Packaging as an imperfection signature: what deployable shells lack that welded ones have

Result

For welded metallic launch-vehicle cylinders the signature is characterised end to end. The weld land is located by design, measured, Fourier-decomposed and published; one NASA study reports that it reduces buckling load by 31 percent, and knockdown factors derived from the work are in use on the Space Launch System.

For a shell that is folded, stowed and deployed, that chain does not exist. The public record holds a single-specimen buckling test — Barker and Guest (2000), where the folded tube failed at about 78 percent of the virgin load, with buckling initiating at the valley folds — a qualitative programme report (ISAT, 2004), boom-specific analyses from NASA Langley and DLR, and a 2014 review that names the quantification gap. Neither SP-8007 Revision 2 nor ECSS-E-HB-32-24A treats folding or stowage as an imperfection source.

The packaging case differs from the welded one along three axes that have nothing to do with the design:

• Fold count. ISAT found severe compressive-strength loss in the first few packaging cycles, then rapidly decreasing effect, so the article that flies is not the article that was characterised unless the cycle count matches.
• Stowage duration. Stowed deployables relax: DLR projects roughly 22 percent relaxation-modulus reduction over 20 years for a deployable sail boom.
• Temperature history. Relaxation is temperature-dependent, and the thermal environment between integration and deployment is mission-specific, not design-specific.

At a crease the material has also yielded, so the imperfection arrives with a local stiffness change; residual creases can reduce the effective membrane modulus by up to two orders of magnitude. What a characterisation programme would have to produce is therefore not a knockdown factor but a family indexed by cycles, time and temperature, with a crease constitutive model alongside the geometry.