In this article an on-line method is proposed to monitor the health of a structure during its operational loading. A 5 volts amplitude multi-sine Surface Acoustic Wave (SAW) is continuously transmitted and sets of typically 100 ultrasonic measurements are recorded. The nonlinearity of the amplitudes within the measurement set appeared to be sensitive (0.5mm cracks could be detected) and robust (with respect to changes in the operational conditions) in order to detect the presence of a fatigue crack. Of course the applicability is limited to those situations where the crack opens and closes during operation. This is the case for many airplane components as for instance the slat track (that is used to enlarge the surface of the wing at take-off and landing). The method was applied to a growing fatigue crack while an aircraft was operating. In contrast to classical ultrasonic measurement methods using high voltage pulse, an optimized multi-sine excitation signal with an amplitude of a few volts only was applied to agree with safety regulations for aircraft. Furthermore, an indicator quantifying the nonlinearity of the ultrasonic surface wave propagation is used. By using a nonlinearity index the influence of changing operation conditions that can be observed with most linear methods is eliminated. The proposed method is validated on a steel beam that is fatigue loaded with a force signal obtained from in-flight data.
Tracking of cracks in airplane components using nonlinear surface wave propagation techniques
Risssuche in Flugzeugteilen mit nichtlinearer Oberflächenwellenfortpflanzung
2005
8 Seiten, 8 Bilder, 14 Quellen
Conference paper
English
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