FEM stress

[NikonF6]

When calculating stress with at a notch, FEM give us nominal stress (time) theoretical stress concentration alpha (or form factor). Do you guys recalculate that stress using fatigue-stress-concentration and other corrections not accounted with FEM?

Thanks

 

[BlasMolero]

Dear NikonF6,
The answer depends of the fatigue method used.
To do a fatigue life calculation in any fatigue code (for instance, winLIFE) for "not welded components" the following three basic methods are available:
• The Nominal Stress Method
• The Local Stress Method (also named the Elastic Notch Stress)
• The Local Strain Approach.

In the Nominal Stress Method the user has to calculate the nominal stress acting in the critical cross section of the component (the noth is not meshed in detail in the case of using FEA). This means, that the user must have an idea of the critical point(s) of a component (for components with simple cross sections this can be done easily, for complex geometry a simplification is required to define a suitable nominal stress). A nominal stress is defined by simple equations and the elastic stress in a component can be calculated by multiplying with a stress concentration factor. A calculation can be done, if the stress concentration factor Kt is known. Then the value of stress concentration factor Kt is used to define the S-N curve of the component, as well as material properties, treatment of the material, surface, etc...

The Elastic Stress Method uses local stress to perform a fatigue life prediction according to S-N-curves. The procedure is similar to the Nominal Stress Method but the S-N-curves are not related to the component but to the local material. This means that the stress concentration factor is not needed, because the local stress is used. Local effects can be considered by using the surface roughness, stress gradient and supporting effect, then Kt = 1.0. Usually FE results are used for the notch stress analysis and you get a stress tensor with 6 components at each node. Therefore it is necessary to take one size from these single stresses, one that is suitable for the fatigue life, an equivalent stress or a damage parameter.

In the Local Strain Approach -contrary to the Nominal Stress Method- the real stress and strain are needed at the locus of interest. This can be calculated by the FE-method. Because of the calculation performance, an elastic stress-strain analysis is usually done and the stresses are transformed to the real stresses using Neuber’s rule.

The use of Local Strain Approach has the following advantages:
• An automatic procedure using this method in combination with finite elements is possible and means that the user does not have to search for the critical point.
• The data describing fatigue life are pure material data. This results in a very universal usability and a limited number of data sets.
• The real stress-strain behaviour of the material is included, so that plastic stresses are modelled correctly.

Some people find it a disadvantage of this method that the theory is much more complex and that the calculation results are difficult for the user to follow. Due to the much greater experience in the use of the Nominal Stress Method, it seems in many cases to be a good idea to use both methods: Nominal Stress Method and Local Strain Approach. The differences in the results will help to make a critical analysis of the parameters and finally give an idea of the scatter range of the calculation results.

Best regards,
Blas.

~~~~~~~~~~~~~~~~~~~~~~
Blas Molero Hidalgo
Ingeniero Industrial
Director

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