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Die zuverlässige Bestimmung der mechanischen Eigenschaften der Grenzfläche zwischen zwei Betonen unterschiedlichen Alters ist von großer Bedeutung für die Bemessung. Die Bewertung der Rauigkeit einer Betonoberfläche erfolgt über den Vergleichswert der mittleren Rautiefe. Als Standardverfahren für die Ermittlung der Rautiefe gilt das Sandflächenverfahren nach Kaufmann. Zur Verbesserung der Zuverlässigkeit bei der Ermittlung der Rautiefe von Betonoberflächen sowie zur Erweiterung der Anwendung auf vertikale und geneigte Oberflächen wurden alternative Messverfahren entwickelt, die auf optischen Messmethoden basieren. Solche Verfahren sind beispielsweise die Laser-Triangulation oder die Photogrammetrie. Im vorliegenden Beitrag wird die Anwendung der zuvor genannten Verfahren zur Ermittlung der Rauigkeit beschrieben. Im Rahmen von Untersuchungen an Deckenplatten mit Gitterträgern werden die Verfahren anschließend an aufgerauten Betonoberflächen vergleichend angewendet.
Lock-in- and flash thermography are standard methods in active thermography. They are widely used in industrial inspection tasks e.g. for the detection of delaminations, cracks or pores. The requirements for the light sources of these two methods are substantially different. While lock-in thermography requires sources that can be easily and above all fast modulated, the use of flash thermography requires sources that release a very high optical energy in the very short time.
By introducing high-power vertical cavity surface emitting lasers (VCSELs) arrays to the field of thermography a source is now available that covers these two areas. VCSEL arrays combine the fast temporal behavior of a diode laser with the high optical irradiance and the wide illumination range of flash lamps or LEDs and can thus potentially replace all conventional light sources of thermography.
However, the main advantage of this laser technology lies in the independent control of individual array areas. It is therefore possible to heat not only in terms of time, but also in terms of space. This new degree of freedom allows the development of new NDT methods. We demonstrate this approach using a test problem that can only be solved to a limited extent in active thermography, namely the detection of very thin, hidden defects in metallic materials that are aligned vertically to the surface. For this purpose, we generate destructively interfering thermal wave fields, which make it possible to detect defects within the range of the thermal wave field high sensitivity. This is done without pre-treatment of the surface and without using a reference area to depths beyond the usual thermographic rule of thumb.
Structured illumination using high-power diode lasers generates a two-dimensional interference of thermal waves. In addition, the sources and the sample are moving relative to each other. Using different configurations, we investigate the validity of the temporal and spatial superposition principle of the heat diffusion equation for these cases both experimentally and by numerical-analytical modelling. Furthermore, we investigate the potential of this approach for non-destructive testing.
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.
Laser tissue soldering (LTS) based on indocyanine green (ICG)-mediated heat- denaturation of proteins might be a promising alternative technique for micro-suturing, but up to now the Problem of too weak shear strength of the solder welds in comparison to sutures is not solved. Earlier reports gave promising results showing that solder supported by carrier materials can enhance the cohesive strength of the liquid solder. In these studies, the solder was applied to the carriers by dip coating. Higher reliability of the connection between the solder and the carrier material is expected when the solder is bound covalently to the carrier material. In the present study a poly(ether imide) (PEI) membrane served as carrier material and ICG- supplemented albumin as solder substrate. The latter was covalently coupled to the carrier membrane under physiological conditions to prevent structural protein changes. As laser source a diode continuous-wave laser emitting at 808 nm with intensities between 250mW and 1500mW was utilized. The Albumin functionalized carrier membrane was placed onto the tunica media of explanted pig thoracic aortae forming an overlapping area of approximately 0.5×0.5 cm2. All tests were performed in a dry state to prevent laser light absorption by water. Infrared spectroscopy, spectro-photometrical determination of the secondary and Primary amine groups after acid orange II staining, contact angle measurements, and atomic force microscopy proved the successful functionalization of the PEI membrane with albumin. A laser power of 450mW LTS could generate a membrane-blood vessel connection which was characterized by a shear strength of 0.08±0.002MPa, corresponding to 15% of the tensile strength of the native blood vessel. Theoretically, an overlapping zone of 4.1mmaround the entire circumference of the blood vessel could have provided shear strength of the PEI membrane-blood vessel compound identical to the tensile strength of the native blood vessel. These in-vitro results confirmed the beneficial effects of solder reinforcement by carrier membranes, and suggest LTS with covalently bound solders on PEI substrates for further studies in animal models.