Existing tests for assessing the formability of sheet metal samples can create strain ratios in the range from uniaxial to biaxial stretching, −1/2 ≤ ɛ2/ɛ1 ≤ 1. A novel sample design is proposed, with shaped cut-outs in a rectangular sheet specimen, intended to produce with strain ratios in the range, −1 ≤ ɛ2/ɛ1 ≤ 1/2 in a small zone, while the sample is uniaxially extended. The strain ratio is controlled by changing the geometry of the cut-outs.
The behaviour of the new sample design is examined by finite element modelling showing nearly proportional behaviour in the test zone, with the best results occurring near to pure shear. Two experimental methods are considered for the validation of these predictions: measurement of the distortion of a grid of small circles created on the sample surface by laser-scribing; use of a commercial strain measurement system based on an applied speckle pattern. Both techniques demonstrate that the evolution of experimentally measured strains during the test closely follows that predicted numerically.
The novel sample is applied to test the formability of a range of materials known to be difficult to form. The tests on aluminium alloys Al 2024, Al 7075, Al 2198 and commercially pure titanium demonstrate that significantly enhanced deformation prior to failure is possible with loading near to pure shear. The implication of these results is that it may be possible to design novel forming processes capable of creating more dramatic deformation in “difficult to form” materials through the creation of strain paths close to pure shear.