Ductile to brittle fracture analysis based on improved application of proposed crack opening criterion deduced by micro-crack evolution equation

In the previous paper (Kobayashi, 2017), the new crack opening criterion deduced by a micro-crack evolution equation was proposed and verified by examination of the fracture mechanisms of metallic and ceramic materials using those well-known data, i.e., fracture toughness and so forth, based on the stresses, strains and strain gradients substituted by the stress intensity factor (SIF). However, discrepancies between experimental and estimated results in A533B steel, Al2024-T4 and SiAlon cases were caused by the plastic/damage energy dissipation. In the present paper, the improved application of the crack opening criterion to be able to apply ductile materials such as A533B steel, Al2024-T4 and SiAlon by considering the plastic/damage energy dissipation based on ASTM standard E1820-01 is proposed and the plastic/damage process zone area at the crack-tip caused by the crack growth is estimated by the crack opening criterion; even though SiAlon is ceramic material, the fracture progress of A533B steel, Al2024-T4 and SiAlon are characterized by stable crack growth (ductile fracture) in the limited range of the crack extension. Moreover, significance of the theoretically estimated yielding stress and/or fracture toughness under the predicted fracture mode is pointed out to identify the phenomenological fracture mode experimentally observed by comparison of the experimental ones; the proposed criterion of micro-crack nucleation for the ductile fracture (Kobayashi, 2012) derived by the vanishing velocity condition is examined by the improved application of the crack opening criterion in the physical point of view. Also the fracture progress of an elastic-perfect plastic material is schematically interpreted by the improved application of the crack opening criterion.