Finite Element Shock Analysis of a Cryogenic Refrigerator

Abstract

The U.S. Navy is investigating Gifford-McMahon refrigerators for cooling superconducting magnets for shipboard use in minesweeping. The refrigerators will be subjected to severe shock in these applications. If a refrigerator is to continue operating efficiently after experiencing shock loads, the cylinder walls must not be permanently deformed when subjected to these loads, and therefore must not approach their elastic limit. The stress in the cylinder walls due to specified shocks was determined by axisymmetric finite element shock analyses of the two-stage cylinder and displacer assembly. Constant horizontal and vertical accelerations and a time-dependent acceleration were applied to the model. The analysis for the vertical shock loading produced a maximum stress of 4 percent of yield of the 304-type stainless steel used for the cylinder walls. The horizontal shock loading produced a maximum stress of 14 percent of yield. The maximum magnitude of the time-dependent shock load equaled that of the constant load and it produced a maximum stress of 1.8 percent of yield. These stress values are consistent with the expectation that if resonate responses are not excited, a constant acceleration will produce larger stresses than a time-varying acceleration of equal maximum amplitude. Given the anticipated values of horizontal and vertical shock to be applied during testing, it is expected that the refrigerator will not experience damaging strains.