Comment on “A framework to practical predictive maintenance modeling for multi-state systems” by Tan C.M. and Raghavan N. [Reliab Eng Syst Saf 2008;93(8):1138-50]

“System-perspective” proposed by “A framework to practical predictive maintenance modeling for multi-state systems” by Tan C.M. and Raghavan N. [A framework to practical predictive maintenance modeling for multi-state systems. Reliab Eng Syst Saf 2008;93(8):1138-50] is a very useful method to evaluate and optimize the maintenance strategy for complex systems, especially for multi-state systems (MSS). The commented paper proposes an innovative process and modeling method to present imperfect maintenance effects on MSS, but there exist some incorrect points and misunderstandings. In this paper, these problems are pointed out and are attempted to be corrected under the original framework of the commented paper.     

A practical procedure for the selection of time-to-failure models based on the assessment of trends in maintenance data

Many times, reliability studies rely on false premises such as independent and identically distributed time between failures assumption (renewal process). This can lead to erroneous model selection for the time to failure of a particular component or system, which can in turn lead to wrong conclusions and decisions. A strong statistical focus, a lack of a systematic approach and sometimes inadequate theoretical background seem to have made it difficult for maintenance analysts to adopt the necessary stage of data testing before the selection of a suitable model. In this paper, a framework for model selection to represent the failure process for a component or system is presented, based on a review of available trend tests. The paper focuses only on single-time-variable models and is primarily directed to analysts responsible for reliability analyses in an industrial maintenance environment. The model selection framework is directed towards the discrimination between the use of statistical distributions to represent the time to failure (“renewal approach”); and the use of stochastic point processes (“repairable systems approach”), when there may be the presence of system ageing or reliability growth. An illustrative example based on failure data from a fleet of backhoes is included.     

The Method of Manufactured Universes for validating uncertainty quantification methods

The Method of Manufactured Universes is presented as a validation framework for uncertainty quantification (UQ) methodologies and as a tool for exploring the effects of statistical and modeling assumptions embedded in these methods. The framework calls for a manufactured reality from which “experimental” data are created (possibly with experimental error), an imperfect model (with uncertain inputs) from which simulation results are created (possibly with numerical error), the application of a system for quantifying uncertainties in model predictions, and an assessment of how accurately those uncertainties are quantified. The application presented in this paper manufactures a particle-transport “universe”, models it using diffusion theory with uncertain material parameters, and applies both Gaussian process and Bayesian MARS algorithms to make quantitative predictions about new “experiments” within the manufactured reality. The results of this preliminary study indicate that, even in a simple problem, the improper application of a specific UQ method or unrealized effects of a modeling assumption may produce inaccurate predictions. We conclude that the validation framework presented in this paper is a powerful and flexible tool for the investigation and understanding of UQ methodologies.     

Thin-film solar cells

The rapid progress that is being made with inorganic thin-film photovoltaic (PV) technologies, both in the laboratory and in industry, is reviewed. While amorphous silicon based PV modules have been around for more than 20 years, recent industrial developments include the first polycrystalline silicon thin-film solar cells on glass and the first tandem solar cells based on stacks of amorphous and microcrystalline silicon films (“micromorph cells”). Significant thin-film PV production levels are also being set up for cadmium telluride and copper indium diselenide.     

Cultural values and governance quality as correlates of road traffic fatalities: A nation level analysis

This study investigated the relationships between governance quality, cultural dimensions and road traffic fatality rates in a sample of 46 countries. Government quality was measured with six World Governance Indicators (WGI) published by World Bank, and the cultural factors included Hofstede's four cultural dimensions and seven Schwartz value dimensions. Both direct and moderator effects of the WGI on traffic fatality rates per million vehicles were found. Each of the six WGI scores correlated negatively with traffic fatalities indicating that the quality of governance and institutions contribute to traffic safety. Hofstede's “power distance” dimension and Schwartz value dimensions “embeddedness”, “hierarchy” and “mastery” were positively and “intellectual autonomy” and “egalitarianism” negatively related to traffic fatalities. The WGI score moderated the effects of “hierarchy” and “mastery” on traffic fatalities so that in countries with low governance quality these cultural factors had stronger impact on traffic fatalities. It was concluded that improvement of the quality of governance and institutions would also result in improvement in traffic safety.     

