Structure parameters governing fracture toughness of engineering materials

1. Of the two concepts of brittle fracture of engineering materials based on the "decohesion" and "coalescence" fracture mechanisms, respectively, the first reflects the threshold fracture toughness for materials of perfect structure, the second - the minimal fracture toughness of the material of a real structure. Since the "coalescence" fracture mechanism is observed with most engineering materials and requires higher fracture energy and the "decohesion" mechanism is a part of the "coalescence" mechanism, it is necessary to investigate both of them in order to study the nature of the fracture process and to optimize the material structure. 2. The model parameters on which the "coalescence" fracture mechanism is based, namely, characteristic distance X_c and microcleavage stress σ_f^*, are directly related to the material minimal fracture toughness and are defined by the weakest elements of its microstructure. 3. Rigorous physical interpretation of the characteristic distance and microcleavage stress requires statistical (dimensional and orientation) consideration, yet modeling of the fracture process in mean values of the above parameters seems to be useful. 4. Fracture toughness dependences on the temperature and loading rate both for a number of ceramic materials and for steels in the brittle-to-ductile transition region have much in common. For this reason, it is possible to use some fracture models, and, in particular, the K_μ-model, to analyze fracture of ceramic materials and to optimize their structure. 5. The main ways of enhancing fracture toughness of engineering materials are associated not only with the plasticization of the latter but also with the creation of such structures that would contribute to an increase of their minimal fracture toughness values. This can be achieved by increasing each of the two fundamental parameters of the material fracture micromechanism: characteristic distance and cleavage stress.