A non-linear acoustic, vibro-modulation technique for the detection and monitoring of contact-type defects, including those associated with corrosion

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 side-band 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.