Examples for size effects

There are many different reasons why components of different size show different fatigue behavior. Following are some of the reasons according to Kloos. For a detailed discussion, the reader is referred to the relevant literature (see Part 4 Appendix: References).

Technological size effects

Quenched and tempered steel

Here, the hardness profile depends on the diameter because of the hardening process. The endurance limit at the surface depends on the surface hardness, therefore the endurance limit depends on the diameter:

Hardness Profile in Tempered or Quenched Steel

High-strength steel

The fatigue behavior depends on the size, shape and distribution of non-metallic inclusions since they lead to local changes in the stress distribution. Note that higher the probability for inclusions the larger the surface is:

Stress Distribution in High-Strength Steel Due to Non- Metallic Inclusions

Graphitic cast iron

Here, we have to handle similar effects due to the morphology of the graphite:

Stress Distribution in Graphitic Cast Iron Due to the Morphology of the Graphite

Size effect of surface treatment

The relative depth of the strengthened surface layer depends on the size of the component and influences the pre-stress for the inner layer:

Different Pre-Stresses at the Inner Layer After Surface Treatment

Statistical size effect

In certain instances, the size effect can be attributed to the internal defect size.

If the defect size is not constant, then a size effect on fatigue life will be noticed. This is due to the statistical distribution of the internal defect size. In large volumes of material, there is a greater probability of finding a large defect where the stresses are relatively high, as illustrated in the figure The Statistical Size Effect Occurs if Defect Size Is Not Constant.

The Statistical Size Effect Occurs if Defect Size Is Not Constant

How to account for size effects

In the literature, you can find many approaches to account for size effects. They are commonly based on modifying the endurance limit for the original fatigue data using correction factors.

Literature

Kloos, K. H.; Buch, A.; Zankov, D. Pure geometrical size effect in fatigue tests with constant stress amplitude and in programme tests. Materialwissenschaft und Werkstofftechnik, 12. Jg., Nr. 2, S. 40-50, 1981.

Learn more

The fatigue notch factor

Stress gradients

Theoretical concepts

Application of the theory

Macroscopic yielding

Neuber's approach to micro-yielding

Summary for size effects

Surface effects

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Examples for size effects, Simcenter 3D 2021.1 Series

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