REFERENCE STRESS ESTIMATION FOR ANISOTROPIC MATERIALS USING LINEAR ELASTIC FINITE ELEMENT RESULTS

摘要

Components in the hot section of a gas turbine engine experience extended high temperature dwells and cycles composed of multiple starts, changes in load, and variable duration. These loading profiles can lead to damage from cyclic viscoplasticity which is heavily path dependent as dwell stress, yield strength, and stress range change constantly during operation. Since an accurate prediction of accumulated damage is critical to managing an engine, reduced order methods for tracking material behavior over complex operation cycles are necessary tools to help avoid unplanned down time and optimize cost over the operational period. One method for tracking the material behavior during path dependent cyclic viscoplasticity requires the use of reference stress. Reference stress is a bulk representative stress that can be used in conjunction with various lifing methodologies to determine component durability. Previous papers provided a method for calculating reference stress for isotropic materials using limit load estimation. The goal of this paper is to extend these methodologies to a reference stress estimation method for anisotropic materials to estimate life for single crystal turbine blades. Derived equations will be shown and results from simple Finite Element (FE) test cases will be discussed to demonstrate the accuracy of the anisotropic reference stress estimation. Once reference stress is obtained, the long term forward creep stress of a component can be estimated for any given initial stress state. This approach can be used to calculate damage during shakedown resulting from redistribution and relaxation due to plasticity and creep, which can be critical for accurately predicting remaining useful life and optimizing engine management.