Analyzing maintenance strategies by agent-based simulations: A feasibility study

Thoroughly planned and implemented maintenance strategies save time and cost. However, the integration of maintenance work into reliability analysis is difficult as common modeling techniques are often not applicable due to state explosion which calls for restrictive model assumptions and oversimplification. From authors’ point of view, agent-based modeling (ABM) of technical and organizational systems is a promising approach to overcome such problems. But since ABM is not well established in reliability analysis its feasibility in this area still has to be demonstrated. For this purpose ABM is compared with Markov chains, namely by analyzing the reliability of a maintained n-unit system with dependent repair events, applying both modeling approaches. Although ABM and Markov chains lead to the same numerical results, the former points out the potentiality of an improved system state handling. This is demonstrated by extending the ABM with operators as additional “agents” featuring their location (x;y) availability (0;1) and different maintenance strategies. This extension highlights the capability of ABM to analyze complex emergent system behavior and allows a systematic refinement and optimization of the maintenance strategies.     

Reducing air-pollution: A new argument for getting drivers to abide by the speed limit?

Speeding is one of the main factors of car crash-risk, but it also contributes to increasing air-pollution. In two studies we attempted to lead drivers to abide by speed limits using “reducing air-pollution” as a new argument. We presented prevention messages that highlighted the role of speeding in increasing “crash-risk”, “air-pollution”, or both (Studies 1 and 2). The messages were also positively or negatively framed (Study 2). Given that women are more concerned with environmental issues than are men, we expected the following hypotheses to be validated for women. The message with the “air-pollution” argument was expected to be evaluated more positively than the “crash-risk” message (H1). The “air-pollution” and “crash-risk and air-pollution” messages were expected to be more effective than the “crash-risk” message on the behavioral intention to observe speed limits (H2a) and on the perceived efficacy of speed-limit observance in reducing air-pollution (H2b; Studies 1 and 2). Furthermore, positive framing was expected to be more effective than negative framing (H3), and presenting a message to be more effective than presenting no message (H4; Study 2).Broadly, our results argue in favor of our hypotheses. However in Study 2, the effects of message framing did not allow us to conclude that negative or positive framing was superior. All in all, messages with the “air-pollution” argument were more effective at leading drivers to observe speed limits. Thus, environmental protection may be a fruitful route to explore for increasing road safety.     

Incorporating organizational factors into Probabilistic Risk Assessment (PRA) of complex socio-technical systems: A hybrid technique formalization

This paper is a result of a research with the primary purpose of extending Probabilistic Risk Assessment (PRA) modeling frameworks to include the effects of organizational factors as the deeper, more fundamental causes of accidents and incidents. There have been significant improvements in the sophistication of quantitative methods of safety and risk assessment, but the progress on techniques most suitable for organizational safety risk frameworks has been limited. The focus of this paper is on the choice of “representational schemes” and “techniques.” A methodology for selecting appropriate candidate techniques and their integration in the form of a “hybrid” approach is proposed. Then an example is given through an integration of System Dynamics (SD), Bayesian Belief Network (BBN), Event Sequence Diagram (ESD), and Fault Tree (FT) in order to demonstrate the feasibility and value of hybrid techniques. The proposed hybrid approach integrates deterministic and probabilistic modeling perspectives, and provides a flexible risk management tool for complex socio-technical systems. An application of the hybrid technique is provided in the aviation safety domain, focusing on airline maintenance systems. The example demonstrates how the hybrid method can be used to analyze the dynamic effects of organizational factors on system risk.     

Flow into an Arterial Branch Model

Arterial branches are found to be a major site for formation of arterial plaque. In this study, several of the main parameters that influence the local flow into an arterial branch model are investigated. In particular, the role of the local geometric parameters of the bifurcation on the overall flow is thought to be interesting. How the changes in the bifurcation geometry influence the distribution of axial wall shear and pressure in the model, is investigated. The major geometric factors influencing this flow are the bifurcation area ratio and angle. The flow in a large number of geometric variations of the branch model is numerically simulated. The models at several branch area ratios in the range of 0·4≤ AR≤ 2·0 are considered. In the above range of area ratios, a range of branch opening half-angle of is also studied. The flow in the above models is calculated for the inlet-flow Reynolds numbers of 250, 500, 1000, and 2000. The asymmetric mass flow into the branches by imposing different exit pressures at the branch outlets is also investigated. Area ratio seems to have the largest influence on the flow within the physiologically relevant range of the parameters considered. Increasing the area ratio can lead to relatively large flow separation in the vicinity of the bifurcation region. At higher values of the opening angle of the bifurcation, the possibility and severity of flow separation at the appropriate wall location increases. Having asymmetric mass flow into different branches also increases the chance of separation at the opening of the constricted branch. The relative influence of the convective acceleration of the flow, as represented by the value of the flow Reynolds number, is also investigated. The particular value of the area ratio or bifurcation angle, necessary to initiate flow separation, is influenced by the Reynolds number of the incoming flow in the mother tube. In particular, the influence of all these parameters on flow properties and their relative importance is quantified. The relation between the influence of these parameters on the flow and the formation of some vascular diseases reported in the literature, is also examined.     

