Singular value decomposition (SVD) has been proven to be an effective de-noising tool for flaw echo signal feature detection in ultrasonic non-destructive evaluation (NDE). However, the uncertainty in the arbitrary manner of the selection of an effective singular value weakens the robustness of this technique. Improper selection of effective singular values will lead to bad performance of SVD de-noising. What is more, the computational complexity of SVD is too large for it to be applied in real-time applications. In this paper, to eliminate the uncertainty in SVD de-noising, a novel flaw indicator, named the maximum singular value indicator (MSI), based on short-time SVD (STSVD), is proposed for flaw feature detection from a measured signal in ultrasonic NDE. In this technique, the measured signal is first truncated into overlapping short-time data segments to put feature information of a transient flaw echo signal in local field, and then the MSI can be obtained from the SVD of each short-time data segment. Research shows that this indicator can clearly indicate the location of ultrasonic flaw signals, and the computational complexity of this STSVD-based indicator is significantly reduced with the algorithm proposed in this paper. Both simulation and experiments show that this technique is very efficient for real-time application in flaw detection from noisy data.

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In order to illustrate influence of surface roughness on the energy change of excitation process in thickness measurement of electromagnetic acoustic transducer (EMAT), the change of Lorentz force during the excitation process is analyzed theoretically. A finite element simulation model of EMAT is established with different surface roughness. The distribution of eddy current density and Lorentz force density of different rough surface is discussed. The results show that the eddy current density gets nonuniform due to the irregular profile of the surface. The average value of Lorentz force density decreases with the increase of surface roughness Ra, which makes the excitation energy of EMAT get weaker. Finally, the experiment results verify the analysis of the theory and finite element simulation.