TERRAIN 3D - TERmographic Reconstruction At INner 3D Interfaces

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Active thermography is now a very well established non-destructive testing and measuring method due to its non-contact and cost-effective application. Two methods are used almost exclusively for this purpose. Both impulse thermography with flash-like energy input and lockin thermography with sinusoidally modulated energy input are categorized as near-surface testing methods and have so far only been able to test thin components. Since the test is carried out over a given area, the results are available quickly and directly in the form of an image. A quantitative reconstruction of the measurement results for a depth-resolved representation of the specimen interior is extremely time-consuming and has so far only been implemented for very simple geometries on a laboratory scale.In the planned project, two approaches to 3D imaging of the specimen interior will be pursued in order to elegantly overcome the mathematically complex regularization required to solve the ill-posed inverse reconstruction problem. A flexible infrared system will be used to provide additional information for the testing.The flexible infrared system makes it for the first time possible to use a spatially and temporally flexible IR radiation source of high intensity for active thermography. Starting from each pixel of the projector-like infrared system (more than one million pixels), a thermal wave encoded in arbitrary order can be emitted. In the first approach, the thermal waves are modeled in such a way that a 1D heat flux is established over the complete component and the known approaches of photothermics now hold for complex structures in a two-dimensional test. In this case, the reconstruction is simplified to a 1D problem. In the second approach, we take advantage of the linear behavior of thermal waves and consider each pixel of the infrared system simultaneously as an individually coded thermal wave. After decoding, known reconstruction methods of ultrasonic testing can be applied from the time-of-flight information obtained for each image point in the radial direction. In both cases, 3D imaging representations of the inner interfaces are output directly from the measurements. The test method is converted into a measurement method.If successful, the project will help to transform active thermography into a direct 3D imaging quantitative measurement method and increase the defect detection rate.