Abstract:

Ultrasonic inspection techniques using circumferential creeping waves can readily detect radial fatigue cracks on the fair side of thin airframe stiffener holes in empty aircraft wings. It is desired to utilize similar techniques in detecting fatigue cracks in the case of fluid-filled fuel tanks. The dispersion equation of a fluid-filled cylindrical cavity in an infinite elastic medium has been studied numerically. It was found that two distinct waves can propagate. One is the circumferential creeping wave that is attenuated by radiating energy into both the fluid and the surrounding solid. The other is the essentially nonattenuated halo wave caused by the resonances in the fluid within the cavity. The slower halo wave is the strongest of the two, but unfortunately it is not sensitive to the existence of any cracks at the surface because it propagates mostly in the fluid. It is the Rayleigh-type creeping wave that is important in the detection of surface breaking or near-surface cracks. It can be concluded that the adverse effect of fluid loading on the inspection technique is significant but not prohibitive. Additional finite-element calculations as well as experimental measurements are being conducted. [See NOISE-CON Proceedings for full paper.]