Some aspects of fracture mechanics research during the last 25 years

A review is given of the various concepts of fracture mechanics, furthermore of their application to assess the toughness of steels and to guarantee the safely of structures containing cracks. The three most important parameters are the stress-intensity factor K-I for linear elastic fracture mechanics (LEFM), the crack tip opening displacement CTOD and the J-integral for elastic-plastic fracture mechanics (EPFM). The ASTM designation E 1820-96 provides a common method for determining all applicable toughness parameters from a single test, including R-curves, Many investigations dealt with the numerous influences on these parameters, e.g. specimen geometry and temperature and strain rate. Various failure concepts have been developed. The method of LEFM based on plane strain fracture toughness K-Ic is well established for high strength steels or low temperatures, For modern steels with high toughness the more complicated concepts of EPFM have to be applied. Because of the availability of commercial software for FE-calculations the application of the J-integral has become widespread in recent years. A number of approximative methods has been elaborated, e.g. CTOD-design-curve, BSI PD6493:1991, CEGB-R6-method, ETM, Eurocode 3 Annex C. Although the theory of fracture mechanics regards the material as an isotropic continuum the question is of great importance, how its materials parameters depend on the microstructure of the steels, Systematic investigations with the aid of the hot deformation simulator Wumsi showed the favourable material properties of fracture mechanics resulting from thermomechanical treatment. Many models were published for the quantitative correlation between microstructure and toughness parameters, mainly K-Ic. At the moment the modified Gurson model is in widespread use, which allows the prediction of J-R-curves.