Imperfection Terminology
Glossary of imperfection types and terms used in buckling analysis — geometric, eigenmode-based, measured, amplitude, sensitivity.
Buckling & Stability4 min read
bucklingreferenceterminologyimperfectionsglossary
Imperfection types
| Term | Definition |
|---|
| Geometric imperfection | Deviation of the actual shape from the nominal design shape — out-of-roundness, dents, waviness, initial crookedness |
| Eigenmode-based imperfection | A numerically generated imperfection whose shape matches a buckling eigenmode, scaled to a target amplitude |
| Measured imperfection | Actual geometric deviation obtained from scanning, CMM or photogrammetry of a real component |
| Equivalent imperfection | A simplified imperfection shape (e.g. a half-sine wave) representing the effect of manufacturing tolerance |
| Residual stress | Self-equilibrating stress locked into the structure from manufacturing — effectively a material imperfection |
| Load eccentricity | Applied load not passing through the centroid — acts as an equivalent geometric imperfection |
Key parameters
| Parameter | Meaning |
|---|
| Amplitude | The maximum deviation of the imperfection from the nominal shape |
| Shape | The spatial pattern of the imperfection — e.g. eigenmode, measured, sinusoidal |
| Sensitivity | The degree to which the collapse load changes with imperfection amplitude |
| Knock-down | The reduction in collapse load relative to the perfect-structure (classical) load |
Typical imperfection amplitudes
| Structure | Typical amplitude |
|---|
| Column (initial crookedness) | L/1000 to L/500 |
| Plate (out-of-flatness) | t/10 to t/2 |
| Cylindrical shell (out-of-roundness) | t/10 to t |
| Spherical shell (out-of-sphericity) | t/10 to t/2 |