BRITTLENESS, DUCTILITY, AND THE GRIFFITH CRACK

The material properties which determine whether a solid will fail by b rittle fracture or ductile deformation are theoretically investigated by including the lending-order dissipative term describing plastic flo w in the equations of elasticity. In particular, this term yields a ti me scale for the blunting of the elastic crock tip described by Griffi th. This time scale is compared to the elastic response time of a soli d to determine whether the solid will sustain the high stresses charac teristic of a crack tip or will plastically relax the elastic strains. These dissipative equations of elasticity also describe the ''work-ha rdened'' state of a solid, where brittle fracture becomes possible in an originally ductile solid.