Why Material Models Matter for Metal Forming Simulation?

We know that simulation often assume design conditions that are ideal, and there is always certain amount of variation present in the results when compared with the actual conditions.  This is due to the fact that production reality is often different from ideal conditions. Considering the case of sheet metal forming, for developing panels for automotive body, presses and beds aren’t always parallel, lubrication might not be uniform and material properties are not always isotropic. The validation of assumptions in finite element simulation using such material models is then questionable, as the realistic behavior of the stamping process or advanced feasibility cannot be determined accurately.

One of the important factors to conquer this issue is to determine the similarity between the material used for sheet metal production and assumed in the simulation process for sheet metal design. This requires understanding the material model effectiveness used to model the behavior and how the material properties defined in the model fit in the actual sheet metal coil to be used for production.

Usually, in FEA of sheet metal stamping, the material is either imported from the existing library or defined by the analyst using the material file available in simulation tools like ANSYS. However, these files are often developed considering average, nominal or typical observed property values. Modeling the formability using these numbers characterized using simplified factors, constants and exponents would yield give spurious results when compared with the reality.

These constitutive models are good for sheet metal design specification of acceptable mechanical properties or compliance, but would not help in representing the material behavior accurately. For instance, considering minimum values of yield strength and n-value of a material may not accurately represent the reality, since the actual material behavior is different.

In actual scenario, yield strength and n-value are inversely proportional. The formability results would then differ significantly; the Ludwik extrapolation based on yield strength and n-value of minimums will lead to incorrect predictions of strains and stresses, which would directly give impact on the springback effect.

The use of material models that uses the raw tested material data and results on three different surface directions for r-values of a rolled sheet metal helps in predicting realistic behavior more accurately. The errors generated due to extrapolation can be avoided since the material is not assumed as isotropic but anisotropic.

Based on the full material test data, the flow curves or hardening curves can be developed with full behavior using empirical data to develop more meaningful material model for flow and stretching. This material model in finite element provides better accuracy as compared to constitutive models that assume values derived from a curve fit to extrapolate the full curve behavior.

It is important then for finite element analysts to understand the importance of these tested material models and utilize them in their simulation for sheet metal product design to avoid ambiguity in results. Although, simulation results should not be considered as a gold standard, bringing it closer to realistic behavior helps in predicting the sheet metal forming more meaningfully. This requires analysts to realize the difference between the material models for engineering and for the actual sheet metal coil shipped for production.

About Author:

Kashyap Vyas is an Engineer at Hi-Tech Engineering Services and holds a Master’s degree in Thermal Engineering with several research papers to his credit. He covers CAD and CAE topics for the engineering industry. His contributions are primarily focused on encouraging manufacturers and suppliers to adopt virtual product development tools to build efficient products with reduced time-to-market.

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