Influence of size

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The influence of size describes the relationships between the transferability of material parameters , which were determined under standardized conditions, to conditions that deviate from the test conditions.

The cause of the size influence can be microscopic and macroscopic causes such as grain sizes and voids . In the literature, the influence of size is divided into four different areas:

1. Statistical size influence: If you compare the fatigue strength of two components that are geometrically identical but different in size, the smaller component has a higher strength . This is due to the fact that the highly stressed component volume or the highly stressed component surface is also larger in the larger component. From a statistical point of view, the probability is higher that there are defects in this area where cracks can initiate under stress.
2. Stress -mechanical / geometric size influence: The stress-mechanical size influence is related to the statistical size influence. Here, too, the different fatigue strength is based on a higher stressed volume of material. The cause, however, is due to different stress gradients in the material . As an example, a beam under flexion and tension with the same stress maxima is mentioned here.
3. Technological influence of size: The technological influence of size is based on the differences in the structure of the material that result from the manufacture or processing. It differ z. As size and shape of the voids and cavities as well as size of the crystalline grains.
4. Surface size influence: The limited penetration depth of methods for introducing compressive residual stresses forms the surface size influence.

In the design of components , therefore, appropriate correction factors must be considered, taking into account the influence of the size of the component. Alternatively, you can also determine a lower limit value: The statistical influence of size increases with the increasing probability of breakage. It disappears at the probability zero, that is, at the zero break line.

• Sample dimension : The size of a sample also influences its material behavior. The smaller a sample becomes, the more the material behavior changes towards that of individual atoms or molecules. This effect sometimes has a strong influence on material constants such as the specific resistance or the melting temperature .