Cause Of Catastrophic Failure Research Articles

Закономерности износа плазмотронов при плазменной резке толстолистового проката на токе обратной полярности

The introduction describes the features of the process of plasma cutting of various metals and alloys using reverse-polarity plasma torches with and the features of cutting thick sheets. The purpose of the work is to study the wear process of plasma torches operating on reverse polarity current when cutting thick rolled sheets of aluminum and titanium alloys. Research methods include optical and scanning electron microscopy, filming of the cutting process and visual inspection of plasma torch elements after receiving specimens. Results and discussion. The section shows the appearance of the main working elements of the plasma torch after cutting in various modes, which led to both stable and gradual wear and to catastrophic failure of the plasma torch. The results of structural studies of the main characteristic zones of nozzles and electrodes after cutting are presented. The studies carried out made it possible to establish the main reasons for the failure of the working elements reverse-polarity plasma torches. The causes of catastrophic failure of plasma torches include failure to maintain the gap between the nozzle and the electrode and melting of the channel of gas supply into the discharge chamber. The wear of nozzles and electrodes in a stable mode can be intensified due to abnormal operation of the starting arc, the presence of manufacturing inaccuracies and excess gas pressure. In conclusion, the main conclusions based on the results of the research are formulated. The process of wear of electrodes, nozzles and body elements of plasma torches during operation at high electric arc power values is described.

Practical method for localizing low-velocity impact on a UAV composite wingbox structure under loading conditions

A low-velocity impact event can produce undetectable damages inside laminated composite structures. Such damages have been introduced as one of the main causes of catastrophic failure of whole structure, thus impact event detection and localization should be essential for many kinds of composite structures. In this study, low-velocity impact localization method was proposed using six multiplexed fiber Bragg grating (FBG) sensors on the composite wingbox structure. The principle is to find the location which has the most similar set of reference signals from the database. Firstly, the required reference signals were measured to construct the database for the test section which is located on the upper skin of the wingbox. Then, the localization was implemented with the process of detecting the time of arrival (TOA) and finding the impact location. The similarities between the impact-induced signals and reference signals are assessed by calculating the root mean squared (RMS) values. Through the impact localizations on the non-reference locations, this method was validated. Also, this method was applied to the impact localizations when the wingbox structure was under static loads. From the acceptable results of such experiments under static loading conditions, the practicality of this method was successfully verified.

Seismic fragility of non-ductile RC frames for pounding risk assessment

Building structures subjected to seismic excitation tend to vibrate with a unique set of dynamic characteristics, such as natural frequencies, fundamental mode shapes etc. In series-configured structural systems, a difference in such characteristics induces a phase difference between the structures’ oscillations, leading to structural collisions or pounding. Pounding often becomes a cause of catastrophic failures, hence an accurate estimation of structural integrity with respect to pounding is of paramount importance. This study aims to examine different fragility curve development methodologies available in the literature to assess the seismic performance of adjacent Reinforced Concrete (RC) structures subjected to structural pounding. The study primarily focuses on displacement-based fragility curves for the highly critical floor-to-column interactions of an eight-storey non-ductile RC frame, non-eccentrically adjacent to a three-storey rigid RC frame having different storey heights. Nonlinear time–history analyses have been performed to simulate pounding, and fragility curves have been generated using five methodologies. The direct Monte-Carlo Simulation technique has been used to benchmark against the obtained results to identify the suitability of each fragility assessment technique. Results indicate that even though there is no effect of the choice of assessment methodology on the estimated risk at low performance levels and intensity measures, the disparity becomes clearly visible at higher levels. The study presents alternate ways of developing accurate fragility curves by keeping in mind the need for computational efficiency without compromising the accuracy of results. The High Dimensional Model Representation meta-model is found to efficiently represent target pounding fragilities while incurring minimal computational costs.

Numerical Study on the Critical Frequency Response of Jet Engine Rotors for Blade-Off Conditions against Bird Strike

