BENDING CHARACTERISTICS AND STRETCH BENDABILITY OF MONOLITHIC AND LAMINATED SHEET MATERIALS

Abstract

Bending deformation characteristics of monolithic, bi-layer and tri-layer laminate sheet materials are studied using Analytical and FE models in this work. The analytical model, based on advanced theory of pure bending considers von Mises yielding, Ludwik hardening law and Bauschinger effect for various laminate constituent thickness ratios. The principal stresses and strains through the thickness and, change in relative thickness at specified bend curvatures are obtained as a function of increasing curvature during bending. Additionally, 2D and 3D finite element (FE) based models for bending are developed to overcome simplifications of the analytical models such as the effect of specimen width on strain distribution. Further, to experimentally assess and validate bending characteristics from the analytical models, a new experimental bend test-jig that is closer to pure bending is developed. The experimental set-up is an open concept design that allows access to the tensile surface as well as through-thickness region for recording and analyzing strains using an online strain mapping system based on digital image correction (DIC) method. Experimental bending is carried out on annealed AA2024 monolithic aluminum alloy sheet and Steel/Aluminum (SS400/AA1050) bi-layer laminate sheet at different thickness ratios. The model and experiments are studied in terms of stress and strain distribution as a function of relative thickness for different clad to matrix thickness ratios. Further the case of simultaneous bending and stretching over small radius bending is analyzed for limit strain prediction using an existing limit strain criterion based on major strain acceleration. An angular stretch bend test is used to subject an hour-glass shaped

AA20240-O aluminum sheet specimen to simultaneous stretching and bending deformation while continuously imaging the critical tensile surface region using an optical camera. The strain development in the critical region is subsequently analyzed using digital image correlation (DIC) method. The effect of DIC parameters such as facet size, facet step, and effect of curve fitting procedures on limit strain are studied. An average limit strain of 0.2 is obtained for AA2024 for a facet size of 9x9 pixels, a facet step of 5 pixels and by applying a 5th order polynomial curve fit to the strain data. The results obtained are comparable with a limit strain of the material. The results are compared with a commercially available tri-layer laminate sheet material Alclad 2024 that has 80 μm thin layer of soft AA1100 on both surfaces of harder AA2024 core material. An improved stretch bendability limit strain of 0.24 for Alclad 2024 tri-layer specimen was predicted by utilizing the major strain acceleration criterion. The thin AA1100 protective layer produced a positive effect on the stretch bendability of Alclad 2024 when compared with monolithic AA2024 specimen.