SURFACE INSTABILITIES IN COMPRESSED OR BENT RUBBER BLOCKS

Biot carried out theoretical analyses of the critical strains at which surface instabilities would be encountered in large blocks of a neo-Hookcan material subjected to different degrees of compression in two directions. Three .special cases are reviewed here: simple compression in one direction, compression in one direction with the perpendicular direction constrained (pure shear), and equibiaxial compression. In all cases a surface instability is predicted to occur at a moderate compressive strain, ranging from about 33% lo about 55%. Because unidirectional compression occurs on the inner surface of a rubber block subjected to simple bending, a similar surface instability would be expected at a critical degree of bending, when the surface compression is about 46%. Observations on bent rubber blocks are compared with this theoretical prediction. Sharp creases occurred on the inner surface at a critical degree of bending but the critical compressive strain there was only about 35% and the bending curvature was less than predicted, only about onc-half as severe. Tile cause of this discrepancy is not known.