Life Prediction Of Turbine Blades Research Articles

Fatigue life prediction of turbine blades with geometric imperfections made of stainless steel

This research addresses critical challenges faced by steam turbine blades, particularly in low-pressure (LP) turbines, where premature failures are common due to stress concentrations at the blade root area. The study introduces a numerical methodology aimed at predicting the life of mistuned steam turbine blades, with a focus on variations in blade geometry which have received limited exploration in existing literature. A simplified, scaled-down mistuned steam turbine bladed disc model was developed using Abaqus finite element software. Acquisition of steady-state stress response of the disc models was performed through finite element analysis (FEA). Thereafter, numerical stress distributions were obtained. Subsequently, within Companion software, a Monte Carlo simulation-based probabilistic approach was applied to evaluate and quantify uncertainties in fatigue life for 17 cases. This analysis considered an accepted manufacturing percentage scatter of ± 5 % for the steam turbine bladed disc. That was conducted by selecting mistuning (geometry variation) percentages as the random variables. The methodology demonstrated reliability, correlating well with literature-based and discrete fatigue life results. This study establishes the potential for accurately predicting the fatigue life of mistuned steam turbine blades using the developed methodology.

Threshold damage-based fatigue life prediction of turbine blades under combined high and low cycle fatigue

Life prediction based on the damage mechanics of aero-engine turbine blades is crucial for performing their strength design and ensuring the operational reliability under combined high and low cycle fatigue (CCF) loadings. In view of this, a simple and efficient life prediction method is developed to consider the interaction of high cycle fatigue (HCF) and low cycle fatigue (LCF) without any additional material constants, and as a basis of that a new nonlinear damage accumulation model is proposed by introducing threshold damage of the component related to material type and current HCF damage. In order to validate the presented methodology, model predictions were compared with the experimental data sets of turbine blades and its alloy materials. The predicted results indicate that the simple life prediction model is capable of estimating the fatigue life quickly with an acceptable accuracy in contrast to other four existing ones. Moreover, the threshold damage-based method provides the higher prediction accuracy than others due to improved HCF damage determined by the damage threshold.

A fatigue damage accumulation model for reliability analysis of engine components under combined cycle loadings

AbstractRotor components of an aircraft engine in service are usually subjected to combined high and low cycle fatigue (CCF) loadings. In this work, combining with the load spectrum of CCF, a modified damage accumulation model for CCF life prediction of turbine blades is first put forward to take into account the effects of load consequence and load interaction caused by high‐cycle fatigue (HCF) loads and low‐cycle fatigue (LCF) loads under CCF loading conditions. The predicted results demonstrate that the proposed model presents a higher prediction accuracy than Miner, Manson‐Halford model does. Moreover, to evaluate the fatigue reliability of rotor components, reliability model with the failure mode of CCF is proposed on the basis of the stress‐strength interference method when considering the strength degeneration, and its results show that the reliability model with CCF is more suitable for aero‐engine components than that with the failure mode of single fatigue.

Fatigue life prediction of turbine blades based on a modified equivalent strain model

Mean stress is known to exert significant effects on fatigue life prediction. Although numerous adjustments have been developed to explain the influence of mean stress, only a few of such adjustments account for mean stress sensitivity. The Smith-Watson-Topper (SWT) model is one of the most widely used models that can provide satisfactory predictions. It is regarded as a case of a Walker model when the material parameter γ = 0.5. The Walker equation considers the mean stress effect and sensitivity, and it can generate accurate predictions in many fatigue programs. In this work, a modified model that accounts for the mean stress effect and sensitivity is proposed to estimate fatigue life. Several sets of experimental data are used to validate the applicability of the proposed model. The proposed model is also compared with the SWT model and the Morrow model. Results show that the proposed model yields more accurate predictions than the other models. The proposed model is applied to predict the fatigue life of a low-pressure turbine blade.

A Combined High and Low Cycle Fatigue Model for Life Prediction of Turbine Blades.

Combined high and low cycle fatigue (CCF) generally induces the failure of aircraft gas turbine attachments. Based on the aero-engine load spectrum, accurate assessment of fatigue damage due to the interaction of high cycle fatigue (HCF) resulting from high frequency vibrations and low cycle fatigue (LCF) from ground-air-ground engine cycles is of critical importance for ensuring structural integrity of engine components, like turbine blades. In this paper, the influence of combined damage accumulation on the expected CCF life are investigated for turbine blades. The CCF behavior of a turbine blade is usually studied by testing with four load-controlled parameters, including high cycle stress amplitude and frequency, and low cycle stress amplitude and frequency. According to this, a new damage accumulation model is proposed based on Miner’s rule to consider the coupled damage due to HCF-LCF interaction by introducing the four load parameters. Five experimental datasets of turbine blade alloys and turbine blades were introduced for model validation and comparison between the proposed Miner, Manson-Halford, and Trufyakov-Kovalchuk models. Results show that the proposed model provides more accurate predictions than others with lower mean and standard deviation values of model prediction errors.

