Photothermal models for lock-in thermographic evaluation of plates with finite thickness under convection conditions

Abstract

Lock-in thermography is a technique which is increasingly being used for the evaluation of relatively thin materials such as composite materials in aircraft structures. In such cases, the models available in the literature cannot accurately predict the difference in phase values between defective and nondefective regions. This is because the models do not consider the convection conditions and the fact that the thickness of the sample is finite. This article proposes two models (a single-layer model and a multilayer model) to rectify this situation. The single-layer model was used to analyze the thermal wave behavior in homogeneous media and the multilayer model was used to analyze the thermal wave behavior in inhomogeneous media. Based on the theoretical and experimental results, the influence of the combined heat transfer coefficient was discussed. The results predicted by the models were consistent with that obtained experimentally.

The effect of size on the quantitative estimation of defect depth in steel structures using lock-in thermography

“…Also plotted in Fig. 6 are the one-dimensional solutions proposed by Bai and Wong 38 and that of Bennet and Patty. 40 A further discussion comparing the one-dimensional solution to the present experimental results will be presented later in the text.…”

“…One approach to determining quantitative information about the depth of a defect is to use a relationship based on a one-dimensional approximation. [38][39][40] However, in real world structures this approximation can be an oversimplification as three-dimensional heat flow effects will tend to dominate. Lock-in thermography has a great potential as a nondestructive inspection ͑NDI͒ tool but requires a good understanding of the limitations with respect to the detectable depth and size of defects.…”

NDI of Rail Squats and Estimating Defect Size and Location Using Lock-In Thermography

“…A special piece of equipment known as a lock-in box is connected to the infrared camera, and is able to compare the data received by the camera with the signal used to generate the periodic input. Let's use a one dimensional mathematical model [13] describing the lockin process incorporating convective effects. The mathematical derivation is reproduced as follows: They describe the heat flux (q) of the heat source with the following equation:…”

“…The phase angle refers to the measured phase difference Δ between the sinusoidal input signal and the measured thermal signal response of an object. The use of lock-in thermography technique as a nondestructive evaluation technique is becoming increasingly attractive in the detection of surface or sub-surface defects in many diverse applications [7][8][9][10][11][12][13][14]. Lock-in thermography offers several advantages over other non-destructive techniques in that it is non-contact, able to inspect wide areas and produce easily interpreted results.…”

Systematic errors in the evaluation of uncorrected data from thermographic lock-in measurements

“…But, if correlation of the temperature response of the surface is realized via harmonic functions (e. g. Sine and Cosine) or via FFT, any kind of DC component leads to deviations in the corresponding amplitude and phase values describing the correlation. The operator usually tries to minimize this DC heating effect by a longer heating time until the DC heating and heat losses due to convection and radiation are similar and the slope of the DC component reaches zero [6] [7].…”