An ultrasonic wheel-array sensor and its application to aerospace structures

This paper describes the development of an ultrasonic wheel-array sensor scanning system. The system operates at 10 MHz using a 64-element array transducer which is 50 mm in length and located in a fluid-filled wheel. The wheel is coupled to the test structure dry, or with a small amount of liquid couplant. An aperture is swept along the array forming a B-scan at each measurement position. The wheel is rolled over the surface of the test structure and a defect map (C-scan) is generated in real-time. The tyre is made from a soft, durable polymer which has very little acoustic loss. The rapid scanning is facilitated by an embedded PC which controls all of the functions of the array transmit and receive module as well as undertaking signal processing of the received waveforms. Three application studies are presented. These are the detection of embedded defects in a thick-section carbon composite test block, the detection of cracking and disbonding in a carbon composite skin-stringer sample and the inspection of sealant layers in an aluminium aircraft wing structure. In each application, the wheel-array sensor was found to produce C-scans of comparable quality to an immersion system with scanning times reduced by factors of 25-50.