Calculation of Stress Intensity Factors for semi-elliptical surface cracks in a turbine rotor

Stress intensity factors based on linear elastic behaviour were calculated for semi-elliptical surface cracks in the front face of a cylindrical disk. The small semi-axis of the cracks coincides with the axis of rotation of the disk. The disk represents a part of a turbine rotor and is used to simplify the calculations. The uncracked rotor is loaded by a radial varying hoop stress distribution, caused by rotation, thermal gradients and the influence of the turbine blades. Following a procedure proposed by Mattheck at al. [1] the stress points of the cracks were calculated by means of the weight functions method.     

Failure Analysis of Steam Turbine Rotor Disk

This article presents an investigation into causes of failure of rotor disk of an 8.25-MW capacity steam turbine, which failed catastrophically. Four pieces of the rotor disk detached from the tenth-stage disk of the turbine rotor during this failure. Visual inspection, chemical analysis, macro- and microscopic analysis of the failed rotor disk, analysis of the operational data, and the history of the rotor operation indicated that the failure could be attributed to stress concentrations at macropores and regions of segregation in the disk. Stress corrosion cracking (SCC) subsequently took place in the regions of stress concentration because the steam in the turbine contained chloride and potassium. The SCC produced a network of cracks associated with the macro- and microporosities. It was recommended that the disk fabrication processes ensure a high-quality microstructure and that operational monitoring of the composition of steam be initiated to ensure that the chlorides and potassium concentrations are maintained below a specified level.     

Fracture analysis of turbine disks and computational–experimental background of the operational decisions

In this paper, fatigue crack growth under operation conditions for rotating disks of aircraft gas turbine engines is analyzed. Initiation and growth of surface cracks for compressor disks made from two-phase titanium alloy has occurred in a disk and blade attachment. Damage accumulation and growth for turbine disks made from steel took place on the inner surface of hole in a hub of wheel. Suggested approach of simulation modeling is used for an analysis and prevention of operation failures of engine rotating components. In the approach described, finite-element models (FEMs) in two and three dimensions were applied to the study of stress–strain state and stress intensity factors for the basic configurations of compressor and turbine disks and their operational damage. Proposed design modifications and repair technologies to existing in-service aircraft gas-turbine engine rotating components are analyzed and substantiated on a static strength and fatigue life basis.     

Estimating working life for aircraft gas turbine engine disks at the stage of fatigue crack development. Part 2

It is shown that the cyclic cracking resistance in turbine disk materials is substantially affected by temperature, frequency, loading-cycle shape, and specimen thickness. Formulas describing fatigue crack growth rates are considered and calculations are done on the periods needed for fatigue cracks to grow to critical sizes from stress concentrators at various temperatures. Deceased. Translated from Problemy Prochnosti, No. 12, pp. 3–15, December, 1994.     

镍基高温合金微型涡轮盘热塑性成形工艺

目的 研究GH4698微型涡轮盘热塑性成形工艺。方法 采用有限元软件Deform-3D对镍基高温合金微型涡轮盘模锻过程进行数值模拟分析,研究坯料不同高径比、不同模锻温度下涡轮盘成形的最大载荷值、等效应力、等效应变、速度场的变化规律。结果 微型涡轮盘模锻过程载荷最大值随模锻温度升高而降低,温度和高径比对涡轮盘的凸台、直榫等部位的等效应力、等效应变影响有明显的差异。在涡轮盘路径1上,随着温度的升高,等效应力逐渐降低,变形更加均匀;随着高径比的增加,变形不均匀程度增大,高径比为1时等效应力的极大值最小;温度和高径比对速度场的影响较小。结论 温度和高径比对GH4698微型涡轮盘锻造变形行为有显著的影响,选择合适的模锻工艺参数可以有效降低成形载荷,并获得使用性能较好的微型涡轮盘。     

Gas turbine disk-blade attachment crack

Evaluation of a gas turbine disk revealed a crack in the blade attachment area. The subsequent effort to understand the origin of this crack led to a series of analyses that included computing the stresses on the attachment, characterization of fatigue crack growth, and a model for fretting fatigue crack growth. These elements were brought together to simulate the conditions that led to the cracking. It is concluded that the crack was probably caused by fretting fatigue induced by the stresses related to normal takeoff and landing cycles and exacerbated by aircraft maneuvers, and that short periods of blade resonance may have contributed to the cracking. If material had not been removed from the attachment surface of the disk by service-induced wear, it is likely more cracks would have been found.     

