Ravibabu Mulaveesala

Theory of frequency modulated thermal wave imaging for nondestructive subsurface defect detection

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.

Pulse compression approach to infrared nondestructive characterization

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.

Frequency-modulated thermal wave imaging for non-destructive testing of carbon fiber-reinforced plastic materials

Phase-based methods of active thermographic studies provide deeper subsurface details and reduce non-uniform emissivity problems in defect detection. In this contribution analysis of subsurface anomalies has been carried out by probing a suitable frequency component with sufficient energy. This paper highlights the comparative analysis of different thermographic schemes on the basis of supplying equal energy to the chosen frequency used for the analysis of a given carbon fiber-reinforced plastic sample used in experimentation. Experiments have been carried out to find the detection ability of different excitation schemes, and comparisons have been made by taking the signal-to-noise ratio of the defects into consideration.

Coded excitation for infrared non-destructive testing of carbon fiber reinforced plastics

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.

Three-Dimensional Pulse Compression for Infrared Nondestructive Testing

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.

Defect detection by pulse compression in frequency modulated thermal wave imaging

A new, quantitative, whole field, non-contact and non-destructive technique for sub-surface defect detection is presented based on frequency modulated thermal wave imaging (FMTWI). Electro-thermal modeling and MATLAB-SIMULINK simulation has been carried out for the proposed technique. Experimental results of frequency modulated thermal wave imaging are reported, and defect detection by a correlation approach demonstrated.

QUADRATIC FREQUENCY MODULATED THERMAL WAVE IMAGING FOR NON-DESTRUCTIVE TESTING

Thermal non-destructive testing and evaluation of glass flbre reinforced plastic materials has gained more importance in aerospace industry due to low weight and high strength capabilities in severe environmental conditions. More recently, pulse compression favorable non-stationary excitation schemes have been exhibiting reliable defect detection capabilities in infrared non-destructive testing. This paper introduces a novel infrared non-destructive testing method based on quadratic frequency modulated thermal wave imaging with pulse compression for characterization of glass flbre reinforced plastic materials. Defect detection capability of the proposed method has been experimentally validated using a glass flber reinforced plastic (GFRP) sample with embedded Te∞on inserts. Experimental results proved the enhanced depth resolution capability of the proposed excitation method as compared to the linear frequency modulation with pulse compression.

Pulse Compression with Gaussian Weighted Chirp Modulated Excitation for Infrared Thermal Wave Imaging

This paper proposes a novel signal processing approach to thermal non-destructive testing by incorporating Gaussian window function onto the linear frequency modulated incident heat ∞ux to achieve better pulse compression properties. The present work highlights a flnite element analysis based modeling and simulation technique in order to test the capabilities of the proposed windowing scheme over the conventional frequency modulated thermal wave imaging method. It is shown that by using Gaussian weighted chirp thermal stimulus, high depth resolution can be achieved.

Pulse Compression Approach to Nonstationary Infrared Thermal Wave Imaging for Nondestructive Testing of Carbon Fiber Reinforced Polymers

Infrared thermography (IRT) is one of the promising remote and whole field inspection techniques for nondestructive characterization of various solids. This technique relies on the mapping of surface temperature response to detect the presence of surface and subsurface anomalies within the material. Due to its fast and quantitative testing capabilities, the IRT has gained significant importance in the testing of fiber reinforced polymers (FRP). A carbon FRP sample with flat bottom holes is considered for inspection using nonstationary digitized frequency modulated thermal wave imaging technique. Furthermore, depth scanning performance using frequency domain-based phase approach has been compared with recently proposed time domain phase approach.

Suitability of frequency modulated thermal wave imaging for skin cancer detection—A theoretical prediction

A theoretical study on the quantification of surface thermal response of cancerous human skin using the frequency modulated thermal wave imaging (FMTWI) technique has been presented in this article. For the first time, the use of the FMTWI technique for the detection and the differentiation of skin cancer has been demonstrated in this article. A three dimensional multilayered skin has been considered with the counter-current blood vessels in individual skin layers along with different stages of cancerous lesions based on geometrical, thermal and physical parameters available in the literature. Transient surface thermal responses of melanoma during FMTWI of skin cancer have been obtained by integrating the heat transfer model for biological tissue along with the flow model for blood vessels. It has been observed from the numerical results that, flow of blood in the subsurface region leads to a substantial alteration on the surface thermal response of the human skin. The alteration due to blood flow further causes a reduction in the performance of the thermal imaging technique during the thermal evaluation of earliest melanoma stages (small volume) compared to relatively large volume. Based on theoretical study, it has been predicted that the method is suitable for detection and differentiation of melanoma with comparatively large volume than the earliest development stages (small volume). The study has also performed phase based image analysis of the raw thermograms to resolve the different stages of melanoma volume. The phase images have been found to be clearly individuate the different development stages of melanoma compared to raw thermograms.