SOME ASPECTS OF FRACTURE-MECHANICS RESEARCH DURING THE LAST 25 YEARS

A review is given of the various concepts of fracture mechanics, furth ermore of their application to assess the toughness of steels and to g uarantee the safely of structures containing cracks. The three most im portant parameters are the stress-intensity factor K-I for linear elas tic 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 a ll applicable toughness parameters from a single test, including R-cur ves, Many investigations dealt with the numerous influences on these p arameters, e.g. specimen geometry and temperature and strain rate. Var ious failure concepts have been developed. The method of LEFM based on plane strain fracture toughness K-Ic is well established for high str ength steels or low temperatures, For modern steels with high toughnes s the more complicated concepts of EPFM have to be applied. Because of the availability of commercial software for FE-calculations the appli cation of the J-integral has become widespread in recent years. A numb er of approximative methods has been elaborated, e.g. CTOD-design-curv e, 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 param eters depend on the microstructure of the steels, Systematic investiga tions with the aid of the hot deformation simulator Wumsi showed the f avourable material properties of fracture mechanics resulting from the rmomechanical treatment. Many models were published for the quantitati ve correlation between microstructure and toughness parameters, mainly K-Ic. At the moment the modified Gurson model is in widespread use, w hich allows the prediction of J-R-curves.