Non-Contact Inspection of Railhead via Laser-Generated Rayleigh Waves and an Enhanced Matching Pursuit to Assist Detection of Surface and Subsurface Defects

Speckle Measurement for Small In-Plane Vibration Using GaAs

In this study, the measurement characteristics of speckles based on the photoinduced electromotive force (photo-emf) effect for high-frequency, small-amplitude, and in-plane vibration were theoretically and experimentally studied. The relevant theoretical models were utilized. A GaAs crystal was used as the photo-emf detector for experimental research, as well as to study the influence of the amplitude and frequency of the vibration, the imaging magnification of the measuring system, and the average speckle size of the measuring light on the first harmonic of the induced photocurrent in the experiments. The correctness of the supplemented theoretical model was verified, and a theoretical and experimental basis was provided for the feasibility of using GaAs to measure in-plane vibrations with nanoscale amplitudes.

Laser ultrasonic nondestructive evaluation of sub-millimeter-level crack growth in the rail foot weld

Laser-generated ultrasonic wave characteristics in the rail foot weld were simulated and reported for qualitative analysis and evaluation of sub-millimeter-level crack growth. Numerical analyses using the finite element method (FEM), the propagation characteristics, and displacement field distribution of a laser-generated ultrasonic wave after the interaction with cracks were fully demonstrated. By calculating displacement amplitude distribution, the optimal sensing position and area were the laser incident point and the upper surface, respectively. Crack growth degree toward the rail bottom and axial direction can be confirmed by analyzing time and amplitude of the echoes originating from the rail bottom and crack surface reflection. By combining time with peak intensity of the echo reflection from the rail bottom, the sub-millimeter-level crack growth process inside the rail foot weld is capable of acquiring and evaluating. The results justify that the laser ultrasonic technique, characterized by laser excitation and laser detection, is a competitive nondestructive testing technique for sub-millimeter-level crack growth evaluation and detection inside the rail foot weld.

Surface Roughness Effects on Self-Interacting and Mutually Interacting Rayleigh Waves

Rayleigh waves are very useful for ultrasonic nondestructive evaluation of structural and mechanical components. Nonlinear Rayleigh waves have unique sensitivity to the early stages of material degradation because material nonlinearity causes distortion of the waveforms. The self-interaction of a sinusoidal waveform causes second harmonic generation, while the mutual interaction of waves creates disturbances at the sum and difference frequencies that can potentially be detected with minimal interaction with the nonlinearities in the sensing system. While the effect of surface roughness on attenuation and dispersion is well documented, its effects on the nonlinear aspects of Rayleigh wave propagation have not been investigated. Therefore, Rayleigh waves are sent along aluminum surfaces having small, but different, surface roughness values. The relative nonlinearity parameter increased significantly with surface roughness (average asperity heights 0.027–3.992 μm and Rayleigh wavelengths 0.29–1.9 mm). The relative nonlinearity parameter should be decreased by the presence of attenuation, but here it actually increased with roughness (which increases the attenuation). Thus, an attenuation-based correction was unsuccessful. Since the distortion from material nonlinearity and surface roughness occur over the same surface, it is necessary to make material nonlinearity measurements over surfaces having the same roughness or in the future develop a quantitative understanding of the roughness effect on wave distortion.