The Mullins effect in equibiaxial deformation

Common experience teaches that the pressure required to inflate a balloon i s noticeably reduced by prestretching it several times prior to its primary inflation. This preconditioning phenomenon is known as stress-softening an d often is called the Mullins effect. A theory of stress-softening in incom pressible isotropic materials is applied to study this effect in an equibia xial extension for which some general results are presented. It is shown, f or example,, that effects of stress-softening in a simple uniaxial compress ion can be adduced from those demonstrated for equibiaxial extension under plane stress. The general equibiaxial results ate applied to study the Mull ins effect in the inflation of a spherical membrane. The stress-softening p henomenon in cyclic inflation and deflation of the balloon is investigated for Mooney-Rivlin and biotissue parent material models. Tt is shown analyti cally that the effect of preconditioning in uniaxial extension is to signif icantly reduce the pressure required to first inflate a balloon to an equiv alent strain intensity. This result characterizes the familiar softening ph enomenon associated with balloon inflation.