Specialists from the School of Non-Destructive Testing and Safety Engineering at Tomsk Polytechnic University (TPU) have developed an effective method for conducting non-destructive thermal testing of composites for the aerospace industry, the university’s press service reported on 16 April.
To eliminate the influence of thermal noise due to surface roughness in the process of finding defects in the structure of carbon fiber or fiberglass parts, scientists used forced cooling in combination with the main heating pulse.
The authors of the development presented the results obtained by studying the new method in the article “Detection of defects in composites by combined heating and cooling: theory and experiments”, published in the Journal of Nondestructive Assessment (Q2; IF: 2.5). Based on these results, scientists are developing a portable flaw detector to monitor the composite structure of aerospace parts.
Currently, the procedure for non-destructive thermal testing is to subject the surface of the controlled object to brief heating and, as it cools, record its temperature using a thermal imaging camera.
However, the roughness and uneven emissivity of the surface of polymeric composites create a number of difficulties for such thermal control. When such a material is heated, for example, by an optical source, its temperature will change unevenly over time, creating thermal interference, against which the temperature signal of an internal defect can be “lost.”
A new control method developed by scientists at the Tomsk Polytechnic is based on sequential heating and cooling of the material surface. To select the optimal parameters for this process, the researchers first developed a mathematical model of the process and then conducted experiments using the line scan method.
The samples studied were a multilayer plexiglass plate painted with matte black paint to ensure a low level of thermal interference of its surface, as well as a carbon fiber product with a very rough surface. Furthermore, the samples had hidden internal defects.
During the experiment, the surfaces of the samples were first heated with a halogen lamp, then cooled, and the temperature field was recorded. The resulting thermal images were analyzed.
An interesting phenomenon was discovered: forced cooling caused a decrease in surface temperature to a pre-warming state, while the internal structure still “gave off” heat and hidden defects still produced significant temperature signals.
Furthermore, the magnitude of the temperature contrast (the ratio between the temperature signal and the temperature in the defect-free zone) increases significantly in this experiment, and the temperature signatures of the defects are better visible, said Arseny Chulkov, lead researcher. of the TPU Center for Industrial Tomography.
In addition to increasing the probability of detecting internal defects, the combined forced heating and cooling procedure, unlike the conventional thermal inspection procedure, does not require high heating of the tested material to provide stable signals in the defective areas.
Currently, TPU specialists are developing a prototype of a portable flaw detector using a new inspection method. Its characteristic will be that it will be able to detect defects in optically transparent and translucent compounds, since it will not use an optical source, but rather a convective, heating and cooling source.
“Radiation in the optical range, when passing through a transparent material, heats it slightly. Thermal control requires the material to absorb heating energy. An air heating and cooling system will solve this problem. It is also planned to implement a combination of thermal scanning tests and the classic ultrasonic method of non-destructive testing in the flaw detector. “This will allow flaws to be identified over a wide range of depths.”points out Arseny Chulkov.
TPU scientists aim to create a device capable of detecting defects in products with horizontal and vertical structural orientation, as well as in products with a curved surface. And its prototype, according to its plans, will be ready at the end of 2024.
Source: Rossa Primavera
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