Fatigue fracture results from the simultaneous action of repeated or fluctuating cyclic stress, tensile stress, and plastic strain. No fatigue crack starts or grows in the absence of any of these three active components. Cyclic stress initiates the crack and tensile stress produces the crack growth. There are many variables in service that influence the fatigue behavior or characteristic pattern. These include the magnitude and frequency of application of the fluctuating stress, the presence of a mean stress, temperature, environment, part size and shape, state of stress and residual stresses, surface finish, surface damages, and microstructure. The occurrence of fatigue may be considered as a three‐stage process.
1 Initiation of the surface or sub‐surface fatigue crack under a fluctuating load.
2 Crack propagation under tensile stress until the section thickness critically bears the imposed load.
3 Sudden fracture under overload
In the presence of a severe stress concentration, multiple crack origins are formed which eventually unite to form a single crack front. When the stress concentration on the surface is absent, cracks propagate more readily near the center of a section than at the surface. While there is a stress concentrating ‘notch, W‐shaped crack fronts are‐observed. For a given material, zone of the final fracture increases with increasing applied load. On rotating or bending the final fracture’, region is often rotated or offset toward the origin in the direction opposite to the direction of rotation. Also, with increasing nominal stress, the final fracture moves toward the center.
Figure 5.9 Schematic representation of the fatigue crack (three stage) phenomenon.
Stress Concentration
A stress concentration is a physical or metallurgical condition that increases the local stress in the part by some factor. A good example is the shaft shown in Figure 5.10. We see that the stress in the area of the radius varies depending on the size of the radius. A small radius can increase the stress dramatically
Thermal Fatigue Failure
Thermal fatigue failure is the result of repeated thermal cycling which generates repeated stress gradients within a free component or stress cycling within a component constrained to fixed dimensions. Thermal fatigue fractures are characterized by surface cracking formed by multiple initiation sites that join randomly by edge sliding to form the main crack. Other features to identify thermal fatigue are as follows:
Figure 5.10 Stress concentration at corners.
Fractures are planner and transverse with no visible plastic deformation.
Fracture is mostly transgranular.
Oxidized fracture surfaces and oxide wedge filled cracks further characterize thermal fatigue failures
Fatigue resistance is affected by a number of controllable factors:
The chemistry of the material and its resultant microstructure have a profound effect on fatigue strength. In fact, they can equally influence on mechanical strength (tensile and yield). Alloying elements, such as chromium, nickel, and moly, have the greatest effect on the iron base system. Solid solution alloys show the maximum increase in fatigue strength.
Grain size appears to be a strong determining factor in inhibiting the plastic deformation process that occurs with crack propagation.
Environmental factors such as cyclic temperature, temperature gradient, and corrosion pitting that result in stress concentrations. The thermal fatigue failure shown in was due to the temperature gradient across the thick wall section.
Reduction of localized surface stress concentrations by such techniques as case hardening, shot peening, auto frottage, and thread rolling.
Proper heat treatment can markedly improve fatigue resistance. As an example, for steels, a tempered martensitic.
Stress Corrosion Cracking
Stress corrosion cracking (SCC) is a fracture process that involves the combined and simultaneous action of a tensile stress and a corrosive environment. SCC occurs when the tensile stress and a specific environment are able to cause failure by their combined action, but are insufficient to cause failure by either one acting alone. In fact, the tensile stresses are usually below the metal’s yield strength. Furthermore, the metal would suffer only minimal corrosion in the absence of the applied stress. The problem itself can be quite complex. The situation with buried pipelines is a good example of such complexity.
There are three requirements for SCC to occur:
1 A susceptible metal.
2 Tensile stresses applied to the metal.
3 A specific environment containing an aggressive species that promotes SCC.
This form of corrosion is particularly dangerous because it may not occur under a particular set of conditions until there is an applied stress. The corrosion is not clearly visible prior to fracture and can result in catastrophic failure. Many alloys can experience stress corrosion, and the applied stress may also be due to a residual stress in the material. An example of a residual stress could be a stress remaining in a material after forming, or a stress due to welding. Stress corrosion cracking will usually cause the material to fail in a brittle manner, which can have grave consequences as there is usually little or no warning before the failure occurs.
The impact of SCC on a material usually falls between dry cracking and the fatigue threshold of that material. The required tensile stresses may be in the form of directly applied stresses or in the form of residual stresses. Cold deformation and forming, welding, heat treatment, machining, and grinding can introduce residual stresses. The magnitude and importance of such stresses is often underestimated. The residual stresses set up as a result of welding operations tend to approach the yield strength. The buildup of corrosion products in confined spaces can also generate significant stresses and should not be overlooked.
How to Prevent SCC
SCC can be controlled by any of the following three approaches:
1 Design, which includes selection of the mechanical and materials aspects of components,
2 Controlling the materials, and
3 Controlling the environment.
Figure 5.11 Stress corrosion on a bar.
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