COMBINED MODE I - MODE III FRACTURE TOUGHNESS OF A HIGH-STRENGTH LOW-ALLOY STEEL

Combined mode I - mode III fracture behavior was investigated for two heat treated conditions of a copper-bearing high-strength low-alloy steel. Special formulations of the J integral, denoted J(,i) and J(,iii) for the opening mode and transverse shear mode respectively, were used to characterize crack initiation while Paris' tearing modulus was used to characterize J-resistance behavior during stable crack extension. Combined mode fracture initiation data define a linear fracture envelope in J(,i)-J(,iii) space. This behavior is predicted qualitatively by a superdislocation model of combined mode elastic-plastic fracture in which the plastic zone is modeled by two superdislocations ahead of the crack tip; one of edge character with Burgers vector equal to the mode I CTOD, the other of screw configuration with Burgers vector equal to the mode III CTOD. For all loading modes, the overaged condition containing incoherent face-centered-cubic copper precipitates exhibited superior toughness as compared to the peak aged condition which had a similar grain size and carbide distribution, but contained coherent body-centered-cubic copper-containing clusters. For all loading geometries, large J-component initiation values were accompanied by large tearing modulus values (large resistance to crack growth). This correspondence between crack initiation resistance and growth resistance agrees with commonly observed mode I behavior. Both heat treated conditions displayed tendencies for transverse shear flow and the overall greater toughness of the overaged material probably results from its greater resistance to plastic instability as evidenced by its greater elongation at fracture in uniaxial tensile tests. Fractographic examination reveals that crack extension occurred by dimpled rupture in all loading modes, but that dimple morphologies changed markedly. For pure mode III loading, friction between the specimen halves may have contributed significantly to measured toughness values.