The effects of fretting on fatigue characteristics of a mechanically fastened aircraft joint.

摘要

A research study to investigate the effects of fretting on fatigue characteristics of an aircraft joint was carried out. The selected joint for this study simulates the rotor head of an aircraft capable of taking off vertically. The primary function of this hub-spindle joint is to retain the main rotor blade against the centrifugal forces, both in-plane and out-of-plane bending moments and torsion caused due to the lift, drag and other aerodynamic forces imposed on the rotor blades while the aircraft is in forward flight.; The primary objectives of this study were twofold; (a) Verify that the average lives of mechanically fastened joints with combined effects of fretting and fatigue will be lower compared to the average lives due to plain fatigue. (b) Discover whether fretting causes cracks to nucleate and fatigue causes those cracks to propagate.; In order to verify the validity of the first hypothesis, seven test joints were tested to failure. Several S/N curves were generated against Mil-Handbook 5H data for comparable plain fatigue response of the same material. Out of the seven specimens that were tested, five were machined from Aluminum 7075-T6, and the other two were machined from Aluminum 7050-T7451. An average fretting fatigue life reduction factor K_ff, of 21 was found for all these seven joints.; In order to validate the second hypothesis, a detailed investigation under a scanning electron microscope of the fretted/failed surfaces was conducted. Severe fretting damage was observed in all test specimens. It was found that fretting-induced damage provided the crack nucleation sites in all test specimens that failed. These nucleation sites were in the form of fretting scars, pits and gouges providing several regions of stress concentration. Under the influence of high tensile stress fields, these sites allowed several small embryonic cracks to form, coalesce and link up to form primary and multiple cracks, which subsequently propagated under the applied cyclic loads leading to final instability, i.e., fracture.; Fretting fatigue should be treated as a major threat against structural integrity of joints. Currently, there are no known analytical models available to design against fretting fatigue. It is therefore recommended that in areas of critical applications full scale tests should be conducted prior to actual use.