the role of both crack depth and a/W ratio on the toughness behavior of laboratory specimens will help clarify the role of constraint on fracture toughness and better enable engineers to model the effect of constraint on a flaw in an actual structure. An experimental study of the effect of crack depth and a/W ratio on the fracture toughness of an A533-B steel was conducted and results were compared with large-scale specimens tested at Oak Ridge National Labs (ORNL). Smaller size specimens tested at the University of Kansas (KU) were taken from the actual ends of the specimens tested at ORNL. The specimens tested at both KU and ORNL were square single-edge-notched bend (SENB) specimens with widths ranging from 20.3 to 100.0 mm (0.8 to 4.0 in.), crack depths ranging from 2.0 to 50.0 mm (0.08 to 2.0 in.), and a/W ratios ranging from 0.1 to 0.5. The geometries of the specimens tested at KU were chosen such that comparisons of the toughness of specimens with constant crack depth and varying a/W ratio, as well as comparisons of the toughness of specimens with constant a/W ratio and varying crack depths, could be made. A forthcoming paper, containing finite element analysis results, will compare the analytical basis for the behavior of these various size specimens. The results indicate that both crack depth and a/W ratio affect the fracture toughness of the steel. For deep crack geometries (a/W = 0.5), crack depth has limited effect on the fracture toughness. However, for shallow crack geometries (a/W = 0.1), crack depth has a significant effect on the fracture toughness. For constant crack depth, varying the a/W ratio does affect the fracture toughness. Thus, crack depth and a/W ratio are interdependent with respect to fracture toughness. The findings of this study are significant in helping to understand the role of both crack depth and a/W ratio on fracture toughness and serve as a basis for understanding the effect of constraint on the behavior of actual structures with cracks.