Conventional fracture mechanics assessment procedures for evaluating the integrity of flawed components have existed since the 1970s. For precise analyses, finite element calculations are required to determine the applied conditions in the cracked structure. If numerical computations have established the stress/strain conditions in the structure, an alternative approach is the application of micromechanical failure models, often referred to as the 'Local Approach', which are based on the micromechanisms of the failure event. For fracture, these models assume that failure occurs in a given volume of material when a critical stress or strain state is reached in the vicinity of the flaw under consideration. Since the fracture event is described locally, the mechanical factors affecting fracture are included in the predictions of the model.
The correct application of micromechanical failure models includes the determination of relevant material parameters using a series of conventional fracture toughness tests or special notched tensile tests and allows precise predictions of structural fracture behaviour to be made. However, these models require considerable computational effort and rely on thorough numerical analyses of the structure studied.
There are micromechanical failure models for both cleavage , or brittle, and ductile [2,3] fracture and the ductile to brittle fracture toughness transition behaviour has also been described by Local Approach methods via the coupling of cleavage and ductile models [4,5]. More details are given in several review articles [6,7,8].
- Beremin F M: 'A local criterion for cleavage fracture of a nuclear pressure vessel steels'. Met. Trans. 1983 Vol. 14A, pp2277-2287.
- Rousselier G: 'Finite deformation constitutive relations including ductile fracture damage'. Proc. Conf. 'Three-dimensional constitutive relations and ductile fracture' (S Nemat-Nasser, Ed.), North Holland, 1981, pp331-355.
- Gurson A L: 'Continuum theory of ductile rupture by void nucleation and growth: Part 1 - Yield criteria and flow rules for porous ductile media', J. Engng. Mat. Tech., 1977, Vol.99, pp2-15.
- Eripret C, Buisine D and Rousselier G: 'The transition regime modelled by local approach to fracture', Proc. Conf. 'Advances in local fracture/damage models for the analysis of engineering problems'. AMD-Vol 137, ASME, 1992, pp195-200.
- Goldthorpe M R and Wiesner C S: 'Micromechanical predictions of fracture toughness for pressure vessel steel using a coupled model', ASTM STP 1332, 29 th National Symposium on Fatigue and Fracture, June 1997, Stanford, CA, 22-24 June 1997, ASTM, 1999, pp341-363.
- Wiesner C S: 'The application of the 'Local Approach' to cleavage fracture of ferritic steels and their weldments', TWI Research Report No. 478/93, November 1993 (only available to TWI Industrial Members).
- Wiesner C S and Andrews R M: 'A Review of Micromechanical Failure Models for Cleavage and Ductile Fracture', TWI Research Report 592/97, February 1997 (only available to TWI Industrial Members).
- Wiesner C S: 'The 'Local Approach' to cleavage fracture', TWI Research Bulletin, 1994, 35, (2) 35-39 (only available to TWI Industrial Members).