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R-curves are at present recognized as the elastic-plastic fracture toughness property to be used in failure analysis. However, they are useful only if it can be shown that they are geometry independent. The objective of this study was to demonstrate this geometry independence using four specimen types—the compact, single-edge-notched tension, center-cracked tension, and double-edge-notched tension types. The material used was an A553B Class 1 steel taken from a nuclear reactor nozzle cutout. To eliminate variable constraint between geometries, the thickness of all the specimens was 2.5 mm (0.1 in.) so that plane stress was maintained throughout.
Fracture toughness was characterized in terms of modified J and the Hellmann-Schwalbetype crack-tip opening displacement. Both parameters demonstrated that the R-curve is geometry independent except for the center-cracked specimens. In that case, a slip-line deformation field peculiar to center-cracked tension specimens was identified as the cause of geometry dependence. Double-edge-notched tension specimens proved that the R-curve difference was not due to pure tension loading.
An experimental calibration was made for the single-edge-notched geometry using a series of blunt notch specimens with varying a/W. From this, J can be determined from the area under the experimental load versus displacement records. A normalizing procedure for load versus displacement was developed from this calibration which allows flow properties to be examined for any size, thickness, or a/W. The normalized load displacement curves can be used for J calibration.
elastic-plastic fracture, JR, -curve, modified , J, fracture mechanics, nonlinear fracture mechanics
Professor, University of Tennessee, Louisville, TN
Senior scientist, Materials Engineering Associates, Lanham, MD
Associate professor, School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA