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Cotterell and Mai and their co-workers [1][2][3][4] have determined the specific essential work of fracture (We) in ductile materials by utilizing geometrically confined specimens in which the size of the outer plastic zone is proportional to the square of the initial ligament length z. They distinguished the inner fracture process zone and outer plastic deformation region and interpreted the inner fracture process zone as the necked region [4] in the path of the crack comprising essential work of fracture. By extrapolating the straight line relationship between the total specific work of fracture Wf(=Wf/t ~ where Wf is the total work of fracture; t is the thickness; ~is. the initial ligament length) and the initial ligament length z down to zero ligament, they obtained the specific essential work of fracture We from the intercept.Recently, we have reported [5][6][7][8] that the resistance to crack initiation and growth can be determined utilizing the locus line of a set of characteristic points on load-displacement curves of specimens which differ only in initial crack size fromwhere R is the resistance to crack growth; B is the specimen thickness; a is the initial crack length; Ur is the area enclosed by a loading curve, the locus line along the characteristic points, and the displacement axis as shown in Fig. 1. The characteristic points can be any points of interest such as the crack initiation points, the points at which the sane amount of crack growth c occurs, the maximum load points, or the fast rupture points. We also reported [8] that the crack-resistance Rm at the maximum load point of load-displacement curve can have similar values for different initial crack sizes when the R-curve of the specimen has a transition in the slope as shown schematically in Fig. 2. Since the maximum load point on a displacement-controlled load-displacement curve occurs at the tengential point between the corresponding constant-load crack-driving force line and theR-curve, Rm for different initial crack sizes can have similar values if the tangential points for different initial crack sizes occur nearby each other on the R-curve.When the inner processing zone and the outer plastic region can be distinguished as in the case considered by Cotterell et al., the R-curve may reflect Int Journ of Fracture 42 (1990)

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