Narrow Results

We consider two repair-replacement policies for a cold standby system consisting of two components with a single repairman. It is assumed that each component after repair is not "as good as new". With this assumption by using geometric process we developed two replacement policies based on the number of down times of the component-1. Our problem is to choose optimal replacement policy (k) such that the long run expected reward per unit time of the system maximized. The mathematical expressions for the long run expected reward per unit time are evaluated and corresponding optimal replacement policies are obtained theoretically with numerical example and by simulation study. Also we have discussed Newton–Raphson method to find optimal k.

In this paper, we consider the arithmetico-geometric process (AGP) repair model. Here, the system has two nonidentical component cold standby repairable system with one repairman. Under this study, component 1 has given priority in use. It is assumed that component 2 after repair is as good as new, whereas the component 1 follows AGP. Under these assumptions, by using AGP repair model, we present a replacement policy based on number of failures, $N$, of component 1 such that long-run expected reward per unit time is maximized. For this policy, system can be replaced when number of failure of the component 1 reaches to $N$. Working time of the component 1 is AGP and it is stochastically decreasing whereas repair time of the component 1 is AGP which is stochastically increasing. The expression for long-run expected reward per unit time for a renewal cycle is derived and illustrated proposed policy with numerical examples by assuming Weibull distributed working time and repair time of the component 1. Also, proposed AGP repair model is compared with the geometric process repair model.

This paper discusses preventive replacement policies for an independent damage process, in which successive shocks occur at random times, and the independent damages caused by shocks are random variables. It is assumed that a unit fails when the damage has exceeded a prespecified level $K$. We consider three models: Model 1: the unit is replaced at the $M$th shock for damage $Z$, at damage $K$, or at time $T$, whichever occurs first. Model 2: the unit is replaced at the $M$th shock for damage $Z$, at damage $K$ or at shock $N$, whichever occurs first. Model 3: the unit is replaced at the $M$th shock for damage $Z$, at damage $K$, at shock $N$ or at time $T$, whichever occurs first. We obtain the expected cost rates for each model and discuss analytical optimal $T^{\ast }$ and $N^{\ast }$ to minimize their expected cost rates. Numerical examples are given when the damage has an exponential distribution and shocks occur at a gamma distribution.

The Laplace transform order of reversed residual life, introduced by [I. Elbatal, The Laplace order and ordering of reversed residual life, Appl. Math. Sci.36 (2007) 1773–1788.] is used in the stochastic comparison of two life distributions. In this paper, we aim to study new notions of stochastic comparisons based on bivariate Laplace transform order of reversed residual lives. We investigate relationships the new stochastic order has with other existing bivariate orders. The interpretation of the new orders and their applications in different contexts are also pointed out.

This paper investigates the optimal state-dependent replacement policy for semi-Markovian deteriorating systems. It is assumed that the replacement time, replacement cost, operating cost rate and sojourn time distributions are all state-dependent. The optimization criterion is to minimize the expected long-run cost per unit time. Under some sufficient conditions on the cost and time parameters, we show that the optimal policy is of control limit type and has monotonic properties which facilitate the derivation of the optimal policy. Finally, a numerical example is given to illustrate the structure of the optimal policy.

This paper proposes a replacement policy for a cumulative damage model where a unit suffers damage caused by both shocks and inspections, and fails when the total damage has exceeded a failure level. To detect such failure, a unit is inspected at periodic times kT(k = 1, 2, …), and is replaced by a new one at detection of failure or at time nT, whichever occurs first. The expected cost rate is derived, when shocks occur at a non-homogeneous Poisson process, and an optimal number n* which minimizes it is discussed. Numerical examples are finally given.

Location-Dependent Query is becoming more and more important in mobile environments. In this paper, to improve system performance, author proposes a new model for LDQ. Firstly, a new structure of semantic index is defined, which can store semantic segment circle and timestamp information. Then we analyze the detailed query processing based on our semantic model. To overcome the shortage of FAR at location prediction and influence of semantic valid scope to cache hit ratio, we propose a new improved replacement algorithm (RBF-FAR), in which a cost function P based on semantic segment distance and valid scope, is introduced as replacement criterion. The experiment results show that new model can gain better performance at response time, network-load and cache hit ratio, than the model on the basis of FAR algorithm.