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Browsing Industrial Engineering by Author "Eryılmaz, Serkan"
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Item A new extended δ-shock model with the consideration of shock magnitude(Applied Stochastic Models in Business and Industry, 2024-07-14) Lorvand, Hamed; Eryılmaz, SerkanIn this article, a new -shock model that takes into account the magnitude of shocks is introduced and studied from reliability perspective. According to the new model, the system breaks down if either a shock after non-critical shock occurs in a time length less than or a shock after a critical shock occurs in a time length less than where . The distribution of the system's lifetime is studied for both discrete and continuous intershock time distributions. It is shown that a new model is useful to describe a certain cold standby repairable system.Article A new generalized 𝜹-shock model and its application to 1-out-of-(m + 1):G cold standby system(Reliability Engineering and System Safety, 2023-02-28) Eryılmaz, Serkan; Ünlü, Kamil DemirberkAccording to the classical 𝜹-shock model, the system failure occurs upon the occurrence of a new shock that arrives in a time length less than 𝜹; a given positive value. In this paper, a new generalized version of the 𝜹-shock model is introduced. Under the proposed model, the system fails if there are m shocks that arrive in a time length less than 𝜹 after a previous shock, m 1. The mean time to failure of the system is approximated for both discretely and continuously distributed intershock time distributions. The usefulness of the model is also shown to study 1-out-of-(m + 1):G cold standby system. Illustrative numerical results are presented for geometric, exponential, discrete and continuous phase-type intershock time distributions.Article Age based preventive replacement policy for discrete time coherent systems with independent and identical components(Reliability Engineering and System Safety, 2023-11) Eryılmaz, SerkanThe paper is concerned with an age based preventive replacement policy for an arbitrary coherent system that consists of components that are independent and have common discrete lifetime distribution. The system having an arbitrary structure is replaced preventively after a specific number of cycles or correctively at its failure time. The necessary conditions for the unique and finite replacement cycle that minimize the expected cost per unit of time are obtained. The policy is studied for some particular system models including the well-known -out-of- structure. The findings of the paper extend the results in the literature from single unit and parallel systems to an arbitrary coherent system. Numerical results are presented for particular discrete component lifetime distributions.Article Age replacement policy for heterogeneous parallel systems(Journal of Computational and Appleid Mathematics, 2024-03-01) Bulanık Özdemir, İrem; Kılıçoğlu, Sevval; Eryılmaz, SerkanThe optimization policy on age replacement mostly focuses on systems comprised of identical components. In this paper, both discrete and continuous time age replacement policies are considered by relaxing the assumption of identical components and working with heterogeneous parallel system, i.e. system with not necessarily identical components. In particular, necessary conditions are obtained for the existence and uniqueness of optimal replacement cycle/time for the parallel system with two nonidentical components under the proposed policy. The extension of the results to a system with more than two components is also presented.Article Computing waiting time probabilities related to (k1, k2, ..., kl) pattern(Statistical Papers, 2023-11-26) Chadjiconstantinidis, Stathis; Eryılmaz, SerkanFor a sequence of multi-state trials with l possible outcomes denoted by f1; 2; :::; lg, let E be the event that at least k1 consecutive 1s followed by at least k2 consecutive 2s,..., followed by at least kl consecutive ls. Denote by Tr the number of trials for the rth occurrence of the event E in a sequence of multi-state trials. This paper studies the distribution of the waiting time random variable Tr when the sequence consists of independent and identically distributed multi-state trials. In particular, distributional properties of Tr are examined via matrix-geometric distributions.Article On 𝜹-shock model with a change point in intershock time distribution(Statistics & Probability Letters, 2024-06-25) Chadjiconstantinidis, Stathis; Eryılmaz, SerkanIn this paper, we study the reliability of a system that works under 𝛿-shock model. That is, the system failure occurs when the time between two successive shocks is less than a given thresh old 𝛿. In a traditional setup of the 𝛿 shock model, the intershock times are assumed to have the same distribution. In the present setup, a change occurs in the distribution of the intershock times due to an environmental effect. Thus, the distribution of the intershock times changes after a random number of shocks. The reliability of the system is studied under this change point setup.Article Optimal age replacement policy for discrete time parallel systems(TOP, 2023-10) Eryılmaz, Serkan; Tank, FatihIn the case of discrete age replacement policy, a system whose lifetime is measured by the number cycles is replaced preventively after a specific number of cycles or correctively at failure, whichever occurs first. Under the discrete setup, the policy has been mostly considered for single unit systems. In this paper, a discrete time age replacement policy is studied for a parallel system that consists of components having discretely distributed lifetimes. In particular, the necessary conditions for the unique and finite replacement cycle that minimize the expected cost rate are obtained. The theoretical results are illustrated with numerical examples to observe the effect of the cost values and the mean lifetime of the components on the optimal replacement cycle.Article Reliability and performance evaluation of weighted k-out-of-n:G system consisting of components with discrete lifetimes(Reliability Engineering and System Safety, 2024-08-31) Eryılmaz, SerkanFor the k-out-of-n system consisting of components that have di§erent weights, the system is in a good state if the total weight of working components is at least k. Such a system is known to be weighted k-out-of-n:G system. Although the weighted k-out-of-n system that has continuously distributed componentsílifetimes has been extensively studied, the discrete weighted k-out-of-n:G system has not been consid ered yet. The present paper Ölls this gap by modelling and analyzing the weighted k-out-of-n:G system that consists of discretely distributed componentsílifetimes. In particular, the behavior of the total capacity/weight of the system with respect to the component failures is evaluated. An optimization problem that is concerned with the determination of optimal number of spare components is also formulated by utilizing the mean lost capacity of the system.Article Reliability of a mixed 𝜹-shock model with a random change point in shock magnitude distribution and an optimal replacement policy(Reliability Engineering and System Safety, 2023-04) Chadjiconstantinidis, Stathis; Eryılmaz, SerkanA mixed 𝛿-shock model when there is a change in the distribution of the magnitudes of shocks is defined and studied. Such a model which is a combination of the 𝛿-shock model and the extreme shock model with a random change point (studied by Eryilmaz and Kan, 2019), is useful in practice since a sudden change in environmental conditions may cause a larger shock. In particular, the reliability and the mean time to failure of the system are evaluated by assuming that the random change point has a discrete phase-type distribution. Analytical results for eval-uating the reliability function of the system for several joint distributions of the interarrival times and the magnitudes of shocks, are also given. The optimal replacement policy that is based on a control limit is also proposed when the number of shocks until the change point follows geometric distribution. The results are illustrated by numerical examples.Article Statistics and probability theory in renewable energy: Teaching and research(Applied Stochastic Models in Businesss and Industry, 2023-12-03) Eryılmaz, Serkan; Kateri, Maria; Devrim, YılserIn this paper, the key-role and utility of statistics and probability theory in the field of renewable energy are emphasized and illustrated via specific examples. It is demonstrated that renewable energy is a very suitable field to effectively teach and implement many statistical and probabilistic concepts and techniques. From a research point of view, statistical and probabilistic methods have been successfully employed in evaluating renewable energy systems. These methods will continue to be of core interest for the renewable energy sector in the future, as new and more complex renewable energy systems are developed and installed. In this context, some future research directions in relation to the evaluation of renewable energy systems are also presented.