What is Thermography in NDT

What is Thermography, its types, principle, working, advantages & limitations

Thermography is an NDT method of obtaining an image of the heat distribution over the surface of an object. The usual method is to use a special camera made of several infrared sensors (an infrared sensitive detector) and a lens which transmits infrared radiation. Such cameras can operate at normal video rates and measure small temperature differences in a part.

Temperature variations in the parts are then displayed as shades of grey or can be converted into pseudo-color images. The image showing these differences can be downloaded to and displayed on a PC from the camera. Temperature variations are as small as 0.1°C can be detected using the Thermography method. The methods used are totally non-destructive and non-invasive and can be highly cost-effective.

Thermography in NDT

Electromagnetic Spectrum relation with Thermography

Thermography is a type of infrared imaging. Thermal technology  detect radiation in the infrared range of the electromagnetic spectrum and produce images of that radiation. Since infrared radiation is emitted by all objects based on their temperatures, according to the black body radiation law. The electromagnetic spectrum is the collection of frequencies It’s the distribution of electromagnetic radiation emitted or absorbed by that particular object.

The electromagnetic spectrum extends from below the low frequencies used for radio communication to gamma radiation at the short-wavelength high-frequency covering wavelengths from thousands of kilometers down to a fraction of the size of an atom.

Electromagnetic Spectru
Electromagnetic Spectrum
Figure 2. Electromagnetic Spectrum

Principle of Thermography​

Infrared thermography is a thermal imaging technique. Thermal images are captured by infrared camera which detects the infrared light radiation (~1 – 14mm). This radiation distribution image can be converted into temperature scale and produced a temperature distribution image called thermogram, according to the Stefan Boltzmann Law. Therefore, thermography allows us to see the variations in temperature, since the amount of radiation emitted by an object increase or decrease corresponding to the change of temperature.

Thermography Techniques

There are two basic types of thermography:

  1. Passive thermography and 
  2. Active thermography.
Figure 3. Types of Thermography

In passive thermography, the camera is simply pointed at the test piece and from the thermal image a temperature map is constructed. The passive approach is used when the object of interest has enough thermal contrast with respect to the background in order to be detected with an infrared sensor. Typical applications include: surveillance, people tracking, humidity assessment in buildings, liquid levels in storage tanks, insulation problems, electrical components, etc.

Active thermography involves heating the surface of the object rapidly using an external heat source and observing how the temperature decays with time. Flaws in the material show up by variations in the temperature decay rate. Active thermography for NDT is based on the detection and recording by an infrared camera of thermal radiations emitted by object surface. To detect defects, it is sometimes necessary to destabilize the object’s thermal state through heating or cooling i.e. active thermography. The presence of an internal defect reveals itself on the surface as a temperature perturbation above this defect.

Applications of thermography

  • Nondestructive testing (NDT) for practical examples in quality inspection
  • Condition montioring for the moting object
  • Quality inspection in metal or non-metal materials such as alloys or composites
  • Pressure vessel & pipelines fault inspection
  • Electronic devices investigation
  • Building evaluation such as wall tile bonding integrity, concrete surface structure inspection and CFRP concrete quality evaluation
  • Medial imaging
  • Research works in material’s thermal properties

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