Life Prediction Of Components Research Articles

Predictions of External Heat Transfer for Turbine Vanes and Blades With Secondary Flowfields

Detailed heat transfer distributions on the endwall and along the vane/blade surface are essential for component mechanical integrity and life predictions. Due to secondary flows, high gradients in heat transfer are present at the endwall and at the vane or blade surface itself where the passage vortex influences the mainstream flow. This paper documents the benchmarking of three turbulence models: 1) k-ε realizable with wall functions, 2) k-ε realizable with two layer model, and 3) the V2F model for endwall and surface heat transfer and flowfield predictions. Benchmark experimental data from a scaled-up low speed rig for both a stator and rotor geometry are used for comparisons of heat transfer and flowfield. While the k-ε realizable turbulence models give a good prediction of the secondary flow pattern, the heat transfer at the endwall and at the surface is not well predicted due to the inadequate modeling of near wall turbulence. The V2F model gives better agreement with the experiments on the endwall and vane midspan heat transfer is also well predicted, although transition occurs too far upstream on the suction surface. The results from this study represent the feasibility of CFD utilization as a predictive tool for local heat transfer distributions on a vane/blade endwall.

Transient Thermoelastic Stress Fields in a Half-Space

Computing the thermoelastic stress field of a material subjected to frictional heating is essential for component failure prevention and life prediction. However, the analysis for three-dimensional thermoelastic stress field for tribological problems is not well developed. Furthermore, the pressure distribution due to rough surface contact is irregular; hence the frictional heating can hardly be described by an analytical expression. This paper presents a novel set of frequency-domain expressions (frequency response functions) of the thermoelastic stress field of a uniformly moving three-dimensional elastic half-space subjected to arbitrary transient frictional heating, where the velocity of the half-space, its magnitude and direction, can be an arbitrary function of time. General formulas are expressed in the form of time integrals, and important expressions for constant velocities are given for the transient-instantaneous, transient-continuous, and steady-state cases. The thermoelastic stress field inside a translating half-space with constant velocities are illustrated and discussed by using the discrete convolution and fast Fourier transform method when a parabolic type or an irregularly distributed heat source is applied.

Virtual Inspection: Optimum Sample Size for POD Experiment

Engineers responsible for life prediction and inspection of aerospace components are often tasked with demonstrating the capability of an inspection. It is extremely important to establish that the quality of the particular nondestructive inspection procedure is good enough to satisfy the safety requirements specified by the Federal Aviation Administration and/or the Military Organizations. The established and accepted metric for characterizing the capability of a nondestructive evaluation procedure is the probability of detection (POD) as a function of the crack size. Typically, such a demonstration requires the fabrication of a number of specimens with controlled, quantified defects or cracks. Engineers planning a demonstration need to know how many specimens they should include in the test plan. Determining the number of test specimens to be produced has been more guesswork than science. Producing arbitrarily a large number of defect specimens is not a good option because producing such defect specimens is a very costly activity in most cases. On the other hand, if too few defect specimens are produced in an unplanned way the assessment of the POD function will not be good. Therefore, it is important to use a sample of appropriate minimum size that still achieves the quantitative requirement for demonstrating the capability of a nondestructive inspection scheme. In this article, we will develop a simulation-based algorithm that will help users of the nondestructive evaluation procedures to choose an optimal sample size and a test plan such that the “standard” requirement is satisfied.

Measurement of Creep Damage by the Natural Frequency Method

Abstract The natural frequency method (NFM), a novel approach for the measurement of creep damage, is proposed in this paper. The technique, with instrument configuration and experimental procedure, is described in detail. The application of the approach has been made on 25Cr20Ni steel, widely used for furnace tubes. The measured results of creep damage show good agreement with theoretical predictions. The critical damage value ωcr of 25Cr20Ni also has been obtained by the proposed approach. Compared with the current density method and the loading and unloading method, NFM has many advantages, such as higher accuracy, simpler apparatus, and better repeatability. On the basis of NFM, an efficient approach can be given for life prediction and extension of high temperature components.

