New Absolute Contrast for pulsed thermography

Abstract

In this paper, a new absolute thermal contrast method is proposed for pulsed infrared thermography. It is based on the computations of reconstructed defect-free images so that no a priori knowledge of a sound area on the sample is necessary. Moreover, a correction is applied to take into account possible delays in the acquisition time. Results are presented both on Plexiglas TM and graphite-epoxy specimens. Comparisons with Pulsed Phase Thermography phase images are also presented along with a discussion on the advantages of the proposed method.

Automatic interpolated differentiated absolute contrast algorithm for the analysis of pulsed thermographic sequences

“…where k is the thermal conductivity in W/(m·K), Τ(r,t) is the temperature distribution as a function of position and time, g(r,t) is the rate of energy generation per unit volume in the medium (W·m 3 ) and C and ρ the specific heat (J·kg -1 ·K -1 ) and the density (kg·m -3 ) of the material. Assuming that the specimens under test are homogeneous, in a defect free sample at a point far from any edge, lateral heat components cancel and the simplest model used to solve equation 1, assuming a Dirac thermal pulse is been applied to the body, can be expressed by: Considering t' as a time comprised between the time of flash impulse (t0) and the time at which the first defect becomes visible [3],…”

“…Assuming that the specimens under test are homogeneous, in a defect free sample at a point far from any edge, lateral heat components cancel and the simplest model used to solve equation 1, assuming a Dirac thermal pulse is been applied to the body, can be expressed by: Considering t' as a time comprised between the time of flash impulse (t0) and the time at which the first defect becomes visible [3],…”

“…Moreover, if the injected energy is assumed to change relatively smoothly, Q/e can be solved locally and the following relation can be extracted [3]:…”

“…Some attempts have been proposed for avoiding this necessity of sound area knowledge and, even, overcoming other problems such as the non-uniformity of the thermal stimulation or the tilt of the heat pulse source respected to the normal to the specimen surface. Among them, the Differentiated Absolute Contrast (DAC) method is one of the simplest and easily implemented [3]. However, the experience of an operator working in this field still is necessary in order to obtain satisfactory results and clear contrast images.…”

Normalization and sound zone determination in pulse thermographic NDE

“…Among them absolute contrast, normalized contrast, running contrast, and standard contrast are some of the early contrast definitions and they require the knowledge of sound zone [4]. Differential Absolute Contrast (DAC) [5] and modified Differential Absolute Contrast (mDAC) [6] are two of the methods that removes the non-uniformity in the thermal images. The reference sound zone profile in these two methods is analytically determined.…”