Bifurcation of elastoplastic pressure-sensitive spheres

Instability of a full sphere, under uniform compression or tension, is investigated within the framework of linear bifurcation theory. Material response is modeled by a Hookean-type hypoelastic relation with pressure dependent instantaneous moduli. Exploiting a formal analogy with Navier equations of linear elasticity we obtain exact solution for the bifurcated field. Two families of eigenmodes and eigenvalues are identified and discussed. We examine, in particular, surface instabilities in compression, and twisting modes with absence of radial velocity. The results are further specified for pressure-sensitive plastic solids with an elliptic yield surface. The deformation theory prediction of bifurcation loads for that material are much lower than those obtained from the flow theory version.