Part 1: Constraint Effects On Fracture Behavior: 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 Laboratories (ORNL). Smaller-size specimens testes 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 depths 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. Another report in this issue 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.

Part 2: Constraint Effects On Fracture Behavior: An Analytical Investigation of the Effect of Crack Depth (a) and Crack Depth-to-Width Ratio (a/W) on the Fracture Toughness of A533-B Steel

The use of various crack depths (a) and crack depth-to-specimen width (a/W) ratios in laboratory tests to model flawed structural components has led to considerable interest in the role of both the crack depth and the a/W ratio on fracture toughness. To investigate the separate roles of crack depth and a/W ratio, three-dimensional elastic-plastic finite element analyses (FEA) of square SE(B) specimens with a/W ratios ranging from 0.1 to 0.5 (crack depths ranging from 2.0 mm [0.08 in] to 50.8 mm [2.0 in]) have been conducted. The material properties used in this analysis were those for the material used in an experimental study conducted at the University of Kansas and the Oak Ridge National Laboratory (ORNL). Both geometries (experimental and analytical) were chosen such that the results for specimens having a fixed crack depth and varying a/W ratios could be compared to the results for specimens with a fixed a/W ratio and varying crack depths.

The results of the experimental studies had indicated that both the crack depth and the a/W ratio each have a significant effect on the fracture toughness. Both the lower-bound toughness and the transition temperature were affected. The three-dimensional elastic-plastic FEA modeled each specimen geometry and compared the stresses, CTOD, and J integral from the various crack depths and a/W ratios. The results of the FEA support the findings of the previous experimental study. For a high-constraint geometry (a/W=0.5), there is not a significant change in the near-tip stresses for different-size specimens. However, for low-constraint geometries (a/W=0.1), the near-tip stresses are significantly affected by the actual crack depth and specimen size. The findings of this study are significant in helping us understand the relative role of crack depth, a/W ratio, and specimen size on fracture toughness. Future studies are expected to extend the findings of this study to the behavior of actual structures with cracks.

Part 3: Constraint Effects On Fracture Behavior: The Significance of Crack Depth (a) and Crack Depth-to-Width Ratio (a/W) With Respect to the Behavior of Very Large Specimens

Previous studies have shown that there is an increase in cleavage fracture toughness in laboratory specimens with shallow flaws compared with those having deep flaws. Typical crack depths in real structures generally are very small relative to the member width. Therefore, the crack depth to structural member width (a/W) ratios are very small (<0.1). Accordingly, the effect of this observation on the behavior of larger structures that actually represent typical engineering applications could be significant.

Using experimental and analytical results from previous studies on A533-B steel specimens, the effect of the shallow-flaw behavior with respect to very large specimens was examined. Using the Dodds and Anderson constraint correction, predictions of the cleavage fracture toughness of large-scale wide-plate tests and full-thickness clad beams from an actual reactor pressure vessel were shown to compare favorably with test results. The results of these studies suggest the possibility of predicting the increase in fracture toughness for low-constraint structural geometries using high-constraint laboratory test specimen results. The ability to take advantage of this increase in toughness in analysis of actual structures could be very useful in estimating the actual safety and reliability of existing structures with service cracks.