In-service Operation Research Articles

Multiple Fiber Fault Location With Low-Frequency Sub-Carrier Tone Sweep

We present a monitoring technique that can be directly integrated in the sub-carrier multiplexing (SCM) transceiver for PtP and wavelength division multiplexing and sub-carrier multiplexing passive optical networks applications. The technique is based on the detection of the backscattered signal from a baseband tone (100 Hz to 100 kHz) and interpretation with the least absolute shrinkage and selection operator (LASSO) operator, a signal processing technique which identifies the fault positions based on the mathematical model of the received signal. The proposed single-ended fiber probing method enables the location of multiple faults with no a-priori knowledge of the fiber, and offers low-cost in-service operation with negligible impact on data transmission, few meters spatial resolution, and limited dynamic range. Due to all its advantages, but also considering its reach limitation, the SincLASSO method features as an ideal monitoring candidate for short reach sub-carrier multiplexed optical fiber networks, such as the ones intended for analog mobile Fronthaul based on fiber and fiber-extended copper lines.

Lifetime and failure modes of plasma sprayed thermal barrier coatings in thermal gradient rig tests with simultaneous CMAS injection

Degradation of thermal barrier coatings (TBCs) in gas-turbine engines due to calcium–magnesium–aluminosilicate (CMAS) glassy deposits from various sources such as sand, volcanic ash, fly ash, or variable quality fuels has been a tenacious issue during the recent years. This follows from the fact that engines are required to operate under increasingly harsh conditions in all kind of gas turbine applications following the demands for higher efficiency and operational flexibility. While the understanding of the mechanism of CMAS induced degradation of TBCs as well as approaches for mitigation of CMAS attack by means of advanced TBC compositions have grown remarkably, most of the reported results have been obtained from lab testing at isothermal conditions or from evaluation of ex-service components, either. The isothermal tests are not reproducing important thermomechanical effects from service conditions, and it may be hard to figure out the thermal history of the ex-service examples.In this study a burner rig facility has been used for the evaluation of TBC performance, where a thermal gradient is applied across the TBC at cyclic load and CMAS is continuously injected to the combustion environment to simulate the conditions closer to actual service in an engine while allowing control of important degradation limiting parameters. Tests have been performed on the state of the art material YSZ while loading parameters as high temperature dwell time and CMAS deposition rate have been varied. Tests have been evaluated in terms of chemical degradation, failure mode and TBC spallation lifetime by means of SEM, EDS and XRD. Test conditions and relevance to in-service operating conditions are discussed. Major impacts on thermal gradient cycling lifetime and similarities and discrepancies related to reported failure mechanism are reviewed.

Reducing risks in the investigation, design and construction of large concrete dams

An overview of the GeoSafe 2016 Symposium topic is provided using the example of large concrete dams for purposes of illustration. It is essential that the risks associated with large dams be evaluated rigorously and managed proactively at all stages of their lives so that the risk of failure remains As Low As Reasonably Practicable (ALARP). Rock engineering features of large concrete dams that require particular attention, assessment and monitoring during the investigation, design, construction, initial filling, in-service operation, and subsequent repair and upgrade stages of the lives of concrete dams are identified and illustrated by examples from recorded experiences. A number of major concrete dam failures, including that of the St. Francis dam, California, U.S.A., in 1928, have led to significant developments in rock mechanics and rock engineering knowledge and techniques, as well as in dam design and review processes. More recent advances include a range of analytical, numerical modelling, probabilistic, reliability, failure mode and risk assessment approaches.

Aeroelastic measurements and simulations of a small wind turbine operating in the built environment

Small wind turbines, when compared to large commercial scale wind turbines, often lag behind with respect to research investment, technological development, and experimental verification of design standards. In this study we assess the simplified load equations outlined in IEC 61400.2-2013 for use in determining fatigue loading of small wind turbine blades. We compare these calculated loads to fatigue damage cycles from both measured in-service operation, and aeroelastic modelling of a small 5 kW Aerogenesis wind turbine. Damage cycle ranges and corresponding stress ratios show good agreement when comparing both aeroelastic simulations and operational measurements. Loads calculated from simplified load equations were shown to significantly overpredict load ranges while underpredicting the occurrence of damage cycles per minute of operation by 89%. Due to the difficulty in measuring and acquiring operational loading, we recommend the use of aeroelastic modelling as a method of mitigating the over-conservative simplified load equation for fatigue loading.

