Compressive behaviour of thin layered thermally insulating structural supports

Abstract

Stacks, each consisting of a large number of thin layers of the same material, were compressed under loads of up to 5 × 103 lbf. Within experimental error, the compression c per stack layer is independent of the number of layers but varies as a power m of the layer thickness: m equals 0.48 for freshly assembled stacks of flat brass discs. Changes of c upon increasing the applied load W from 5 × 102 to 103 lbf are of the order of the centre line average values of the surfaces in contact. For loads greater than 8 × 102 lbf the effective moduli exceed 105 lbf in−2, increase with W and load cycling, and are within two orders of magnitude of the bulk elastic moduli. Considerable hysteresis occurs in the first load-unload cycle, but further irreversibility rapidly decreases in subsequent cycles provided W is never reduced below 50 lbf. The W-c curves for freshly assembled stacks of flat layers are approximately exponential: changes in slope of the graph of log W against c suggest that asperity deformation processes predominate at high loads (> 5.5 × 102 lbf) and layer flattening at low loads (<2.5×102 lbf). A stack possessing lateral as well as compressive strength is described in Appendix 1: this design has been adopted for a mechanically strong thermal insulator.