Investigating formability enhancement in double side incremental forming by developing a new test method of tension under cyclic bending and compression

Incremental sheet forming (ISF), including single point incremental forming (SPIF) and double side incremental forming (DSIF), has demonstrated significantly enhanced material formability compared to traditional sheet metal forming processes. However, the material deformation mechanisms that lead to the enhanced formability in DSIF are not fully understood. In this study, a new test method, named as Tension under Cyclic Bending and Compression (TCBC), has been developed to investigate four deformation modes observed in DSIF, including tension, compression, bending and cyclic loading, on the material formability enhancement. An analytical model based on the elementary plasticity theory has been developed to characterize the effect of tension, compression, bending and cyclic loading on the initiation of material plastic deformation. A TCBC rig has been manufactured and experimental tests of two aluminium alloys have been conducted by applying the Design of the Experiments to investigate the significant effects of different deformation modes on the material formability. Finite element modelling of TCBC test as well as DSIF process has also been developed to compare their plastic strain evolution and strain paths during the cyclic deformation process. The material formability has been found to be improved significantly under TCBC condition and the existence of compression loading leads to strengthened localized material plastic deformation, which contributes to the enhanced material formability and delayed fracture. The new TCBC test method developed in this study has demonstrated its potential to replace the current testing method using the DSIF process itself for material formability studies.