The effect of in-service steam temperature transients on the damage behavior of a steam turbine rotor

This study was aimed at identifying the influence of the temperature transient on the creep–fatigue behavior of a steam turbine rotor. Especially, the steam temperature fluctuations during the steady state operation phase could lead the stress to the oscillation, which also contributes to the fatigue damage. Toward this end, in-service operating data during startup, shutdown and steady state operation phases was chosen to investigate the creep–fatigue damage of a 1000MW steam turbine rotor. A viscoplastic constitutive model with damage was presented to describe creep–fatigue deformation behavior. A significant growth in creep damage was found under the influence of the temperature fluctuation compared to the creep damage under the stable temperature condition. In addition, steam temperature fluctuation induced the larger thermal gradient and temperature difference at the location where is closer to the rotor surface. The correlation of the stress range due to these temperature fluctuations has been established by carrying out the transient thermo-mechanical FE analysis. Frequent thermal fluctuations during the steady state operation phase were identified as one of the most influential factors for the creep–fatigue damage of the steam turbine rotor.

Simulation of C-Component Dislocation Loops by Ions and Protons Irradiation. A Tool to Understand the Breakaway Growth of Recrystallized Zr-Based Alloys

During normal in-service operation, the PWR fuel assemblies exhibit a macroscopic elongation. This dimensional change is the result of three different phenomena: thermal creep, irradiation-creep and irradiation growth which occurs in the absence of any applied stress. This last phenomenon, referred as stress-free growth, is related to irradiation-induced microstructural changes taking place in the hexagonal close packed grains and corresponding to an elongation along the basal plane and a shortening along the c-axis, the volume remaining constant (Fig 1). Under neutron irradiation, recrystallized Zr-based alloys undergo stress-free growth which accelerates for high irradiation doses (Fig 2). This acceleration is correlated to specific c-component vacancy dislocation loops.

Modal testing for model validation of structures with discrete nonlinearities.

Model validation using data from modal tests is now widely practiced in many industries for advanced structural dynamic design analysis, especially where structural integrity is a primary requirement. These industries tend to demand highly efficient designs for their critical structures which, as a result, are increasingly operating in regimes where traditional linearity assumptions are no longer adequate. In particular, many modern structures are found to contain localized areas, often around joints or boundaries, where the actual mechanical behaviour is far from linear. Such structures need to have appropriate representation of these nonlinear features incorporated into the otherwise largely linear models that are used for design and operation. This paper proposes an approach to this task which is an extension of existing linear techniques, especially in the testing phase, involving only just as much nonlinear analysis as is necessary to construct a model which is good enough, or ‘valid’: i.e. capable of predicting the nonlinear response behaviour of the structure under all in-service operating and test conditions with a prescribed accuracy. A short-list of methods described in the recent literature categorized using our framework is given, which identifies those areas in which further development is most urgently required.

Comparative analysis of SOFC–GT freight locomotive fueled by natural gas and diesel with onboard reformation

Due to increasing awareness of the deleterious environmental and health effects of diesel combustion emissions, major regulatory action and policy measures are focused on reducing emissions from diesel engines. Freight operations, including rail-based freight transportation, have received special attention as an industry where major change can be affected, especially in neighborhoods located near operations centers. A FORTRAN-based dynamic simulation model of an SOFC–GT (Solid Oxide Fuel Cell–Gas Turbine) system from a prior feasibility study has been adapted to analyze system operation along a representative but demanding route in southern California. In previous simulations with the model, the basic operational feasibility of the system has been demonstrated as well as the in-service operation for pre-reformed fuels. In the current study, the analysis is extended to include reformation of two fuels (diesel and natural gas) onboard the locomotive and analyses of system efficiency, fuel consumption, CO2 emission, and NOx emission that can be attained through careful thermal integration of the reformer unit. Route-averaged fuel-to-wheels system efficiencies of 60% and 52% are predicted for natural gas and diesel fuel, respectively. Additionally, SOFC–GT operation could provide (1) a reduction approaching 98% in NOx for both fuels; (2) a 54% savings in CO2 for operation on natural gas; and (3) a 30% CO2 reduction for operation on diesel fuel compared to state-of-the-art locomotive technology. These gains may be offset by design challenges, especially for the diesel case, due to the requirement for large volumes of water to support the reformation process even for medium-length freight hauling trips.