On using available environmental data in service life estimations

In the process of service life prediction or estimation of building and construction components and materials, data of the prevailing exposure environment, (the conditions at and around a building or construction) is required. However, most environmental data is measured by and available from meteorological and air quality research communities. This data is collected at macro and meso levels, some distance from the object studied, and raises the need to transform data in order to describe the specific, local conditions adjacent to that object. To estimate levels of degradation agents in the exposure environment, especially those close to the building or construction at local and micro levels have to be considered. This paper will show and discuss useful environmental data sources, and how to transform such data by means of available distribution models. Lorsqu'il s'agit de faire des prévisions ou des estimations de durée de vie de bâtiments ainsi que de composants et matériaux de construction, il faut disposer des données sur les conditions prévalantes d'exposition et d'environnement (conditions qui règnent dans un bâtiment ou à proximité). Or, la plupart des données environnementales sont mesurées et fournies par des instituts de recherche en météorologie et qualité de l'air. Ces données sont collectées à grande et à moyenne échelle, à une certaine distance de l'objet étudié, et elles doivent donc être transformées si l'on veut décrire les conditions spécifiques locales à proximité dudit objet. Pour estimer le niveau de dégradation dans l'environnement d'exposition, notamment à proximité des bâtiments ou constructions, il faut considérer les agents au micro-niveau et au niveau local. Cet article présente et examine des sources de données environnementales utiles et montre comment transformer ces données en utilisant les modèles de distribution existants.

Transient reliability of ceramic structures

Present capabilities of the NASA Ceramic Analysis and Reliability Evaluation of Structures/Life (CARES/Life) code include probabilistic life prediction of ceramic components subjected to fast fracture, slow crack growth (SCG) (stress corrosion), and cyclic fatigue failure modes. Currently, this code has the capability to compute the time‐dependent reliability of ceramic structures subjected to simple time‐dependent loading. For example, in SCG type failure conditions CARES/Life can handle the cases of sustained and linearly increasing time‐dependent loads, whereas for cyclic fatigue applications, it can account for various types of repetitive constant amplitude loads. In real applications applied loads are rarely that simple, but rather vary with time in more complex ways such as engine start up and shut down and dynamic and vibrational loads. In addition, when a given component is subjected to transient environmental and/or thermal conditions, the material properties also vary with time. The objective of this paper is to demonstrate a methodology capable of predicting the time‐dependent reliability of components subjected to transient thermomechanical loads that take into account the change in material response with time. In this article, the dominant delayed failure mechanism is assumed to be SCG. This capability has been added to the NASA CARES/Life code, which has also been modified to have the ability of interfacing with commercially available finite element analysis codes executed for transient load histories. An example involving a ceramic exhaust valve subjected to combustion cycle loads is presented to demonstrate the viability of this methodology and the CARES/Life program.

Fatigue life estimation of cast components

An engineering method based on fracture mechanics is suggested for life predictions of components of nodular cast iron. The method should be applicable to any structure containing defects which can cause crack initiation. Component testing was performed and the observed results were compared with model predictions. Fatigue crack growth data were obtained by testing on CT specimens even for load ratios R < 0. By accounting for crack closure the R-effect on the growth rates can also be effectively eliminated for this type of material.

Life Prediction and Monitoring of Nuclear Power Plant Components for Service-Related Degradation

This paper describes industry programs to manage structural degradation and to justify continued operation of nuclear components when unexpected degradation has been encountered due to design materials and/or operational problems. Other issues have been related to operation of components beyond their original design life in cases where there is no evidence of fatigue crack initiation or other forms of structural degradation. Data from plant operating experience have been applied in combination with inservice inspections and degradation management programs to ensure that the degradation mechanisms do not adversely impact plant safety. Probabilistic fracture mechanics calculations are presented to demonstrate how component failure probabilities can be managed through augmented inservice inspection programs.

Failure Mechanisms of High Temperature Components in Power Plants

The principal mechanisms of failure of high temperature components include creep, fatigue, creep-fatigue, and thermal fatigue. In heavy section components, although cracks may initiate and grow by these mechanisms, ultimate failure may occur at low temperatures during startup-shutdown transients. Hence, fracture toughness is also a key consideration. Considerable advances have been made both with respect to crack initiation and crack growth by the above mechanisms. Applying laboratory data to predict component life has often been thwarted by inability to simulate actual stresses, strain cycles, section size effects, environmental effects, and long term degradation effects. This paper will provide a broad perspective on the failure mechanisms and life prediction methods and their significance in the context utility deregulation. [S0094-4289(00)00103-1]

Diagnosing Engineering Problems with Neutrons

In the past, many unexpected failures of components were due to poor quality control or a failure to calculate—or to miscalculate—the stresses or fatigue stresses a component would experience in service. Today, improved manufacturing, fracture mechanics, and computational finite element methods combine to provide a solid framework for reducing safety factors, enabling leaner design. In this context, residual stress—that is, stress that equilibrates within the structure and is always present at some level due to manufacturing—presents a real problem. It is difficult to predict and as hard to measure. If unaccounted for in design, these stresses can superimpose upon in-service stresses to result in unexpected failures.Neutron diffraction is one of the few methods able to provide maps of residual stress distributions deep within crystalline materials and engineering components. Neutron strain scanning, as the technique is called, is becoming an increasingly important tool for the materials scientist and engineer alike. Point, line-scan, area-scan, and full three-dimensional (3D) maps are being used to design new materials, optimize engineering processes, validate finite element modeis, predict component life, and diagnose engineering failures.

