Experiment results of failure progression from low power wires

Despite various studies that have been conducted so far on the failure of high power cables, failure progression in low power cables, wires and interconnections have not been well understood yet. In general, it is hypothesised that failures of wires are progressed from random intermittent failures that are gradually developed as hard faults. This paper aims to present a test rig and possible test techniques that can be used for testing the failure progression of wires and interconnections. Research presented in this paper is based on tools, equipment and techniques that facilitate various ageing mechanisms needed to capture proper and right failure patterns from low power cables, wires and interconnections. Paper originally presented at the 5th International Conference in Through-life Engineering Services Cranfield University, 1st and 2nd November 2016

Bayesian Network Approach to Assessing System Reliability for Improving System Design and Optimizing System Maintenance

abstract: A quantitative analysis of a system that has a complex reliability structure always involves considerable challenges. This dissertation mainly addresses uncertainty in- herent in complicated reliability structures that may cause unexpected and undesired results. The reliability structure uncertainty cannot be handled by the traditional relia- bility analysis tools such as Fault Tree and Reliability Block Diagram due to their deterministic Boolean logic. Therefore, I employ Bayesian network that provides a flexible modeling method for building a multivariate distribution. By representing a system reliability structure as a joint distribution, the uncertainty and correlations existing between system’s elements can effectively be modeled in a probabilistic man- ner. This dissertation focuses on analyzing system reliability for the entire system life cycle, particularly, production stage and early design stages. In production stage, the research investigates a system that is continuously mon- itored by on-board sensors. With modeling the complex reliability structure by Bayesian network integrated with various stochastic processes, I propose several methodologies that evaluate system reliability on real-time basis and optimize main- tenance schedules. In early design stages, the research aims to predict system reliability based on the current system design and to improve the design if necessary. The three main challenges in this research are: 1) the lack of field failure data, 2) the complex reliability structure and 3) how to effectively improve the design. To tackle the difficulties, I present several modeling approaches using Bayesian inference and nonparametric Bayesian network where the system is explicitly analyzed through the sensitivity analysis. In addition, this modeling approach is enhanced by incorporating a temporal dimension. However, the nonparametric Bayesian network approach generally accompanies with high computational efforts, especially, when a complex and large system is modeled. To alleviate this computational burden, I also suggest to building a surrogate model with quantile regression. In summary, this dissertation studies and explores the use of Bayesian network in analyzing complex systems. All proposed methodologies are demonstrated by case studies.Dissertation/ThesisDoctoral Dissertation Industrial Engineering 201

Mathematical maintenance models of vehicles’ equipment

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