The strength values for the fatigue strength verification are usually determined on standardized test specimens, which are usually not directly comparable either in their shape, dimension and manufacturing process with the component, which is to be evaluated on the basis of the material properties obtained. Therefore, correction factors are used in the fatigue strength evaluation in order to take into account the inevitable imperfections in the determination of characteristic values and in the comparison with the real component as best as possible.
The term material influence takes into account the effects of possible metallurgical deviations between sample and component. These are among other things influences by the alloy composition, the grain size, the heat treatment or by the uniformity or unevenness of the microstructure.
Another aspect is the different influences of size, which cause a deviation to the characteristic values from the test specimen. For example, the probability of breakage due to a statistically distributed material defect increases with increasing sample size. The same applies to processes that produce surface hardening that only have a certain layer thickness due to the technology and process. The supporting influence of this layer decreases with increasing wall thickness of the component. The strength values also depend on the number, size and arrangement of non-metallic inclusions.
An influence that is often particularly significant results from the residual stresses or even previous damage to a component caused by production. A heavily formed sheet or a component quenched during heat treatment shows a significantly different fatigue behaviour locally. The superposition of the residual stresses with the operating stresses can lead to both a shortening or an extension of the life expectancy, depending on whether the mean stress increases or decreases.
Direct & indirect consideration
In some cases, these influences can be better estimated by upstream process simulations. For aluminium alloys, for example, there is a known relationship between the cooling rate and the distance between the dendrite arms, the so called dendrite arm spacing. In the case of cast components, this can be determined with a mould filling and solidification simulation; in this way, the grain size can be taken into account with a local factor instead of a global one. A similar procedure also exists for other influencing factors, e.g. the distribution of non-metallic inclusions or the fibre orientation in fibre-reinforced plastic parts. The already quoted residual stress condition could also be included in the evaluation in this way. However, the effort involved in this procedure is sometimes very considerable, especially in the case of internal stresses. In many cases, the upstream process simulation requires separate models and good metallurgical and process engineering knowledge.