Finite Element simulations of rubbers and biological soft tissue usually assume that the material being deformed is slightly compressible. It is shown here that in shearing deformations the corresponding normal stress distribution can exhibit extreme
sensitivity to changes in Poissons ratio. These changes can even lead to a reversal of the usual Poynting effect. Therefore the usual practice of arbitrarily choosing a value of Poissons ratio when numerically modelling rubbers and soft tissue will, almost certainly, lead to a significant difference between the simulated and actual normal stresses in a sheared block because of the difference between the assumed and actual value of Poissons ratio. The worrying conclusion is that simulations based on arbitrarily specifying Poissons ratio close to 1/2 cannot accurately predict the normal stress distribution even for the simplest of shearing deformations. It is shown analytically that this sensitivity is due to the small volume changes which inevitably accompany all deformations of rubber-like materials. To minimise these effects, great care should be exercised to accurately determine Poissons ratio before simulations begin.
