ON PATTERNS OF DEFORMATION IN PHASE-TRANSFORMATIONS AND LUDERS BANDS

Materials which contain an unstable portion in the stress-strain relat ion frequently exhibit patterns of deformation characterized by altern ating bands of large and small deformations. Certain metals produce th ese deformations as they undergo plastic straining or phase transforma tions. A rate-dependent material law including a field of imperfection s is employed in a finite element model to produce a pattern of large and small deformations which resembles those observed experimentally. An essential feature of the formulation is the inclusion of a softenin g branch to the stress-strain curve to trigger the localization of str ains, followed by a hardening branch to arrest it.