D3 Thermography and Radiography

Topic

Pulsed thermography and radiography are the two complementary non-destructive inspection methods combined in this model, with each detecting a different aspect of potential internal defects within the part. Because each method relies on a distinct physical principle—heat diffusion in one case and radiation attenuation in the other—together they can detect defects that a single method might miss. We begin by discussing pulsed thermography.

Pulsed thermography involves applying a heat pulse to the part's surface and using an infrared camera to monitor how the surface cools as heat diffuses into the material. If a subsurface defect exists with a thermal diffusivity different from that of the surrounding material, heat diffuses anomalously at that location; consequently, the surface area above the defect cools at a different rate—either faster or slower—than the surface area above sound material. This diffusivity anomaly, detected at the surface, reveals the presence of the underlying defect.

Radiography, by contrast, involves passing radiation through the part and measuring the amount of radiation that emerges, in accordance with the Beer-Lambert attenuation law. This law states that radiation is attenuated exponentially as it passes through a material and that this attenuation depends directly on the thickness traversed and the material's density at that point. If the material density at a specific point is lower than that of the surrounding material, less radiation is attenuated, and more radiation reaches the detector; conversely, if the density is higher, more radiation is attenuated, and less reaches the detector. That point-by-point difference in the amount of radiation reaching the detector is what produces the radiographic contrast in the resulting image: a point on the part receiving more radiation than its surroundings appears lighter, while one receiving less appears darker. It is precisely this radiographic contrast that reveals porosity or inclusions within the part, as both a pore and an inclusion of a different composition possess a density distinct from that of the surrounding material.

For this reason, this model—incorporating both pulsed thermography and radiography—allows these complementary methods to jointly cover two distinct ways of detecting an internal defect: pulsed thermography, via the thermal diffusivity anomaly the defect creates beneath the surface; and radiography, via the radiographic contrast produced in the image by the defect's density difference.

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