Catastrophic Component Failure Research Articles

On material-agnostic fatigue life prediction using Buckingham Pi Theorem

A robust, quantitative analysis is needed to prevent frequent, catastrophic failure of components in powerplants and aircraft, to cite but two examples, due to cyclic loading that could lead to fatigue failure. The model presented in this research will be useful in the reliable design and safe operation of components and equipment. A fatigue life (the dependent variable) prediction model, which employs both stress-based, and stress–strain based criteria is presented that uses the Buckingham Pi Theorem with the experimental parameters influencing fatigue behaviour taken as the independent variables. The validation of the model is done using reliable, frequently used uniaxial fatigue data, culled from the NIMS (National Institute for Materials Science, Tsukuba, Japan) fatigue database on important industrial alloys that experience fatigue loading in service. (No reliable data on such a scale were available to test the validity of this model under multi-axial fatigue loading conditions.) It is demonstrated that the model predicts the experimental results accurately. The accuracy of prediction is examined for every case and compared with that of some other existing methods. Moreover, the potential applicability of this analysis in areas not yet validated is examined.

Modeling and experimental validation of a Wf/W-fabrication by chemical vapor deposition and infiltration

Tungsten (W) has a unique combination of excellent thermal properties, low sputter yield, low hydrogen retention, and acceptable activation. Therefore, W is presently the main candidate for the first wall material in future fusion devices. However, its intrinsic brittleness and its further embrittlement during operation bears the risk of a sudden and catastrophic component failure. As a countermeasure, tungsten fiber-reinforced tungsten (W f /W) with extrinsic toughening is being developed. A possible synthesis route is chemical vapor deposition (CVD) using heated W fabrics as substrate. The challenge is that the growing CVD-W can isolate domains from precursor access leading to strength-reducing pores. To deepen the process understanding and to optimize the CVD parameters, models were developed with COMSOL Multiphysics and validated experimentally. W deposition rate equations as function of the temperature and the partial pressures of the precursors H 2 and W F 6 were experimentally validated in previous work. In the present article, these equations are applied to obtain partial pressures within the CVD reactor. The results are taken as input for transient simulations in the microscale, in which W coatings, growing onto multiple adjacent W fibers, were simulated via mesh deformation and remeshing. The surface-to-surface contact of the W coatings and the corresponding potential pore formation were simulated by implementing sophisticated deposition rate stop conditions. Within the measuring uncertainties of ≃ ± 1%, the models are validated successfully by experimental comparison regarding the deposition rate, pore structure, and relative densities ranging from 0.6 to 0.9. • CVD/CVI of W on multiple adjacent fibers was modeled using COMSOL Multiphysics. • Coating growth simulated by transient FEM mesh deformation and remeshing. • Gas-to-surface and surface-to-surface interactions were considered. • Partial pressure inputs obtained from a stationary macroscaled reactor simulation. • Experimental validation via coating thickness, pore structure and relative density.

Improving the W Coating Uniformity by a COMSOL Model-Based CVD Parameter Study for Denser Wf/W Composites

Tungsten (W) has the unique combination of excellent thermal properties, low sputter yield, low hydrogen retention, and acceptable activation. Therefore, W is presently the main candidate for the first wall and armor material for future fusion devices. However, its intrinsic brittleness and its embrittlement during operation bears the risk of a sudden and catastrophic component failure. As a countermeasure, tungsten fiber-reinforced tungsten (Wf/W) composites exhibiting extrinsic toughening are being developed. A possible Wf/W production route is chemical vapor deposition (CVD) by reducing WF6 with H2 on heated W fabrics. The challenge here is that the growing CVD-W can seal gaseous domains leading to strength reducing pores. In previous work, CVD models for Wf/W synthesis were developed with COMSOL Multiphysics and validated experimentally. In the present article, these models were applied to conduct a parameter study to optimize the coating uniformity, the relative density, the WF6 demand, and the process time. A low temperature and a low total pressure increase the process time, but in return lead to very uniform W layers at the micro and macro scales and thus to an optimized relative density of the Wf/W composite. High H2 and low WF6 gas flow rates lead to a slightly shorter process time and an improved coating uniformity as long as WF6 is not depleted, which can be avoided by applying the presented reactor model.

