Gas turbine rotor stress analysis using continuum damage mechanics

Master thesis in mechanical engineering (applied design)

Abstract

Analysis of gas turbine rotor stress using continuous damage mechanics

In this thesis, Lometer's unified model based on continuous damage mechanics is used to model the stress-strain behavior of a gas turbine rotor sample. The unified damage law, which is used to model creep-fatigue interaction, is based on the cumulative plastic strain component caused by creep damage and low-cycle fatigue damage. With the help of periodic stretching and release tests, the parameters of the united damage model, the viscous Norton model and the constants of the nonlinear kinematic stiffness model of the rotor material were determined. Also, in order to validate the finite element method, the conducted tests were modeled completely identically by ABAQUS finite element software. The stress-strain behavior of the gas turbine rotor was modeled with the help of ABAQUS finite element software and the growth of damage at critical points was calculated. The replica test was also performed at four points on the rotor surface and the formation of creep holes was checked with the help of SEM. Based on the analysis of continuous damage mechanics, the creep-fatigue interaction was investigated and the rotor life was estimated

1-1 Introduction

Gas turbines are one of the most important components for energy production in industries such as aerospace, marine, oil and thermal power plants, and they are used in various industries. It is expanding day by day. Therefore, the study and investigation of different dimensions of the gas turbine for the purpose of its optimal use and development has gained special importance in research centers of the world today. Due to the fact that gas turbines are subjected to very high temperatures and forces in working conditions, they have a limited life. Therefore, it is necessary to be able to predict the life of its components. The ability to perform life estimation enables us to use engineering equipment optimally, which has many economic benefits.

One ??of the most important and fundamental components of a gas turbine is its rotor, which is exposed to very high stresses and temperatures. These critical working conditions of high temperature and stress cause various destruction mechanisms to be applied on the rotor and as a result, the rotor deteriorates and loses properties over time.

In the field of the causes of stalling [1] of the rotor, numerous researches have been conducted and the most important mechanisms of its destruction, including creep, fatigue, oxidation and corrosion, have been investigated from a microstructural and physical point of view. Also, the mutual effect of these inhibitions, which can be caused by the simultaneous effect of two or more of these factors, has been investigated. According to the results, the creep-fatigue interaction[2] is one of the most important causes of stalling in the gas turbine rotor. This phenomenon, which is caused by the harsh working conditions of high temperature and high stresses, limits the life of the rotor. The combination of stress and high temperature causes creep phenomenon and extreme temperature gradients cause thermal fatigue. Therefore, the most important destruction mechanisms that play a role in the deterioration of the rotor and as a result reducing its life are thermal fatigue, creep and their interaction.

Unlike other turbine parts such as blades and connections, rotor failure during operation can cause irreparable and heavy damages to the entire turbine assembly. Therefore, the manufacturers and users of turbines have always been trying to be able to recognize the useful life of the rotor and repair it at the right time and replace it if necessary. In addition, rotor replacement can be very costly for power plants. According to these materials, it becomes clear that the more accurate estimation of the rotor life in order to use it optimally has always been one of the things researched by researchers and it can significantly help to reduce costs in the industry. Therefore, complete and accurate knowledge of the mechanisms of failure and failure of turbine parts, especially the rotor, is considered a necessity and can lead to significant economic savings with optimal life estimation. From this point of view, the importance of gas turbine rotor life estimation becomes clear.

It should be mentioned that significant advances in the field of gas turbine manufacturing technology have caused the important and rotating parts of power plant components, such as the rotor and turbine components, to be used under much higher working loads and temperatures than in the past, which indicates the need to expand new research in this field.

1-2 Damage Mechanics Continuity[3]

Material damage is a physical process during which the material undergoes a reduction and deterioration of mechanical properties under loading and eventually breaks. The weakening of the material is caused by the appearance and growth of microcracks [4] and microholes [5] in the fabric of the material. The science of damage mechanics is the science of studying the mechanical variables involved in these processes in the material under loading. Contrary to the discontinuous nature of damage, the theory of continuous damage mechanics tries to model the growth and expansion of these discontinuities in a continuous framework, which it does by defining an internal variable in the continuous environment [1]. It can be said that if fracture mechanics [6], which is the science of examining and modeling discontinuities, can be expressed in the framework of classical continuous mechanics, we will be guided towards continuous damage mechanics. In fact, the purpose of developing continuous damage mechanics is to fill the gap between classical continuous mechanics and fracture mechanics. In recent decades, a lot of research has been done on modeling the damage process, and so far, various continuous damage models have been presented to describe such a phenomenon in the framework of damage mechanics.

Although the basic principles and concepts of damage mechanics have a long history, but its expansion has occurred especially for soft materials in recent decades, and for this reason, it is considered a relatively new field in mechanical sciences. Currently, damage mechanics is known as one of the most appropriate methods for evaluating failure in soft materials [2]. Mechanical and thermal stresses for a gas turbine rotor sample. In working conditions, the gas turbine rotor is subjected to extreme temperature gradients and very high stresses, which lead to the development of deterioration mechanisms in the rotor. The phenomenon of creep due to the combination of temperature and high stresses as well as the phenomenon of thermal fatigue due to temperature changes can be mentioned as the most important damage mechanisms in the rotor. Considering that the two phenomena of creep and fatigue occur simultaneously, it is necessary to consider the effects of these two phenomena simultaneously in rotor stress analysis. In this research, the theory of continuous damage mechanics has been used for stress analysis, because this theory has the ability to consider the creep-fatigue interaction.

Regarding the complex geometry of the rotor and its mixed loading, in this research, ABAQUS finite element software has been used to analyze the mechanical and thermal stresses of the rotor.

Also, considering that for stress analysis, the need for Determining the mechanical properties of the desired rotor is done by performing various tests on the desired material. The mechanical properties of the material have been determined. mechanics (CDM) model of Lemaitre was used to model the stress-strain behavior of a gas turbine rotor. The unified damage law, which is used to model creep-fatigue interaction, is based on the increment of the accumulated inelastic strain which can be due to creep damage and the low cycle fatigue damage. The parameters of the unified CDM model, Norton viscous model and constants of the nonlinear kinematic hardening model were determined by cyclic tension and relaxation tests. In order to validate FE method, all tests were simulated exactly by ABAQUS. Stress-strain behavior in the gas turbine rotor was modeled by ABAQUS finite element software and damage evolution in critical points was obtained.