A.M.S. Hamouda

Condition-based maintenance for continuously monitored degrading systems with multiple failure modes

This article develops an optimum Condition-Based Maintenance (CBM) policy for continuously monitored degrading systems with multiple failure modes. The degradation of system state is described by a stochastic process, and a maintenance alarm is used to signal when the degradation reaches a threshold level. Unlike existing CBM models, this article considers multiple sudden failures that can occur during a systems degradation. The failure rate corresponding to each failure mode is influenced by either the age of the system, the state degradation of the system, or both. A joint model is constructed for the statistically dependent time-to-maintenance due to system degradation and time-to-failure of different failure modes. This model is then utilized to obtain the optimum maintenance threshold level that maximizes the systems limiting availability over its life cycle or minimizes the long-run cost per unit time. A numerical example, using real-life data from a reliability test of communication systems, is provided to demonstrate the application of the proposed approach.

Hierarchical honeycomb auxetic metamaterials

Most conventional materials expand in transverse directions when they are compressed uniaxially resulting in the familiar positive Poisson’s ratio. Here we develop a new class of two dimensional (2D) metamaterials with negative Poisson’s ratio that contract in transverse directions under uniaxial compressive loads leading to auxeticity. This is achieved through mechanical instabilities (i.e., buckling) introduced by structural hierarchy and retained over a wide range of applied compression. This unusual behavior is demonstrated experimentally and analyzed computationally. The work provides new insights into the role of structural organization and hierarchy in designing 2D auxetic metamaterials, and new opportunities for developing energy absorbing materials, tunable membrane filters, and acoustic dampeners.

Synthesis and Characterization of Nano Boron Nitride Reinforced Magnesium Composites Produced by the Microwave Sintering Method

In this study, magnesium composites with nano-size boron nitride (BN) particulates of varying contents were synthesized using the powder metallurgy (PM) technique incorporating microwave-assisted two-directional sintering followed by hot extrusion. The effect of nano-BN addition on the microstructural and the mechanical behavior of the developed Mg/BN composites were studied in comparison with pure Mg using the structure-property correlation. Microstructural characterization revealed uniform distribution of nano-BN particulates and marginal grain refinement. The coefficient of thermal expansion (CTE) value of the magnesium matrix was improved with the addition of nano-sized BN particulates. The results of XRD studies indicate basal texture weakening with an increase in nano-BN addition. The composites showed improved mechanical properties measured under micro-indentation, tension and compression loading. While the tensile yield strength improvement was marginal, a significant increase in compressive yield strength was observed. This resulted in the reduction of tension-compression yield asymmetry and can be attributed to the weakening of the strong basal texture.

A Review on Fatigue Life Prediction Methods for Metals

Metallic materials are extensively used in engineering structures and fatigue failure is one of the most common failure modes of metal structures. Fatigue phenomena occur when a material is subjected to fluctuating stresses and strains, which lead to failure due to damage accumulation. Different methods, including the Palmgren-Miner linear damage rule- (LDR-) based, multiaxial and variable amplitude loading, stochastic-based, energy-based, and continuum damage mechanics methods, forecast fatigue life. This paper reviews fatigue life prediction techniques for metallic materials. An ideal fatigue life prediction model should include the main features of those already established methods, and its implementation in simulation systems could help engineers and scientists in different applications. In conclusion, LDR-based, multiaxial and variable amplitude loading, stochastic-based, continuum damage mechanics, and energy-based methods are easy, realistic, microstructure dependent, well timed, and damage connected, respectively, for the ideal prediction model.

Evaluation of the mechanical properties of AA 6063 processed by severe plastic deformation

In this study, the mechanical properties, including surface hardness, tensile strength, fatigue, and fretting fatigue behavior of AA 6063 processed by equal channel angular pressing as the most efficient severe shear plastic deformation (SPD) technique, were investigated. Following the SPD process, samples were subjected to heat treatment (HT), hard anodizing (HA), and a combination of HT and HA. Rotating–bending fretting fatigue tests were performed to explore the samples’ response to the fretting condition. From the experimental fatigue and fretting fatigue tests, it was apparent that the SPD treatment had a positive effect on enhancing the fatigue and fretting fatigue lives of the samples at low and high-cyclic loads compared with the HT technique by 78 and 67xa0pct, and 131 and 154xa0pct respectively. The results also indicate that the SPDxa0+xa0HT technique significantly increased the fatigue and fretting fatigue lives of the samples at high and low cycles by 15.56 and 8.33xa0pct, and 14.4 and 5.1xa0pct respectively, compared with the SPD method. HA of AA6063 increased the fatigue and fretting fatigue lives of SPDxa0+xa0HT-processed samples at low cycle by 15.5 and 18.4xa0pct respectively; however, at high cycle, HA had reverse effects, whereby the fatigue and fretting fatigue lives of SPDxa0+xa0HT-processed samples decreased by 16.7 and 30xa0pct, respectively.

