Prediction of hole expansion ratio for automotive grade steels

• FE simulation revealed that the edge of the hole deforms in a uniaxial tensile mode during hole expansion.
• A fracture based failure criterion is employed to predict the HER.
• A methodology is proposed to calculate the true fracture strain from uniaxial tensile data.
• A correlation is proposed between the true fracture strain during uniaxial tensile test and the HER.

The objective of this work is to characterize experimentally and predict analytically the hole expansion ratio of steel sheets from uniaxial tensile properties. Firstly, finite element analysis is performed to understand the deformation modes (i.e. stress state) at the edge of the hole during its expansion by a conical punch. Finite element simulation study confirms that during the hole expansion process, the edge of the hole undergoes uniaxial tensile deformation. Secondly, a fracture based failure criterion is employed to predict the hole expansion ratio, considering that the test stop criteria of the hole expansion process is the formation of a through thickness crack. Next, a methodology is proposed to compute the true fracture strain during uniaxial tensile test, following which a model is proposed to predict the hole expansion ratio analytically from the true fracture strain. The proposed model is validated with eight different steel grades.