Different FEA results of the same element

In summary, the conversation discusses using FEA to solve a problem with a constrained plane element and a concentrated load. Two different meshes were used, but the results obtained were not the same. The question arises whether the differences are in value or in sign. An example with fewer finite elements is compared and it is suggested that using a different material modulus value for tension and compression on diagonal elements may be the cause of the difference in deflection. The limitations of Constant Strain Triangle (CST) elements are also mentioned, including their use in areas with small strain gradients and in mesh transition areas. It is generally not recommended to use CSTs for general analysis purposes unless a large number of elements are used for reasonable accuracy.
  • #1
mohamadh95
45
0
Hello. Consider the model shown below. The problem is to be solved by FEA. The plane element is constrained from moving at the red line. A concentrated load P is applied at the red point and pointing downward.
We solve the problem using the following meshes.
Untitled-1.png
f.png

Same elements are being used and the number of elements is the same in both meshes. Why the results obtained are not the same?
 
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  • #2
Are the answers different in value or in the sign?
 
  • #3
JBA said:
Are the answers different in value or in the sign?
I did an example similar to the one above with less finite elements. Here are the displacement results for an example:
IMPORTANT FEA ASSEMBLY TRIANGULAR ELEMENTS.png

The black rectangle was modeled with two triangular cst elements.
 
  • #4
Did you use a different material modulus value for tension and compression on the diagonal elements? A stiffer modulus for compression than tension on the diagonal might create the type deflection difference seen. In the blue example, both the top element and the diagonal element are in tension; but, in the red example, only the top element is in tension, the diagonal element is in compression.
 
  • #5
Constant Strain Triangle (CST) elements have fundamental limitations with their formulation that limits their accuracy in low-density meshes. This is due to the fact that the element's formulation assumes constant strain across the element (hence the name), sometimes resulting in reduced accuracy. CST's should only be used in models that have low strain gradients, or in high mesh densities so as to minimize strain gradients across individual elements.

See here: http://www.rpi.edu/~des/CST.ppt
rpi.edu said:
1. Use in areas where strain gradients are small
2. Use in mesh transition areas (fine mesh to coarse mesh)
3. Avoid CST in critical areas of structures (e.g., stress concentrations, edges of holes, corners)
4. In general CSTs are not recommended for general analysis purposes as a very large number of these elements are required for reasonable accuracy.

And here: https://engineering.purdue.edu/~ahvarma/CE595/CE595 Section 5.ppt
purdue.edu said:
- The CST gives good results in regions of the FE model where there is little strain gradient
- Otherwise it does not work well.
 

FAQ: Different FEA results of the same element

1. What are the possible reasons for getting different FEA results of the same element?

There can be several reasons for getting different FEA results of the same element, including:

  • Variation in input parameters such as material properties, boundary conditions, loading conditions, etc.
  • Differences in the meshing techniques and element size used in the FEA analysis.
  • Errors or inaccuracies in the finite element model geometry or element orientation.
  • Inadequate convergence of the solution due to improper selection of solution settings or element types.
  • Numerical instabilities caused by nonlinearities, contact, or other complex behaviors in the model.

2. How can I determine which FEA results are more accurate in case of discrepancies?

To determine which FEA results are more accurate, you can perform a sensitivity analysis by varying the input parameters and comparing the results. You can also validate the results by conducting physical experiments or comparing them with analytical solutions for simpler cases. Additionally, it is important to ensure that the finite element model is properly converged and the solution settings are appropriate for the problem being analyzed.

3. Can different FEA software packages produce different results for the same element?

Yes, it is possible for different FEA software packages to produce different results for the same element. This is because different software packages may use different algorithms, element formulations, and solution techniques, which can affect the accuracy and convergence of the results. It is important to carefully validate and compare the results from different software packages before drawing any conclusions.

4. What should I do if I get significantly different FEA results from different element types?

If you get significantly different FEA results from different element types, it is important to carefully review your model and check for any errors or inaccuracies. You can also try refining the mesh and using smaller elements to see if there is any improvement. Additionally, it may be helpful to consult with a specialist or conduct a literature review to understand the typical behavior of the element types being used.

5. How can I improve the consistency of FEA results for the same element in different models?

To improve the consistency of FEA results for the same element in different models, you can follow some best practices such as using the same meshing technique and element size, ensuring proper convergence of the solution, and validating the results with analytical or experimental data. It is also important to carefully review and compare the input parameters and solution settings used in the different models to ensure consistency. Additionally, using the same FEA software package can help improve the consistency of results.

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