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Major strain vs Strain_Y for a uniaxial tensile test


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Dears, 

I am conducting a uniaxial tensile test on an aluminium sheet. I create one Surface component with Facet 19 and point distance of 16. I check for this surface component Strain_Y and Major Strain. For a uniaxial test the major strain and the strain in the loading direction (Y) are the same (verified by an FE model of the sample). The DIC Major_strain deviates from the Strain_Y, please see the exported stage with both values. At some nodes the difference is zero to negligible, but at some other nodes it exceeds 500 µstrains. 

Would be great if someone has an insights into this. I though the issue could be with placing the coordinates system, but I tried to change it but still the same results. 

 

Thank you.

Surface_component_f19_g16_0023.csv

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Hi Nedaa,

please be aware that an simulation is just a mathematical model with an optimized setup and boundary conditions. The real measurement especially the ones in the optical measurement can be affected by many factors. 

For example: 

  • The Clamping
  • The Force direction. If the force is not exactly uniaxial
  • The finding of the facet points during the image correlation 

I guess your scenario is based on the last point. The major strain is calculated by information of both epsilon x & y. It could be possible that the finding of the center of gravity facet, so the 3D-coordinate, could be found slightly wrong. Because of illumination, the stochastic pattern, high deformation and so on. This leads to an inspection of the strain which is not exactly corresponding to the material behavior. We are talking about few µm. But this can lead to a mistake which results in a higher major strain or a different major strain in comparison to the strain in the force direction. 

Can the difference be defined in a specific area? Maybe you can share a screenshot of this stage you exported as a csv file in your question. 

Greetings, Ivan 

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  • 4 weeks later...

Hi Nedaa,

what you asked for isn't the difference between FEA and DIC but the difference between EpsilonY and MajorStrain both from DIC, right?

So Ivan already mentioned a good point: It's about noise.
Every real world measurement will have some noise from several sources.
EpsilonY is defined as the surface strain in Y direction. MajorStrain is defined as the largest surface strain independent on the direction. So if there is some noise, MajorStrain "picks" the highest available strain value in some direction. Mathematically it means: from the provided 2x2 surface strain tensor, EpsilonY uses the element in the lower right corner whereas MajorStrain uses the greater Eigenvalue. These values may be different even on a pure tensile test due to noise.

You can visualize the directions used for EpsilonY and MajorStrain.
Use:
INSPECTION -> Check Dimension -> Strains -> Direction epsilon (X)...
or
INSPECTION -> Check Dimension -> Strains -> Direction major strain...

You will see: The largest differences between EpsilonY and MajorStrain values will appear at the points where the directions are most different.
At these points EpsilonY and MajorStrain represent strain values in different directions and are not similar therefore (and don't have the same meaning).

So for small strain tests (e.g. to determine the Young's modulus) it's always recommended to use EpsilonY instead of MajorStrain.

Best regards,
Theo

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