Loading and local stresses

If loads are applied to a component they create local stresses and strains at each point in the component. For loads that are a function of time, the stresses that are created are also a function of time and can cause fatigue damage.

Load Applied to a Component Induces a Nominal Stress and a Local Stress at the Notch

In the figure we see three terms that all may be used to base a fatigue analysis on. First, the load itself may be used, which is represented by a force applied to the component in this picture. Secondly, the nominal stress could be used, which is defined as the average stress over a cross section of interest. In our example

where A denotes the area of the cross section. Finally elastic stress at the notch (the "pseudo stress") could be used. It may be calculated from the nominal stress if the geometry of the notch is known:

We call Kt the stress concentration factor. It may also be called in the literature.

You may of course calculate directly from the load:

We call c the load influence factor or influence coefficient.

Note:

The influence factors are the values read from the (unit-load case) finite-element calculations in Specialist Durability. In finite elements, we handle stress tensors, therefore also the influence coefficients are handled as influence tensors.

The concept of influence coefficients may also be used for calculation of local pseudo stresses for multiple non-proportional loading:

Pseudo Stress for Non-Proportional Loading

To calculate the (elastic) pseudo stress, you may use the individual influence factors ci and calculate the pseudo stress by superposition:

Property	Symbol	Formulae
Load
Nominal stress
Pseudo stress
Stress concentration factor	Ktor
Load influence factor	c

Learn more

Types of loading and the definition of a cycle

Fatigue life and fatigue damage

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Loading and local stresses, Simcenter 3D 2021.1 Series

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