Safety countermeasures and crash reduction in New York City—Experience and lessons learned

Traffic fatalities and injuries constitute a major global public health problem and the United States has fallen behind other developed countries in traffic safety. Yet, New York City stands out as a traffic safety model in the nation with its low fatality rate and its significant reductions in various types of crashes. This study develops a safety framework that considers three principal axes that affect crashes: why, who, and where. While “why” concerns exposure, conflict, and speed, “who” and “where” consider the unique characteristics of the road users and the surrounding built environment. Grounded in this safety framework, the effectiveness of 13 safety countermeasures and street designs installed in New York City between 1990 and 2008 are evaluated using a two group pretest–posttest design. The potential regression-to-the-mean problem is addressed by applying the ANCOVA regression approach. The results show that signal related countermeasures that are designed to reduce conflicts: split phase timing, signal installations, all pedestrian phase, and increasing pedestrian crossing time, reduce crashes. Traffic calming measures, including road diets, are also found to have significant safety benefits. Countermeasures that are designed to alert drivers’ cognitive attention, such as high visibility crosswalks and posted speed limit reduction signs, appear to have a lesser effect. The various safety countermeasures implemented in New York City considered all three important dimensions in the safety framework: why, who, and where. The study suggests these strategies are likely to contribute to the large reductions in crashes in New York City. We also demonstrate that a rigorous quasi-experimental design can be readily deployed in transportation safety evaluation studies.     

Selection and experimental evaluation of low-cost porous materials for regenerator applications in thermoacoustic engines

This paper aims at evaluating three selected low-cost porous materials from the point of view of their suitability as regenerator materials in the design of thermoacoustic travelling-wave engines. The materials tested include: a cellular ceramic substrate with regular square channels; steel “scourers”; and stainless steel “wool”. Comparisons are made against a widely used regenerator material: stainless steel woven wire mesh screen. For meaningful comparisons, the materials are selected to have similar hydraulic radii. One set of regenerators was designed around the hydraulic radius of 200 μm. This included the ceramic substrate, steel “scourers”, stainless steel “wool” and stacked wire screens (as a reference). This set was complemented by steel “scourers” and stacked wire screens (as a reference) with hydraulic radii of 120 μm. Therefore six regenerators were produced to carry out the testing. Initial tests were made in a steady air flow to estimate their relative pressure drop due to viscous dissipation. Subsequently, they were installed in a looped-tube travelling-wave thermoacoustic engine to test their relative performance. Testing included the onset temperature difference, the maximum pressure amplitude generated and the acoustic power output as a function of mean pressure between 0 and 10 bar above atmospheric. It appears that the performance of regenerators made out of “scourers” and steel “wool” is much worse than their mesh-screen counterparts of the same hydraulic radius. However cellular ceramics may offer an alternative to traditional regenerator materials to reduce the overall system costs. Detailed discussions are provided.     

Sequences of regressions and their independences

Ordered sequences of univariate or multivariate regressions provide statistical models for analysing data from randomized, possibly sequential interventions, from cohort or multi-wave panel studies, but also from cross-sectional or retrospective studies. Conditional independences are captured by what we name regression graphs, provided the generated distribution shares some properties with a joint Gaussian distribution. Regression graphs extend purely directed, acyclic graphs by two types of undirected graph, one type for components of joint responses and the other for components of the context vector variable. We review the special features and the history of regression graphs, prove criteria for Markov equivalence and discuss the notion of a?simpler statistical covering model. Knowledge of Markov equivalence provides alternative interpretations of a given sequence of regressions, is essential for machine learning strategies and permits to use the simple graphical criteria of regression graphs on graphs for which the corresponding criteria are in general more complex. Under the known conditions that a Markov equivalent directed acyclic graph exists for any given regression graph, we give a polynomial time algorithm to find one such graph.     

