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AQTIVE - Active quantitative thermography using innovative vertical emitting lasers

Subject Area Measurement Systems
Coating and Surface Technology
Plastics Engineering
Lightweight Construction, Textile Technology
Materials in Sintering Processes and Generative Manufacturing Processes
Term from 2018 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 400857558
 
Final Report Year 2023

Final Report Abstract

In the AQTIVE project, the possibilities and limitations for active thermography were investigated that arise due to the good spatial and temporal flexibility in the irradiance distribution of high-power VCSEL arrays. As a substitute for halogen lamps, ordinary laser systems, LEDs, and flash lamps, a VCSEL array can be used very well for optically excited thermography for nondestructive testing and allows complete spectral separation from infrared radiometric temperature determination, as well as fast controllability while maintaining high irradiance. It was possible to set up 2-dim. photothermal testing systems with which coating thicknesses can be determined quickly and over a wide area with an accuracy of up to +- 3 µm. The wide modulation bandwidth from cw up to 600 Hz was also used in signal forming. Thus, with the help of coded Barker or Legendre sequences, the detection sensitivity of defects could be improved by more than 10 dB in some cases compared to pulse-phase thermography. However, the deeper the defect was, the smaller the advantage of a fast excitation source became. Combined with novel approaches such as the virtual wave concept and neural networks, the depth range and resolution of thermographic inspection could be increased even further, which is particularly important for metallic materials. The VCSEL array, which consists of 8 or 12 individually controllable linear laser zones, offers the possibility of spatially structured heating compared to the otherwise common homogeneous light sources. It has been demonstrated that this newly gained spatial degree of freedom can be productively used in active thermography. With the aid of multiple measurements that varied in their spatial irradiance distribution, laser-based super-resolution thermography could be established for metallic materials. In addition to the aforementioned temporal structuring, the limits of active thermography could be pushed even further, allowing even deep-lying defects to be precisely resolved in terms of size and position. For plastics, the first advantages of phase-delayed spatial excitation over 2-dim. homogeneous lockin-thermography for vertical cracks have so far been found in the model. The VCSEL array offers a real price advantage over alternative high-power laser sources but does not allow true homogeneous illumination due to patchy sub-structuring in laser cells. Among other things, this property requires precise characterization of the overall measurement system prior to productive use. In order to further develop spatially structured laser thermography into a commercially viable method, further technical revisions of the present VCSEL arrays with respect to improved drive-ability are likely to be required.

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