Vibrations are usually induced in aero engines under their normal operating conditions. Therefore, it is necessary to predict the critical frequencies of the rotating components carefully. Blade deformation of a jet engine under its normal operating conditions due to fatigue or bird strike is a realistic possibility. This puts the deformed blade as one of the major safety concerns in commercially operating civil aviation. A bird strike introduces unbalanced forces and non-linearities into the engine rotor system. Such dynamic behavior is a primary cause of catastrophic failures. The introduction of unbalanced forces due to a deformed blade, as a result of a bird strike, can change the critical frequency behavior of engine rotor systems. Therefore, it is necessary to predict their critical frequencies and dynamic behavior carefully. The simplified approach of the one-dimensional and two-dimensional elements can be used to predict critical frequencies and critical mode shapes in many cases, but the use of three-dimensional elements is the best method to achieve the goals of a modal analysis. This research explores the effect of a bird strike on the critical frequencies of an engine rotor. The changes in critical mode shapes and critical frequencies as a result of a bird strike on an engine blade are studied in this research. Commercially available analysis software ANSYS version 18.2 is used in this study. In order to account for the material nonlinearities, a Johnson Cook material model is used for the fan blades and an isotropic–elastic–plastic–hydrodynamic material model is used for modeling the bird. The bird strike event is analyzed using Eularian and smoothed particle hydrodynamics (SPH) techniques. A difference of 0.1% is noted in the results of both techniques. In the modal analysis simulation of the engine rotor before and after the bird strike event, the critical failure modes remain same. However, a change in the critical frequencies of the modes is observed. An increase in the critical frequencies and excitation RPMs (revolution per minute) of each mode are observed. As the mode order is increased, the higher the rise in critical frequency and excitation RPMs. Also, a change in the whirl direction of the different modes is noted.

Active control of hydraulic oil contamination to extend the service life of aviation hydraulic system

The hydraulic system is the control center of aircraft, the performance of which is critically dependent on the purity of the oil that flows through it. Contamination of hydraulic system oil has been known to be the primary cause of catastrophic failure and accidents. A review of literature in the area of hydraulic oil contamination reveals three facts: (i) there is a dearth of effective methods to prevent intrusion of external contaminants into the hydraulic oil, (ii) there are no effective purification methods for liquid and gaseous contaminants of oil, and (iii) there is an absence of real-time monitoring for major contaminants. This work systematically reviews the hazards, sources, and removal methods of major contaminants, especially of gaseous contaminants. An active method to control oil contamination is proposed instead of the existing passive maintenance techniques that are ridden with problems. In a case study of aviation hydraulic system, our active control system comprises (i) a closed hydraulic circuit and a closed oil tank to prevent the intrusion of external contaminants, (ii) a vacuum centrifuge to efficiently remove contaminants without creating consumption costs, and (iii) monitoring devices installed to actively monitor the contaminant content in oil during operation. The results from the purification experiments and three-dimensional modeling indicate that the active control of hydraulic oil contamination can effectively remove 98.4% solid contaminants and reduce the water content below 50 ppm, and in particular reduce the gas content below 2%. It is obvious that this method can improve the continuous operation capability of aviation hydraulic systems and extend the intermaintenance period of aircrafts by reducing the contaminant content in oil.

Gear tooth root fatigue test monitoring with continuous acoustic emission: Advanced signal processing techniques for detection of incipient failure

The phenomenon of fatigue in gears at the tooth root can be a cause of catastrophic failure if not detected in time. Where traditional low-frequency vibration may help in detecting a well-developed crack or a completely failed tooth, a system for early detection of the nucleation and initial propagation of a fatigue crack can be of great use in condition monitoring. Acoustic emission is a potentially suitable technique, as it is sensitive to the higher frequencies generated by crack propagation and is not affected by low-frequency noise. In this article, a static gear pair is tested where a crack was initiated at a tooth root. Continuous acoustic emission was periodically recorded throughout the test. Data were processed in multiple ways to support the early detection of crack initiation. Initially, traditional feature–based acoustic emission was employed. This showed qualitative results indicating fracture initiation around 8000 cycles. A rolling cross-correlation was then employed to compare two given system states, showing a sensitivity to large changes towards the final phases of crack propagation. A banded fast Fourier transform approach showed that the 110- to 120-kHz band was sensitive to the observed crack initiation at 8000 cycles, and to the later larger propagation events at 22,000 cycles. Two advanced data processing techniques were then used to further support these observations. First, a technique based on Chebyshev polynomial decomposition was used to reduce each wavestream data to a vector of 25 descriptors; these were used to track the system deviation from a baseline state and confirmed the previously observed deviations with a higher sensitivity. Further confirmation came from the analysis of wavestream entropy content, providing support from multiple data analysis techniques on the feasibility of system state tracking using continuous acoustic emission.

NUMERICAL INVESTIGATION OF MECHANICAL AND THERMAL DYNAMIC PROPERTIES OF THE INDUSTRIAL TRANSFORMER

Industrial transformer is one of the most critical assets in the power and heavy industry. Failures of transformers can cause enormous losses. The poor joints of the electrical circuit on transformers can cause overheating and results in stress concentration on the structure which is the major cause of catastrophic failure. Few researches have been focused on the mechanical properties of industrial transformers under overheating thermal conditions. In this paper, both mechanical and thermal properties of industrial transformers are jointly investigated using finite element analysis (FEA). Dynamic response analysis is conducted on a modified transformer FEA model, and the computational results are compared with experimental results from literature to validate this simulation model. Based on the FEA model, thermal stress is calculated under different temperature conditions. These analysis results can provide insights to the understanding of the failure of transformers due to overheating, therefore are significant to assess winding fault, especially to the manufacturing and maintenance of large transformers.

