The constrained deformation of an aluminium alloy foam sandwiched between steel substrateshas been investigated. The sandwich plates are subjected to through-thickness shear and normalloading, and it is found that the face sheets constrain the foam against plastic deformation andresult in a size e$ect: the yield strength increases with diminishing thickness of foam layer.The strain distribution across the foam core has been measured by a visual strain mappingtechnique, and a boundary layer of reduced straining was observed adjacent to the face sheets.The deformation response of the aluminium foam layer was modelled by the elastic-plastic6nite element analysis of regular and irregular two dimensional honeycombs, bonded to rigidface sheets; in the simulations, the rotation of the boundary nodes of the cell-wall beam elementswas set to zero to simulate full constraint from the rigid face sheets. It is found that the regularhoneycomb under-estimates the size e$ect whereas the irregular honeycomb provides a faithfulrepresentation of both the observed size e$ect and the observed strain pro6le through the foamlayer. Additionally, a compressible version of the Fleck–Hutchinson strain gradient theory wasused to predict the size e$ect; by identifying the cell edge length as the relevant microstructurallength scale the strain gradient model is able to reproduce the observed strain pro6les across thelayer and the thickness dependence of strength.