APPLICATION OF DAMAGE TOLERANCE METHODOLOGY TO HELICOPTER DYNAMIC COMPONENTS USING THE EXAMPLE OF A MAIN ROTOR PITCH LINK FOR THE AH-1S HELICOPTER

The helicopter community’s growing interest in damage tolerance design and substantiation stems from existing military requirements and new commercial requirements. This interest is reinforced by the successful application of damage tolerance philosophy to fixed-wing structures. Experience in damage tolerance comes mainly from its application to the simple geometries of cracks in large, thin shells, which are common for fixed-wing structures, ship’s hulls, and pressure vessels. Rigorous application of damage tolerance methodology to new types of structural geometry and complex loading, which are characteristic of helicopter components requires expensive and time consuming research and development efforts. This paper presents a simplified engineering approach to damage tolerance substantiation based on the concept of equivalent structure. This approach allows the user to replace complex structures, their loadings, the geometry of the crack plane and the crack itself with simplified counterparts for which the elements of the crack growth model are known, documented, and verified. One result of using the simplification criteria is moderate conservatism in the crack growth predictions. The proposed approach is illustrated by its application to the analysis of a rod end bearing housing for the AH-1S main rotor pitch link. Crack growth testing was conducted for the critical cross sections which were identified by analysis. The prediction of crack growth which resulted from the analysis was verified by the test. The material parameters associated with crack growth prediction were verified by testing center-cracked panel coupons. The accuracy of the equivalent structure method for determining the Stress Intensity Factor (SIF) has been evaluated by comparing the results with available numerical solutions.