Evaluating the Effect of Vane Trailing Edge Flow on Turbine Rim Sealing

Abstract Modern gas turbine development continues to move toward increased overall efficiency, driven in part by higher firing temperatures that point to a need for more cooling air to prevent catastrophic component failure. However, using additional cooling flow bled from the upstream compressor causes a corresponding detriment to overall efficiency. A primary candidate for cooling flow optimization is purge flow, which contributes to sealing the stator–rotor cavity and prevents ingestion of hot main gas path (MGP) flow into the wheelspace. Previous research has identified that the external main gas path flow physics play a significant role in driving rim seal ingestion. However, the potential impact of other cooling flow features on ingestion behavior, such as vane trailing edge (VTE) flow, is absent in the open literature. This paper presents experimental measurements of rim cavity cooling effectiveness collected from a one-stage turbine operating at engine-representative Reynolds and Mach numbers. Carbon dioxide (CO2) was used as a tracer gas in both the purge flow and vane trailing edge flow to investigate flow migration into and out of the wheelspace. Results show that the vane trailing edge flow does in fact migrate into the rim seal and that there is a superposition relationship between individual cooling flow contributions. Computational fluid dynamics (CFD) simulations using unsteady Reynolds-averaged Navier–Stokes (URANS) were used to confirm VTE flow ingestion into the rim seal cavity. Radial and circumferential traverse surveys were performed to quantify cooling flow radial migration through the main gas path with and without vane trailing edge flow. The surveys confirmed that vane trailing edge flow is entrained into the wheelspace as purge flow is reduced. Local CO2 measurements also confirmed the presence of VTE flow deep in the wheelspace cavity.

Analysis of Electromechanical Interaction in Aircraft Generator Systems

The supply of electrical power is usually achieved by a generator driven from a prime mover by some form of mechanical drivetrain. Such an electromechanical system will have natural resonant modes in both the electrical and mechanical subsystems. The electrical generator provides a coupling between the subsystems, transferring not only useful power, but also disturbances between the electrical and mechanical domains. These disturbances may excite resonances resulting in cross-domain (electromechanical) interaction. This can lead to lifetime reduction in the mechanical components and instability in the electrical network, resulting in poor reliability for the wider system and potentially catastrophic component failure. Electromechanical interaction is particularly critical in power generation systems onboard aircraft, because the generator is driven by a gas turbine via an inherently low-stiffness drive train. It is then critical to identify electromechanical interaction at the design stage so that these issues can be avoided. However, predicting the occurrence of interaction through simulation is challenging, requiring multidomain models operating with different time scales. This paper analyzes an aircraft auxiliary power offtake to produce a reduced-order mechanical drivetrain model, allowing the modal frequencies to be predicted and cross-domain interactions to be modeled. A purpose-built electromechanical test platform is used to validate the model and demonstrate how electrical disturbances are passed through the generator to the mechanical system and affect the electrical network. Future research will use the test bed to demonstrate strategies for avoiding or suppressing unwanted interactions.

Homogenization of partial debond effect on the effective thermal conductivities of FRP composite using finite element analysis

Accurate prediction of effective properties of composites prevents catastrophic failure of components designed, which depends on the non-similarities such as voids, debond, cracks and fibre orientation. Prediction of effective properties of composites with the inclusion of non-similarities in composites by micromechanics approach is prohibitively expensive; Homogenization (macro-mechanics) approach solves the problem by reducing computational capabilities. In this study, finite element models are presented for predicting effective thermal conductivities of a partially debonded unidirectional square-packed array composite. Partial debond angle effect and its location effect are studied. Also applicability of Homogenization approach is tested for three different cases and is found that maximum error between macro- and micro-mechanics approaches (homogenization) is 2.1 %. Effective longitudinal thermal conductivity of the composite is not affected by partial debond.

Modeling the Influence of Microstructure on Corrosion Pit Growth and Resulting Stress Concentration

Localized corrosion like pitting corrosion can result in catastrophic failure of components due to transition of pits to stress corrosion cracks under loading. This transition is a product of pit morphology and mechanical behavior of the metal leading to regions of stress concentration from where cracks emanate. Fundamentally, both factors are dependent on the underlying microstructure, with pit morphology resulting from differences in growth rates across grains, and mechanical behavior resulting from elastic and plastic anisotropies across crystal orientations. This work presents a decoupled stress corrosion analysis, where at first pit growth simulations are performed at various locations in a steel microstructure, while taking into account the effect of microstructure through variation in corrosion potential. Subsequently the pit geometries are accommodated in mechanical loading simulations along with the underlying microstructure and anisotropic material properties to calculate stress distributions around the pit.

