Crack detection on hot test samples with thermography

Broberg, Patrik; Hogskolan Vast Institutionen for ingenjorsvetenskap; Sweden

Broberg, P.; University West; Sweden
Runnmalm, A.; University West; Sweden

ID: ECNDT-0133-2018
Session: Thermography and Thermosonics 1
Room: H1
Date: 2018-06-14
Time: 09:20 - 09:40

Surface breaking cracks in metal structures can be detected using thermography with a range of different excitation methods. With optical excitation, illumination by UV light for example, the difference between the absorptivity and emissivity between a crack and the surrounding surface is used to produce a temperature difference. The same theory has here been used, but without the optical excitation. Instead an apparent temperature difference is created by either heating the sample or by lowering the reflected background temperature.
It has previously been shown that surface breaking cracks have large emissivity and absorptivity due to multiple reflections inside the crack. The surrounding metal surface, on the other hand, is generally highly reflective in the infrared region. This means that a crack will radiate as a black body and will show the real temperature of the sample, while the surrounding surface will show mostly the reflected radiation from the surroundings. If the sample temperature is the same as room temperature there will not be any difference in the measured radiation between a crack and the rest of the sample. If the sample temperature is raised, the apparent temperature from a crack will be higher than that from the rest of the sample, which mostly reflects room temperature. The same effect can be created by lowering the room temperature in relation to the sample temperature, by for example reflecting a cold object in the sample surface.
The advantage of this excitation method compared to optical excitation is that the excitation equipment can be simplified. In the case where a cold object is reflected in the surface, there is no energy input at all into the test piece. This method can also be used for inspecting hot samples without excitation.