عنوان مقاله [English]
Fatigue design of helicopter components has proven to be a difficult and challenging task. The purpose of the fatigue evaluation of helicopter components, especially the main rotor blade, is to show that catastrophic failure expectation during its service life due to fatigue is completely gone; in addition, occurrence of Non-catastrophic cracking remains low enough so that the economic use of the helicopter structure can be achieved. The fatigue design methodology, most commonly applied by the helicopter community, was based on the Safe-Life philosophy. In spite of safety and the overall success in helicopter applications, this method does not account for any component strength that deviates from the strength distribution assumption made during the fatigue substantiation process. In recent years, due to low efficiency of Safe-Life method, using other fatigue life analysis methods, such as flaw tolerance, damage tolerance, and fault tolerance, have been felt to be more necessary.The Flaw Tolerant Safe-Life or Enhanced Safe-Life method provides component management requirements based on the assumption of the existence of flaws in the components critical areas. This approach is adopted only in the helicopter community. Flaw Tolerant method is based on traditional safe-life design methodology that uses special S-N curves obtained from specimens containing some defects (flaws). Damage tolerance requires that the structures retain, after a partial failure, the capability to withstand the limit load. Fault tolerance is a new damage tolerance design method which is based on equivalent initial flaw-sized concept. Since 1989, the Airworthiness Regulation evolved towards the application of Damage Tolerance principles to rotorcraft. This change forced the helicopter industries to review their design methodologies and to face new problems, linked with fracture mechanics applications to their typical structures.In this paper, various approaches to helicopter fatigue design are reviewed, and the advantages and disadvantages of each method are discussed. Then, different methods of helicopter fatigue design are compared. At the end, other aspects of helicopter fatigue design, such as influence of material on helicopter fatigue design, fatigue design of helicopter components against impact loads, and conducting tests in relation to the helicopter fatigue design are all discussed.