Reliable Server Assignment in Networks Using Nature Inspired Metaheuristics

In this paper, a reliable server assignment problem in networks is defined as determining a deployment of identical servers to maximize a measure of service availability, and solved using nature-inspired metaheuristic approaches, namely Ant Colony Optimization, Particle Swarm Optimization, and Clonal Selection Principle of Artificial Immune Systems. In networks, the communication between a client and a server might be interrupted because the server itself is offline or unreachable as a result of catastrophic network failures. Therefore, it is very important to deploy servers at critical network nodes so that the reliability of the system is maximized. A new reliability measure, called critical service rate, is defined to evaluate alternative server assignments with respect to the network's ability to provide services in the case of catastrophic component failures. The structure of the optimal server assignments is studied, and the performances of three nature inspired metaheuristics are investigated in a rigorous experimental study. Based on the computational studies, their advantages and disadvantages are discussed.

The Genetic Signatures of Noncoding RNAs

The majority of the genome in animals and plants is transcribed in a developmentally regulated manner to produce large numbers of non–protein-coding RNAs (ncRNAs), whose incidence increases with developmental complexity. There is growing evidence that these transcripts are functional, particularly in the regulation of epigenetic processes, leading to the suggestion that they compose a hitherto hidden layer of genomic programming in humans and other complex organisms. However, to date, very few have been identified in genetic screens. Here I show that this is explicable by an historic emphasis, both phenotypically and technically, on mutations in protein-coding sequences, and by presumptions about the nature of regulatory mutations. Most variations in regulatory sequences produce relatively subtle phenotypic changes, in contrast to mutations in protein-coding sequences that frequently cause catastrophic component failure. Until recently, most mapping projects have focused on protein-coding sequences, and the limited number of identified regulatory mutations have been interpreted as affecting conventional cis-acting promoter and enhancer elements, although these regions are often themselves transcribed. Moreover, ncRNA-directed regulatory circuits underpin most, if not all, complex genetic phenomena in eukaryotes, including RNA interference-related processes such as transcriptional and post-transcriptional gene silencing, position effect variegation, hybrid dysgenesis, chromosome dosage compensation, parental imprinting and allelic exclusion, paramutation, and possibly transvection and transinduction. The next frontier is the identification and functional characterization of the myriad sequence variations that influence quantitative traits, disease susceptibility, and other complex characteristics, which are being shown by genome-wide association studies to lie mostly in noncoding, presumably regulatory, regions. There is every possibility that many of these variations will alter the interactions between regulatory RNAs and their targets, a prospect that should be borne in mind in future functional analyses.

Normal people working in normal organizations with normal equipment: System safety and cognition in a mid-air collision

A fundamental challenge in improving the safety of complex systems is to understand how accidents emerge in normal working situations, with equipment functioning normally in normally structured organizations. We present a field study of the en route mid-air collision between a commercial carrier and an executive jet, in the clear afternoon Amazon sky in which 154 people lost their lives, that illustrates one response to this challenge. Our focus was on how and why the several safety barriers of a well structured air traffic system melted down enabling the occurrence of this tragedy, without any catastrophic component failure, and in a situation where everything was functioning normally. We identify strong consistencies and feedbacks regarding factors of system day-to-day functioning that made monitoring and awareness difficult, and the cognitive strategies that operators have developed to deal with overall system behavior. These findings emphasize the active problem-solving behavior needed in air traffic control work, and highlight how the day-to-day functioning of the system can jeopardize such behavior. An immediate consequence is that safety managers and engineers should review their traditional safety approach and accident models based on equipment failure probability, linear combinations of failures, rules and procedures, and human errors, to deal with complex patterns of coincidence possibilities, unexpected links, resonance among system functions and activities, and system cognition.

Reconfigurable control system design for fault diagnosis and accommodation.

