Stress Linearization and Primary Membrane + Bending Stress Intensity Evaluation in LS-DYNA Post-Processing

[musen lin]

Dear LS-DYNA Community,

Hello everyone,

I am currently conducting a structural analysis for a component and need to perform a stress evaluation according to relevant engineering standards (e.g., similar to ASME BPVC or other specific industry standards). Based on the standard's requirements, I need to calculate the stress intensity for the primary membrane plus bending stress under specific load conditions and compare it against allowable limits. The standard specifically states that this stress intensity should be calculated according to the maximum shear stress theory.

After building and running my model in LS-DYNA, I am finding it difficult to understand how to perform this specific stress evaluation in the post-processing phase (primarily using LS-PrePost), especially regarding the steps for stress classification and subsequent calculations. My model may contain both solid and shell elements.

Specifically, my questions or areas of confusion include:

1. In LS-PrePost, how do I perform the required Stress Linearization for results from solid and/or shell elements to accurately decompose and extract the primary membrane stress tensor and primary bending stress tensor across a section?
2. Once I have extracted the membrane and bending stress tensors, how do I superimpose (add) these two tensors in the post-processor to get the combined stress tensor?
3. How do I then calculate the stress intensity (Stress Intensity) based on the maximum shear stress theory from this combined stress tensor? (i.e., max(∣σ1​−σ2​∣,∣σ2​−σ3​∣,∣σ3​−σ1​∣))
4. For models containing both solid and shell elements, are there any particular considerations or recommended workflows when performing stress linearization and subsequent processing?

I understand that stress classification is necessary and that I cannot simply use the total stress values. However, I am not very familiar with the specific steps and data flow within the LS-DYNA/LS-PrePost environment for this process.

If anyone has experience performing similar stress evaluations (like those required by ASME or for high-reliability components) in LS-DYNA, especially regarding stress linearization or membrane/bending stress extraction, could you please share some guidance or resources (tutorials, script examples, etc.)?

Thank you very much for any help!

Best regards,

Musen Lin

[Lucas Mackey]

I know of no way to extract those results directly from LS-PrePost, though it may be possible.

Instead, for solid elements, you can extract stress in the x, y, and z directions and post-process them using another software (e.g., Excel, MATLAB). Results can be extracted as CSV files (go to FEM > Post > History). To perform stress linearization, you'll need to know the element IDs for element sets through the wall thickness, as well as relative locations of those element centroids within each element set. See ASME BPVC Section VIII Division 2, Annex 5-A for details on how to perform the necessary calculations. For additional details, see Hechmer et al., The ASME Code and 3D Stress Evaluation, 1991; Hollinger et al., Three-Dimensional Stress Criteria-Summary of the PVRC Project, 2000; or WRC Bulletin 429, 3D Stress Criteria Guidelines for Application.

Lucas