Towards Weight Savings for Damage Tolerant Masts and Driveshafts

Abstract

The design of a rotorcraft main rotor mast or tail rotor driveshaft is critical to the sizing of many drive system parts, including gearboxes and attachments to the airframe. A reduction in diameter of these parts could result in significant weight savings for the aircraft. Due to the criticality of masts and driveshafts, a damage tolerant design is highly desirable. However, damage tolerance requirements for single load path structures tend to produce heavier parts than those designed using safe-life methods. To alleviate the weight impact without sacrificing safety, the damage tolerant design of the mast and driveshaft must consider all aspects of design. This may include non-planar crack propagation, asymmetrical part-through crack growth, and the effect of surface treatments such as shot peening, carburization, etc. The present research incorporates effects that slow crack propagation into fatigue crack growth analysis, supported by a building-block test program of coupon and element level specimens. The test results show that surface condition influences crack front shape and aspect ratio, and that complex geometry can cause asymmetrical and non-planar crack growth. A NASGRO® simulation of a fillet radius element specimen showed a good correlation with testing results for complex geometry without surface residual stress, and future work is proposed to conduct a fatigue crack growth simulation with the effects of typical surface treatments. Successful prediction of crack growth in masts and driveshafts may lead to substantial weight savings in those parts, and the potential for similar improvements in other affected drive system components.