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Wells has proposed using crack tip opening displacement as a measure of the fracture toughness of cracked plates in general yielding. The concept is applicable to thin plates loaded to extensive plastic deformation. The method is used to determine the fracture toughnesses of HY-80 steel plates. (Author)

Earlier crack tip strain measurements have shown a definitive three dimensional effect upon crack tip deformation. In this study, a pseudo three dimensional Finite element method (FEM) calculation is made with a composite stiffness matrix. The calculated strains agree exceedingly well with the experimental measurements. The degree of crack tip stiffening is equivalent to only 4% of the plane strain plastic zone along the crack front in a 1/4 inch thick aluminum speciment, and 8% for a 1/2 inch thick speciment. Both specimens are loaded considerably beyond general yielding. The results cast serious doubt upon the premise that the specimen, with extensive plastic deformation, can be used to determine plane strain fracture toughness.

Near tip strains and crack tip opening displacements were both measured with the moire method and calculated with Finite Element Analysis. The equivalent stress intensity factor of a small specimen loaded into general yielding is related to its crack tip strains and stresses. Its relations to the overall deformation of a specimen and the applied stress are also obtained by the near tip stress and strain correlation. Its application to a ductile aluminum is illustrated. In addition, crack tip opening and crack tip advancing were calculated based on the shear separation mode of the unzipping model.