For economical reasons one needs to examine how service life of aircrafts can be extended without inducing any risk. For such aging aircrafts a major concern is integrity of structures. Critical flaws such as disbonding, area delamination, and corrosion can largely reduce structure strength and thereby limit the lifetime of the structure.
There are increasing interests to develop new NDT techniques which are capable of inspecting the defects quickly and remotely. Lock-in thermography [1, 2] as one of these new NDT techniques has a number of inspection advanges which provide useful informations about subsurface structures and subsurface defects of aircraft components. In the case of lock-in thermography thermal waves can be generated with heat modulation on the front surface. The monitoring of a large surface area is achieved with an IR-camera which is synchronized to the excitation source. Since local phase change is independent of the illumination distribution and surface optical structures, the phase image reveals only local subsurface thermal inhomogenities.
Results obtained on aircraft structural components will demonstrate the applicability of lockin thermography for inspection of stringer disbonding (Fig.1), quality of repaired area, and delamination.
|Fig.1 Phase image of a CFRP-component with rear side stringer disbonding areas revealed by interruption of horizontal white lines.|
1. L G. Busse, D. Wu, W. Karpen, J. Appl. Phys. 71 (1992), p.3962-3965
2. P. K. Kuo, Z. J. Feng, T. Ahmed, L. D. Favro, R. L. Thomas, J. Hartikainen, in Photoacoustic and Photothermal Phenomena (P. Hess, J. Pelzl, Ed) Springer Ser. Topics in Optical Sciences, 58 (Springer, Berlin 1987), p.415-418
Steady-state "thermal ellipsometry" was proposed long time ago for the evaluation of thermal anisotropy in homogeneous materials. In this method, the sample surface is point heated and one measures the ellipticity of the isotherms which asymptotically develop around the heated point. In the case of orthotropic materials, this parameter is very simply related to the ratio of the principal in-plane conductivities (it is equal to the square root of this ratio).
Of course, the transient temperature distribution (as for example with step heating) as well as the amplitude and phase maps obtained with modulated heating can be similarly exploited to provide some information on the anisotropic thermal properties. By the way, the transient isotherms, the iso-amplitude curves, as well as the iso-phase carves, all exhibit the same aspect ratio which, of course, corresponds to the one pertaining to steady-state.
Nevertheless, the aforementionned procedures only lead to a relative quantity, i.e. the anisotropy coefficient. By analyzing the phase profiles, i.e. the slope of the phase retardation vs. the distance from the heated spot along the two in-plane principal directions, one can infer the corresponding principal diffusivities.
We implemented an experimental set-up to perform these measurements: LOCK-IT (LockIn Thermography) for thermal ellipsometry. Basically, a chopped focused laser beam is used to periodically heat the sample surface and an infrared camera monitors the modulated temperature distribution induced therefrom (either a monodetector camera or a focal plane array camera were used for this purpose). A procedure was specifically developped to perform a 2-D synchronous detection. The advantage of the adopted method is that nearly any value can be chosen for the shutter driving frequency (indeed, the only forbidden values are the multiples of the Nyquist frequency, i.e. half the camera image frequency).
Amplitude and phase maps were finally retrieved, and both could then be analyzed. However, it is well known that phase images are immune to surface emissivity variations. In the case of modulated heating, we thus preferentially considered the latter. In a first step, elliptical curves were fitted to a series of selected iso-phase domains and their aspect ratio as well as their orientation were retrieved. The anisotropy coefficient and the preferred fibre orientation (in the case of a fibre reinforced composite sample) were thus obtained. Then, in a second step, the slope of the profiles in both main directions were evaluated through linear regression (by omitting the central portion of the phase distribution because of laser beam "pollution" - gaussian heating instead of point heating), or by global image analysis. The values of the principal diffusivities were separately measured by this way.
LOCK-IT approach for in-plane diffusivities measurement was first assessed on isotropic samples (duraluminium plates), and then on unidirectionnal carbon-epoxy plates. In the first case, the retrieved value for the diffusivity corresponded well to the one obtained by transmission, i.e. by applying the classical laser flash method. In the second case, all obtained data were coherent: the square root of the ratio of the measured principal diffusivities corresponded fairly well to the aspect ratio of the iso-phase curves, on one hand, and to the aspect ratio of the isotherms obtained in steady-state, on the other hand.
Previous considerations dealt with homogeneous materials. However, we recently showed that by applying thermal ellipsometry on stratified materials one could obtain some information about the eventual in-depth laminate orientation changes. A theoretical analysis was performed to assess this method. A 3-layer model was introduced to simulate the behaviour of short fibre injection moulded composites (such materials exhibit a 3-layer structure: skin layers where the fibres are oriented in the mean flow direction which prevails during the moulding, and a core layer where the fibres are essentially at 90° to this direction).
An inversion procedure was then developped to infer both the core layer thickness and its depth from an IR image obtained by thermal ellipsometry in steady-state regime. This procedure is based on the evolution of the isotherm aspect ratio, vs. the isotherm area.
Some experimental results obtained with carbon-epoxy laminates and short fibres injection moulded composites will be finally presented.
Thermal waves are produced when a modulated heat source generates a temperature field which is periodic in time and space.
Heat deposition can be achieved from outside, e. g. by periodical illumination or hot air, but also from inside the sample, by periodical loading and unloading, where heat is generated by hysteresis effect. Power density achieved this way depends on how much power is converted into heat in every cycle and on how many cycles are in the unit time, so that the product of frequency, loss angle and the square of stress is involved. This is the base for vibrothermography  and also for the SPATE technique  (which, in fact, is based on the thermoelastic effect) showing stress concentration in components under cyclic load.
While these techniques investigate stationary situations with essentially constant heat flow, lockin thermography requires a modulation technique where depth range is related to frequency in such a way that lower frequencies allow for a deeper penetration in the material.
To achieve enough heat generation per time at a low frequency one needs a high stress level. Alternatively, one can use a high frequency as carrier frequency (providing the heat source) and a low frequency amplitude modulation (providing the modulation of the heat source).
The high carrier frequency, around 50 kHz, allows for low stress levels while the low frequency thermal waves have enough depth range to bring information from inside the sample to the surface, where temperature modulation is analysed by an infrared camera.
Basically this is a technique where regions of stress concentration or of enhanced internal friction (e.g. cracks and delaminations) appear as bright areas in the magnitude image. The phase image contains the information about the depth at which these areas are located.
Loss angle imaging could be considered as a modification of conventional ultrasonics. However there is one significant difference: only part of the sample needs to be in contact with the ultrasonic transmitter, and the detector does not have any contact with the inspected component. So this technique allows for remote mapping of mechanical losses.
Applications will be demonstrated for materials having high losses (e. g. polymers) or low loss materials having defects with high friction losses (e. g. cracks in ceramics). From these results we conclude that the technique should find its place in the quality control of these materials.
 Busse G., Bauer M., Rippel W., and Wu D., Lockin vibrothermal inspection of polymer composites, Quantitative Infrared Thermography QIRT (1992) pp. 154-159.
 Patent No. PCT/GB 79/00081 and DE 2952809 C2 (SPATE).
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