The separation of the fracture energy in metallic materials

The total plastic strain energy which is consumed during fracture of a plain-sided CT specimen is separated into several components. These are the energies required for deforming the specimen until the point of fracture initiation, for forming the flat-fracture surfaces, for forming the shear-lip fracture surfaces, and for the lateral contraction and the blunting at the side-surfaces, W-lat. Characteristic crack growth resistance terms, R-flat and R-slant, are determined describing the energies dissipated in a unit area of flat fracture and slant-fracture surface, respectively. R-flat is further subdivided into the term R-surf, to form the micro-ductile fracture surface, and into the subsurface term, R-sub, which produces the global crack opening angle. Two different approaches are used to determine the fracture energy components. The first approach is a single-specimen technique for recording the total crack growth resistance (also called energy dissipation rate). Plain-sided and side-grooved specimens are tested. The second approach rests on the fact that the local plastic deformation energy can be evaluated from the shape of the fracture surfaces. A digital image analysis system is used to generate height models from stereophotograms of corresponding fracture surface regions on the two specimen halves. Two materials are investigated: a solution annealed maraging steel V 720 and a nitrogen alloyed ferritic-austenitic duplex steel A 905. For the steel V 720 the following values are measured: J(i) = 65 kJ/m(2), R-surf = 20 kJ/m(2), R-flat = 280 kJ/m(2), R-slant = 1000 kJ/m(2), W-lat = 30J. For the steel A 905 which has no shear lips, the measured values are: J(i) = 190 kJ/m(2), R-flat = 1000 kJ/m(2), and W-lat = 45 J. Apart from materials characterization, these values could be useful for predicting the influence of specimen geometry and size on the crack growth resistance curves.