Sensitivity of Freight and Passenger Rail Fuel Efficiency to Infrastructure, Equipment, and Operating Factors

After labor expenses, the cost of fuel is the largest operating budget expense item for freight and passenger rail operations in the United States. Because fuel is such a costly component of operations, the railroad industry is constantly researching and testing new methods either to reduce the volume of fuel consumed or to switch to less-expensive fuels and sources of energy. An understanding of the factors that affect fuel consumption and their interactions is valuable for analyzing the feasibility of a given technology. Such knowledge allows for more intelligent extrapolation of simulation and laboratory results across a range of routes and in-service operating conditions. This research investigates the relative effects of infrastructure, equipment, and operating parameters on fuel efficiency for freight and passenger railroads on a mixed-use corridor. Partial factorial experiments investigate the effects of multiple factors on freight and passenger train fuel efficiency. Rail simulation software is used to run trial cases of single-track lines with heterogeneous traffic and to calculate energy-efficiency metrics. Results from the simulations are used to create two multivariate regression models for freight and passenger rail fuel efficiency. A sensitivity analysis identifies the relative effects of these factors on freight and passenger train fuel efficiency. By understanding the relative influence of various parameters on fuel efficiency, practitioners can focus data collection, modeling, and other fuel-saving efforts on the most significant factors.

Investigating in-service failures of water pipes from a multiaxial notch fatigue point of view: A conceptual study

Many mechanisms and processes can cause deterioration and ultimately failure of water distribution pipes during in-service operation, amongst these is damage caused by metal fatigue. This paper summarises an attempt at formalising a novel methodology suitable for estimating the number of years taken for a through thickness fatigue crack to form in this complex scenario. The devised method is based on the so-called modified Wöhler curve method and can be applied to estimate fatigue damage of water pipes independently from the degree of multiaxiality and non-proportionality of the load history. The computational approach of the proposed fatigue life estimation technique makes full use of an incremental procedure: fatigue damage is evaluated year by year by assuming that all variable involved in the process can change over time. The detrimental effect of corrosion pits is directly accounted for by treating them as conventional notches whose size increases with time. Finally, by taking as reference information the number of years for a blowout hole to form, the proposed approach is used to show how the lifetime of grey cast iron pipes can be remarkably shortened by fatigue.

Fretting fatigue crack analysis of the turbine blade from nuclear power plant

The root cause of the failure of a cracked blade in the low pressure turbine of a nuclear power plant was investigated. The low clearance between the blade and the disk caused a sliding motion, which led to fretting wear during in-service operations. The crack in the blade was determined to have initiated under fretting wear and propagated with high cycle fatigue by means of analyzing the fracture surface of the failed blade. Micro oxide particles were detected from the Electron Dispersive Spectroscopy data analysis at the fatigue initiation position as typical evidence of fretting wear. The stress distribution on the blade in normal service was calculated with a non-linear elastic–plastic finite element method. The analysis results show that the crack initiation location was different with the location of the highest Mises stress. The fretting fatigue crack initiated under a combination of fretting wear and relatively high Mises stress. Designed fretting fatigue experiments were performed with the results that the fatigue life was greatly reduced by the interaction of the loading history and local fretting wear.

Survival Life Analysis of the Cutting Tools During Turning Titanium Metal Matrix Composites (Ti-MMCs)

Metal matrix composites (MMCs), as a new generation of materials; have proven to be viable materials in various industrial fields such as biomedical and aerospace. In order to achieve a valuable modification in various properties of materials, metallic matrices are reinforced with additional phases based on the chemical and/or physical properties required in the in-service operating conditions. The presence of the reinforcements in MMCs improves the physical, mechanical and thermal properties of the composite; however it induces significant issues in the domain of machining, such as high tool wear and inferior surface finish. The interaction between the tool and abrasive hard reinforcing particles induces complex deformation behaviour in the MMC structure. Sever tool wear is technically the most important drawback of machining MMCs.In this study a statistical model is developed to estimate the mean residual life (MRL) of the cutting tool during machining Ti-MMCs. Initial wear, steady wear and rapid wear regions in the tool wear curve are regarded as the different states in the statistical model. Hence, the valuable information regarding the estimated total time spent in each state, called the sojourn time, and the transition times between the states are obtained from the model.In this paper the standard cutting conditions, based on the recommendation of the tool supplier, are adopted. Based on a Weibull model, the reliability and hazard functions are obtained and are utilized in order to calculate the MRL and the sojourn times.

