Metallurgy for Dummies

When considering the response of metallic materials to cyclic loading, it is essential to distinguish between components such as machined parts, which are initially free of defects, and those such as castings and welded structures, which inevitably contain pre-existing defects. The fatigue behaviour of these two types of component is quite different. In the former case, the major part of the fatigue life is spent in initiating a crack; such fatigue is ‘initiation-controlled‘. In the second type of component, cracks are already present and all of the fatigue life is spent in crack propagation, such fatigue is ‘propagation-controlled’.

Two way bending fatigue in a bolt. Large Arrow at 1 o’clock shows area of fatigue crack initiation.

For a given material, the fatigue strength is quite different depending on whether the application is initiation- or propagation-controlled. Also the most appropriate material solution may be quite different depending on the application. For example with initiation-controlled fatigue, the fatigue strength increases with tensile strength and hence it is usually beneficial to utilise high strength materials. On the other hand, with propagation-controlled fatigue, the fatigue resistance may actually decrease if a higher strength material is employed.

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Preceding sections have described the influence of the micro structure on strength and toughness using metallurgical mechanisms. Chemical and physical metallurgy can change microstructural characteristics so that optimum strength and toughness requirements may be obtained. By combining the various treatments it is possible to achieve a wide range of steel properties (Figure 13) :

• Chemical metallurgy treatments

Variation of the chemical composition of a steel by adding alloying elements aims to increase strength and/or increase resistance to brittle fracture. Solid solution hardening generally lowers toughness and is not widely employed.

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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.

Fracture Mechanic

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:

• K-concept

The K-concept can be applied in the case of linear-elastic component behaviour. The crack driving force, the so-called stress intensity factor K_I, 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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Fracture mechanics provides a quantitative description of the resistance of a material to fracture. The fracture toughness is a material property which can be used to predict the behaviour of components containing cracks or sharp notches. The fracture toughness properties are obtained by tests on specimens containing deliberately introduced cracks or notches and subjected to prescribed loading conditions.

Cracking Pipe

Depending on the strength of the material and the thickness of the section, either linear-elastic (LEFM) or elastic-plastic fracture mechanics (EPFM) concepts are applied.

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