Measurement of Young’s Modulus of Thermal Barrier Coatings by Suspended Coupled Flexural Resonance Method

Thermal barrier coatings(TBCs) has been extensively used on hot section components of gas turbine engine, especially turbine blades. Young’s modulus of TBCs is a significant mechanical parameter in life prediction research of turbine blades, but it is hard to measure the EB-PVD thermal barrier coatings by conventional tensile/compression method for its brittleness and porous microstructure. Therefore, Young’s modulus mathematical model of double-layer beam structure specimen was deduced on the basis of the first-order beam bending vibration equation and composite beam bending equation. An experimental platform with dynamic signal acquisition system was set up for measuring Young's modulus of thermal barrier coatings under high temperature. Young's modulus of ceramic coat was measured from ambient temperature to 1100°C to provide material data for subsequent research on life prediction of turbine blades with thermal barrier coatings.

Power-exponent function model for low-cycle fatigue life prediction and its applications – Part II: Life prediction of turbine blades under creep–fatigue interaction

This paper is concerned with the applications of the proposed models, namely the modified linear damage summation (MLDS) method and the modified strain range partitioning (MSRP) method described in Part I of the series papers, to life prediction of turbine blades under creep–fatigue interaction. To begin with, a detailed FEM analysis is conducted considering peak loading of thermal load, centrifugal force and airflow force to determine life assessment positions. Secondly, according to the blades rig testing load, a stress–strain response analysis is performed for pure low-cycle fatigue (LCF) and creep–fatigue interaction, in which steady-state temperature field, plastic kinematic hardening and contact between the blade body and hoop segment are taken into consideration to achieve accordance with practical conditions. Finally, based on FEM simulation results, the life of turbine blades is predicted using the five different models for creep–fatigue loading. A minimum safe and conservative life of 1293 h is given by the MLDS method based on power-exponent function model. The predicted lives by the MLDS and MSRP methods show reasonable agreement with the rig testing life, which further illustrates the validity of power-exponent function model and its applications.

Power-exponent function model for low-cycle fatigue life prediction and its applications – Part I: Models and validations

Based on power transformation method, this paper advances a power-exponent function model for low-cycle fatigue (LCF) life prediction and then introduces it to creep–fatigue life prediction methods – the strain range partitioning (SRP) method and the linear damage summation (LDS) method for comprehensive assessment of power-exponent function model. Power transformation exponents ( p) are given for some typical materials. Through LCF and creep–fatigue life predictions for these materials, we illustrate the validity of power-exponent function model and its applications. The comparisons between different models show that the power-exponent function model has better performances for LCF life prediction. In nature, Manson–Coffin law is the first order Taylor expansion approximation of power-exponent function model in log–log scale coordinates, corresponding to p = 1. The enhancement of nonlinearity makes the power-exponent function model and its applications to creep–fatigue life prediction methods more precise than the corresponding unmodified methods, whereas these models still use linear parameter regression method and remain simple for engineering applications. An example of these models’ applications to life prediction of turbine blades under creep–fatigue interaction is given in the accompanying paper.

Residual Life Prediction of Turbine Blades of Aeroderivative Gas Turbines

Various methods have been proposed for predicting residual creep life of turbine blades. However, these conventional methods cannot be applied to predict residual lives with appropriate accuracy. This communication describes an alternative method, which is non-destructive and primarily based on blade extension measurements. Blade elongation measurements can be utilized in creep-life prediction as a non-destructive method and they are believed to give reasonable criteria for blade retirement. (orig.)

Dynamic stress analysis and a fracture mechanics approach to life prediction of turbine blades

Emerging blade technologies are finding it increasingly essential to correlate blade vibrations to blade fatigue in order to assess the residual life of existing blading and for development of newer designs. In this paper an analytical code for dynamic stress analysis and fatigue life prediction of blades is presented. The life prediction algorithm is based on a combination method, which combines the local strain approach to predict the initiation life and fracture mechanics approach to predict the propagation life, to estimate the total fatigue life. The conventional stress based approach involving von Mises theory along with S-N-Mean stress diagram suffers from the drawback that it does not make allowance for the possibility of development of plastic strain zones, especially in cases of low cycle fatigue. In the present paper, strain life concepts are employed to analyse the crack initiation phenomenon. Dynamic and static stresses incurred by the blade form inputs to the life estimation algorithm. The modeling is done for a general tapered, twisted and asymmetric cross section blade mounted on a rotating disc at a stagger angle. Blade damping is non-linear in nature and a numerical technique is employed for estimation of blade stresses under typical nozzle excitation. Critical cases of resonant conditions of blade operation are considered. Neuber's rule is applied to the dynamic stresses to obtain the elasto-plastic strains and then the material hysteresis curve is used to iteratively solve for the plastic stress. Static stress effects are accounted for and crack initiation life is estimated by solving the strain life equation. Crack growth formulations are then applied to the initiated crack to analyse the propagation of crack leading to failure. The engineering approximations involved are stated and the algorithm is numerically demonstrated for typical conditions of blade operations.