Parameterization of Spatial Ultrasonic Wavefront via Laser Ultrasonic Technique

Abstract

Ultrasonic wavefield imaging (UWI) provides insightful spatial information about ultrasonic wave propagation in 2D space. The guided waves propagate with unique patterns in relation to the wave speed and boundary condition for 2D (plate-like) structures. In both isotropic and anisotropic materials, the wavefronts of the incident and reflected waves propagate with unique curves that are distinguishable with UWI. These curve patterns are representable by, and similar to, parameterized polar curves in 2D geometric space. In this paper, several parametric curves (circular, hyperbolic, and cyclic-harmonic curves) were considered. The respective parameters of the parametric curve were set to obtain the closest curve pattern to the wavefront pattern. Then, the 2D laser mirror scanner performed the scanning pattern according to the Cartesian coordinates calculated by the parametric equation as coded into the laser ultrasonic interrogation system (LUIS). An aluminum plate and a cross-ply CFRP plate were used in this paper. The laser scanning patterns of circle, hyperbola, and cyclic-harmonic curve were considered on the aluminum and CFPR plates. Then, the signal responses were plotted as a UWI in angle vs. time domain. The results show that the circular and cyclic-harmonic scanning patterns can tune the incident wavefronts of the aluminum plate and the CFRP plate to arrive at the same arrival times along the scanning path respectively. The hyperbolic scanning pattern demonstrated also the ability to tune the reflected wavefronts to arrive at the same arrival times. With these parameters of the parametric curve, the proposed method may serve an alternative approach for sensing placement implementation in SHM applications. Future work will consider studying the 2D frequency response of the measured ultrasounds in angle vs. time domain to investigate the possible features for damage diagnosis strategies development.