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A fractographic study of crack extension in plane strain, fully plastic Zircaloy® was conducted. Three specimen geometries were studied: double-edge notched, compact tension, and center notched specimens. Slip line solutions for these geometries predict high, moderate, and no stress elevation, respectively, ahead of the crack tip. These differences might be expected to produce different modes of crack extension. However, crack tip blunting effects were found to overwhelm geometry induced differences. Blunting was found in all three geometries. Stretched zone width was found to be ∼J/σy, or about 0.01 in. (0.254 mm), regardless of geometry. Several variations of blunted crack tip shape were observed for all three specimen types. Two methods of crack extension were observed for all three geometries studied. In the first method ellipsoidal holes were found to nucleate and grow ahead of the main crack tip. Growth continued until the ligament between the main crack and the hole was less than the major axis length of the hole. The hole then joins the main crack by means of local strain instability. The second method consists of alternate sliding off in a zigzag fashion. JIC was measured and found to be equivalent for all three geometries.
Engineer, General Electric Co., San Jose, Calif.
Senior engineer, Westinghouse Research and Development Center, Pittsburgh, Pa.
Assistant professor, University of Pittsburgh, Pittsburgh, Pa.
Stock #: JTE10111J