Scientists of the Engineering School of Non-Destructive Testing and Safety of TPU have developed a new effective method of thermal non-destructive testing of composites used in the aerospace industry. The Polytechnic proposed using forced cooling in combination with the main heating pulse when detecting defects in carbon fiber and fiberglass. This made it possible to make more reliable control of materials that have a high level of thermal interference caused by the roughness of the object's surface and the uneven radiation coefficient. Based on the results obtained, a portable flaw detector is being developed, which can be used to assess the quality of composite parts of aerospace equipment.
The research was supported by a grant from the Russian Science Foundation. The results of the scientists' work are published in the Journal of Non-Destructive Evaluation (Q2; IF: 2.5).
The classical procedure of thermal non-destructive testing consists in short-term heating of the surface of the object of control and recording its temperature at the cooling stage using a thermal imager. Thermal control of polymer composites, the surface of which has roughness and uneven emissivity, has a number of difficulties. They are related to the fact that when such a material is heated, for example, by an optical source, its temperature will vary unevenly over time. The deviation of the recorded temperature is a thermal interference, against which the temperature "signal" from the internal defect can be missed.
Scientists at Tomsk Polytechnic University have proposed a new method of thermal non-destructive testing of composites. It is based on sequential heating and cooling of the surface of the material. As part of the study, the scientists first numerically modeled various parameters of the proposed method, after which they conducted experiments using the linear scanning method. A multilayer plexiglass plate painted with black matte paint was used as experimental samples, which provided a low level of thermal interference to its surface, as well as a carbon fiber product with a significantly rough surface. Both samples had internal hidden defects. During the experiment, the surface was heated with a halogen lamp, then forced cooling of the surface of the object of control and registration of the temperature field was carried out. After that, the scientists performed an analysis of the thermal images obtained.
The use of forced cooling of the controlled surface at a certain point in time after its heating causes an interesting phenomenon — the excessive surface temperature of the sample drops to its initial temperature, while the internal structure still "gives off" heat, and hidden defects still produce significant temperature signals. At the same time, the value of the temperature contrast, which is the ratio of the temperature signal to the temperature in the defect-free zone, increases significantly. As a result, against the background of "suppressed" surface noise, the temperature marks of defects are more visible," comments Arseniy Chulkov, senior researcher at the TPU Center for Industrial Tomography.
Thus, artificially increasing the temperature signal above the internal defects increases the probability of their detection. In addition, the combined heating and forced cooling procedure, unlike the classical thermal control procedure, does not require the application of a high thermal load to the controlled material to ensure a high level of signals in defective areas.
At this stage of the project, polytechnics are developing a prototype of a portable flaw detector that implements control in the proposed way. In addition, it will be suitable for defect control in optically transparent and translucent composites. The peculiarity of the device being created is that instead of an optical source, a convective source will be used for heating and cooling.
Radiation in the optical range, passing through a transparent material, weakly heats it. For thermal control, it is necessary that the heating energy is absorbed by the material. An air heating and cooling system will solve this problem. It is also planned to implement a combination of scanning thermal control and the classical ultrasonic method of non-destructive testing in the flaw detector. This will make it possible to identify defects in a wide range of depths," notes Arseny Chulkov.
The polytechnics plan to create a device that will detect defects in products with horizontal and vertical orientation, as well as in products with a curved surface. It is expected that a prototype portable flaw detector will be ready by the end of 2024.
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