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

A fracture prediction criterion for brittle materials has been introduced i n 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 t o the simulation of forging operations, initially developed by INSA (Lyon). The chosen probabilistic fracture model is based on the weakest-link theor y and the statistical theory of Weibull. A surface approach or a volume app roach can be retained on the basis of the type of critical flaws in the cer amic. Two different criteria are available in order to characterise the str ess state, considering the tensile normal stresses and neglecting the compr essive stresses. Aa identification procedure of the critical flaw type is p resented for a particular ceramic material. Statistical parameters of ceram ic fracture have been determined experimentally using bending tests perform ed 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 subjecte d to multi-axial stress states, a particular configuration has been defined to compare the simulation predictions with the experimental results. A for ging test has then been developed, in which a billet of superalloy is forme d in a ceramic tool up to its fracture at the temperature of 1423 K. The ex perimental distribution of tool fracture, according to the strain of the bi llet, is in good agreement with fracture predictions computed by the simula tion. (C) 1999 Elsevier Science S.A. All rights reserved.