Simultaneous Ultrasonic Monitoring of Crack Growth and Dynamic Loads during a Full Scale Fatigue Test of an Aircraft Wing

An in situ ultrasonic angle beam method has been developed to detect and size fatigue cracks emanating from fastener holes. The method employs transducers mounted in a pitch-catch configuration on each side of the hole, and the modulation of received signals with load is used to detect and size small fatigue cracks. The focus of this paper is the monitoring of a series of critical fastener holes during a full scale fatigue test of an aircraft wing. Essential to the success of this method is the ability to use the same ultrasonic signal as is used for crack monitoring to estimate the instantaneous load. The applied load causes a time shift in ultrasonic echo arrivals resulting from both dimensional changes and the acoustoelastic effect. Waveforms are recorded randomly during fatiguing, and the dynamically applied load associated with each waveform is estimated from the measured time shift. Additionally, small cracks near the hole open and close during fatigue loading, causing a modulation of the amplitude of the received signals with load. The amount of energy reduction as a function of load is analyzed to estimate the cumulative area of cracks near the fastener hole. The load monitoring capability of the ultrasonic method is also valuable for quantitatively assessing the degree of local loading experienced by a specific fastener hole, which may not be possible with strain gauges. The wing panel test reported here took place over about seven weeks with near real time reporting of results, which has demonstrated the robustness of the ultrasonic method and its ability to both detect cracks and monitor their growth.