  Rubber components and soft tissues are often subjected to large bending
deformations "in service". The circumferential line elements on the inner face
of a bent block can contract up to a certain critical stretch ratio lambda_cr
(say) before bifurcation occurs and axial creases appear. For several models
used to describe rubber, it is found that lambda_cr = 0.56, allowing for a 44%
contraction. For models used to describe arteries it is found, somewhat
surprisingly, that the strain-stiffening effect promotes instability. For
example, the models used for the artery of a 70 year old human predict that
lambda_cr = 0.73, allowing only for a 27% contraction. Tensile experiments
conducted on pig skin indicate that bending instabilities should occur even
earlier there.
