Stress Linearisation Best Practices
Principles for stress linearisation in pressure vessel FEA — SCL placement, component extraction, classification, singularity avoidance and interpretation.
Place SCLs at critical locations
Place stress classification lines at the locations where the stress is highest: nozzle inner corners, shell-to-head junctions, support attachments and weld toes. Place additional SCLs away from discontinuities for comparison with the nominal stress.
Make SCLs perpendicular to the surfaces
The SCL should be a straight line through the wall, perpendicular to both the inner and outer surfaces. At curved geometries, the SCL may follow a curved path. Non-perpendicular SCLs give incorrect linearisation results.
Extract all six stress components
The stress tensor has six independent components. All six must be extracted along the SCL and linearised separately. The principal stresses are then calculated from the linearised tensor, not by linearising the principal stresses directly.
Avoid singularities
Do not place SCLs at singularities (sharp re-entrant corners, zero-radius weld toes). The linearised stress at a singularity is mesh-dependent and does not converge. Model a realistic radius or use structural stress methods instead.
Classify correctly
Classify the linearised stresses as primary, secondary or peak based on the location and load type, not just the stress value. A stress at a nozzle junction may be classified as local membrane, not general membrane. The classification determines the applicable allowable.
Interpret, do not just compare numbers
Do not simply compare the linearised stress with an allowable. Interpret the result in the context of the physical behaviour. Is the stress load-controlled or displacement-controlled? Is it at a singularity? Does it converge? Engineering judgement is required.