Pre-Stressed Modal Analysis Best Practices
Best practices for pre-stressed modal analysis of rotating blades — pre-stress application, frequency verification and Campbell diagram construction.
Pre-stress application
Run the static analysis with the centrifugal load first. Save the stress stiffening matrix. Run the modal analysis using the elastic plus stress stiffening matrix. Confirm the frequency increases with speed — if it does not, the pre-stress is not being applied.
Frequency verification
Check the zero-speed frequency against a non-rotating modal analysis. Check the first bending frequency against an analytical cantilever estimate. Check that the frequency shift is plausible (20-50% for a typical blade).
Campbell diagram
Run the pre-stressed modal at several speeds. Plot frequency vs speed for each mode. Add the engine order lines. Identify all crossings within the operating range. Do not assess a crossing as critical without forced response analysis.