TY - CONF A1 - Ziegler, Mathias A1 - Ahmadi, Samim A1 - Hirsch, Philipp Daniel A1 - Lecompagnon, Julien A1 - Hassenstein, Christian A1 - Thiel, Erik A1 - Pech May, Nelson Wilbur T1 - Using spatial and temporal shaping of laser-induced diffuse thermal wave fields in thermography T2 - SMSI 2020 Proceedings N2 - The diffuse nature of thermal waves is a fun-damental limitation in thermographic nonde-structive testing. In our studies we investigated different approaches by shaping the thermal wave fields which result from heating. We have used high-power laser sources to heat metallic samples. Using these spatial and temporal shaping techniques leads to a higher detection sensitivity in our measurements with the infra-red camera. In this contribution we show our implementation of shaping laser-induced diffuse thermal wave fields and the effect on the defect reconstruction quality. T2 - SMSI 2020 Conference CY - Online meeting DA - 22.06.2020 KW - Thermal wave KW - Diffusion KW - High-power laser KW - Thermography KW - Spatiotemporal shaping PY - 2020 DO - https://doi.org/10.5162/SMSI2020/C5.1 SP - 179 EP - 180 AN - OPUS4-50897 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Thiel, Erik A1 - Ziegler, Mathias A1 - Studemund, Taarna T1 - Localization of subsurface defects in uncoated aluminum with structured heating using high-power VCSEL laser arrays JF - International Journal of Thermophysics N2 - We report on photothermal detection of subsurface defects by coherent superposition of thermal wave fields. This is made possible by structured heating using high-power VCSEL laser arrays whose individual emitter groups can be arbitrarily controlled. In order to locate the defects, we have developed a scanning method based on the continuous wavelet transformation with complex Morlet wavelet using the destructive interference effect of thermal waves. This approach can also be used for thermally very fast and highly reflective materials such as uncoated aluminum. We show that subsurface defects at an aspect ratio of defect width to defect depth down to 1/3 are still detectable in this material. KW - Thermography KW - Heat diffusion KW - Laser thermography KW - Structured heating KW - NDT KW - Subsurface defects KW - Thermal wave KW - VCSEL KW - Wavelet transformation PY - 2019 DO - https://doi.org/10.1007/s10765-018-2478-9 SN - 1572-9567 SN - 0195-928X VL - 40 IS - 2 SP - 17, 1 EP - 13 PB - Springer Science+Business Media, LLC, part of Springer Nature 2019 AN - OPUS4-47208 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Ahmadi, Samim A1 - Thiel, Erik A1 - Ziegler, Mathias T1 - Thermal wave field engineering using structured 1D laser illumination N2 - Thermal waves are solutions of the heat diffusion equation for periodic boundary conditions and can be seen analogously to strongly damped waves. Although the underlying differential equation differs from the wave equation, the essential property for analogy between both equations is linearity such that superposition applies. This linearity is maintained even after a linear transformation, such as the Fourier transform from time to frequency domain. It follows that the temporal superposition principle is already used in active thermography, e.g. in pulsed thermography, as a superposition of many individual frequencies. However, the systematic spatial superposition has not yet been fully exploited, mainly due to a lack of suitable energy sources. As a first step, we are investigating how thermal wave fields of arbitrary space-time structures can be engineered using structured laser illumination. The proof of principle was shown using a laser coupled projector. Unfortunately, the available optical output power was limited due to the thermal stress limit of the device. That is why we are working towards a more sophisticated moving 1D array of high-power diode lasers. We characterized the novel light source and believe that apart from the benefit of spatial and temporal illumination it can combine the temporal regimes of impulse and lock-in thermography. In a second step, we investigate moving and oscillating line sources with different line shapes. We use a Green’s Function ansatz to analytically model the thermal wave propagation of structured 1D laser illumination in isotropic materials. Furthermore, we show some methods how they can be implemented. With this technique, we were able to accelerate our detection method firstly presented in for vertical narrow defects by factor three. Generally, we believe that this technique opens up similar opportunities than in other NDE methods. High-resolution ultrasound, for example, is also based on the superposition of single emitters and a recent concept suggests an option to deal with the diffusion wave character of the thermal waves. T2 - 62. International School of Quantum Electronics die Tagung “Progress in Photoacoustic & Photothermal Phenomena” CY - Erice, Italy DA - 06.09.2018 KW - Thermal wave KW - Thermal wave field KW - Thermal engineering KW - Structured laser illumination KW - 1d laser KW - Laser array KW - Laser thermography PY - 2018 AN - OPUS4-46193 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Ziegler, Mathias A1 - Thiel, Erik A1 - Studemund, T. T1 - Thermography using a 1D laser array – From planar to structured heating JF - Materials Testing N2 - In the field of optically excited thermography, flash lamps (impulse shaped planar heating) and halogen lamps (modulated planar heating) have become established for the specific regimes of impulse and lock-in thermography. Flying-spot laser thermography is implemented by means of a rasterized focused laser, e. g. for crack detection (continuous wave operation) and photothermal material characterization (high-frequency modulated). The availability of novel technologies, i. e. fast and high-resolution IR cameras, brilliant innovative light sources and high-performance data acquisition and processing technology will enable a paradigm shift from stand-alone photothermal and thermographic techniques to uniform quantitative measurement and testing technology that is faster and more precise. Similar to an LED array, but with irradiance two orders of magnitude higher, a new type of brilliant laser source, i. e. the VCSEL array (vertical-cavity surface-emitting laser), is now available. This novel optical energy source eliminates the strong limitation to the temporal dynamics of established light sources and