A review of recent advances in the application of blank-holder force towards improving the forming limits of sheet metal parts

Abstract

This paper presents a review of current industrial research and development in blank-holding technology, and its effect on the formability of sheet metals. The first section of the paper introduces a brief historical background of how and why the formability of sheet metal is defined in terms of a two-dimensional strain map. It also describes how the measure of formability depends on the state of strain, which in turn depends on the specific experimental technique used. Section 2briefly reviews research work dealing with both experimental and theoretical determination of the forming-limit diagram (FLD), the focus being to demonstrate the significance of the strain path on the FLD by showing how the calculated limit strain depends on the strain history. Sections 3 and 4demonstrate that for a given sheet material and part geometry, the parameters involved in stamping practice can significantly affect the formability of the material. It is shown how the blank-holder force and the way it is applied on the blank affects the state of stress and strain within the part, and also influences the strain path. Section 3reviews open- and closed-loop control of the blank-holder force, and demonstrates that by properly adjusting this parameter the working window between the tearing and wrinkling boundaries can be expanded. The research work reviewed in this section employed a uniformly-distributed blank-holder force, which varied with time (punch depth). The application of this technology to large, industry-size panels is mostly reported by the Japanese automobile industry. Section 4describes how the real-time control of the blank-holder force has been expanded to include spacial as well as time variations. The papers reviewed in this section deal with the multi-force variable-blank-holding technique and report considerable improvement in formability. Significant development in this area has been made in Germany, leading to the construction of production-size presses. So far as an optimum trajectory of blank-holder force variation is concerned, the results are inconclusive. It appears that an ideal trajectory depends on both part geometry and material type. Considering that different materials show different sensitivity to various modes of failure, the above observation is not unexpected.