Transient Eddy current Thermography technique uses transient electromagnetic apparatus to effect eddy-currents that are induced in a conducting media for generating local heating inside materials/structures and measure the ^effect of the heat generated and heat diffusion in the material through measurement of temperature at the outer boundaries of the material/structure. The temperature is measured, as a fiinction of time, and utilized to maice measurements and diagnostics about the material state, in an imaging mode. The transient diffusion of the heat inside the material is imaged by measuring the transient temperature profiles at the surface of the materials using devices such as IR Camera, IR^* thermometer, and other forms of temperature measurement devices. The presence of anomalies^ and the characteristics of the materials, change the surface temperature transients and thus can be used for the nondestructive evaluation (NDE) of conducting and non-conducting materials. The material state that may be measured in material includes, but not limited to: (a) Electrical Conductivity, (b) Thermal Diffusivity, (c) Flaws and anomalies, (d) thickness of layers and materials, (e) material non-uniformity, (f) depth profiles of material properties, (h) Stress and stress profiles, (i)material degradation including plastic deformation, hydride embritlement, creep, fatigue, etc., (k) magnetic properties, (1) process parameters, and (m) Surface properties such as roughness, treatments, hardness, etc.
Related Background
Induction/Eddy-current heating relies on the currents generated within the materialto produce heat. Many of Induction heating applications are found in manufacturing

industry, especially for applications involving bonding or changing the properties of metals or other electrically conductive materials, as it provides fast, consistent heat which is cost effective. For a more detailed description of Induction heating process and its application the reader may refer to these books [1, 2]. Many attempts were made to model Induction heating [3-5].
The basic premise of thermographic NDT is that the heat diffusion phenomenon into the material is perturbed by the presence of a subsurface defect, causing a temperature contrast at the surface, this temperature contrast is piclced up by thermal imaging equipment which can be further processed for extraction the internal flaw information which it holds. [6].
Eddy current heating as a thermal exciter for thermography has being explored recently for surface and subsurface crack detection[7-13], and for the application in lock-in thermography [14, 15], An attempt was made to imderstand the underlying principles of Eddy current heating which for NDT and standardizing the procedure [16].
The transient thermography NDE technique requires monitoring of subtle changes in the surface temperature of the test sample body for a rise of few degrees. The surface temperature monitoring demands the Usage of thermal detectors of very high resolution, frame rates, and accuracy. This is made possible by recent IR camera, which can capture thermograms at high speeds with great resolution and accuracy (~±0.1K). IR camera has the advantage of sub-windowing which will help to the zoom the monitoring area. Sub windowing also increases the speed of frame capture.
The mechanism of pulsed thermography and defect detection process is explained in [6]. where the heat pulse is given to the sample from one side and the temperature is monitored on the same side of the sample (reflection mode thermography).
Apparatus Description
The experimental setup primarily involves (a) a means for the generation of electromagnetic induction mto the conducting material, and (b) a means for the

measurement of temperatures at the boundaries of the material. A typical apparatus is illustrated in Figure 1. Here an electromagnetic coil is employed for the generation of eddy currents in the conducting material sample which is responsible for the heat generation, and an Infra-red (IR) camera is utilized for the measurement of temperatures at the surface boundaries of the material. Here, the coil is placed on one side of the sample and the camera is placed on the other side (called the transmission, mode). In this arrangement, the heat is generated in the coil side, and the heat diffuses into the material and is recorded on the opposite surface of the sample. Other methods can be adopted where the coil and the camera are on the same side, and called the reflection mode. These two modes are shown in Figure 3.
The input signal to the coil was a transient excitation that can be any of the possible types, (a) Tone Burst, (b) Spike Pulse, (c) Chirp, (d) Coded pulses (like the Golay codes, etc.), and (f) arbitrary. The input signal is generated using a function generation circuit that is amplified by a power amplifier and fed to the coil. The input signal characteristics is the primary novelty of the proposed techniques, since all previously reported work in this approach involves DC excitation and continuous heating. The transient mode of excitation provides opportunities for improved information mcluding depth based of material properties and measurements based on both electrical conductivity and thermal diffusivity of materials. For mstance, in the case of Tone Burst mode of excitation, the frequency (due to the skin depth phenomena of the eddy current induction) and the length of the burst excitation (amplitude of heating) can be varied to provide information about thickness of the media, conductivity variations as a function of depth of the material, among other applications. A typical tone burst excitation signal is shown in Figure 4( and a typical bobbin coil used for electromagnetic induction is shown in Figure 5.
Typical results are shown in Figures 6 and 7. Both experimental results were conducted using transmission mode. In Figure 6, the bobbin coil was used to image the defects inbetween aluminum plate that was bonded to a mbber layer. The different sizes of defects were imaged successfully. In Figure 7, the defects in a metal plate was examined using transient pulsed flash lamp heating thermography and the transient tone burst eddy