The growing demand in system reliability and survivability under failures has urged ever-increasing research effort on the development of fault diagnosis and accommodation. In this paper, the on-line fault tolerant control problem for dynamic systems under unanticipated failures is investigated from a realistic point of view without any specific assumption on the type of system dynamical structure or failure scenarios. The sufficient conditions for system on-line stability under catastrophic failures have been derived using the discrete-time Lyapunov stability theory. Based upon the existing control theory and the modern computational intelligence techniques, an on-line fault accommodation control strategy is proposed to deal with the desired trajectory-tracking problems for systems suffering from various unknown and unanticipated catastrophic component failures. Theoretical analysis indicates that the control problem of interest can be solved on-line without a complete realization of the unknown failure dynamics provided an on-line estimator satisfies certain conditions. Through the on-line estimator, effective control signals to accommodate the dynamic failures can be computed using only the partially available information of the faults. Several on-line simulation studies have been presented to demonstrate the effectiveness of the proposed strategy. To investigate the feasibility of using the developed technique for unanticipated fault accommodation in hardware under the real-time environment, an on-line fault tolerant control test bed has been constructed to validate the proposed technology. Both on-line simulations and the real-time experiment show encouraging results and promising futures of on-line real-time fault tolerant control based solely upon insufficient information of the system dynamics and the failure dynamics.

Modeling interfacial conditions in nominally flat contacts for application to fretting fatigue of turbine engine components

The area of contact between the blade root and disk in high-performance turbomachinery has been identified as a critical area for the nucleation of fatigue damage leading to premature and often catastrophic component failures. The interaction of small-scale relative displacements or microslip at the contact surfaces and sharp gradients in the near-surface contact stress field drives a damage process known as fretting. This paper presents and compares two computational approaches — one relying on a quasi-analytical formulation and another employing the finite element method — for characterizing the interfacial conditions arising in experimental set-ups used to simulate conditions of fretting in blade/disk contacts. The quasi-analytical formulation provides a computationally efficient approach for analyzing arbitrary contacting profiles. It is also shown that predictions from the analyses compare favorably with interfacial damage observed in an on-going series of fretting fatigue tests of a titanium alloy used commonly in turbomachinery components, Ti–6Al–4V.

Using built-in-test to reduce TPS run times and improve TPS reliability

This paper addresses using information derived from Built-in-Test (BIT) to fault diagnose Units Under Test (UUTs), wherever possible. This philosophic approach to diagnostic testing is not new. It has been studied over the past 20 years under the visor of "Integrated Diagnostics", but it has yet to be truly implemented in a "real life" military diagnostic test environment. The mindset of Test Program Set design engineering, along with customer and contractor management alike, remains "complete diagnostic testing based upon single catastrophic component failure modes". If we are to generate cost efficient Test Program Sets (TPSs) under reduced military budget constraints, this will have to change! The test engineer must be encouraged to use methodologies to speed up development time and decrease TPS run times. Using present technology, this is possible now, and as the technology matures, will become a truly viable approach in the future. For the purpose of this paper, the author relies heavily on his extensive US Navy Automatic Test Equipment (ATE) and Test Program Set (TPS) experience, as well as on previous studies performed on using BIT to fault diagnose Unit Under Test failures on US Naval Air weapon systems.

Integrity assessment of nuclear containment vessel steels due to missile impact load

The impact behavior of the steel plate of BWR containment vessels against missiles, that are caused by the postulated catastrophic failure of components with a high kinematic energy has been investigated. The research started in 1977 and was completed in 1979 by a project team of the Technical Research Association for Integrity of Structures at Elevated Service Temperatures, sponsored by three Electric Power Companies (Tokyo, Chubu, and Chugoku). Although the probability of the occurrence of missiles inside and outside of containment vessels is extremely low, the following items are required to maintain the integrity of containment vessels. 1. (1) The probability of the occurrence of missiles. 2. (2) The weight and energy of the missiles. 3. (3) The impact behavior of containment vessel steel plate against postulated missiles. In connection with the above item (3), an actual-scale missile test was conducted. In addition, a computation analysis was performed to confirm the impact behavior against the missiles, in order to search for wide applicability to the various kinds of postulated missiles. This research has finally aimed at obtaining a new empirical formula which carries out the assessment of the integrity of containment vessels.

Use of Passive Redundancy in Electronic Systems

Circuit design predicated solely upon the premise of providing circuits with greater and greater immunity against component tolerances at some point results in an increase in catastrophic failure rates. Thus, circuits which are overdesigned to provide maximum protection against drift failures may actually contribute to a lower system reliability than do those circuits which are designed to operate with tighter component tolerances. These considerations lead to the conclusion that as systems grow in complexity or as reliability requirements are significantly increased for current equipment, a point is reached where further improvement in design procedures will not necessarily lead to further increases in system reliability. A way out of this dilemma appears to be the introduction of some form of redundancy on the component or circuit level in order to overcome the effects of the inevitable occurrence of catastrophic component failures. This paper discusses the application of redundancy techniques, of a nonadaptive and passive nature, to electronic circuits and systems.