Abstract
Metal fatigue is a weakened condition induced by repeated stresses which ultimately results in cracks or even fracture and, thus, has to be understood in terms of its influencing factors. In this chapter, the fatigue strength of materials and components will be reviewed including the major parameters which have an effect on the number of cycles-to-failure . Material- and process-related effects will be discussed as well as stress concentration and surface conditions . A major proportion of this chapter is related to the effects of mean stresses and residual stresses to demonstrate effective measures for improving fatigue strength and, thus, optimizing lightweight design.
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Notes
- 1.
In uncertainty quantification, this type of uncertainty—often called aleatoric uncertainty—causes results which differ each time the same experiment is performed. Hence, probability distributions, or alternatively Monte Carlo experiments have to be used for the quantification of this type of uncertainty.
- 2.
Here the term ‘confidence limit’ denotes a scatter band of the test data of a given probability of occurrence which has to be differentiated from the confidence limit for a certain percentile of the statistical analysis expressing the uncertainty associated with the calculation of this percentile.
- 3.
That is certainly not true in a narrow sense, because we will see that specimen may include geometrical features too. But such a simplified statement helps us to understand that it cannot be the final stage to use specimen test results for a durability performance evaluation of parts and assemblies.
- 4.
Welded joints are an important part of engineered structures and, thus, need some special remarks which will serve a dedicated chapter later.
- 5.
It is important to make a difference between a defect and a fatigue relevant defect: A structure may have a large number of defects but is not influenced by them in terms of fatigue life, if the defects are not in a highly loaded area of the structure.
- 6.
Please have a look at the various different designs you easily find for aluminum alloy wheels and count the spokes: The uneven number that is found for the majority of the wheels is part of Birkhoff’s concept of certain harmony, symmetry or order.
- 7.
Hence using strain gages applied to the structure at the change in local geometry, or finite element analysis can be a measure to get an information about the actual stress which results from stress concentration and loading.
- 8.
The concept of the stress concentration factor is more related to macroscopic features than to microscopically small changes of the local geometry. Hence, the stress at a crack tip is not part of that concept and actually replaced by the so-called stress intensity factor that is linearly related to the stress and directly related to the square root of a characteristic crack dimension. We’ll come back to that when looking at crack growth and fracture mechanics.
- 9.
But it has to be clear that it is not a proper concept to introduce sharp notches in engineering design to make use of the notch support—that would be not appropriately because the huge increase of the peak stress is what limits the fatigue life of a poorly designed product.
- 10.
Again, that relation follows a power function pretty nicely and that is the reason to show it in a semi-log diagram here.
- 11.
Fiber-reinforced plastics and composites have the opposite behavior: Since the fibers have their load carrying capability under tensile loading, the reinforced plastics get better fatigue strength when there is positive mean stress. Having compressive mean stress the fibers may start to bend or crush which then limits the load carrying capability of the material significantly.
- 12.
It is easy to understand that a destructive technique for measuring residual stresses is applied if it is expected to have unintentional and unwanted residual stresses.
- 13.
Though the hole-drilling method looks like an old-fashioned method, it is even today successfully used and has many applications.
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Heim, R. (2020). Influencing Factors for Fatigue Strength. In: Structural Durability: Methods and Concepts. Structural Integrity, vol 17. Springer, Cham. https://doi.org/10.1007/978-3-030-48173-5_3
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