  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 Poisson's ratio. These changes
can even lead to a reversal of the usual Poynting effect. Therefore the usual
practice of arbitrarily choosing a value of Poisson's 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 Poisson's
ratio. The worrying conclusion is that simulations based on arbitrarily
specifying Poisson's 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 Poisson's
ratio before simulations begin.
