Frequency Modulated (FM) Time Delay-Domain Thermal Wave Techniques, Instrumentation and Detection: A Review of the Emerging State of the Art in QNDE Applications

摘要

In this work, the concept of the frequency modulation (FM) technique is applied to thermal wave systems. Heyser [1] introduced this technique in the field of acoustical measurements of loudspeakers and named it time delay spectrometry (TDS). Through its implementation and long-term use in acoustic engineering, TDS has been shown to outperform any other time selective technique with respect to noise rejection and nonlinearity suppression from measurements of systems with linear behavior [2]. The time delay technique, based on a linear frequency sweep of the excitation function, has been specifically compared to the impulse response transformation method and the wide-band random noise method [3] and has been proven to have superior measurement dynamic range properties. The present first application of TDS to a thermal wave system and subsequent comparison with a pseudo-random noise method has shown that our FM technique is superior to the more conventional types of excitation and signal analysis, consistently with conclusions presented in Ref. [3].