Suppression of front and back surface reflections in ultrasonic analytic-signal responses from composites

Abstract

Pulse-echo ultrasound testing is the most prevalent method for inspection of composite materials in industry although evolving designs combined with the anisotropic nature of composites demands the constant development of more advanced signal-processing techniques and testing equipment. One problem that is frequently encountered in ultrasonic inspection, in pulse-echo mode, is the masking effect that occurs due to the strong surface reflections. This can prove critical for the detection of near-surface defects and accurate ply-tracking of the first and last two plies. The purpose of this study is to suppress the front- and back-surface reflections by first removing them from the measured ultrasonic response using the analytic signal and its instantaneous parameters. The first in the series of key steps the method includes is determining the shape of the input pulse using the measured front-surface reflection. After obtaining the input pulse, the front- and back-surface reflections are constructed artificially. The front-surface reflected pulse is the product of a complex reflection coefficient and the input pulse, while the back-surface reflected pulse is the product of a complex reflection coefficient, an attenuation term, and the incident pulse at the back surface. The next step involves subtracting the constructed surface pulses from the original response and substituting a reflection from a resin layer embedded in composite at the front and back surfaces. Those reflections are added back to the signal in order to make the ply extraction work consistently in the internal layers. The method has been tested using both simulated and real data. Subtraction of the front-surface was highly successful in a range of material configurations, but subtraction of the back-surface required algorithm refinements to cope automatically with all the scenarios tested. The ability of the method to improve detectability of defects and tracking of near-surface plies is demonstrated using data from real samples with near-surface delaminations, tape gaps and overlaps, and internal wrinkling.