Pulse thermography (PT) is one of the most popular thermal stimulation method in IR thermography. One reason for this popularity is the quickness of the inspection relying on a thermal stimulation pulse, with duration going from a few ms for high thermal conductivity material inspection (such as metal parts) to a few seconds for low thermal conductivity specimens (such as plastics, graphite epoxy components). Such quick thermal stimulation allows direct deployment on the plant floor with convenient heating sources. Moreover, the brief heating prevents damage to the component (heating is generally limited to a few degrees above the initial component temperature).

Basically, PT consists of briefly heating the specimen and then recording its temperature decay curve. Qualitatively, the phenomenon is as follows. The temperature of the material first rises during the pulse. After the pulse, it then decays because the energy - the thermal front - propagates by diffusion under the surface. Later, the presence of a subsurface defect (example: a disbonding) reduces the diffusion rate so that when observing the surface temperature, such a subsurface defect appears as an area of higher temperature with respect to the surrounding sound area. In fact in such a case the reduced diffusion rate caused by the subsurface defect presence translates into “heat accumulation” and hence higher surface temperature just over the defect. Moreover, such phenomenon occurs in time so that, deeper defects are observed later and with a reduced “diluted” or “spread” thermal contrast. An interesting relationship relates (in a first approximation) the observation time t as a function of the square of the subsurface defect depth z:

t~z^2/α

where α is the thermal diffusivity of the material.

A widely used rule of thumb says that the radius of the smallest detectable defect should be at least one to two times larger than its depth under the surface That rule of thumb is a useful guideline for basic PT in homogeneous isotropic materials. However, better results may be possible through the use of advanced signal processing methods.

Various configurations are possible:

• Point inspection: heating with a laser or a focused light beam; advantages: repeatable heating, uniformity; drawback: the necessity to move the inspection head to fully inspect a surface slows down the inspection process.
• Line inspection: heating using line lamps, heated wire, scanning laser, line of air jets (cool or hot); advantages: fast inspection rate (up to 1 m^2/s) and good uniformity thanks to the lateral motion; drawback: only part of the temperature history curve is available due to the lateral motion of the specimen and the fixed distance between thermal stimulation and temperature signal pick-up. Projecttion of a series of line heating strips is also used to detect surface cracks.
• Surface inspection: heating using lamps, flash lamps, scanning laser; advantages: the complete analysis of the phenomenon is possible since the whole temperature history curve is recorded; drawback: concerns about non-uniformity of the heating (lamps, flashes, heat gun, laser, microwave).

If the temperature of the part to inspect is already higher than ambient temperature, it can be of interest to make use of a cold thermal source such as a line of air jets (or water jets; sudden contact with ice, snow, etc.). In fact, a thermal front propagates the same way whether being hot or cold: what is important is the temperature differential between the thermal source and the specimen. An advantage of a cold thermal source is that it does not induce spurious thermal reflections into the IR camera as in the case of a hot thermal source. The main limitations of cold stimulation sources are related to practical considerations as for instance it is generally easier and more efficient, to heat rather then to cool a part.

In case of microwave heating, direct internal heating of the part is achieved and since travel time is reduced from subsurface defect to surface with respect to surface to subsurface defect to surface in case of surface heating, defects are delineated better with less thermal contrast “spreading”.

Possible observation methods are as follow:

• in reflection: the thermal source and detector are located on the same side of the inspected component,
• in transmission, the heating source and the detector are located one on each side of the component to inspect.

Generally, the reflection approach is used for detection of defects located close to the heated surface while the transmission approach allows to detect defect close to the rear surface (due to the spreading effect of the energy propagating within the specimen). Obviously, if the rear surface is not accessible, the transmission approach is not possible. Finally, in the transmission approach, the defect depth can not be estimated due to the same travel distance whatever is the defect depth (the transit time of the thermal front through the total material thickness is the same).

Nowadays, fully integrated systems combining acquisition head and heating unit are commercially available. Such systems are convenient on the plant floor since deployment is fast. For laboratory experiments table-top apparatus are preferred because various configurations can be tested. In both cases, image processing allows improved visibility of subsurface defects, extended depth range and quantification of thermal properties (e.g.: depth).

Some of the recent popular image processing techniques are: Neural Networks (NN), Pulsed Phase Thermography (PPT) or Synthetic Data (SD). Interestingly, combinations of processing techniques are also possible.

PPT consists to observe images in the frequency rather than in the time domain, in particular phase images related to travel time are interesting since they are less affected by unwanted effects such as non-uniform heating.

SD fits the raw data with a polynomial (of fourth order) for significant denoising and data reduction. Simple thermal contrast processing is also common.