Influence of ductile damage on the bending behaviour of aluminium alloy thin sheets

Sheet metal forming involves planar stress states, in the sheet plane, like in tension and simple shear, or stress states characterized by a gradient in the thickness, like in bending. In this latter case, material limit prediction derived from an instability criterion is no longer valid. In this work, a criterion based on a critical void volume fraction, identified from macroscopic tests, is applied to the case of bending of square samples of aluminium alloy AA6016-T4. Mechanical tests are performed at two aging times to quantify its influence on the mechanical behaviour and only the hardening law is modified to take it into account. It is shown that a good correlation is obtained between the critical void volume fraction obtained from tension on notched samples and biaxial expansion, and the onset of crack development in the bent zone. Moreover, macroscopic load recorded during bending is sensitive to ductile damage, which makes this test particularly interesting for damage investigation. (C) 2010 Elsevier Ltd. All rights reserved.