Mechanical behavior and forming of commercially-pure niobium sheet

This work investigates the mechanical and forming behavior of pure niobium sheet, which has broad applications in low-temperature physics including the proposed International Linear Collider. An extensive suite of mechanical tests is performed on commercially-pure niobium (CP-Nb), including uniaxial tension, strain-rate-jump, biaxial tension and disc compression, which are used to characterize the plastic behavior of the CP-Nb sheet. The material is found to have significant plastic anisotropy, high R-values, significant ductility, and non-negligible rate-dependent behavior. Subsequently, two types of forming experiments, deep-drawing and stretch-forming, are conducted with CP-Nb blanks of different diameters. These two experiments subject the CP-Nb sheet to different modes of deformation and stress states, and are meant to highlight the strengths and deficiencies of constitutive models. To model the forming behavior of the CP-Nb sheet, the material hardening at large strains is extracted from the uniaxial tension test, including the strain-rate effect. The plastic anisotropy of the CP-Nb sheet is modeled with the Yld2000-2D and Yld2004-3D non-quadratic yield functions. The friction coefficient of 0.25 is inversely identified from the deep-drawing experiments, by matching the predicted and measured punch force–displacement responses. While the features of the deep-drawing experiment are captured well using the constitutive framework described above, those of the stretch-forming experiment are not. Agreement with the latter is accomplished only after a targeted recalibration of the anisotropic yield function. This underscores the general conclusion that successful modeling of plastic deformation processes requires an accurately calibrated yield function which captures the dominant, process-specific stress states.