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Top1. Introduction
In the past, many companies considered maintenance as an inevitable source of cost. For these companies, maintenance operations have a corrective function (fix it on failure) and are only executed in emergency conditions. Today, this form of intervention is not acceptable because of certain critical elements such as product quality, plant safety, and increased costs of maintenance departments (Bevilacqua and Braglia, 2000). Furthermore, in the current competitive world, manufacturing firms attempt to improve their performance in terms of cost, quality, and flexibility, in an effort to compete with other firms of the global marketplace (Ertugrul Karsak and Tolga, 2001). In the manufacturing firms, various problems such as manufacturing technology selection, maintenance strategy selection, machine location, and evaluation of quality function would influence production cost, product quality, and product delivery-time (Bashiri, Badri, and Hejazi, 2011). As indicated by Mobley (2002), one third of all of the maintenance costs are wasted as a result of unnecessary maintenance. The use of inefficient maintenance policies considerably increases the direct maintenance costs (Rastegari and Bengtsson, 2014). On the other hand, the manufacturing firms are under a great pressure to continuously reduce their production costs. One of the main expenditure items for these firms is their maintenance costs, which can comprise up to 15%-70% of the overall production costs according to the type of industry (Bevilacqua and Braglia, 2000; Rastegari and Bengtsson, 2015).
In recent years, the importance of maintenance strategy selection has been increased due to the critical role it has for increasing the availability, the safety, and the mean time to failures, improvement of system reliability and product quality, reducing shutdown time of factory, and preventing unnecessary investments. Various methodologies have been used for selecting a suitable maintenance strategy in the literature as presented as follows. Triantaphyllou, et al. (1997) have suggested a method to address criticality of the criteria related to the problem of maintenance strategy selection. They first prioritized the maintenance strategies using the AHP and then performed a sensitivity analysis on different criteria to identify the most important one. The maintenance strategy selection was presented by Luce (1999), Okumura and Okino (2003) based on different production loss and maintenance costs incurred by different maintenance strategies. Azadivar and Shu (1999) proposed a method for selecting an appropriate strategy for each class of systems in a just in time environment exploring 16 characteristic factors that could play a role in the maintenance strategy selection. Using the AHP method for the maintenance strategy selection was suggested by Bevilacqua and Braglia (2000). Al-Najjar and Alsyouf (2003) and Sharma, et al. (2005) evaluated the maintenance strategies through the fuzzy interference system (FIS) and the fuzzy multi-criteria decision-making methods. Mechefske and Wang (2003) used the fuzzy linguistic variables to evaluate and select the optimum maintenance strategy and condition monitoring technique. Bertolini and Bevilacqua (2006) presented a combined model based on the AHP and the goal programming (GP) to identify the best maintenance strategy for the critical centrifugal pumps in an Italian oil refinery.