Experimental study on the nonlinear mixing of ultrasonic waves in concrete using an array technique

This research develops a procedure that combines ultrasonic array technology with non-collinear ultrasonic wave mixing to detect and scan internal microscale damage in concrete. Exploiting the underlying mechanics of nonlinear wave mixing, two incident shear wave beams generated by two ultrasonic transducer arrays (and with frequencies low enough to propagate without being overly scattered by the aggregate) are used to create a longitudinal mixed wave, the amplitude of which is a direct measure for the amount of localized, internal damage. The arrays enable beam steering, making it possible to scan for damage along an arc. The effectiveness of the proposed technique is demonstrated by characterizing different types of microscale damage embedded at known locations in a concrete prism specimen. These results establish the feasibility of using a nonlinear wave mixing array technique for the efficient inspection of internal microscale damage in a large concrete structure.