Efficient computational modelling of carbon fibre reinforced laminated composite panels subjected to low velocity drop-weight impact

This paper reports on the actual and virtual low velocity impact response of carbon fibre composite laminates. It utilises the contribution of through-thickness stresses, in the prediction of the onset of internal damage created by this type impact scenario.The paper focuses on the damage imparted by the flat nose impactor since this induces a different type of damage and structural response compared to that of the standard test method of using a round nose impactor.Vulnerability of the fibrous composites to vertical drop-weight impact can result in premature failure which is a major concern in their widespread usage. The topic has been of intense research to design more damage tolerant and resistant materials. However, due to materials’ anisotropic and three-dimensional nature and complicated damage mechanisms no standard model could have been achieved. Designers predict consequences of a local impact within the global structural context without full-scale testing.Majority of the existing simulation models neglect through-thickness stresses that are regarded as the major cause of catastrophic failures. Efficient and reliable investigations are required to reduce testing and include through-thickness stresses. Drop-weight impact simulation models were developed herein using ABAQUS™ software. Simulations were carried out to compute in-plane stresses subjected to flat and round nose impacts on laminates of differing thicknesses. These stresses once computed were numerically integrated employing the equilibrium equations to efficiently predict through-thickness stresses. The predicted stresses were then utilised in failure criteria to quantify the coupled and embedded damage. This provides a quick insight into the status and contribution of through-thickness stresses in failure predictions. The computed values were compared to the experimental results and found to be in good agreement.

Identification of two simultaneous partial discharge sources in an oil-pressboard insulation system using acoustic emission techniques

Insulation failure is one of the major causes of catastrophic failure of transformers. It is established that partial discharge (PD) causes insulation degradation and premature failure of insulation. In power apparatus, more than one PD source may be active simultaneously. The nature of insulation degradation for multiple PD sources is different from that due to single PD source. Therefore, it will be helpful for severity assessment of insulation degradation, if the number of active PD sources are identified and classified. This paper presents a method for identification and classification of two simultaneously active PD sources using acoustic emission techniques. The acoustic emission (AE) signals are measured for laboratory simulated PD in an oil-pressboard insulation system for three different electrode systems. The measurements of partial discharge acoustic emission (PDAE) signals are carried out for single PD source and for two simultaneous PD sources. The measured signals are analyzed using discrete wavelet transform (DWT), box counting fractal dimension and lacunarity. Box counting fractal dimension and lacunarity are calculated for DWT decomposed signal of major frequency band. Energy distribution in different frequency bands of DWT decomposed signal along with box counting fractal dimension and lacunarity is used for classification of two simultaneous PD sources.

A reliability modeling framework for the hard disk drive development process

Motivated by the fact that the major causes of catastrophic failure in micro hard disk drives are mostly induced by the presence of particles, a new particle-induced failure susceptibility metric, called the Cumulative Particle Counts (CPC), is proposed for managing reliability risk in a fast-paced hard disk drive product development process. This work is thought to represent the first successful attempt to predict particle-induced failure through an accelerated testing framework which leverages on existing streams of research for both particle-injection-based and inherent-particle-generation laboratory experiments to produce a practical reliability prediction framework. In particular, a new testing technique that injects particles into hard disk drives so as to increase the susceptibility of failure is introduced. The experimental results are then analyzed through a proposed framework which comprises the modeling of a CPC-to-failure distribution. The framework also requires the estimation of the growth curve for the CPC in a prime hard disk drive under normal operating conditions without particle injection. Both parametric and non-parametric inferences are presented for the estimation of the CPC growth curve. Statistical inferential procedures are developed in relation to a proposed non-linear CPC growth curve with a change-point. Finally, two applications of the framework to design selection during an actual hard disk drive development project and the subsequent assessment of reliability growth are discussed.

