Implement base transformation from local to global for stress and strain tensors (shell elements)

[alexandrehazo]

Problem Description

Radioss ANIM files show stress and strain results for shell elements in the element-local coordinate system, whereas LS-Dyna d3plot files expect these values to be expressed in the global coordinate system.

Currently, the anim_to_d3plot.py converter extracts stress and strain data from Radioss results and passes them directly to the D3PLOT format, treating values expressed in the local element coordinate system as if they were in the global system.

Without a proper transformation, the converted data is interpreted incorrectly in the d3plot file, leading to inconsistencies in visualization and analysis.

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Solution

This pull request introduces the logic to convert stress and strain tensors from the local Radioss elemental system into the global coordinate system expected by LS-Dyna. This is done by:

• Reconstructing the local axes for each shell element
• Applying a rotation to the in-plane tensors

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Visual Demonstration

Here are two screenshots showing the changes introduced by these modifications, as visualized in HyperView and LS-PrePost.

Each comparison (in order on the screenshots) includes:

1. The D3PLOT generated from the previous converter
2. The D3PLOT from the updated converter with the basis transformation
3. The ANIM file

All are displayed in the global frame, with both X-stress and stress tensors shown.

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Consistency Check

We can also observe consistent results in LS-PrePost, where the values now align with those shown in the ANIM file.