Ultrafast Pulsed Terahertz Sensing for the Contact-free Control of Composites and Adhesive Assemblies

We present a ground breaking Terahertz Time-Domain spectrometer (THz-TDS) and illustrate its performance by the non-destructive evaluation of fibre reinforced plastics and adhesively joined assemblies. Operated as imaging device this technology gives in a direct way access to the inner structure of dielectric materials and reveals manufacturing defects or fatigue damage in short acquisition time. Electromagnetic radiation of THz frequencies allows the contact-free testing of dielectric volumes and buried or coated metallic surfaces without ionizing effect. THz-TDS is a particular powerful tool for NDT as it provides a multitude of information useful at the same time for imaging, material characterisation and spectroscopic analysis. Amplitude and time of flight of the ultrashort THz pulse in the time domain, as well as the spectrally resolved amplitude and phase information available by Fourier-Transformation, enable the analysis of complex objects. In many cases, three dimensional monitoring is possible with one-sided access to the object. THz-TDS has been applied to aircraft composites, and its ability to detect mechanical and heat damage, voids, delamination, water or moisture contamination has been shown. The single shot THz pulsed sensor “STRIPP” developed in our group reduces the acquisition time drastically relative to the repetitive solutions: the ultrashort detection window of some tens of picoseconds is recorded within one single laser shot. The measurements are performed at high stability and acquisition of kHz rates, without moving parts. With this leap in technology, the potential of terahertz radiation can be further exploited in industrial environments. We recently analysed an object and the integrity of its structural sub-units during free fall. The here presented applications illustrate further the performance of the technology, particular its ability to determine the thickness of substructures with micrometre precision, to localize precisely defects as delamination and inclusions. So, the high contrast of ultrafast THz pulses on dielectrics enables the localization and analysis of polymers that are buried in another polymer. In general, “STRIPP” can be used for the design, quality control and inspection of aerospace components. Electromagnetic waves with Terahertz (THz) frequencies or corresponding submillimetre wavelengths are situated in the range between the infrared and microwaves. They combine advantages of these neighboured spectral regions as high penetration depth in dielectrics and low scattering on micro-structured media. With optical generation schemes, free space propagation with good beam quality and accordingly diffraction