• Voreux, Olivier

Abstract

This PhD project aims at assessing the capabilities associated with the local approach to fracture to simulate the propagation of a long fatigue crack in structural components. To this end, a three-step approach is considered. First, the cyclic non-linear behavior of the Nickel-based superalloy AD730™ is studied using dedicated cyclic characterization tests at three target temperatures (20, 550 and 700°C). Crack propagation tests on laboratory specimens are then performed in order to evidence the main crack driving mechanisms. Next, a set of constitutive equations for the cyclic non-linear behavior of AD730™ is proposed and calibrated. A strong behavior-damage coupling is settled leading to a time-incremental damage model for fatigue. The model is implemented in a finite element code using a fully implicit resolution scheme. In order to solve for the mesh-dependency issue, a non-local extension of the damage model is proposed using an implicit gradient formulation. Finally, an error-based mesh adaption procedure is considered in order to refine the mesh in the fracture process zone, close to the crack-tip where the non-linear phenomena occur. Once crack onset is achieved, a crack path tracking algorithm is used to evaluate the geometry and the direction of the crack increment. Then, a damage-to-crack transition consisting in remeshing steps, fields transfer and equilibrium recovery is performed. This way, crack growth kinetics can be captured. The whole numerical loop is assessed on calculations conducted on a SEN-T specimen subjected to complex fatigue and creep-fatigue loading conditions. The capabilities of the proposed approach and its limitations are finally discussed.

Topics

• impedance spectroscopy
• nickel
• crack
• fatigue
• creep
• superalloy