Abstract
The effects of prestrain by rolling on the formability of AA6016 sheets in the biaxial stretching region are studied in this paper. An experimental program including formability experiments has been carried out for as-received and prestrained sheets. Forming limit strains are determined by the digital image correlation technique and then mapped into stress space using an anisotropic plasticity model in order to assess the path-independence of forming limit stresses. Different diagrams for description of formability such as the traditional strain-based forming limit diagram (FLD), the forming limit stress diagram (FLSD) and various equivalent plastic strain based (EPS-based) diagrams are applied in order to study the effect of prestrain. It is shown that prestrain by rolling has the same effect on the forming limit strains as prestrain by plane strain tension. Furthermore, the forming limits of the virgin material nearly coincide with the forming limits of the prestrained material in the FLSD and EPS-based diagrams, serving as a conservative estimate for the prestrained material. Due to the roping phenomenon, the material displays significant anisotropy in forming limit strains. At the same time, the presence of roping does not affect the path-independence of the forming limits in the FLSD and EPS-based diagrams.
Highlights
Sheet metal formability is commonly described by means of the forming limit diagram (FLD) or forming limit curve (FLC), dividing safe and unsafe strain combinations
Marciniak-Kuczynski formability tests [7] of virgin and prestrained sheets of aluminium alloy AA6016 were performed and different forming limit diagrams were compared with respect to path independence
The samples experienced roping and multiple local necks, so the forming limit strains were detected by the means of the thickness-control method [18,19,32]
Summary
Sheet metal formability is commonly described by means of the forming limit diagram (FLD) or forming limit curve (FLC), dividing safe and unsafe strain combinations. The uniaxial tension tests were carried out on dog-bone samples with 70 mm gauge length and 12.5 mm width taken out in seven directions (0 , 15 , 30 , 45 , 60 , 75 , and 90 ) oriented to the rolling direction, both for the virgin and the prestrained material. The formability of the material was investigated by means of Marciniak-Kuczynski tests [7], that were performed in a BUP600 forming machine using a flat punch with a diameter of 100 mm. The rate of the major strain was about 0.001-0.002 s-1 during the uniform deformation and went up to the 0.05 s-1 in the local necking areas prior to failure
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