30Cr1Mo1V钢汽轮机转子高应力段的疲劳裂纹扩展速率

在某已服役了16 a的30Cr1Mo1V钢汽轮机转子的高应力段取样制作成紧凑拉伸试样,用MTS 810.50试验机进行室温和538℃下的疲劳裂纹扩展速率试验。结果表明：该钢疲劳裂纹稳定扩展阶段的疲劳裂纹扩展速率适用于Paris公式,室温下的疲劳裂纹扩展速率方程为da/dN=2.2101×10-8（ΔK）2.9163,538℃下的疲劳裂纹扩展速率方程为da/dN=9.8794×10-8（ΔK）2.6844;对于30Cr1Mo1V转子钢,温度升高,疲劳裂纹扩展速率加快;30Cr1Mo1V转子钢在疲劳裂纹稳定扩展阶段存在转折点,将该阶段又细分为两段,经过转折点后疲劳裂纹扩展速率的增速减慢;与原始材料相比,已服役16 a的30Cr1Mo1V钢汽轮机转子高应力段材料的疲劳裂纹扩展速率增大。     

Challenges in modelling the evolution of stress corrosion cracks from pits

The evolution of stress corrosion cracks from pits is important in many industrial applications but continues to be a challenge in both measurement and prediction. Life prediction in these circumstances has to account for pit growth kinetics, the conditions for the transition from pits to cracks, and the growth rate of cracks in the short and long crack domain. An example of importance is the performance of steam turbine rotors in power generation. Although stress corrosion failures are comparatively rare, the consequences can be severe and occasionally catastrophic. Consequently, considerable effort is being focused on evaluating the effect of operational variables on pitting and crack growth and in developing an improved basis for structural integrity assessment. A preliminary mathematical model based on deterministic equations with statistically variable input parameters was developed for simulating the evolution of the pit depth distribution at different exposure times, and the transformation to stress corrosion cracks was based on the Kondo criteria. The model predicted some features of the damage well but recent novel measurements of the evolution of stress corrosion cracks from pits combined with finite element analysis of the strain distribution suggest that the transition from a pit to a crack is more complex than can be accounted for by the Kondo approach.     

Basic diagram of fatigue fracture for 34KhN1M steel under conditions of phase transition in water

We construct a basic diagram of fatigue fracture for 34KhN1M steel aimed at computing the durability of a steam turbine disk working in the steam-water transition zone. In plotting this diagram, we took into account the service and extreme electrochemical conditions at the tip of a corrosion fatigue crack. The diagram was applied to estimate the serviceability of a disk with cracks of various types depending on the intensity of tensile stresses and the geometry of cracks.Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 30, No. 2, pp. 90–99, March–April, 1994.     

Failure Analysis and Repair of a Catastrophically Damaged Gas Turbine Compressor Disk Using SEM Technique and CFD Analysis

During a major overhaul of an 85?MW gas turbine unit in Iran-Rey power plant, 39 cracks were detected with different lengths and locations on the compressor disk of stage 11. All of the cracks initiated from the dovetail regions. Preliminary visual inspections and further micro-fractography using the scanning electron microscope demonstrated that the fretting fatigue phenomenon was the main cause of failure. Four repair methods were suggested to restart the unit. The first one was to remove all of the cracks from the disk by machining, or the so-called blending. The second, third, and fourth ways were to remove the entire rotor blades of stage 11, to remove the entire rotor and stator blades of the stage 11 simultaneously, and to remove those rotor blades of stage 11 corresponding to the damaged dovetails, respectively. Although the first way of solution was initially carried out on the damaged disk, the first author offered that restarting the unit with the blended disk is not reliable enough because of the presence of a large number of repair points on the disk. Using the numerical investigations based on the computational fluid dynamics, it was found that only the second suggestion (i.e., removing the entire rotor blades of the stage 11) might be applicable. Ultimately, the entire stage 11 rotor blades were removed from the blended disk, and the gas turbine unit was successfully restarted without encountering abnormal operation. Although the performed process resulted in approximately 20% output power loss compared with the unit's power before the blades' removal, the unit was quickly restored to be ready to restart, and the electric power could be generated during the period of peak consumption.     

