Crack Growth Analysis and Test of a Main Rotor Damper Attachment Lug

This paper describes the crack growth analysis and verification test for an Al7075-T73 Main Rotor Damper Attachment Lug. The analysis involved 2D and 3D stress analysis using the MARC finite element code, stress intensity analysis using the FRANC3D and NASGRO codes, crack growth rate data fit with a closure model, and crack growth analysis using the NASGRO crack growth code. The crack growth testing involved fatigue testing using a load cylinder to introduce the vibratory damper loads, the introduction of notches at high stress locations as crack initiators, and the tracking of cracks using acetate replicates examined in a Scanning Electron Microscope (SEM). The predicted crack growth compared well with the measured crack growth on the lug surfaces. However the predicted and measured final crack shapes were somewhat different. Conclusions include emphasis on good stress analysis, stress intensity analysis, and crack growth rate data to provide accurate crack growth predictions.