A flexible sheet material useful as an energy absorbing material is impregnated with a dilatant silicone composition comprising the reaction product of a polydiorganosiloxane and a boron compound selected from boric oxide, a boric acid, a boric acid precursor, a borate or a partially hydrolysed borate. The silicone composition can be modified by reaction with a hydrophobic compound reactive with silanol groups to improve the resistance to washing. The flexible sheet can be a material, e.g. a fabric, having a negative Poisson's ratio.