Quantification of structural loading during off-road cycling.

To provide data for fatigue life prediction and testing of structural components in off-road bicycles, the objective of the research described herein was to quantify the loads input to an off-road bicycle as a result of surface-induced loads. A fully instrumented test bicycle was equipped with dynamometers at the pedals, handlebars, and hubs to measure all in-plane structural loads acting through points of contact between the bicycle and both the rider and the ground. A portable data acquisition system carried by the standing rider allowed, for the first time, this loading information to be collected during extended off-road testing. In all, seven experienced riders rode a downhill trial test section with the test bicycle in both front-suspension and full-suspension configurations. The load histories were used quantitatively to describe the load components through the computation of means, standard deviations, amplitude probability density functions, and power spectral density functions. For the standing position, the coefficients of variation for the load components normal to the ground were greater than 1.2 for handlebar forces and 0.3 and 0.5-0.6 for the pedal and hub forces, respectively. Thus, the relative contribution of the dynamic loading was much greater than the static loading at the handlebars but less so at the pedals and hubs. As indicated by the rainflow count, high amplitude loading was developed approaching 3 and 5 times the weight of the test subjects at the front and rear wheels, respectively. The power spectral densities showed that energy was concentrated in the band 0-50 Hz. Through stress computations and knowledge of material properties, the data can be used analytically to predict the fatigue life of important structural components such as those for steering. The data can also be used to develop a fatigue testing protocol for verifying analytical predictions of fatigue life.

On the estimation of the fatigue cycle distribution from spectral density data

This paper deals with the fatigue life prediction of components and structures subjected to random fatigue, i.e. to cyclic loading whose amplitude varies in an essentially random manner. In particular, this study concentrates on the general problem of directly relating fatigue cycle distribution to the power spectral density (PSD) by means of closed-form expressions that avoid expensive digital simulations of the stress process. At present, all the methods proposed to achieve this objective are based on the use of a single parameter of the PSD. In this work, by numerical simulations and theoretical considerations, it is shown that the statistical distribution of fatigue cycles depends on four parameters of the PSD and the methods proposed in the literature provide reliable results only in particular cases. The proposed approach lays the foundation for a more accurate evaluation of the fatigue cycle distribution. As a first result, a closed-form solution for the case of broad-band processes with narrow-band time derivative has been obtained in this paper.

Modelling plasticity in finite bodies containing stress concentrations by a distributed dislocation method

An efficient method for the analysis of limited plasticity at stress raising features such as notches and holes in finite bodies has been developed. A network of stationary dislocations is used to simulate the plasticity and simple constant displacement boundary elements form the borders of the geometries. The notch or hole itself is implicitly included in the formulation by using specialized kernels for these features in an infinite plane, thereby improving the numerical efficiency. The cyclic plastic behaviour of an edge-notch in an infinite plane and a finite rectangular plate are analysed under elastic-perfectly plastic, plane strain conditions. Examples of the resulting stress state after stress redistribution and plastic shakedown are displayed which aid in the reliable prediction of component life.

Cumulative Fatigue Damage Mechanisms and Quantifying Parameters: A Literature Review

Abstract Cumulative fatigue damage analysis plays an important role in fatigue life prediction of components and structures which are subjected to field loading histories. Understanding of cumulative damage mechanisms is essential since it provides the necessary physical bases for modeling the cumulative damage process. A damage measure that can reflect and quantify the real damage state the material undergoes is also a key issue for successful modeling of cumulative fatigue damage. This review paper provides a comprehensive overview of research activities highlighting the recent findings and progress on phenomenological observations and mechanisms, as well as quantification measures of cumulative fatigue damage. Depending on the definition of failure or the characteristics of failure experienced in a material, the effectiveness of a damage parameter could vary from case to case. Many damage parameters have been proposed and many of them are in use. Those to be reviewed are sorted into categories of metallurgical parameters, surface crack quantifications, mechanical measures, and physical parameters. Early studies on cumulative damage mechanisms and quantifying measures are reviewed only briefly, since they have been covered in the existing literature.