A mixed-mode crack analysis of rotating disk using finite element method

This paper presents a rigorous elastodynamic hybrid-displacement finite element procedure for a safety analysis of fast rotating disks with mixed-mode cracks. Based on a modified Hamilton's principle, the finite element model is derived such that the proper crack-tip singularities are taken into consideration and the interelement displacement compatibility conditions are still satisfied. Thus, the specimen can be represented by a finite element assemblage in which “singular” elements are used around the crack-tip and high-order isoparametric “regular” elements are taken elsewhere.To determine the mixed-mode stress intensity factors, the modified $\text{J}\text{?}{}_{\mathrm{k}}$ integrals for rotating cracked disks have been established taking into account the effect of centrifugal force. Using the “strain-energy-density factor” concept, the direction of crack growth of a rotating disk with an arbitrary internal crack is predicted. To provide a method of non-destructive testing in evaluating the integrity of structures, natural vibrations of cracked disk are then studied. Lastly, the influence of inertia effects due to rotating speed changes in determining the dynamic stress intensity factors is examined.For verification purposes, the simple case of a rotating disk with radial cracks is first solved. Excellent correlations between the computed results and available referenced solutions are drawn. New solutions for the circular disk with circumferential or arbitrarily-oriented cracks are also presented.     

Pit-to-crack transition under cyclic loading in 12% Cr steam turbine blade steel

Fatigue measurements were performed up to the very high cycle fatigue regime in order to investigate pit-to-crack transition in 12% Cr steam turbine blade steel. Pre-pitted and smooth specimens were tested in air and aerated 6 ppm Cl⁻ solution. S–N curves for different stress ratios were determined and a stress ratio dependent critical pit size for pit-to-crack transition with subsequent failure was found. Early crack initiation and small crack growth were observed in the process of development and by fractography using field emission scanning electron microscopy. Fatigue crack growth rates (FCGR) for cracks emanating from pits were determined. Good similarity of FCGR curves for short and long cracks was obtained by normalising the stress intensity factor ranges with the threshold values.     

Fatigue of a nickel base superalloy with bimodal grain size

Room temperature fatigue tests in the form of four point bending were performed at a frequency of 20 Hz and a load ratio R= +0·1 on electropolished Waspaloy specimens taken from a forged turbine disc. Samples, which had a partially recrystallised microstructure with a bimodal grain size, were removed from the outer rim of the disc. The S-N curve was similar to that reported for a fully recrystallised structure with a coarse grain size, from the same turbine disc. The dominant crack initiation sites were found to be inclusions with a subsequent stage I crack growth generally along slip bands. Other crack initiation sites observed included slip bands, annealing twins, and grain boundaries. Fatigue deformation occurred by the propagation of planar slip bands. Slip band cracks formed more frequently in coarse grains than expected from their volume fraction. Fluctuations in short crack growth rate were observed and were associated with microstructural features such as grain boundaries and twin boundaries, both of which acted as barriers in the early stages of crack growth. The short crack growth rate versus stress intensity range graph was similar to those from uniform fine grained Waspaloy.