Development and use of a new burner rig facility to mimic service loading conditions of Ni-based single crystal superalloys

Performing representative experiments of in-service operating conditions of Ni-based superalloys used as high pressure turbine blades in aeroengines is a challenging issue due to the complex environmental, mechanical and thermal solicitations encountered by those components. A new burner rig test facility called MAATRE (French acronym for Mechanics and Aerothermics of Cooled Turbine Blades) has been developed at ENSMA – Pprime Institute to mimic as close as possible those operating conditions. This new test bench has been used to perform complex non-isothermal creep tests representative of thermomechanical solicitations seen by some sections of HP turbine blades during engine certification procedure.

Heat Treatment Development for a Rapidly Solidified Heat Resistant Cast Al-Si Alloy

Existing heat treatment standards do not properly define tempers for thin-walled castings that solidified with high solidification rates. Recently emerged casting processes such as vacuum high pressure die casting should not require long solution treatment times due to the fine microstructures arising from rapid solidification rates. The heat treatment studies involving rapidly solidified samples with secondary dendrite arm spacing between 10 and 35 μm were conducted for solution times between 30 min and 9 h and temperatures of 510 and 525 °C and for various aging parameters. The metallurgical analysis revealed that an increase in microstructure refinement could enable a reduction of solution time up to 88%. Solution treatment resulted in the dissolution of Al2Cu and Al5Mg8Si6Cu2, while Fe- and TiZrV-based phases remained partially in the microstructure. The highest strength of approximately 351 ± 9.7 and 309 ± 3.4 MPa for the UTS and YS, respectively, was achieved for a 2-step solution treatment at 510 and 525 °C in the T6 peak aging conditions, i.e., 150 °C for 100 h. The T6 temper did not yield dimensionally stable microstructure since exceeding 250 °C during in-service operation could result in phase transformation corresponding to the over-aging reaction. The microstructure refinement had a statistically stronger effect on the alloy strength than the increase in solutionizing time. Additionally, thermal analysis and dilatometer results were presented to assess the dissolution of phases during solution treatment, aging kinetics as well as dimensional stability.

Modelling the manufacturing history, through life creep-fatigue damage and limiting defect sizes of a pipework joint using finite element based methods

The work reported in this paper describes a simulation of the manufacture, through-life operation and limiting defect size assessment of a pipework joint in a nuclear powerplant boiler. The objective of this work is to understand the critical factors that influence the integrity of the joint in-service and support accurate predictions of service life. This work differs from typical structural integrity assessments in that advanced modelling techniques have been used throughout the assessment process and include detailed simulations of the manufacturing process, a simulation of the entire in-service operating history including predictions of creep-fatigue damage and cracked body analysis to determine limiting defect sizes.Residual stresses resulting from the manufacturing process can be a key driver for creep and creep-fatigue damage. The calculation of creep-fatigue damage for assessment purposes is typically undertaken within the framework of an appropriate assessment code, such as EDF Energy's R5. The standard assessment approach usually requires calculation of stresses using elastic finite element analysis followed by hand calculations to calculate the damage.A combination of explicit and implicit finite element methods are employed to simulate a range of manufacturing processes which influence the in-service structural integrity of a branched pipework joint. Where available, test data have been compared to the results to assess the validity of the simulation. The simulation results then feed into a finite element based structural integrity assessment. The methods follow the principles outlined in the EDF Energy R5 assessment code but use the inelastic strains calculated directly from analysis. The methods are based around the general purpose finite element code Abaqus.The residual stresses generated during manufacture may adversely affect the critical defect sizes for the pipework joint. However, the complex geometry and loading complicate the assessment of the cracked body. Therefore, a finite element analysis representing the cracked body has been carried out on the pipework joint to evaluate the J-integrals at the locations of interest and hence calculate the critical defect sizes. The analysis considers the residual stresses determined from the finite element analysis of the manufacturing processes.

A hybrid approach for nondestructive assessment and design optimisation and testing of in-service machinery

Complex rotating machinery requires regular condition monitoring inspections to assess their running conditions and their structural integrity to prevent catastrophic failures. Machine failures can be divided into two categories. First is the wear and tear during operation, they range from bearing defects, gear damage, misalignment, imbalance or mechanical looseness, for which simple condition-based maintenance techniques can easily detect the root cause and trigger remedial action process. The second factor in machine failure is caused by the inherent design faults that usually happened due to many reasons such as improper installation, poor servicing, bad workmanship and structural dynamics design deficiency. In fact, individual machines components are generally dynamically well designed and rigorously tested. However, when these machines are assembled on sight and linked together, their dynamic characteristics will change causing unexpected behaviour of the system. Since nondestructive evaluation provides an excellent alternative to the classical monitoring and proved attractive due to the possibility of performing reliable assessments of all types of machinery, the novel dynamic design verification procedure – based on the combination of in-service operation deflection shape measurement, experimental modal analysis and iterative inverse finite element analysis – proposed here allows quick identification of structural weakness, and helps to provide and verify the solutions.

