Fatigue Strength Improvement of Helicopter Steel and Titanium Lugs with the ForceMate&[reg] Process: Analytical Prediction versus Test Results

Lug type joints are used extensively in helicopter rotor control systems. These lug joints operate in a high vibratory load and frequency environment and are susceptible to fatigue damage, especially fretting fatigue. Traditionally, shrink fit bushings are used to reduce effects of fretting and improve fatigue strength and life. In the latest rotor designs, expanded bushings are installed using Fatigue Technology Inc.'s (FTI) ForceMate® process to achieve further improvement in fatigue strength and life. FTI has developed finite element analysis (FEA) methods to predict the improvements in fatigue strength of control system lug joints with ForceMate® bushings installed compared to shrink fit bushings under various loading conditions. FTI has presented a paper comparing test results with analytical results in a paper presented at AHS in 2008 for aluminum lug type of joints. This paper presents findings of similar study for titanium and steel lugs. FEM Analysis was performed by FTI and testing was conducted by Bell Helicopter. The test coupons blanks were provided by Bell and coupons were prepared by FTI. Test results and analytical predictions for titanium and steel lugs are presented in this paper.