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TBET- Tone Burst Eddy Current Thermography |
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Tone Burst Eddy-current Thermography (TBET) uses the tone burst AC pulses to locally heat a conducting material and uses a thermal imaging IR camera to map the surface temperatures in a themography mode. The eddy-currents produced due to the induction coil placed above the sample causes of rise in the near surface temperature of the sample due to the Joule heating effect. The pattern of the heat on the surface of the sample, as a function of time of observance, will depend on if and type of the defect and its position.
Thus the heated surface of the sample due to the coil is to be monitored and a rise of few degrees of temperature in the area of monitoring is expected to help the detection of defect presence. This method can be used for both nondestructive testing applications for defect detection as well as materials characterization for the measurement of thermal diffusivity constants as well as electrical conductivity properties of conducting materials and components.
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Advantages of TBET over Conventional Pulsed Thermography :
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Low power is utilized compared to conventional thermography as the temperature rise to contrast ratio is lower for TBET. |
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Parametric optimization is possible to identify defects at different locations, with ease i.e. Possibility of tuning the defect depth accuracy. |
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Thermal excitation miniaturization helps in local testing of the sample. |
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No flooding of sample with energy. |
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The whole setup can be made portable. |
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No necessity to paint the samples when dealing with highly reflecting surfaces in flash thermography applications. |
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Frequency tuning to identify flaws at different levels. |
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Fig. 1 Simulation model of TBET |
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| Fig. 2 Thermal Contrast data for TBET |
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Fig. 3 Temperature Profiles for TBET |
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Reference: Ch. N. Kiran Kumar, C.V. Krishnamurthy, and Krishnan Balasubramaniam, “Tone Burst Eddy-current
Thermography (TBET)”,(Under Preparation). |
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Magneto-Optics NDE Measurements |
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A new non-destructive, non contact, surface magneto optic (MO) Kerr magnetometer utilizing changes in the polarization of light reflected from a magnetic sample in the form of thin strips caused due to the magnetism of the material has been developed. This new technique can be utilized to measure nondestructively the change in local magnetic hysteresis of a magnetic metal averaged over the area of the sample covered by the laser spot size. It is quite known that the hysteretic curve changes due to micro structural changes caused by many mechanical processes, such as plastic deformation, fatigue and also creep. Therefore, this technique may also lead to life cycle assessment of many of the structural components that are magnetic in nature. However, the application of this method is restricted to magnetic materials.
Though AC Kerr effect or modulated Kerr effect technique will give a better signal to noise ratio, if one can suppress the noise from various sources, a dc measurement technique itself will give sufficient enhancement in signal to noise ratio. By suppressing various electromagnetic and mechanical noise sources to get better signal to noise ratio is carried out.
Magneto- optic Kerr effect measurements were carried out using the in house built Kerr magnetometer with two different spot sizes on 430 G stainless steel strip samples. The results were found to be affected by a small drift due to the higher magnetic force experienced by the sample. This is because the thickness of the sample is higher compared to the thickness of the samples on which Kerr effect measurements generally have been done.
A correction to the data arising from this drift was given and new hysteresis loops obtained after the correction look to be a typical hysteresis loop obtained from a thin magnetic sample. The hysteresis loops obtained using smaller spot size show that the sample does not show saturation behavior, compared to the one obtained using bigger spot size.
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