Interaction of conductor with polymeric materials (XLPE/EPR) at partial discharges

A novel investigation into the behavior of the interface between an insulating material and a conductor subjected to sinusoidal stimulus is presented. Both copper and aluminum wires are used in the windings of electrical machines and also as power cable conductors. During in-service operation, partial discharges (PD) may be initiated in the insulation systems. The paper presents experimental results showing the interaction of different kinds of conductors and dielectric materials on the mechanism of partial discharges in the insulation system. The dedicated test setup is designed to perform experiments with various electrodes, with special focus on copper. Two kinds of specially prepared insulation samples were tested, ethylene propylene rubber (EPR), cross-linked polyethylene (XLPE). These insulating materials were subjected to a sinusoidal stimulus at predefined time intervals and were exposed to PD action. The paper comprises the results of PD measurements and microstructural observations performed by means of scanning electron microscope (SEM) coupled with energy dispersive analysis (EDS). The morphological and elemental analysis reveals the interaction of conductor on polymeric material degradation. Two zones were distinguished, migration and erosion. The migration of atoms of electrode material into the dielectric substrate due to PD activity was observed.

Quantification of Reduction of Nitrogen Oxides by Nitrate Accumulation on Titanium Dioxide Photocatalytic Concrete Pavement

Field trials of photocatalytic pavements were recently initiated and are being considered by many states (e.g., Virginia, Texas, New York, and Missouri). Results from this study are from the country's first air-purifying asphalt and concrete photocatalytic pavements on December 20, 2010. The test area was a pavement site located on the Louisiana State University campus in Baton Rouge. The objective of this study was validation of photocatalytic degradation of nitrogen oxides (NOx) at the test site by measuring nitrate salts (NO3) deposited on the pavement surface. With quantification of the nitrate levels produced in the field attributable to photocatalytic activity, measurements were correlated to laboratory test results of NOx reduction efficiency. A field sampling procedure of NO3 deposited on the pavement surface is presented. On the basis of the results of the experimental program, the proposed method to quantify photocatalytic efficiency through nitrate measurements was successful. There was definite evidence that photocatalytic degradation of NOx was taking place in the treated section. In addition, the photocatalytic process was active during the first 4 days followed by a slight decrease in degradation of NOx. Full regeneration of photo catalytic activity took place through a self-cleaning process during a rain event. Six months of traffic and in-service operating conditions had negligible effects on the efficiency of the photocatalytic coating. In addition, there was good agreement between nitric oxide removal efficiency measured in the field after one day of nitrate accumulation and in the laboratory at the same relative humidity.

Influence of electric current on the mechanical characteristics of rail steel

We present the results of investigations into the influence of high-density electric current pulses on the mechanical characteristics of rail steels in the initial state and after a long in-service operating time. The effect of in-service loading conditions and modes of exposure to electric current pulses on the steel mechanical properties and their anisotropy is shown.

Structural Health Monitoring, In-service Experience, Benefit and Way Ahead

The structural health monitoring (SHM) is an important requirement to handle military aircraft safety. In contrast with civil aircraft, the missions, configurations and environments are changing frequently and therefore also the load spectra, which lead to a various life consumption. To ensure the structural integrity of the airframe and structural systems by SHM systems the following four functions are essential: Event and fatigue monitoring, including remaining life assessment and damage detection monitoring. This is the basis for an economic and safety in-service operation of a flying weapon system. This article shows the way from a simple monitoring system to a state-of-the-art support system based on examples from the German Air Force (GAF). From a military aircraft manufacturer’s point of view, the pro and contra of these systems based on in-service experience of various aircraft types are listed. Referring to this experience the benefit of the modernization of SHM-System will be presented and the requirements on a modern SHM-System especially for the ground support system and the logistic and in addition for new monitoring areas will be worked out. The approach for hot spot monitoring for damage detection is explained. A special view will be given on the possible integration of modern damage detection methods in existing and new SHM-Systems. Furthermore the combination of damage detection system and existing usage monitoring systems opens the way for enhanced monitoring systems.