Pulse compression approach to infrared nondestructive characterization

Abstract

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.

Non-Destructive Evaluation of Concrete Structures by Non-Stationary Thermal Wave Imaging

“…Pulse compression technique prevalent in RADAR allows the transmission of a medium peak power, long duration modulated wave to improve the target detection range and resolution comparable to that achieved with a short duration high peak power pulsed based techniques [11,12,14]. This can be achieved with a correlation based pulse compression technique by cross correlation of the temporal temperature distribution over the chosen reference pixel s (t) over the sample, with the time delayed attenuated version of the pixel h (t) for an imposed linear frequency modulated incident heat flux over the sample.…”

“…Insight of the above mentioned advantages and limitations of the widely used conventional methods, this paper focuses on an experimental investigation for detection of corrosion of rebar in reinforced concrete structures by frequency modulated non-stationary thermal wave imaging [7][8][9][10][11][12][13][14][15][16][17][18].…”

“…This requires an external thermal stimulus to the inspected specimen in order to obtain significant temperature differences witnessing the presence of subsurface anomalies. The present work is mainly focused on the active approach [8][9][10][11][12][13][14][15][16][17][18].…”

“…In active approach [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18], external heat stimulus is provided to the test sample of whose thermal response is to be observed. Known characteristics of the external thermal stimulation applied onto the specimen (i.e., nature of excitation, its time duration and its band width etc), facilitates the qualitative and quantitative characterization of sub-surface defects.…”

Non-Destructive Testing by Means of Frequency Modulated Infrared Imaging

“…However, none has so far been free from certain limitations: Pulse thermography(PT) requires high peak power heat sources and being sensitive to surface emissivity variations and nonuniform heating on the surface of test sample, lock in Thermography (LT) suffers with limited depth resolution and long processing time [5], [6] and Pulse Phase Thermography (PPT) needs high peak power heat sources to detect deeper subsurface defects, which however may damage the surface of the test sample. Being prevalent in RADAR, pulse compression methodology allows more energy imposition with low power, long duration stimulation and resolving the detail using compressed profiles [7]- [9] Which can subsequently enhances signal to noise ratio, provides better resolution and facilitates deeper depth probing as well. This paper highlights the defect detection capability of pulse compression method and its edge over conventional phase based detection for FMTWI, with the experimentation carried over a mild steel specimen containing flat bottom holes.…”