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International Journal of Reliability and Safety (3 papers in press)
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.