This research will complete specific analysis for cavity nucleation in nonlinear solids and related fundamental issues associated with ductile fracture of metals and investigations on Saint-Venant's principle in nonlinear elasticity and related fundamental issues in mechanical testing of polymers and composite materials. The research on cavity nucleation involving the formation and subsequent growth and coalescence of voids from microscopic inclusions which leads to ductile fracture in metals and alloys, has long been of concern to material scientists. The studies on the extension of investigations on Saint Venant's principle will help to resolve several critical issues related to material instability.