General Mechanical

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Non Linear vs Linear static structural analysis

• sachin kumar
Subscriber

I have performed two analysis as given below:

1) Linear analysis: material properties are defined for elastic regions only, and maximum stress comes out to be 1200Mpa@18000N which is well above the yield strength of materials, but as per experimental results or field results material is not facing any challenge regarding failure or bending of material which means as per experment material is ok but simulation is saying it is fail.

2) Non-linear: In second analysis, I have defined material properties in plastic regions as well. now the max stress comes out to be below 200Mpa which means material is ok, but as we know that for plastic region failure criterion is strain so I have checked strain value which comes out to be less than 1.5% which is again below the failure criteron (5% as per industry norm or literature). Note: Although it is not fully converged but i have taken mid value results i hope it is ok?

In this project we had the experimental results basis that we have gone for non-linear material propertiy analysis. in general we used to do linear analysis

Suppose we do not have experimental data in that situation how to decide which analysis to perform in order to get best and economical design.

• Sampat Kumar
Ansys Employee

Hi Sachin,
In real life, the material follows the non-linear curve but for the linear analysis, the software follows the same linear path after crossing the yield point. In the linear analysis, the failure is concluded by comparing the maximum stress value with the yield or ultimate value.

For the linear analysis, you have defined the material property for the elastic region only. Since you applied the load of 1800N and the stress exceeds the defined elastic limit of the material then failure would occur in the model. you have to perform the Non-linear analysis for this.

For Non-linear analysis, Since your model is not converging then you can check the following points that might be the reason for this strange behavior.
1. Increase the sub-steps in the analysis setting.
2. Verify whether the stress developed in the model is localized. there might be a chance of stress singularity in the model. I would suggest you to check how much area is showing stress more than the yielding.
3. Will you please check that the model you are using for the simulation is the same as your experimental specimen? If the model is different then the maximum stress contour developed in the model is distributed over the area or not as per your expectation.
4. Please verify that the large deformation is on or off for the non-linear analysis.

Since the result is not fully converged, taking the midpoint result depends on your expectations and the analysis you are performing.

Regards,
Sampat

• sachin kumar
Subscriber

Hi Sampat

I have made some changes in model settings and now Non-linear model is fully converged.

Result shows that strain value is 0.013 and max von mises stress = 270Mpa.

and material yield strength = 200Mpa. failure strain = 0.05

Basis above results and material strength, could you explain whether this material will fail or not.

• Sampat Kumar
Ansys Employee

Hi Sachin,

The stress, as you mentioned, exceeded the material’s yield point, but it can’t simply imply that the material may fail. As we consider the developed stress of 200 MPa obtained in the larger part of the component then we can assume the material has undergone plastic deformation, but it still depends upon you which point you consider for the design and FOS you have used. If the developed stress exceeded the yield stress at the pointed location or on a very small part, then we can’t compare this with the experimental result. The stress developed in the component depends upon various reasons, like geometric irregularities, and loading conditions, whether they are tensile, compressive, fluctuating, etc.

For strain 0.013, which is below the failure strain (0.05),  according to your data, it indicates that the material has not exceeded its maximum strain capacity and can be considered to have not failed. But again, if you have the above-mentioned geometry irregularities or loading conditions, then the strain may not develop as in the experimental, so you can’t compare this.

In summary, the developed stress has exceeded the yield point of the material but has not crossed its failure strain or is not close to that. Based on the above data, we may consider that the material may not fail, but still, it depends upon your experience and the design point that you have selected. It also depends upon the geometric irregularities, loading condition, material, and FEA numerical error.

Regards,
Sampat