Characterisation and modelling of ductile fracture in incremental sheet forming of an aluminium alloy with consideration of temperature variations.

Although extensive studies have been conducted to predict ductile fracture at room temperature, no attempt has been made to use Gurson-Tvergaard-Needleman (GTN) model to capture the effect of temperature due to spindle speed induced friction heat on damage accumulation in incremental sheet forming (ISF). In this work, the temperature effect is incorporated into a shear modified GTN model to characterise ductile fracture behaviour of an aluminium alloy (AA1050) at varying temperatures in ISF processing under different strain states. The correlation between void evolution and damage accumulation at different temperatures is revealed, and the void volume fraction (VVF) at different deformation stages is determined by conducting microscopic in-situ tensile test. By introducing a new temperature-dependent VVF function and determining appropriate GTN damage parameters, ductile fracture under different ISF processing temperatures is evaluated using FE simulation and validated by ISF experimental testing. The results show that the proposed temperature-dependent VVF function in GTN modelling is capable of predicting the ductile fracture of both the set temperatures in in-situ tensile tests and the varying temperature conditions in ISF processes. This study unveils the underlying mechanisms behind material deformation and damage evolution with consideration of temperature variations caused by tool spindle speed at various ISF forming conditions, which provides guidance for process optimisation and formability improvement in ISF process.