Fracture and Fatigue in Graphene Nanocomposites

Graphene, a single-atom-thick sheet of sp-bonded carbon atoms, has generatedmuch interest due to its high specific area and novel mechanical, electrical, and thermal properties. Recent advances in the production of bulk quantities of exfoliated graphene sheets from graphite have enabled the fabrication of graphene–polymer composites. Such composites show tremendous potential for mechanical-property enhancement due to their combination of high specific surface area, strong nanofiller–matrix adhesion and the outstanding mechanical properties of the sp carbon bonding network in graphene. Graphene fillers have been successfully dispersed in poly(styrene), poly(acrylonitrile) and poly(methyl methacrylate) matrices and the responses of their Young’s modulus, ultimate tensile strength, andglass-transition temperaturehave been characterized. However, to the best of our knowledge there is no report on the fracture toughness and fatigue properties of graphene–polymer composites. Fracture toughness describes the ability of a material containing a crack to resist fracture and it is a critically important material property for design applications. Fatigue involves dynamic propagation of cracks under cyclic loading and it is one of the primary causes of catastrophic failure in structural materials. Consequently, the material’s resistance to fracture and fatigue crack propagation are of paramount importance to prevent failure. Herein we report the fracture toughness, fracture energy, and fatigue properties of an epoxy polymer reinforced with various weight fractions of functionalized graphene sheets. Remarkably, only 0.125% weight of functionalized graphene sheets was observed to increase the fracture toughness of the pristine (unfilled) epoxy by 65% and the fracture energy by 115%.Toachievecomparableenhancement,carbonnanotube (CNT) and nanoparticle epoxy composites require one to two orders of magnitude larger weight fraction of nanofillers. Under fatigue conditions, incorporation of 0.125% weight of functionalized graphene sheets drastically reduced the rate of crack propagation in the epoxy 25-fold. Fractography analysis

Failure analysis of lock-pin in a gas turbine engine

Due to severe working condition in gas turbines, there are many different cases on the failure of turbine components. In this paper, one of the common catastrophic events of hot section is investigated. The failure was occurred on 1 July, 2005, in a domestic power plant at Bushehr port in the south of Iran. A 25 MW gas turbo generator after 2228 start–stops and 52,586 h operation failed abruptly and caused many detrimental results. Formerly similar cases have been reported in different power plants. Visual inspections showed a typical rubbing due to irregular ruptures of buckets and an instantly rotor unbalancing. Fracture surface investigation, and impact test results showed that there was not any problem in the microstructure and mechanical properties of disc. The metallography of blades showed that there is no significant problem in the microstructure due to fatigue, creep or hot corrosion. The preliminary results determined the mechanical specifications of lock-pins were satisfied. However, it was found some problems in metallographic inspection results of lock-pins. Fractography investigation showed typical fatigue fracture surfaces in the neck of failed lock-pins and localized pitting signs near the head of lock-pin, where crack had been nucleated. In addition, there were extensive fretting corrosion effects in the serration areas of disc and blades. It seemed the failure has been started by fatigue fracture in a lock-pin and then the bucket could move out side of disc serrations. As a correlated conclusion the likely causes of catastrophic failure in the hot section, is mainly considered to be a HCF due to blade root/head of lock-pin impact. The cause of such impact may be increases in clearance of disc serrations because of fretting wear and local microscopic fusions between blade root and lock-pins stem, after an extended period in service. In this case, the risk of failure has been clearly exacerbated by improper operation due to destroying of sealing and hot gasses leakage in these areas and finally inadequate design of lock-pins to sustain a severe hot condition in the first stage of turbine section.

Safe worst-case execution time analysis by abstract interpretation of executable code

To verify that a safety-critical application meets all requirements lies in the responsibility of the system designers. Classical software validation methods like code review and testing with debugging are expensive and cannot really guarantee the absence of errors. In constrast, abstract interpretation provides a methodology for static program analysis that yields statements valid for all program runs with all inputs. Employing static analyzers is thus orthogonal to classical testing. A possible cause of catastrophic failure is the violation of timing constraints. Recent trends, e.g. in automotive industries (X-by-wire, time-triggered protocols) require knowledge on the Worst-Case Execution Time (WCET) of each task. Yet WCET determination is a difficult problem because of the characteristics of modern software and hardware. This talk focuses on AbsInt's tool aiT that combines abstract interpretation and integer linear programming to determine upper bounds for the WCETs of code snippets given as routines in executables. The hardware architecture of the underlying processor is precisely taken into account, including cache and pipeline behavior. Bounds are determined without the need to change the code and hold for all executions. aiT is used among others by Airbus in the development of various safety-critical applications for the A380.

Hydride formation in pure titanium

Hydrogen embrittlement of metals is a subject of importance because it is often the cause of catastrophic failure. The loss in ductility occurs when the hydrogen concentration exceeds a percent or few. In titanium, the embrittlement effect depends on the alloy composition. α-Ti, the low temperature hexagonal phase transforms to bcc β-Ti at 882°C in pure Ti. The transformation temperature is suppressed by β-stabilizing impurities such as Fe ie 4% Fe reduces the temperature to 400°C. This paper examines the hydride formation in iodide-Ti and 99% pure Ti.Thin foils of iodide Ti (.1Sn-.05 Si-.04Fe-.01Cu) and 99% pure Ti(Fe,Mo,Cu,Al) were encapsulated in vacuum tubes, annealed for 2 hours at 800°C, and quenched in ice water. The titanium was then charged in an H2SO4.H2O solution with an applied voltage of +4.5V for one hour. The samples were encapsulated in vacuum tubes and aged for 3 or 4 hours at 200°C.