Effects of Material Properties on the Elastoplastic Buckling of an SRF Cavity Under External Pressure

An SRF cavity is generally manufactured with a shell structure to decrease the temperature of the inner surface and consequently to decrease the rf surface resistance. During operation, the SRF cavity is immersed in a bath of liquid helium while its interior is maintained under ultra-high vacuum. To be loaded under a condition of external pressure at a cryogenic temperature, a pressure test at room temperature is requested for safety examination. Explicit calculation and estimation of buckling to prove its structural strength is thus essential. With a great stress on the cavity, the nonlinear behavior of the stress-strain curve of niobium generates the elastoplastic buckling behavior different from elastic buckling. The stress-strain curve of niobium depends on the formation, fabrication, and treatment, thus modifying the buckling behavior. We investigated the buckling behavior of a 500-MHz SRF cavity under external pressure, for which various stress-strain curves were applied. Finite-element software (ANSYS) was used to calculate the limit pressure and post-buckling behavior, with an incremental arc-length control scheme to include effects of nonlinearities of both the geometry and the material property. Not only the limit pressure but also the buckling mode vary with the assigned material property and boundary conditions.