Zerstörungsfreie Prüfung von faserverstärkten Kunststoffen mit der Dauerstrich THz-Spektroskopie (English shop)

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For many centuries, humankind has known how to exploit the properties of materials consisting of a combination of fibres and a base substance, the matrix. However, the fibre-reinforced composite materials resulting from this combination only became industrially relevant in the 1940s, when glass fibres could be produced on a large industrial scale. Today there is a wide variety of fibre-reinforced composites, among which glass-fibre-reinforced plastics with an epoxy resin matrix are one of the most important representatives.

This dissertation provides an insight into the production of conventional polyolefins, such as polyethylene, as well as of fibre-reinforced plastics, which may contain various fibre materials. In the manufacture of components from the aforementioned semi-finished products, quality assurance and hence the associated measurement technology play an important role. To this end, an overview is given of the currently industrially relevant measurement techniques, both destructive and non-destructive. At this point, the non-destructive and cost-effective continuous-wave THz measurement technique can be employed as an imaging measurement system together with the data evaluation developed within the framework of this work. The system used for this purpose exploits the principle of photomixing in a semiconductor material bearing a metallised antenna structure on its surface. By using a phase-locked transmitter–receiver combination, coherent signal detection is possible. With the aid of the sinusoidal interferograms obtained, which in some cases consist of only one or two periods, the amplitude and phase values of each image point can be determined, from which material parameters such as refractive index and absorption can be extracted. Furthermore, this work shows how, by evaluating the amplitude and phase progression of the signal, a spatially resolved assessment is also possible, for example of the quality of plastic welded joints, or the detection of defective interlayers in GFRP as well as of the prevailing fibre volume content. The differentiation of inclusions such as air or metal in polyethylene is likewise possible on the basis of the measurement signals. The continuous-wave THz spectrometer thus advances into the application fields of imaging THz systems, which have hitherto been occupied mainly by cost-intensive THz time-domain spectrometers. The performance of the measurement system is demonstrated on industrially relevant components.

In a further step, a novel, cost-effective, narrow-band and tunable THz source was developed, based on parametric frequency conversion, specifically difference frequency mixing, in an optically nonlinear crystal. For this purpose, the crystal is located inside the cavity of a Vertical External Cavity Surface Emitting Laser (VECSEL). Such a THz source offers the possibility, especially at frequencies above a few hundred GHz, of generating an output power of the THz signal that is considerably higher than that of a photomixing system. The laser system set up in the course of this work has the property of emitting two laser colours simultaneously, which is indispensable for intracavity THz generation. Through further optimisation steps of the thermal and optical properties of the VECSEL, the intracavity optical power could be improved to such an extent that, by applying parametric difference frequency generation, the output power of the resulting THz wave could be increased into the milliwatt range. The parametric THz source developed represents a powerful, inexpensive and compact signal source for future industrially applicable non-destructive testing systems. Especially at frequencies from a few hundred GHz upwards, it offers a powerful alternative to the existing, comparatively low-power continuous-wave sources based on the principle of photomixing.