Experimental and numerical analysis of material deformation behavior in sheet metals and its forming process

Simulation-based Design and Manufacturing has shown great effectiveness in shortening design cycle. Industrial applications are now demanding accurate and robust predictions of fine details in manufacturing processes, which require fundamental mechanics understanding, and good experimental characterization methods.This thesis performs analytical, computational, and experimental analysis for the fundamental causes of surface distortion in sheet metal forming. Naturally a stability problem, surface distortion caused by in-plane compression cannot be predicted accurately by available FEM software packages. This research uses an approach that considers surface distortion as local phenomenon. By comparing the virtual work done by in-plane stresses, and the energy necessary to form various possible wave modes in the local area, the theory predicts the initiation and form of surface distortions. Effects of surface contacts and advanced material models such as kinematic hardening are considered. The theory was implemented in a numerical predictor linked with commercial FEM software codes (PAM-STAMP and LS-DYNA), and a user graphic interface was developed to enable practitioners to predict the incurrence of surface distortions and enhance the efficiency of the forming process design.Standard Yoshida Buckling tests were used to verify the wrinkling predictor. A new Contact Buckling test was designed to study the special case of buckling under single surface contact. Two types of material, 0.78mm-thick 180B steel and 1.012mm-thick 6111-T4P aluminum, were tested to verify the accuracy of the wrinkling predictor on the onset of wrinkling.It has been well documented that one important factor for accurate prediction of forming behavior is material characterization. We propose a novel tension-compression test, which is able to characterize the unloading behavior of the material, to address the challenge of experimentally measuring thin sheet's kinematic hardening behavior. This test was shown to be more cost-effective and easier to set up without loss of precision, compared with existing tests. Material properties obtained from the test helps improve accuracy of the wrinkling predictor.This thesis established numerical and experimental foundation for characterizing surface distortion and material's non-linear kinematic hardening behavior. The impact of this work lies in the advancement of the prediction accuracy in sheet metal forming.