1. The Role of Equivalent Stress
When using ANSYS for structural finite element calculations, for most metal materials, equivalent stress is a commonly used criterion. It is a comprehensive stress indicator that considers the combined effect of three principal stresses and can be used to evaluate material yield and failure. So, if the structural equivalent stress obtained through ANSYS calculation exceeds the material’s yield strength—for example, if the yield strength of Q235 is 235MPa and the calculated maximum equivalent stress value is around 300MPa—what should we do? Should we modify the design? Or change the material?
2. What to Do When Equivalent Stress Exceeds Yield?
For this issue, I believe it may be due to design defects in the structure itself, or it could be caused by inadequate model simplification, poor mesh quality, improper load and constraint settings, etc. Therefore, I usually follow the following path to check and correct.
Load and Constraint Not Clear
We need to recheck the force analysis of this structure. Actually, doing a good job of force analysis is very important, but it is often overlooked. Doing a good job of force analysis is the prerequisite for us to do good calculations. We need to sort out various working conditions of this structure and determine the magnitude, direction, and position of external loads.
Material Parameter Input Error
Check whether the material parameters are custom-defined, and whether parameters such as elastic modulus and Poisson’s ratio are incorrectly defined during the customization process. The decimal point of the elastic modulus really should not be wrong.
Poor Mesh Quality
In ANSYS Workbench, use the mesh quality checking tool to check mesh quality Element Quality, and try to ensure that this Min minimum value is greater than 0.2. Use local mesh refinement, hexahedral mesh control, contact region mesh subdivision, and high-order elements to improve mesh quality.
Stress Concentration and Stress Singularity
This is the most likely problem to occur. Does this maximum equivalent stress appear at sharp corners, welded plates, rigid constraints, and other locations? For such problems, it can be solved by adding transition fillets, adding stiffeners, and expanding cross-sectional dimensions.
At the same time, we also need to pay attention to avoiding over-constraints and using fixed constraints cautiously (this constraint method is our favorite to use), as it is prone to cause stress concentration. We can try using remote displacement constraints in ANSYS Workbench instead.
Mesh Element Type Problem
Some structures are not suitable for using solid elements. For example, for thin plate structures, it is recommended to use shell elements. For such thin plate structures using solid elements, the mesh quality will not be too good either. Therefore, in the part connection regions, it is easy to produce some local stress super-large regions. Essentially, this is also a kind of local stress singularity. Therefore, for a structural model, we need to consider before calculation which parts use solid elements, which parts use shell elements, and which parts use beam elements, so as to make our calculations more stable, more accurate, and more efficient.
Saint-Venant’s Principle
If this maximum equivalent stress is near the constraint region, such as this fixed constraint region, you can try to evaluate the stress away from this region. If there is no way to avoid it, such as the bottom region of the support member, then consider not loading the constraint in this region, and also model the features near this region to establish more realistic boundary conditions.
Modify Structure
If the area exceeding the yield strength is very small, such as just the hole edge region or certain weld regions, and these areas are far smaller than the overall structure, then it can also be regarded as local stress concentration, and can be solved by adding fillets locally and adding stiffeners.
If the area exceeding the yield strength is very large, then it can only be redesigned. For example, if the cross-sectional dimensions of certain box girders are too small, resulting in the overall structural strength being too weak, then the cross-sectional dimensions need to be adjusted.
Modify Material
This method is simple: switch to materials with higher yield strength, such as changing from Q235 to Q345.
Consider Nonlinearity
In our calculations, we often use linear elastic materials, that is, without considering material nonlinearity. This linear elastic calculation result will be conservative, meaning it will overestimate stress. This is because it does not consider plastic redistribution. Therefore, we can also incorporate material nonlinearity, defining the elastoplastic parameters completely in the material definition. For some large deformation problems, turn on the large deformation switch, and through calculation, reconfirm whether the actual yield region and residual deformation are within the allowable range.
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