2. Literature review and state of the art
In general, maintenance strategies can be subdivided into two categories: on the one hand, corrective
maintenance, also known as run-to-failure or reactive maintenance, and on the other hand, preventive maintenance.
Corrective maintenance implies that the maintenance activities start with the detection of a failure. With this
strategy, the system or the component will be maintained or even replaced after it breaks. The particular type, timing
and extent of necessary repair measures are unknown and cannot be planned. In contrast, preventive maintenance
strategy implies that the activities will be done as a precaution in order to avoid any possible failures. Besides this,
the preventive maintenance can again be divided into two parts. One part is the time-based maintenance, which
means that the maintenance measures will be carried out after a predefined operating hour’s interval or after a
particular time horizon. It is accepted that the lifetime is usually not completely exploited. The other part is the
condition-based maintenance (CBM), which often leverages a condition monitoring system in order to observe
different indicators, which can describe the condition of the considered system [12]. Hence, on a theoretical basis,
the maintenance program can be planed whenever one or more indicators show that something is going to fail [13].
The basis for the strategy is that 99% of all failures are announced through measurable indicators [14].
An advancement of the main strategies is reliability centered, risk based and total productive maintenance. It
should be mentioned that there are several more derivatives, which generally exist in order to address particular
requirements of the overall systems [15]. The reliability centered maintenance concept focuses on cost-efficiency