The purpose of this article is to revise some concepts on defects nucleation based on bifurcation of equilibrium solutions. Equilibrium solutions are obtained on a homogeneous body and on a body with an infinitesimal defect such as cavity under the same prescirbed dead load. First void formation and growth in non linear mechanics are examined. A branch of radial transformation bifurcates from the undeformed configuration in presence of a small cavity. Two cases of behaviour is examined. One case is the growth of cavity by only the deformation of the matrix. In another modelling the cavity evolves like a damaged zone. The transition between the sound part and the damaged one is governed by a local criterium. Each configuration leads to the definition of a nucleation criterium based on a presence of a bifurcation state, comman state of the homogeneous boby and a body with an infinitesimal defect.