INTERACTIONS BETWEEN TIME AND CYCLIC DEFORMATION PROCESSES IN A NIMONIC ALLOY

Interactive creep–fatigue behaviour of a nickel‐base superalloy (IN 597) has been examined at 850 °C under various strain‐limited, cyclic torsional loading conditions. In one test, forward creep deformation was reversed by creep under equal magnitude stress levels and strain limits. In other tests, forward creep strain was reversed by fast monotonic plasticity with and without a subsequent period of relaxation. These cycles were repeated within each test until fracture. This paper examines empirically the influence of a number of test variables upon cyclic creep curves, and demonstrates the usefulness of predictions based upon continuous low cycle fatigue and simple creep data when used in conjunction with a mechanical equation of state. A cyclic equilibrium condition was not achieved from these tests. Instead, a progressive softening occurred giving reductions to the amount of creep strain, creep time interval and reversed peak stress with each new cycle. Such reductions are expressed from derived formulae that embrace the range of inelastic strain, cycle number, creep dwell stress, reversed peak stress, and times expended in creep and relaxation.Observations made on accumulated creep strain reveal the contribution to a creep–fatigue fracture from cyclic creep. This has led to a modified form of the linear damage rule which can provide conservative life predictions for components operating in service under similar cyclic conditions.

Cumulative fatigue damage and life prediction theories: a survey of the state of the art for homogeneous materials

Abstract Fatigue damage increases with applied load cycles in a cumulative manner. Cumulative fatigue damage analysis plays a key role in life prediction of components and structures subjected to field load histories. Since the introduction of damage accumulation concept by Palmgren about 70 years ago and ‘linear damage rule’ by Miner about 50 years ago, the treatment of cumulative fatigue damage has received increasingly more attention. As a result, many damage models have been developed. Even though early theories on cumulative fatigue damage have been reviewed by several researchers, no comprehensive report has appeared recently to review the considerable efforts made since the late 1970s. This article provides a comprehensive review of cumulative fatigue damage theories for metals and their alloys, emphasizing the approaches developed between the early 1970s to the early 1990s. These theories are grouped into six categories: linear damage rules; nonlinear damage curve and two-stage linearization approaches; life curve modification methods; approaches based on crack growth concepts; continuum damage mechanics models; and energy-based theories.

Environmental interactions with fatigue crack growth in alpha/beta titanium alloys

The impact of different environments on the growth of short through cracks (0.25mm<a<3mm) in thin section DEN test pieces (t=2mm) is evaluated for the titanium alloy IMI318 (Ti–6Al–4V) at atmosphere. Both oxidising and reducing atmospheres have been considered in the form of air, chlorine and hydrogen gas at various levels of partial pressure. Data are presented for crack growth under cyclic and dwell waveforms and for R values of 0.01 and 0.625. It is demonstrated that the measured rates are extremely sensitive to the gaseous species. This sensitivity is reflected in fracture surface features and quasi-cleavage facetting in particular. The implications for engineering component life predictions are discussed.

Das Nennspannungskonzept: Die Ermüdungslebensdauer vorhersagen

Fatigue life prediction for components subjected to random stress is often used. A collection of test data in a large data base was used as the basis for investigation of validity This paper deals with the data base, the known methods of fatigue life prediction and the criteria of validity.

Some characteristics of early stages of stress corrosion cracking

Environment-induced crack growth generally progresses through several stages prior to component failure. Crack initiation, short crack growth, and stage I growth are early stages in crack development that are summarized in this paper. The implications of these stages on component reliability derive from the extended time that the crack exists in the early stages because crack velocity is low. The duration of the early stages provides a greater opportunity for corrective action if the short cracks can be detected. Several important factors about the value of understanding short crack behavior include: (1) component life prediction requires a knowledge of the total life cycle of the crack including the early stages, (2) greater reliability is possible if the transition between short and long crack behavior is known since component life after this transition is short, and (3) remedial actions are often more effective for short than long cracks.

Machine Health Monitoring and Life Management Using Finite-Element-Based Neural Networks

This paper demonstrates a novel approach to condition-based health monitoring for rotating machinery using recent advances in neural network technology and rotordynamic, finite-element modeling. A desktop rotor demonstration rig was used as a proof of concept tool. The approach integrates machinery sensor measurements with detailed, rotordynamic, finite-element models through a neural network that is specifically trained to respond to the machine being monitored. The advantage of this approach over current methods lies in the use of an advanced neural network. The neural network is trained to contain the knowledge of a detailed finite-element model whose results are integrated with system measurements to produce accurate machine fault diagnostics and component stress predictions. This technique takes advantage of recent advances in neural network technology that enable real-time machinery diagnostics and component stress prediction to be performed on a PC with the accuracy of finite-element analysis. The availability of the real-time, finite-element-based knowledge on rotating elements allows for real-time component life prediction as well as accurate and fast fault diagnosis.