A multiaxial viscoplastic model for advanced Si{sub 3}N{sub 4} ceramics

A uniaxial creep/creep rupture model developed previously based on results obtained from uniaxial tensile creep tests of an advanced Si{sub 3}N{sub 4} ceramic has been refined and upgraded in multiaxial form to facilitate general applications. Severity of asymmetric creep behavior in tension compared to that in compression observed in recent experimental results mandates this revision. The tensorial formulation follows the plasticity theories developed for soils. Material sensitivity to pressure, asymmetry between tension and compression, changes of material structure due to hardening of grain boundary phase, and deformation induced volumetric swelling attributable to cavity and void formations have all been considered. Provisions for such features in the proposed multiaxial model are shown to have improved modeling flexibility