Ductility of aluminium alloy AA6016: An experimental and numerical study with application to sheet metal forming
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In the sheet metal forming industry numerical modeling and simulation can be used to study the effect of different parameters in an efficient and economical way. For these purposes proper material models are required. In the present work an experimental and numerical study of sheetmetal subjected to non-proportional strain paths through pre-straining was performed. The main objectives were 1) to examine the general effects of pre-straining with respect to formability and 2)to address an existing finite element modeling strategy combined with a chosen pre-strain modelingapproach in terms of prediction of the formability characteristics of the pre-strained material. Sheets of the aluminium alloy AA6016 were investigated in its original condition and when subjected to two different levels of pre-straining by cold rolling (5 % and 8 % plastic thickness strain). A series of experiments was performed, including uniaxial tension tests, shear tests, plane strain tension tests and Marciniak-Kuczynski (M-K) formability tests, all carried out using digital image correlation(DIC) techniques for strain measurement. Different approaches for the determination of experimental forming limit diagrams (FLDs) using DIC-data from the M-K tests were explored with application to current development of related tools. The material model was calibrated from uniaxial tension tests and shear tests. FE analyses of the uniaxial tension tests, shear tests and plane strain tension tests served as initial evaluation of the material model calibration and pre-strain modeling approach. FLDs were obtained from simulations of the full Marciniak-Kuczynski formability test set-up in order to evaluate the modeling strategy. Forming limits were also obtained through calculations based on the analytical Marciniak-Kuczynskianalysis. No sufficient methods for quantitative determination of experimental FLDs were employed. From qualitative forming limit investigations it was found that the formability of AA6016 was better in the rolling direction than in the transverse direction for all degrees of rolling. Material subjected to equibiaxial stretching was found to fail through fracture, while failure through localized necking was seen in straining closer to plane strain conditions. In addition pre-straining by rolling was seen to imply reduced formability in all directions in the region of biaxial stretching. Although the plane strain tension test analyses on the 8 % rolled material showed possibly related deviations, no secure signs were seen that the chosen modeling approach was insufficient. The forming limits established from simulations did not agree with those from the experiments, possibly indicating insufficient experimental data available for material model calibration. Relatively good agreement was seen between analytically and numerically calculated forming limit diagrams, henceit was concluded that the numerical framework can describe the physics of the rather complex formability process.