A NON-LINEAR ACOUSTIC, VIBRO-MODULATION TECHNIQUE FOR THE DETECTION AND MONITORING OF CONTACT-TYPE DEFECTS, INCLUDING THOSE ASSOCIATED WITH CORROSION

摘要

We describe an acoustic, vibro-modulation technique, based on nonlinear interaction of ultrasound and vibrations in the presence of defects, which has been developed for non-destructive detection of contact-type interfaces such as at cracks, debonding, delaminations and crevice corrosion. Low frequency vibration is employed to cause a variation in the contact area of the defect surfaces leading to modulation of a probing ultrasonic wave. Due to this effect, the spectrum of the received probe signal contains additional sideband spectral components at the combination frequencies of the ultrasound probe and vibration. Most significantly, the presence of these components allows for differentiation of the contact-type defects from other structural and material inhomogeneities. Examples of successful application of this technique include detection of stress corrosion cracks, corrosion-induced delamination of structural elements, fatigue cracks in various materials (aluminum, steel, titanium), disbonding and cracks in composite materials. Recent tests have also demonstrated that the vibro-modulation technique has a potential for assessment of aircraft lap-joint corrosion. The first prototype system of a practical instrument which utilizes the vibro-modulation technique, has been developed and tested. The system has proven to be effective for nondestructive testing of full-scale heterogeneous structures without the need to physically scan a probe over the structural components of interest.