Forming limits of sandwich sheet materials

Failure of sandwich sheet materials by tensile instability and localized necking was studied by performing punch-forming experiments on stainless steel clad aluminum. By using narrow blanks and no lubrication, lateral contraction was possible, and failures could be produced in the drawing area of the forming limit diagram. For this deformation regime, diffuse instability led to localized necking. As in monolithic materials, the development of the localized neck in stainless steel clad aluminum determined the forming limit, and predictions of the strain levels for the onset of local instability correlated well with the observed forming limit strains. By preventing lateral contraction, failures in stretching were produced. The forming limit strains in this case depended on the strains at the onset of diffuse instability in much the same manner as is observed for monolithic materials. The strains at the onset of diffuse instability were predicted using a generalized rule of mixtures, and agreement between measured values and values predicted from component properties was good when the strain-path dependence of the instability strain for the individual components was taken into account. The diffuse necking process in stretching of stainless steel clad aluminum led to local thinning when deformations involved small degrees of biaxiallity. On the other hand, nonuniform through thickness straining of the component layers in specimens strained close to balanced biaxial stretching appeared to control the localization process and gave rise to forming limit strains lower than expected from observations of punch formed monolithic sheet materials. For all deformation modes, localized flow culminated in delamination and fracture. S. L. SEMIATIN, formerly graduate student, Department of Metallurgy and Materials Science, Carnegie-Mellon University