Xiang Zhang presented his research at ASME IMECE 2015 Conference
The title of the Xiang’s presentation is: “Fatigue and Creep-Fatigue Modeling of Alloy 617 at High Temperature”.
Presentation Abstract:
Nickel based Alloy Inconel 617 is considered as a leading candidate for the U.S. very high temperature reactor (VHTR) program. Under the context of VHTR, components are exposed to high temperatures up to 950oC and frequently experience operation startup and shutdown, requiring structural materials with excellent high temperature creep and damage resistance. In order to gain understanding of the creep deformation mechanism of alloy 617 and facilitate the life prediction capability, a computational framework for fatigue and creep fatigue modeling of alloy 617 has been developed. A crystal plasticity finite element (CPFE) model considering isothermal, large deformation and cyclic loading conditions has been established. The formulation is based on the well-known multiplicative decomposition of the deformation gradient and idealizes the deformation in the crystal lattice primarily as glide of dislocations through the crystal lattice. Dislocation motion produces slip in close-packed directions along planes of high atomic density. A physically-based description of the kinetics of slip that considers the evolution of temperature-dependent slip resistance and back stress is adopted to consider deformation under cyclic loading condition. The slip resistance evolution equation is also modified to account for the initial softening at the beginning of first cycle observed from the experiments which is caused by solute drag creep. The microstructure is reconstructed using the DREAM.3D software from EBSD data and is fully resolved with finite elements with RVE size convergence studied conducted to select the appropriate RVE size. Due to the large number of material parameters used in the CPFE model, a three-stage calibration procedure was adopted to obtain an overall good match of the experimental tests. All parameter values initiate from literature values in the calibration process and ranges are provided based on preliminary simulation results. At the first stage, the elastic parameters are calibrated using an optimization process in which the objective function is defined by the simulated and tested bulk modulus and Poisson’s ratio. At the second stage, a subset of the plastic parameters is calibrated using an optimization again in which the deviation of a series of data points between the simulated and tested monotonic stress-strain curve was defined as objective function. A Gaussian process was adopted in which a surrogate model was established for each data point based on a large collection of simulation results prior to the optimization process and the optimization process is conducted based on the GP models for its computational efficiency. In the third stage, a subset of the plastic parameters was fine tuned to get an overall good match of the initial and stabilized response. The calibrated parameters provide well matched stress-strain and stress-time comparison with different strain ranges of fatigue tests as well as different hold time for creep-fatigue tests.