Assessment of the Fracture Toughness of Elastoplastic Materials Based on the Structure State at the Crack Tip

The methodological foundations of assessing the fracture toughness of elastic-plastic materials are considered. A methodology is proposed based on the results of assessing the fracture toughness of a material based on the structure state in the region of the crack tip. It is shown that the state of the material structure in the fracture zone of a uniaxially loaded specimen under the action of a stress close to the material’s ultimate strength is adequate to the structure state in the region of the crack tip before crack initiation and is a structural characteristic of this material. Based on the above correlation of material states, a structural parameter is proposed for assessing the fracture toughness of elastic-plastic materials with a crack. A statistical parameter, determined by the LM-hardness method from the Weibull homogeneity coefficient m, which characterizes the degree of scattering of hardness characteristics obtained by indenting the test portion of the loaded specimen, was taken as the fracture toughness index. This characteristic is invariant to the type of stress state and the loading method, i.e., the state of the material structure in the region of the crack tip does not depend on the loading method, which affects only the rate of reaching the ultimate structural damage in the region of the crack tip and the direction of crack propagation. It has been experimentally proved that the structural criterion of fracture toughness can be the same material characteristic as the force or strain characteristic of material fracture toughness. The difference between materials in their ability to resist crack initiation and propagation is determined by their initial structure. It depends only on the value of the thermal and force load parameters at which the material reaches a level of damage (loosening) of the structure at the crack tip sufficient for crack initiation. For comparison, we consider possible options for assessing the fracture toughness of elastic-plastic materials about the damageability of the structure: based on the scattering of the hardness values of the material of the specimen with a crack, on the state of the material structure in the test portion after the failure of the loaded specimen or after its reloading, as well as based on the state of the material structure of the specimen under loading to the ultimate strength of the material.