In this research is tried to determine the optimal interval for preventive maintenance and repair for a segment in a system according to considered criteria for decision making and also with considering the constraints that exist in the real world for this decision. In order to determine the optimal interval for maintenance and repair regarding to multiple criteria MCDM methods are used. One of the constraints that we encounter with in the real world and is considered in this research is that decision maker tends to have acceptable limits so that, firstly if an alternative doesn't satisfy decision maker's expectations, this alternative must be eliminated and secondly if an alternative lies within the limits but far from the ideal boundary of decision maker, the utility of this alternative must be decreased. For achieving this purpose and considering this requirement in the model multiple choice goal programming is used. Another constraint that is considered in this research is that in a real situations when encountering with an unplanned failure, failure maintenance cost and failure maintenance time are not known clearly and this fact must be considered in a model used for choosing the optimal interval for maintenance and repair. In order to considering these situations in the second model, parameters are assumed to befuzzy and modified linear assignment method is used to solve the model. Some examples are solved for two models in the final section and sensitive analysis for various parameters is investigated.
