Ultrasonic array inspection of the Clifton Suspension Bridge chain-links

Wrought iron chain-links from the Clifton Suspension Bridge have been inspected for fatigue cracking. The inspection uses a 64-element 2 MHz, linear ultrasonic array probe. A shoe is used to transmit ultrasound from the array, through the curved surface of the chain-links and into the inspection region. The full-matrix of transmit-receive time domain signals is captured from the array and stored on a computer for off-line post-processing. This approach allows like-for-like comparison of different imaging algorithms, including standard imaging algorithms such as plane and focused B-scans and sector-scans. Images are also obtained using the total focusing method (TFM) which synthetically focuses the beam. on each pixel in the image and offers high, spatial resolution throughout the imaging plane. The full aperture focusing techniques are shown to perform best on a geometrically identical replica. For example, relative to the slot tip reflection, the background grain noise was at -11.0 dB for the focused sector-scan and -13.1 dB for the TFM. Typical in-situ images obtained from the Clifton Suspension Bridge are then described.