Fracture mechanics based life prediction of hollow extrusion dies

Hot metal extrusion dies, especially for hollow profiles, are quite costly. Repair or replacement of dies and affiliated tooling adds to the cost, brings down productivity, and can contribute to product defects. Reasons for the high production and maintenance cost are stringent material requirements (highstrength and high-hardness tool steels), precision manufacturing methods (spark erosion, wire EDM, etc), and a series of specialized heat treatment and surface hardening operations. Consequently, a major goal for die designers and manufacturers is longer service life. An equally important objective is a reasonably accurate prediction of time-to-failure, to help devise an optimum replacement and inventory strategy, and for good performance evaluation. Majority of hot-work extrusion dies fail by fatigue fracture. Treating the die as a flat plate with edge crack, a fracture mechanics based fatigue life prediction model was developed by the authors in an earlier work. To improve the precision of die life prediction, a hollow (tube) extrusion die is modeled in the current work as a pressurized cylinder with internal crack. Based on this new strategy, a model is developed to forecast fracture failure of extrusion dies. Stochastic nature of various fatigue related die parameters is examined. Monte Carlo simulation is used for die life prediction under a given set of operating conditions and mechanical properties. Simulated fracture life values thus obtained are quite realistic in comparison with actual extrusion die life data from the industry.