Sonic Fatigue of Simulated Riveted Joints

Abstract

The fatigue of a structure clamped to a support with a row of simulated rivets was investigated. The structure consisted of a 2024-T3 aluminum sheet (2 in.×3.5 in.×0.032 in.) cantilevered in a closely fitting slot in a siren test duct. Three aluminum bolts were used to simulate a clamping row of countersunk rivets. This design was chosen because it eliminated membrane stress at the rivet line and provided for control of clamping tension. The cantilever was excited at its fundamental resonance by plane waves of sound propagating at grazing incidence in the duct. The nominal strain at the rivet line was set by controlling the amplitude of motion at the end of the cantilever. Three different rivet diameters were used in fatigue life measurements: 0.125, 0.250, and 0.375 in. Each rivet size was tested at three strain levels. The fatigue lives ranged from 0.4×105 to 4×105 cycles. The fatigue-strain-concentration factors measured in this experiment varied from 1.5 for the 0.125-in.-diam rivet to 1.7 for the 0.375-in.-diam rivet. The concentration factor for the 0.125-in. diam rivet was found to be close to that measured in a subsequent experiment on reversed bending of a cantilever beam with a 0.125-in.-diam hole. An important decrease in fatigue resistance occurred in going from a rivet diameter of 0.125-in. to a diameter of 0.250 in.; a relatively small change was observed from 0.250 in. to 0.375 in. It appears that small rivets will increase sonic fatigue resistance of riveted joints. (Research sponsored by Aeronautical Systems Division, Air Force Systems Command, U. S. Air Force.)