MST/3413     

THE EFFECTS OF STRESS RATIO ON THE GROWTH BEHAVIOUR OF SMALL FATIGUE CRACKS IN AN ALUMINUM ALLOY 7075-T6 (WITH SPECIAL INTEREST IN STAGE I CRACK GROWTH)

Abstract— The growth behaviour of small fatigue cracks has been investigated on aluminum alloy 7075-T6 at stress ratios R of 0, −1 and −2. The effects of stress ratio are discussed with special interest in the stage I region of small crack growth. Cracks which initiated at R =−1 and −2, grew by a stage I mechanism up to a certain depth followed by stage II crack growth. The stage I to stage II transition occurred under a constant maximum stress intensity factor which was approximately consistent with the threshold effective stress intensity range, λ K _eff,th, for large cracks. At R = 0, on the other hand, stage I crack growth was not observed because of crack initiation at inclusions. Small cracks grew more rapidly than large cracks subjected to the same nominal stress intensity ranges at all the stress ratios, and they grew below the threshold stress intensity range, λ K _th, for large cracks. Stage I cracks, in particular, showed much higher growth rates than large cracks and grew even below λ K _eff,th. It is suggested that stage II crack growth rates should be characterized in terms of an effective stress intensity range, while a micromechanics approach will be necessary to evaluate stage I crack growth rates.     

燃汽轮机发电机铸造风扇开裂分析

某意大利进口的燃汽轮机发电机铸造风扇在国内安装使用2 a后发生早期开裂事故。从低倍组织、断口形貌、显微组织和力学性能等方面对风扇开裂原因进行了分析,并利用有限元软件对风扇的小环应力分布进行了计算。结果表明:该风扇开裂的性质为低应力累积损伤所致的疲劳开裂,开裂的主要原因是风扇裂纹源部位存在较多的裂纹、显微疏松孔洞等补焊缺陷以及在补焊后未能进行适当的热处理。     

Failure analysis of gas turbine transition pieces,leading to a solution for prevention

In this study the failure analysis of transition pieces of a gas turbine is investigated. Transition piece connects combustion chamber to the turbine and acts as a nozzle which leads hot gases to stationary blades of turbine. The problem of this transition piece in this gas turbine, which was common in other similar units, was cracks developing on the lower wall near the connection to the turbine. Study of gas turbine operation history, cracks apparent form microstructure analysis and fracture surface, revealed that the thermal fatigue was the main reason for the failure and also oxidation facilitated the crack propagation. In order to prevent the failure of transition pieces, it was proposed to create a row of holes, at 1 cm above the impaired region for local cooling and stopping probable cracks. Due to the implemented solutions, the failure of transition pieces and their annual repair are prevented.     

Initiation and early growth of short fatigue cracks at inclusions

In this study, the initiation and early growth behaviour of short fatigue cracks in En 7A steel with a high content of elongated MnS inclusions was investigated, by generating and evaluating data on the growth of short fatigue cracks under various stress levels and stress ratios for the six principal specimen orientations. Short cracks usually initiated at the debonded interfaces between the matrix and the inclusions. If there was no debonding, cracking sometimes occurred in the inclusions. In the early stages, short cracks propagated by a mechanism of inclusion influenced growth. Under low stress levels, usually one short crack was initiated which dominated most of the fatigue life, while under high stress levels there was multicrack interaction.

MST/3249A     

Cumulative-damage modeling of fatigue crack growth in turbine engine materials

Life predictions of turbine engine structural components utilize fracture mechanics principles to determine fatigue crack growth rates. Fatigue cracks grow under conditions of variable temperature, frequency, hold time, stress ratio and stress level. At elevated temperatures, time-dependent material behavior can play a significant role in the material behavior. Cumulative-damage models must account for all these variables as well as interaction effects. The earliest modeling involved interaction schemes and, primarily, time-independent material behavior. More recent work has focused on time-dependence and creep-fatigue interaction effects. A review of current modeling concepts is presented.     

Method for assessment of the residual life of turbine disks

A method for assessment the durability of turbine disks at the stages of damage removing and development subject to structural and technological changes is developed on the basis of the nonlinear mechanics of deformation and fracture. The proposed method includes numerical analysis of the stressdeformed condition of disks in the initial and damaged states, determination of stress and strain fields in the zones of cutting of the damaged material volume, and calculation of low-cycle fatigue and the duration of crack growth from initial to critical sizes. The theory of the developed method is the concept of a fracture process zone. The method is used for comparative evaluation of stress-deformed conditions and durability parameters for options of changing the geometry of a steam turbine disk by removing the damaged material around slot fillet of key.