Criticality measures for components with multi-dimensional degradation

Failures of engineering structures and equipment are often attributed to the failure of a single component. Hence, it is important to identify critical components in a system and understand how a components criticality changes over time under dynamic environments. This article investigates the criticality analysis for components with multiple competing failure modes due to degradation. The component degradation is modeled as a k-dimensional Wiener process. A component fails when any of the k degradation processes associated with that component attains a certain threshold level. Motivated by Nelsons cumulative exposure model, a relationship between both the mean and diffusion of the degradation process and environmental conditions is established. Expressions of a components criticality measures are derived. Numerical examples are presented to illustrate the criticality analysis.

Adaptive cumulative sum charts with the adaptive runs rule

Adaptive cumulative sum (ACUSUM) charts, which adjust the reference value dynamically based on estimated shift size, provide good performance in detecting a range of mean shifts. However, when the range is wide, ACUSUM may not perform well for small shifts over the range. An adaptive runs rule, which is motivated by the concept of supplementary runs rule, is proposed, in order to make control charts more sensitive to small mean shifts. The adaptive runs rule assigns scores to consecutive runs based on the estimated shift size of the mean. The ACUSUM chart is supplemented with the adaptive runs rule to enhance its sensitivity in detecting small mean shifts. The average run length performance of the ACUSUM chart with the adaptive runs rule is compared with those of cumulative sum and variants of adaptive charts including ACUSUM. The experimental results reveal that the ACUSUM chart with the adaptive runs rule achieves superior detection performance over a wide range of mean shifts.

ISO 9000 performance among the Malaysian companies: the effects of motives

This paper presents the results of a study to identify the real motives behind ISO 9000 Certification. Towards this goal, a survey of some 300 ISO 9000 certified companies has been carried out for Malaysian manufacturing companies. Statistical tests on its relationship with performance improvement and the overall benefits and the barriers on gaining ISO 9000 certification are done. The paper also reveals other factors influencing the performance of ISO 9000, for example, the period of time after the company has been certified and size of company. Finally, factor analysis is applied to identify the main factors in certification motives, benefits, barriers, and their in-between relationship.

Multivariate statistical process control charts based on the approximate sequential χ2 test

Similar to the univariate CUSUM chart, a multivariate CUSUM (MCUSUM) chart can be designed to detect a particular size of the mean shift optimally based on the scheme of a sequential likelihood ratio test for the noncentrality parameter. However, in multivariate case, the probability ratio of a sequential test is intractable mathematically and the test statistic based on the ratio does not have a closed form expression which makes it impractical for real application. We drive an approximate log-likelihood ratio and propose a multivariate statistical process control chart based on a sequential χ2 test to detect a change in the noncentrality parameter. The statistical properties of the proposed test statistic are explored. The average runs length (ARL) performance of the proposed charts is compared with other MCUSUM charts for process mean monitoring. The experimental results reveal that the proposed charts achieve superior, both zero-state and steady-state, ARL performance over a wide range of mean shifts, especially when the dimension of measurements is large.

Economic cost models of integrated APC controlled SPC charts

In this paper, an economic cost model is proposed for processes integrating both automatic process control (APC) and statistical process control (SPC) for quality monitoring and control. Both the special cause and common cause variations are reduced by applying integrated APC and SPC. Traditionally, the integrated processes using APC and SPC are evaluated by the average run length (ARL). However, ARL may not be appropriate as a measurement of the economic design since it does not take into consideration the run length variation. Also, there are few studies that compare the cost models of such an integrated control system and the effect of cost parameters using different APC controllers. Therefore, we develop an economic cost model using non-homogenous Poisson process to describe the occurrence of an APC adjustment and develop a long run expected cost to investigate the use of different controllers in such integrated systems. Numerical examples are presented to demonstrate the applicability of the proposed model.