current thermography, in a transmission mode, demonstrating that the transient eddy current technique has the ability to image the defects in the plate which were not imaged using the flash lamp heating.
1. The technique is a non-destructive means of making measurement using induction based heating through transient means of excitation, and transient imaging/measurement of temperature at the boundaries of the structure/material/component.
2. The technique depends on the two material properties i.e. (a) Electrical Conductivity, and (b) Thermal difEusivity and hence can be used to measure these properties.
3. The technique can be used to measure depth resolved properties such as (a) electrical conductivity, (b) thermal conductivity, (c) stresses, (b) hardness, (c) magnetic permeability, (d) ........ .
4. The technique can be used to measure and characterize material properties and used of sorting of materials and applications thereof.
5. The technique can be used for measuring the layer thicknesses on single and multi-layered structures and components.
6. The technique can be used for material degradation including plastic deformation, fatigue, creep, embrittlement, corrosion, stress corrosion cracking, etc. in materials, structures and components.
7. The technique can be used for testing and evaluation of defects in materials, during manufacture, post manufacture, in-service, life extension, and failure analysis purposes. Defect types includes delamination, disbands, cracks, porosity, voids, corrosion, wide-spread multi-site damage,- etc.
8. The technique can be used.in imaging mode for all of the above properties over a large surface area of a component and/or structure.
9. The technique may also be used in point measurements that can be at single point or a series of points over a surface.

! 0. The technique can be used for all conducting materials including metals, composites, ceramics, etc.
11. The technique can be used for non-conducting materials by application of a layer of conducting material on to the non-conductive material. The conducting layer may be of removal type or non-removal type.
12. The technique may be used for process monitoring during manufacture or processing of materials.
13. The transient excitation that can be any of the possible types includmg, but not
limited to, (a) Tone Burst, (b) Spike Pulse, (c) Chirp, (d) Coded pulses (like the
Golay codes, etc.), and (f) arbitrary ftjnctions.
14. The mode of implementation of the method can be in transmission or reflection mode. The orientation of the excitation source and the sensing device can be at normal or oblique angles.
15. The technique can be implemented usirig direct measurement and/or thought the use of devices such as reflecting mirrors in inaccessible locations.
16. The technique can be implemented through a wide range of induction methods includmg electromagnetic coils of different shapes and sizes made from any material that conducts electricity. Use of cores made from different materials may enhance the performance of heating.
17. The technique can be implemented using a variety of temperature measuring devices including infra-red camera, infra-red thermometers, temperature sensitive coatings, etc.
18. The technique is not limited on the dimension of the material/structiu-e/component
that is being evaluated.
19. The technique is supported by analytical and numerical mathematical models that support the forward solution (prediction of the results) and the inverse solutions (quantitative interpretation of the experimentals) and parametric estimations.
20. The technique may support signal and image processing algorithms that enhance the signal to noise ratio (SNR) of the experimental data and provide enhancements to the image, when used in an imaging mode.

We Claim:
1. A metliod of non-destructive evaluation of the
mechanical, electrical, thermal, magnetic-
characteristics of a specimen as also the
anomalies and other snch internal characteristics
of the specimen by eddy current thermography
characterised by parts of, and over, the entire specimen, to produce heat, the resulting temperatures at each such part being individually measured, and the readings of the individual measurements being combined to give a complete reading for the whole specimen, to enable the mechanical, electrical, thermal, magnetic characteristics of the specimen as also the anomalies and other such internal characteristics of the specimen to be furnished in a known way.
2. A method of non-destructive evaluation of the
mechanical, electrical, thermal, magnetic
characteristics of a specimen as also anomalies and
other such internal characteristics of the specimen
by eddy current thennography substantially as
herein described with reference to and as
illustrated in the accompanying drawings.