The basis of a fracture mechanics safety analysis is the comparison between the crack driving force in a structure and the fracture toughness of the material evaluated in small scale tests. The application of one of the concepts depends on the overall behaviour of the structure which may be linear-elastic (K-concept) or elastic-plastic (CTOD- or J-Integral-concepts). For a safe structure the crack driving force must be less than the fracture toughness.
In general the toughness values of the material are evaluated according to existing standards. The crack driving force can be calculated on the basis of analytical solutions (K-concept), empirical or semi-empirical approaches (CTOD-Design-Curve approach, CEGB-R6-procedures) or using numerical solutions (indirectly: EPRI-handbook, directly: finite-element calculations). The different methods are explained briefly below:
The K-concept can be applied in the case of linear-elastic component behaviour. The crack driving force, the so-called stress intensity factor KI, defined in Section 1.4, has been evaluated for a large range of situations and calculation formulae are for example given in the stress-analysis-of-cracks handbook.
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