Factors influencing worker use of personal protective eyewear

Objective

To identify and describe the array of factors that influence a workers’ decision to wear personal protective eyewear (PPE) and the barriers that exist in preventing their use.

Design, setting and participants

A series of focus groups enrolled workers and supervisors primarily from manufacturing, construction, or service/retail industries that had potential exposure to eye injury hazards in their job tasks. Focus group sessions were facilitated to collect qualitative and quantitative data in two categories, “sought information” and “emergent themes”, related to the factors influencing use of PPE.

Results

We conducted a series of 7 groups with 51 participants, 36 (71%) males and 15 (29%) females ranging in age from 19 to 64 years old, from a variety of occupations including construction (24%), production (22%), installation, repair and maintenance (14%), and healthcare (10%). Most were highly experienced in their occupation (>10 years); males (86%) and females (53%), and had received some safety training in the past (82%). The majority of workers in this study were required to wear PPE on their worksite (78%), however only 55% had a dedicated safety officer. A conceptual model that summarizes the “sought information” and “emergent themes” is presented that depicts the decision making process for the factors influencing use of PPE and consists of three primary branches; perceptions of hazards and risks, “barriers” to PPE usage, and enforcement and reinforcement. Lack of comfort/fit, and fogging and scratching of the eyewear were suggested as the most important barriers to PPE usage. Younger age and lack of safety training were other important factors affecting use of PPE.

Conclusions

Several potentially modifiable factors identified would lead to an increase in workers’ PPE use and encourage supervisors to provide ongoing positive feedback on the continuous use of PPE by workers at risk for an eye injury.     

Efficiency simulations of thin film chalcogenide photovoltaic cells for different indoor lighting conditions

Photovoltaic (PV) energy is an efficient natural energy source for outdoor applications. However, for indoor applications, the efficiency of PV cells is much lower. Typically, the light intensity under artificial lighting conditions is less than 10 W/m² as compared to 100-1000 W/m² under outdoor conditions. Moreover, the spectrum is different from the outdoor solar spectrum. In this context, the question arises whether thin film chalcogenide photovoltaic cells are suitable for indoor use. This paper contributes to answering that question by comparing the power output of different thin film chalcogenide solar cells with the classical crystalline silicon cell as reference. The comparisons are done by efficiency simulation based on the quantum efficiencies of the solar cells and the light spectra of typical artificial light sources i.e. an LED lamp, a “warm” and a “cool” fluorescent tube and a common incandescent and halogen lamp, which are compared to the outdoor AM 1.5 spectrum as reference.     

A simple template method for hierarchical dendrites of silver nanorods and their applications in catalysis

A novel strategy has been put forward to prepare hierarchical dendrites of silver nanorods via a simple integration method using “Devarda's template” as a reducing agent and architecture template with the assistance of ultrasonic waves, in which the template was firstly fabricated and employed. The individual silver dendrite is composed of a long central trunk with secondary branches, which preferentially grew in a parallel direction with a definite angle to the trunk. The results reveal that the dendrites are single crystalline in nature and interestingly prove that the silver single crystal has the preferential orientation in 〈1 1 1〉 direction in normal conditions. The contrast experiments demonstrated that both “Devarda's template” and the ultrasonic irradiation are necessary for building hierarchically silver dendrites in a water system. Moreover, the experimental results show that the dendrites of silver nanorods are the superior electrode materials for the electrochemical sensors to detect directly NO₂⁻ in aqueous solution.     

Intelligent speed adaptation as an assistive device for drivers with acquired brain injury: A single-case field experiment

