119 research outputs found

    Imperfect inspection of a system with unrevealed failure and an unrevealed defective state

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    This paper proposes a model of inspection of a protection system in which the inspection outcome provides imperfect information of the state of the system. The system itself is required to operate on demand typically in emergency situations. The purpose of inspection is to determine the functional state of the system and consequently whether the system requires replacement. The system state is modeled using the delay time concept in which the failed state is preceded by a defective state. Imperfect inspection is quantified by a set of probabilities that relate the system state to the outcome of the inspection. The paper studies the effect of these probabilities on the efficacy of inspection. The analysis indicates that preventive replacement mitigates low quality inspection and that inspection is cost-effective provided the imperfect-inspection probabilities are not too large. Some derivative policies in which replacement is “postponed” following a positive inspection are also studied. An isolation valve in a utility network motivates the modeling

    A two-phase inspection policy with imperfect testing

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    This paper presents an inspection policy to detect failures of a single component system that remain hidden otherwise. Inspection reveals whether the unit is in good or failed state. The possibility of non perfect testing is assumed, thus, successive inspections may fail detecting a failure or result in a false alarm. The occurrence of false alarms is reported in optical fire detectors and inspection of printing circuit boards which are on the basis of electronic systems. A two-phase inspection schedule takes into account the changes in component’s aging. The system may undergo different inspection frequencies to detect both early failures or those due to the natural deterioration in the system as time goes by. The examples reveal the advantages of a two-phase inspection when comparing with the unique interval inspection

    Conditional inspection and maintenance of a system with two interacting components

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    In this paper we consider the inspection and maintenance of a two-component system with stochastic dependence. A failure of component 1 may induce the defective state in component 2 which in turn leads to its failure. A failure of component 1 and a defect in component 2 are detected by inspection. Our model considers a conditional inspection policy: when component 1 is found to have failed, inspection of component 2 is triggered. This opportunistic inspection policy is a natural one to use given this stochastic dependence between the components. The long-run cost per unit time (cost-rate) of the conditional inspection policy is determined generally. A real system that cuts rebar mesh motivates the model development. The numerical examples reveal that when the ratio of the cost of corrective system replacement, that is on failure, to the cost of preventive system replacement is large there exists a finite optimum policy in most cases. Moreover, for the studied system wherein inspections of component 2 are expensive relative to those of component 1, having a reliable indicator of the defective state in component 2 is a good strategy to avoid costly failures of component 2, particularly when its time to failure is short

    Systems reliability for phased missions

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    The concept of a phased mission has been introduced as a sequential set of objectives that operate over different time intervals. During each phase of the mission, the system may alter such that the logic model, system configuration, or system failure characteristics may change to accomplish a required objective. A new fault tree method has been proposed to enable the probability of failure in each phase to be determined in addition to the whole mission unreliability. Phase changes are assumed to be instantaneous, and component failure rates are assumed to be constant through the mission. For any phase, the method combines the causes of success of previous phases with the causes of failure for the phase being considered to allow both qualitative and quantitative analysis of both phase and mission failure. A new set of Boolean laws is introduced to combine component success and failure events through multiple phases so that the expression for each phase failure can be reduced into minimal form. [Continues.

    Mathematical maintenance models of vehicles’ equipment

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    Dissertation for obtaining a scientific degree of Doctor of Philosophy within the specialty 05.22.20 «Maintenance and repair of vehicles». – National Aviation University, Kyiv, 2018.The thesis addresses the critical scientific problem of creating the appropriate maintenance models for digital avionics systems and degrading equipment of vehicles, which increases the operational effectiveness of such systems significantly. The thesis research includes the analysis of the current state and models of digital avionics maintenance. The study describes the necessity for developing the mathematical maintenance models for redundant digital avionics systems, considering the discontinuous nature of their operation, continuous nature of in-flight testing, possibility of both permanent and intermittent failures and organization of several maintenance levels using various diagnostic tools for detecting both failure types. Another focus of the thesis is the analysis of modern trends and mathematical models of condition-based maintenance (CBM) of vehicles’ equipment. The necessity of developing new CBM mathematical models for degrading equipment of vehicles, considering the probabilities of correct and incorrect decisions when checking system suitability for use in the upcoming operation interval, and the possibility of joint determination of the optimum inspection schedule and replacement thresholds for systems that affect and do not affect safety have been substantiated. The scientific novelty of the primary results obtained in the course of the thesis research is as follows: 1. For the first time, mathematical models to evaluate the operational reliability indicators of continuously monitored line replaceable units/line replaceable modules (LRUs/LRMs) and redundant avionics systems over both finite and infinite time interval, which, unlike known models, consider the characteristics of both permanent and intermittent 2failures, have been developed. These models allow evaluating the impact of intermittent failures on the availability and mean time between unscheduled removals (MTBUR) of LRU/LRM. 2. For the first time, generalized expressions to calculate the average maintenance costs of redundant avionics systems, considering the impact of permanent and intermittent failures, have been developed for alternative maintenance options that differ by the number of maintenance levels (one, two or three), which allows choosing the optimal maintenance option during warranty and post-warranty periods. 3. For the first time, a mathematical model of CBM, based on condition monitoring at scheduled times has been developed, which, unlike the known models, considers the probabilities of correct and incorrect decisions made when checking system suitability. This model allows formulating the criteria of determining the optimal replacement threshold for each inspection time and substantially reduce the likelihood of system failure in the forthcoming interval of operation. 4. For the first time, generalized mathematical expressions to calculate the effectiveness indicators of CBM over a finite time interval, as well as the criteria of joint optimization of the inspection schedule and replacement thresholds for systems that affect or do not affect the safety, have been developed. These results allow significantly improve the availability, reduce average maintenance costs and reduce the number of inspections. The practical value of the results obtained in the thesis is as follows: 1. The techniques to calculate probabilistic and time-related indicators of maintenance effectiveness for digital avionics LRUs/LRMs over finite and infinite operating intervals have been developed. The proposed procedures allow to estimate the availability, operational reliability function (ORF), and mean time between unscheduled removals (MTBUR) of LRUs/LRMs during warranty and post-warranty maintenance periods for both federated avionics (FA) and integrated modular avionics (IMA) architectures; 2. A technique for minimizing the warranty maintenance cost of the redundant digital avionics systems has been developed, demonstrating (through the example of the ADIRS system of the Airbus A380 aircraft) that in the case of the optimal option of warranty maintenance, the average maintenance cost per aircraft decreases by 28 %; 33. A technique for minimizing the post-warranty maintenance cost of the redundant digital avionics systems has been developed. It demonstrates (through the example of the ADIRS system of the Airbus A380 aircraft) that a three-level maintenance option with an intermittent fault detector (IFD) at I and D levels, is optimal as it reduces the total expected maintenance costs by 11 times compared to a one-level option, and by over 8.5 times compared to a two-level option without IFD; 4. A technique for determining the optimal replacement thresholds when monitoring the condition of the degrading system at scheduled times has been developed, which allows to significantly reduce the system failure probability in the forthcoming interval of operation. 5. A technique for joint determination of the optimal replacement threshold and periodicity of suitability checking when monitoring the system condition has been developed, which allows to substantially increase the availability of systems while significantly reducing the number of inspections. The results of the thesis research may be used in the development and maintenance of FA and IMA systems, as well as degrading equipment of vehicles

