Biaxial test method for determination of FLCs and FFLCs for sheet metals: validation against standard Nakajima method

Recently, a biaxial test method comprising a cruciform specimen design and spatio-temporal method to determine the limit strains has been proposed for the determination of forming limit curves (FLCs) and fracture forming limit curves (FFLCs) for sheet metals. However, this test method has not yet been validated against the existing standard methods. In the present work, this biaxial test method has been applied to the aluminium alloy AA5754 for formability evaluation at room temperature and results from the biaxial test method have been compared with those from the standard Nakajima method. Theoretical analysis has been carried out to compare equi-biaxial tension cases for the two methods; a similar variation of thickness strain with radial distance normalised by the radius of the gauge area is found between the two methods. In the biaxial tests, decreasing the radius of the through-thickness dome profile, with which the gauge area is thinned, leads to fracture nearer the specimen centre but produces a less uniform strain distribution. Importantly, the major strains at necking on the FLC, as determined using the biaxial and the standard test methods, are almost the same in the plane-strain state, while in other strain states, the major strains are slightly lower for the biaxial method than that for the Nakajima method. An FFLC for AA5754 has also been determined using the biaxial test method, in which the major strain at fracture decreases with increasing strain ratio β from −0.5 to 0, while it changes only slightly when β > 0