[5A2] Removal of front- and back-surface reflections of ultrasonic responses from composites
M Zilidou1 and R Smith2
1University of Bristol, UK
2RCNDE Director, UK
Ultrasonic testing has been one of the most commonly used methods for inspection of composite materials in industry. However, the evolving designs and inhomogeneous nature of composites requires the constant development of more advanced equipment and signal-processing techniques. One problem frequently encountered in ultrasonic testing is the masking effect of the strong surface reflections in pulse-echo mode. This can be critical for detecting the presence of near-surface defects and for accurate tracking of the first and last two plies. The aim of this study is to remove the front- and back-surface reflections from the measured ultrasonic response using the analytic signal and its instantaneous parameters (amplitude, phase, frequency). The key steps include determining the input pulse using the front-surface reflection of the measured response and artificially constructing the front-surface (the product of the input pulse and a complex reflection coefficient) and back-surface reflected pulses (the product of the incident pulse at the back surface, a complex reflection coefficient, and an attenuation term). These are then subtracted from the original response and a reflection from a resin layer embedded in composite is added back in. Subtraction of the front-surface from both simulated and real data proved easier and more straightforward than subtracting the back-surface but refinements to the algorithms overcame the problems. The success of the method will be demonstrated using real data from samples with internal wrinkles, tape gaps and overlaps, near-surface delaminations, and machined front surfaces.
