A CRYSTAL VISCO-PLASTIC MODEL FOR NI-BASE SUPERALLOYS UNDER LOW CYCLE FATIGUE

Components in gas turbines, specifically turbine blades are subjected to extreme loading conditions such as high temperatures and stresses over extended periods of time; therefore, predicting material behavior and life expectancy at these loading conditions are extremely important. The development of simulations that accurately predict monotonic response for these materials are highly desirable. Single crystal Ni-base superalloys used in the design of gas turbine blades exhibit anisotropic behavior resulting from texture development and dislocation substructures. A Crystal Visco-plastic (CVP) model has the capability of capturing both phenomena to accurately predict the deformation response of the material. The rate dependent crystal visco-plastic model consists of a flow rule and internal state variables. This model considers the inelastic mechanism of kinematic hardening which is captured using the Back stress. Crystal graphic slip is taken in to account by the incorporation of 12 Octahedral slip systems. An implicit integration structure that uses Newton Raphson iteration scheme is used to solve the desired solutions. The MATLAB model is developed in two parts, including a routine for the CVP constitutive model along with a separate routine which functions as an emulator. The emulator replicates a finite element analysis model and provides the initial calculations needed for the CVP. A significant advantage of the MATLAB model is its capability to optimize the modelling constants to increase accuracy. The CVP model has the capability to display material behavior for monotonic loading for a variety of material orientations and temperatures.