at the same time is spectrally clearly separated from the detection wavelength. It combines the fast temporal behavior of a diode laser with the high optical irradiance and the wide illumination area of flash lamps. In addition, heating can also be carried out in a structured manner, because individual areas of the VCSEL array can be controlled independently of each other. This new degree of freedom enables the development of completely new thermographic NDT methods. KW - Thermography KW - Laser thermography KW - Laser KW - Lock-in KW - VCSEL KW - Thermal wave KW - Photothermal PY - 2018 UR - https://www.hanser-elibrary.com/doi/abs/10.3139/120.111209 DO - https://doi.org/10.3139/120.111209 SN - 0025-5300 VL - 60 IS - 7-8 SP - 749 EP - 757 PB - Carl Hanser Verlag CY - München AN - OPUS4-45482 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Thiel, Erik T1 - Novel thermographic methods for non-destructive testing using structured illumination N2 - Photothermal imaging is commonly used for the characterization of material properties, the determination of layer thicknesses or the detection of inhomogeneities such as voids or cracks. For this purpose, the solid specimen is externally heated, e.g. by using a light source. The resulting transient heat flows interact with the inner structures of the specimen, which in turn is measured as a transient temperature distribution at the surface. Novel array-shaped, high-power laser light sources allow to control the heating of the surface arbitrarily, both temporally and spatially. This enables us to shape the heat flows within the material in a very specific way. In a first application, we demonstrate how to apply destructively interfering thermal wave fields in order to detect subsurface defects with a very high sensitivity. A similar technique, although originating from a very different physical domain, is already in use for medical 3D imaging showing the high potential of this approach. T2 - Adlershofer For­schungs­forum 2017 CY - Berlin, Germany DA - 10.11.2017 KW - Active thermography KW - Photothermal KW - Crack detection KW - Thermal wave KW - Structured heating PY - 2017 AN - OPUS4-42835 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Götschel, S. A1 - Maierhofer, Christiane A1 - Müller, Jan P. A1 - Rothbart, Nick A1 - Weiser, M. T1 - Quantitative defect reconstruction in active thermography for fiber-reinforced composites T2 - 19th World Conference on Non-Destructive Testing 2016 N2 - Carbon-fiber reinforced composites are becoming more and more important in the production of light-weight structures, e.g., in the automotive and aerospace industry. Thermography is often used for non-destructive testing of These products, especially to detect delaminations between different layers of the composite. In this presentation, we aim at methods for defect reconstruction from thermographic measurements of such carbon-fiber reinforced composites. The reconstruction results shall not only allow to locate defects, but also give a quantitative characterization of the defect properties. We discuss the simulation of the measurement process using finite element methods, as well as the experimental validation on flat bottom holes. Especially in pulse thermography, thin boundary layers with steep temperature gradients occurring at the heated surface need to be resolved. Here we use the combination of a 1D analytical solution combined with numerical solution of the remaining defect equation. We use the simulations to identify material Parameters from the measurements. Finally, fast heuristics for reconstructing defect geometries are applied to the acquired data, and compared for their accuracy and utility in detecting different defects like back surface defects or delaminations. T2 - 19th World Conference on Non-Destructive Testing 2016 CY - München, Germany DA - 13.06.2016 KW - Active thermography KW - Numerical simulation KW - Time-resolved thermogram KW - Thermal wave PY - 2016 UR - https://nbn-resolving.org/urn:nbn:de:kobv:b43-366644 UR - https://www.wcndt2016.com/Programme/show/Th-4-C-4 VL - 2016 SP - Th.4.C.4-1 EP - Th.4.C.4-10 AN - OPUS4-36664 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Thiel, Erik A1 - Kreutzbruck, M. A1 - Ziegler, Mathias T1 - Spatial and temporal control of thermal waves by using DMDs for interference based crack detection T2 - Emerging Digital Micromirror Device Based Systems and Applications VIII, Proc. of SPIE N2 - Active Thermography is a well-established non-destructive testing method and used to detect cracks, voids or material inhomogeneities. It is based on applying thermal energy to a samples’ surface whereas inner defects alter the nonstationary heat flow. Conventional excitation of a sample is hereby done spatially, either planar (e.g. using a lamp) or local (e.g. using a focused laser) and temporally, either pulsed or periodical. In this work we combine a high power laser with a Digital Micromirror Device (DMD) allowing us to merge all degrees of freedom to a spatially and temporally controlled heat source. This enables us to exploit the possibilities of coherent thermal wave shaping. Exciting periodically while controlling at the same time phase and amplitude of the illumination source induces – via Absorption at the sample’s surface - a defined thermal wave propagation through a sample. That means thermal waves can be controlled almost like acoustical or optical waves. However, in contrast to optical or acoustical waves, thermal waves are highly damped due to the diffusive character of the thermal heat flow and therefore limited in penetration depth in relation to the achievable resolution. Nevertheless, the coherence length of thermal waves can be chosen in the mmrange for modulation frequencies below 10 Hz which is perfectly met by DMD technology. This approach gives us the opportunity to transfer known technologies from wave shaping techniques to thermography methods. We will present experiments on spatial and temporal wave shaping, demonstrating interference based crack detection. T2 - Photonics West 2016, OPTO, 9761 CY - San Francisco, CA, USA DA - 15.02.2016 KW - Active thermography KW - Thermal wave KW - Spatial light modulation KW - Crack detection KW - DMD KW - DMD coupled laser PY - 2016 DO - https://doi.org/10.1117/12.2210918 SN - 0277-786X VL - 9761 SP - 97610N-1 EP - 97610N-13 AN - OPUS4-35587 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -