This letter provides the theory and mathematical analysis in support of a recently proposed frequency modulated thermal wave imaging for nondestructive subsurface defect detection in solids. The authors illustrate how the technique simultaneously combines the advantages of both conventional pulse based thermography as well as modulated lock-in thermography. A specimen is heated for launching thermal waves into the sample, not at a single frequency (lock-in) or at all frequencies (pulse), but in a desired range of frequencies. While peak power requirement is reduced, phase images obtained retain known advantages. Experimental results from a carbon fiber reinforced plastic sample are presented in support.

Infrared thermography is a whole field, noncontact, and nondestructive characterization technique widely used for the investigation of subsurface features in various solid materials (conductors, semiconductors, and composites). Increased demand for greater subsurface probing in thermal nondestructive testing is often thwarted by the probing high peak power into the sample, for which narrow pulse operation is usually used. The technique of pulse compression offers a means of increasing the average power available to illuminate test specimen without any loss of the depth resolution needed for the tactical requirements. This is accomplished by transmitting a wide pulse in which the incident heat flux is frequency modulated and then, by proper signal processing methods, causing a time compression of the received signal to a much narrower pulse of high effective peak power. For the demonstration, a mild steel sample having flat bottom holes at various depths is introduced and detection capability of the proposed approach has been studied.

Thermal non-destructive testing (TNDT) is a whole field and non-contact technique for defect detection. The present work describes a variant of TNDT for subsurface defect detection based on frequency modulated thermal wave imaging (FMTWI). Use is made of the frequency dependence of thermal diffusion length, to achieve entire depth scanning of a sample in one run. This novel technique overcomes some of the drawbacks associated with traditional pulse and lock-in thermography. Experimental results are presented in support.

This paper proposes a Barker coded excitation for defect detection using infrared non-destructive testing. Capability of the proposed excitation scheme is highlighted with recently introduced correlation based post processing approach and compared with the existing phase based analysis by taking the signal to noise ratio into consideration. Applicability of the proposed scheme has been experimentally validated on a carbon fiber reinforced plastic specimen containing flat bottom holes located at different depths.

This letter proposes an optimal nondestructive subsurface defect detection method to investigate the capabilities of the infrared thermography through a finite-element analysis-based model. A finite-element analysis (FEA) software was used to generate models and analysis was carried out using MATLAB software. Pulse compression approach has been introduced for subsurface defect detection and its advantages and limitations are compared with existing phase approach-based thermography. Investigations has been carried out on a simulated plain carbon steel specimen with a flat bottom hole defects at various depths of different diameters is introduced. Comparison has been made with the conventional phase-based techniques.