Design and optimization of cruciform tension and shear specimen for forming limit acquisition

The forming limit diagram (FLD) has been widely employed for the formability estimation and prediction of sheet metal. Recently, the cruciform tension test has emerged as a promising strategy to obtain the forming limit curves (FLCs). Compared to conventional bulging test, cruciform tension can readily achieve various strain paths without changing the specimen shape, and effectively avoid the influence caused by the friction and normal stress. However, the design and manufacturing of the cruciform specimen have always hindered the wider application of this technique, since the trade-off dilemma between deformation requirement and machining difficulty is hard to overcome. In this work, based on the comprehensive investigation on existed cruciform specimens, the advantages/disadvantages of different specimens and effect of various specimen features are clarified. Then, combining different specimen features, a series of experiments and finite element (FE) simulations are conducted to obtain the optimum tension and shear specimen suitable for FLC acquisition. Based on the optimum specimens, the FLC covers a large range of strain path can be obtained, improving the applicability of FLC. Moreover, the fracture behavior and morphology are also analyzed for further unveiling of different loading ratios. Finally, the optimum specimens are successfully applied in AA5052-H14 and Q235A steel sheet to obtain their FLCs, indicating a great applicability of the specimen. Therefore, this work conducts a complete and scientific procedure for the cruciform specimen design and experiment implementation, which could offer a universal solution for cruciform design and manufacturing.