Intelligent speed adaptation (ISA) was tested as an assistive device for drivers with an acquired brain injury (ABI). The study was part of the “Pay as You Speed” project (PAYS) and used the same equipment and technology as the main study (0030 and 0035). Two drivers with ABI were recruited as subjects and had ISA equipment installed in their private vehicle. Their speed was logged with ISA equipment for a total of 30 weeks of which 12 weeks were with an active ISA user interface (6 weeks = Baseline 1; 12 weeks = ISA period; 12 weeks = Baseline 2). The subjects participated in two semi-structured interviews concerning their strategies for driving with ABI and for driving with ISA. Furthermore, they gave consent to have data from their clinical journals and be a part of the study. The two subjects did not report any instances of being distracted or confused by ISA, and in general they described driving with ISA as relaxed. ISA reduced the percentage of the total distance that was driven with a speed above the speed limit (PDA), but the subjects relapsed to their previous PDA level in Baseline 2. This suggests that ISA is more suited as a permanent assistive device (i.e. cognitive prosthesis) than as a temporary training device. As ABI is associated with a multitude of cognitive deficits, we developed a conceptual framework, which focused on the cognitive parameters that have been shown to relate to speeding behaviour, namely “intention to speed” and “inattention to speeding”. The subjects’ combined status on the two independent parameters made up their “speeding profile”. A comparison of the speeding profiles and the speed logs indicated that ISA in the present study was more efficient in reducing inattention to speeding than affecting intention to speed. This finding suggests that ISA might be more suited for some neuropsychological profiles than for others, and that customisation of ISA for different neuropsychological profiles may be required. However, further studies with more subjects are needed in order to be conclusive on these issues.     

Non-uniform stress distribution and deformation bifurcation of thin film/substrate system subjected to gradient temperature

Stresses in thin films or coatings control the reliability of the thin film/substrate structure. By considering a circular thin film/substrate system subjected to gradient temperature, we derive relations between the non-uniform stresses in film and temperature, and between the non-uniform system curvatures and temperature. These relations featured a “local” part that involves a direct dependence of the stress or curvature components on the temperature at the same point, and a “non-local” part which reflects the effect of temperature of other points on the location of scrutiny. Furthermore, the deformation bifurcation behavior of the thin film/substrate system is analyzed. As the thermo-mismatch strain in the thin film increases, the system may transit to a biaxial curvature state (non-spherical deformation), in other words, the bifurcation of curvature will occur.     

Fibre cross-section determination and variability in sisal and flax and its effects on fibre performance characterisation

The results of a study on the measurement of fibre cross-section and its variability in flax and sisal fibres are presented. Cross-section values obtained from fibre “diameter” measurements were more than double the values obtained from actual observation of cross-sections of the same individual fibres. The overall conclusion is that fibre “diameter” measurement is not an attractive method for accurate estimation of cross-sectional area of these natural fibres. This conclusion is significant for researchers engaged in micromechanical investigation of natural fibre composites since differences in fibre cross-section translate directly into differences of the same magnitude in the values obtained for the fibre modulus and strength. The error in fibre cross-section introduced by the “diameter” method scales with the average fibre “diameter” which may also result in erroneous observations of fibre modulus and strength scaling inversely with natural fibre “diameter”. The difference in average cross-section observed from fibre to fibre was significantly greater than the variation along the length of each individual fibre. The minimum to maximum cross-section variability of individual flax fibres was found to be approximately twice that observed for sisal fibres.     

T-stress and its implications for crack growth

Some of the “irregular” crack growth behaviour observed in different specimen geometries may not be unrelated. Discrepancies in fatigue crack growth rate have been observed in different specimen geometries of the same material; crack front “tunnelling” and out-of-plane crack growth have been found in mode I tension at elevated temperature. The results presented in this paper seem to indicate the relevance of a crack tip constraint parameter, the elastic T-stress, to the irregular crack growth behaviour that conventional LEFM fails to explain.     

Tele-maintenance “intelligent” system for technical plants result management

The management of technical plant for productivity and safety is generally a complex activity, particularly when many plants distributed in the territory are considered (i.e. the more and more frequent case of outsourced plants maintenance by specialized companies), granted quality and cost results are required (i.e. the case of some rather innovative contract solutions) and the technology involved is heterogeneous and innovative (i.e. electro-mechanical plants).In order to efficiently achieve the above aims an “intelligent” maintenance-management system for the distant monitoring and controlling by a remote control center has been developed. The so-called GrAMS (granted availability management system) system is conceived to give to organizations involved in technical-industrial plants management the possibility to tend to a “well-known availability” and “zero-failures” management.In particular, this study deals with the diagnostic aspects and safety level of technical plants (such as elevators, thermo-technical plants, etc.), and with the involvement of ad hoc designed software analysis tools based on neural networks and reliability indicators.Part of the research dealing with the tele-maintenance intelligent system has been financed by the Italian High Institute for Safety (ISPESL) and led to the development of a pre-industrial prototype whose realization and testing is here described.