Fatigue of reinforced-polyurethane-based, sheet metal forming dies

Fatigue in metal forming dies is an important issue in the manufacturing industry. Fatigue failure leads to repair or replacement of the die, which interrupts and slows down the manufacturing process. Fatigue of bulk metal forming (such as, forging and extrusion) dies has been the focus of most die failure studies because the nature of the process tends to subject the die to high stresses, thus increasing the likelihood of die failure. However, as polymer composite tooling materials are making inroads into rapid tooling technologies, die failure in sheet metal forming, in which the dies are subjected to significantly lower stresses, is drawing attention. This paper presents a fatigue failure analysis of V-bending dies machined from a polyurethane-based tooling board. The mechanical properties of the tooling materials are characterized to identify the underlying failure mechanisms. Various fatigue failure criteria, namely, maximum tensile principal stress, effective stress, Smith–Watson–Topper, and critical plane approaches are investigated as die life prediction methods, among which the latter two approaches were most accurate. Finite element analyses and experiments are performed to verify the results.