International Journal of Reliability and Safety (4 papers in press)

Regular Issues

• Complementary optimal age maintenance policy for repairable systems  
  by Nse Udoh, Effanga Effanga, Christian Onwukwe 
  Abstract: A complementary optimal age maintenance policy is proposed in this work for repairable systems that require a high level of availability and safety standard by exploiting the comparative advantage of minimum expected cost-based optimal age replacement policy and maximum limiting availability-based optimal age replacement policy. The failure distribution, cost of preventive and failure maintenance and downtime of preventive and failure maintenance, which affect the age at which an operating system is replaced, were used to formulate the expected cost and limiting availability functions for repairable systems.
  Keywords: repairable system; failure distribution; cost and availability functions; replacement maintenance.
  
  
• A log third order polynomial normal transformation approach for high reliability estimation with scarce samples  
  by Palaniappan Ramu, Harshal Kaushik 
  Abstract: Normal transformations are often used in reliability analysis. A third order polynomial normal transformation (TPNT) approach is used in this work. The underlying idea is to approximate the cumulative distribution function (CDF) of the response in probit space using a third order polynomial while imposing monotonicity constraints. The current work proposes to apply log transformation to the ordinate of the transformed CDF and hence the approach is called Log-TPNT. The log transformed data assists in improved fitting to the tails of the distribution resulting in better predictions of extreme values. Log-TPNT is demonstrated on a suite of statistical distributions covering all types of tails and analytical examples that cover aspects of high dimensions, nonlinearity and system reliability. The results reveal that Log-TPNT can predict the response values corresponding to high reliability, with samples as scarce as 9. Finally, the variations associated with the response estimates are quantified using bootstrap.
  Keywords: reliability; cumulative distribution function; normal transformation; polynomial fit; bootstrap.
  
  
• Optimal design of accelerated degradation test plan under modified ramp-stress loading  
  by Preeti Wanti Srivastava, Manisha Budhiraja 
  Abstract: This paper deals with the design of optimal modified ramp-stress accelerated degradation test (ADT) using the assumption that the products degradation path follows Wiener Process. In an ADT, failure occurs when the performance characteristic crosses the critical value the first time. The model parameters are estimated using method of maximum likelihood. The optimum plan consists in finding out optimum number of specimens, optimum stress change point(s), and optimum stress rates by minimizing asymptotic variance of estimate of qth quantile life at use condition subject to the constraint that total testing or experimental cost does not exceed a pre-specified budget. A numerical example is given to demonstrate the proposed model. Sensitivity analysis is carried out to examine the robustness of the proposed plan.
  Keywords: accelerated degradation test; modified ramp-stress; Wiener process; inverse Gaussian distribution; variance optimality criterion.
  
  
• A system dynamics simulation model for environmental risk assessment at strategic level in power plants  
  by Sahar Al Mashaqbeh, Jose Eduardo Munive-Hernandez, M.Kurshid Khan, Awni Al Khazaleh 
  Abstract: In a constantly changing business environment, a systematic approach is needed for risk assessment in order to allow for a more long-term strategic view. The System Dynamics (SD) modelling technique can be applied as an effective approach to understanding the dynamic behaviour of a system over time. This understanding can be subsequently explicitly reflected on policies, strategic plans and operational procedures. This paper presents a SD model to assess environmental risks in power plants. The model considers a long-term strategic view. The model helps to understand the long-term behaviour of the system under study. A questionnaire and focus group interviews have been conducted to understand the relationship among various key risks. Causal loop diagrams are developed to identify key risks and define their interdependencies. Stock and flow diagrams are built to quantify these interdependencies. Two scenarios were considered for simulation of the impact of waste handling risk and environmental regulation risk. The SD model has been validated with two power plants in the Middle East. The developed SD model highlighted the impact of key environmental risks on the performance of power plants. This performance considers availability factor, efficiency and operational and maintenance cost. Although the SD model focuses on risk assessment in power plants, it can be easily adapted to other industry sectors.
  Keywords: system dynamics; non-technical risks; environmental risks; FMEA; EWGM.