Numerical simulation of failure prediction for ceramic tools: comparison with forging test results

Abstract : A fracture prediction criterion for brittle materials has been introduced in the POLLUX finite-element code in order to predict the risk-of-rupture of ceramic tools during a forging operation. POLLUX is a software dedicated to the simulation of forging operations, initially developed by INSA (Lyon). The chosen probabilistic fracture model is based on the weakest-link theory and the statistical theory of Weibull. A surface approach or a volume approach can be retained on the basis of the type of critical flaws in the ceramic. Two different criteria are available in order to characterise the stress state, considering the tensile normal stresses and neglecting the compressive stresses. An identification procedure of the critical flaw type is presented for a particular ceramic material. Statistical parameters of ceramic fracture have been determined experimentally using bending tests performed under environmental conditions close to those of forging. A constitutive equation of the workpiece material has been proposed, issued from torsion tests. In order to validate the model in the case of ceramic tools subjected to multi-axial stress states, a particular configuration has been defined to compare the simulation predictions with the experimental results. A forging test has then been developed, in which a billet of superalloy is formed in a ceramic tool up to its fracture at the temperature of 1423 K. The experimental distribution of tool fracture, according to the strain of the billet, is in good agreement with fracture predictions computed by the simulation.