    Delay-time modelling of a critical system subject to random inspections

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    We model the inspection-maintenance of a critical system in which the execution of inspections is random. The models we develop are interesting because they mimic realities in which production is prioritised over maintenance, so that inspections might be impeded or they might be opportunistic. Random maintenance has been modelled by others but there is little in the literature that relates to inspection of a critical system. We suppose that the critical system can be good, defective or failed, and that failure impacts on production, so that a failure is immediately revealed, but a defect does not. A defect, if revealed at inspection, is a trigger for replacement. We compare the cost and reliability of random inspections with scheduled periodic inspections and discuss the implications for practice. Our results indicate that inspections that are performed opportunistically rather than scheduled periodically may offer an economic advantage provided opportunities are sufficiently frequent and convenient. A hybrid inspection and replacement policy, with inspections subject to impediments, is robust to departure from its inspection schedule. Keywords: Maintenance; reliability; random inspection; production; qualit

    A hybrid maintenance policy with fixed periodic structure and opportunistic replacement

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    We model a maintenance policy with fixed periodic structure that is a hybrid of periodic inspection and opportunistic replacement. The policy is applicable to geographically remote systems such as offshore wind farms. The policy has three phases. Initially, there is an inspection phase to identify early defects. This is followed by a wear out phase during which corrective replacements are performed. Preventive replacement occurs at the end of this phase. The novelty of the model is an opportunistic phase, which overlaps with the latter part of the corrective phase, when preventive replacement is executed early if an opportunity arises. In this way, we model the reality in which remote systems with high logistics costs and restricted access may benefit from opportunistic visits for maintenance. Using a numerical example, we analyse the behaviour of the decision variables for a range of values of the parameters common to such systems. These parameters relate to: component heterogeneity; restricted access; default (failure to execute a planned action); arrival of opportunities and other standard parameters in a maintenance cost model. Specifically, our results indicate when opportunities can have a significant impact on the cost-rate of the optimum policy, but that leveraging opportunities cannot achieve a very high availability. Generally, we demonstrate that maintenance planning should be flexible when factors beyond the control of the maintainer impact maintenance effectiveness

    Maintenance of systems with critical components. Prevention of early failures and wear-out

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    We present a model for inspection and maintenance of a system under two types of failures. Early failures (type I), affecting only a proportion p of systems, are due to a weak critical component detected by inspection. Type II failures are the result of the system ageing and preventive maintenance is used against them. The two novelties of this model are: (1) the use of a defective distribution to model strong components free of defects and thus immune to early failures. (2) the removal of the weak critical part once it is detected with no other type of rejuvenation of the system which constitutes an alternative to the minimal repair. We study the conditions under which this model outperforms, from a cost viewpoint, other two classical age-replacement models. The analysis reveals that inspection is advantageous if the system can function with the critical component in the defective state for a long enough time. The proportion of weak units and the quality of inspections also determine the optimum policy. The results about the range of application of the model are useful for decision making in actual maintenance. A case study concerning the timing belt of a four-stroke engine illustrates the model

    Multi-defect modelling of bridge deterioration using truncated inspection records

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    Bridge Management Systems (BMS) are decision support tools that have gained widespread use across the transportation infrastructure management industry. The Whole Life Cycle Cost (WLCC) modelling in a BMS is typically composed of two main components: a deterioration model and a decision model. An accurate deterioration model is fundamental to any quality decision output.There are examples of deterministic and stochastic models for predictive deterioration modelling in the literature, however the condition of a bridge in these models is considered as an ‘overall’ condition which is either the worst condition or some aggregation of all the defects present. This research proposes a predictive bridge deterioration model which computes deterioration profiles for several distinct deterioration mechanisms on a bridge.The predictive deterioration model is composed of multiple Markov Chains, estimated using a method of maximum likelihood applied to panel data. The data available for all the defects types at each inspection is incomplete. As such, the proposed method considers that only the most significant defects are recorded, and inference is required regarding the less severe defects. A portfolio of 9,726 masonry railway bridges, with an average of 2.47 inspections per bridge, in the United Kingdom is the case study considered
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