The influence of freeform bending process parameters on residual stresses for steel tubes

The superordinate goal of this study is to develop a closed-loop control for a property-controlled freeform bending process based on the prediction of the residual stresses and further mechanical properties by a soft sensor. Therefore, it is hypothesized that different combinations of freeform bending process parameters lead to matching geometry pairs with different residual stresses that can be influenced by the bending die position. A novel correlation between the bending die position, residual stresses and hardness provides the basis for the soft sensor and for closed-loop control design.This paper focuses on freeform bending with movable die, for which a novel bending strategy is developed that results in a set geometry while also providing various residual stress states in the tube. A numerical simulation model for freeform bending with movable die successfully models the variation in degrees of freedom of the machine and provides high qualitative and quantitative results regarding the identification of geometry and residual stresses. Real freeform bent validation experiments and HV 10 Vickers hardness measurements were carried out. HV 10 Vickers hardness tests were identified as a suitable parameter for preliminary system identification and validation, hence making it an appropriate parameter for soft sensor development.