The Effect of Crack Opening on Thermosonics and Laser-Spot Thermography

Abstract

Laser-spot thermography is a novel method for detecting surface breaking cracks in metals. Another active thermographic research method is thermosonic (Sonic IR) inspection, which may potentially compete with or complement laser-spot thermography in the detection of surface breaking cracks. Thermosonic inspection generates heat predominantly by friction of crack-faces under high-power ultrasonic excitation, the resulting surface temperature rise being imaged with an infrared camera. Thermosonic inspection is therefore best suited to defects that are closed over part of their length. In contrast, laser-spot thermography detects defects by the perturbation of the radial heat flow from a spot heated with a short-pulse laser, the increased thermal impedance presented by air between the faces of the crack blocking the conduction of heat. This paper presents an experimental comparison of the performance of thermosonics and laser-spot thermography on a series of samples with different crack sizes, the opening of each crack being varied by three-point bend loading.

Two-Stage Automated Defect Recognition Algorithm for the Analysis of Infrared Images

“…A 10 Hz repetition rate Nd:YAG pulse laser operating at 1064 nm wavelength and intensity equal to 3.85 mJ=mm 2 were used to impart a single 8 nanosecond 7 mm diameter pulse on the test specimen. The use of a large circular spot instead of a focused line or point as conventionally done in laser-spot thermography [6,11,[16][17][18] allowed to increase the scanning speed because the region of interest (ROI) associated with each scanning point was greater. The infrared images were recorded with a FLIR SC660 camera operating at 30 Hz frame rate and transferred to a PC via FLIR ExaminIR software.…”

“…The method is attractive for the quick assessment of complex components [7]. Usually, the high power ultrasound is delivered to the structure by means of acoustic horns [7][8][9][10][11][12][13] or piezoelectric exciters [4,14]. Similar to thermosonics, vibrothermography uses continuous vibration excitation to generate a steady state local temperature rise [7,11].…”

“…Directional patterns can be also induced by interposing a cylindrical lens between the laser and the test specimen. A surface breaking crack near to the illumination point prevents the lateral flow of heat and produces a perturbation to the thermal image that can be useful to detect the presence of such cracks [6,11,16,17].…”

“…Most of the existing methods require user input or intervention, and automation is not always possible. A common approach to remove thermal noise is applying spatial filters [10,19,20], whereas a frequent process to enhance the defect signatures consists in subtracting a reference (baseline) frame to every other frame [4,7,9,11,12,15,20]. For instance, Li et al [19] proposed a three-dimensional matched filter to detect cracks from a video sequence collected during a vibrothermographic inspection.…”

Comparative study on submillimeter flaws in stitched T-joint carbon fiber reinforced polymer by infrared thermography, microcomputed tomography, ultrasonic c-scan and microscopic inspection

“…15,16 Fiber orientation detection using laser spot thermography was reported in 2013. 17,18 Further studies on surface cracks [19][20][21] and fiber orientation 22,23 were reported recently. Lock-in thermography, also known as modulated thermography, which is a technique derived from photo thermal radiometry.…”