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Determination of static fracture toughness of coarse-grained and ultrafine-grained materials by the depth of the plastic zone under the fractures surface

In the practice of diagnostics of emergency failure of structures or machine parts, there are cases when information is needed on the fracture toughness ( K 1C or K С ) of the material of products, the dimensions and configuration of which do not allow the fabrication of samples for testing the material for K 1C according to the standard GOST 25.506-85. On an example of a large group of materials with bcc, fcc and hcp lattices, the relationship of the fracture toughness ( K 1C or K C ) of materials with the yield strength, hardness and elongation is considered. It is shown that the fracture toughness of materials both in coarse-grained (CG) and ultrafine-grained (UFG) states is ambiguously related to these mechanical characteristics of materials. Therefore, the fracture toughness of materials cannot be determined by calculation based on the above strength and plastic properties. In this paper, we propose a method for determining fracture toughness of CG and UFG materials by the maximum depth of the plastic zone under the fracture surface ( h max ) of fractured specimens, structures, or machine parts. The depth of the plastic zones under the fracture surface was determined by the X-ray method. If the destruction occurred in the plane stress (PS) condition or in the transition region from plane deformation to plane stress state (PD←→PS), then the depth of the slightly deformed macro zone ( h y ) was taken as h max . Using the example of a large group of materials with bcc, fcc and hcp lattices, it has been shown that it’s possible in principle to determine the fracture toughness of CG and UFG materials with different types of crystal lattice using the plastic zone depth under the fractures surface.