Up to now THz-TDS-systems aren't considered to be nondestructive testing facilities for large scale industrial applications, despite it was proven that they provide a comprehensive set of quality parameters. A practical approach to bring THz-TDS in addition to already existing testing systems into the industrial mainstream is systematic development of future test procedures and test facilities for dielectrics. For this purpose polyethylene test specimen with introduced artefacts were designed, to evaluate the detection sensitivity of Time of Flight measurements based on dielectrics. SAFT reconstructed tomograms are presented which visualize the sizes and location of artificially introduced flaws.

THz Applications for Non-Destructive Testing

Obwohl THz-TDS-Systeme vielfaltige Qualitatsparameter von Dielektrika ermitteln konnen, ist deren industrielle Nutzung im Bereich der zerstorungsfreien Bauteilprufung noch nicht anerkannter Stand der Technik. Ein pragmatischer Weg, TDS-Systeme neben traditionellen Prufverfahren zu etablieren, sind systematische Untersuchungen an Modellbauteilen, mit dem Ziel einer Methodenentwicklung zur zuverlassigen beruhrungslosen Prufung von Dielektrika. Anhand von THz-TDS-Messungen an Polyethylen-Testkorpern mit kunstlich eingebauten Fehlstellen werden anfanglich die Moglichkeiten der Laufzeitmessung zur Ermittlung von Tiefenlagen von Unganzen in Testkorpern prasentiert. Im weitern Teil wird gezeigt, wie durch eine veranderte Versuchsdurchfuhrung und Verwendung eines modifizierten SAFT-Rekonstruktions-Algorithmus Tomogramme berechnet werden konnen, in denen die kunstlich eingebrachten Fehlstellen hinsichtlich ihrer Grose und Lage geometrisch richtig dargestellt werden.

THz-SAFT Rekonstruktion von HDPE-Bauteilen

Today X-ray tomography and Ultrasound Synthetic Aperture Focusing Techniques are representative examples for defect imaging in the field of nondestructive testing. THz time domain spectroscopy (TDS) operates with short electromagnetic pulses. The sensitivity of the pulses to optical material property changes makes THz-TDS attractive for the detection of flaws in dielectric materials. Unfortunately, the X-ray computed tomography (CT) model holds only for THz-TDS reconstruction if minor refraction index differences between the inhomogeneities and the surrounding material matrix exist. A Time Domain SAFT algorithm has been developed to overcome the restrictions. THz time domain measurement on representative sample sets with inherent artefacts were performed to use them for the image reconstructions. The results will be presented and compared with optical surface images of the used test objects to evaluate the SAFT algorithm in relation to the reconstruction quality.

THz-TDS-SAFT for the detection of inherent discontinuities in dielectric materials

Electromagnetic waves with frequencies between 0.1 and 10 THz are described as THz-radiation (T-ray). The ability to penetrate dielectric materials makes T-rays attractive to reveal discontinuities in polymer and ceramic materials. THz-Time Domain Spectroscopy Systems (THz-TDS) are available on the market today which operates with THz-pulses transmitted and received by optically pumped semiconductor antennas. In THz-TDS the travelling time (ToF) and shape of the pulse is changed if it interacts with the dielectric material and its inherent disconti-nuities. A tomogram of the object under the test can be reconstructed from time of flight diffraction (ToFD) scans if a synthetic focusing aperture (SAFT) algorithm is applied. Otherwise, planar discontinuities like cracks in plastics or delaminated lay-ers in composites can be abstracted as layers located at any angle in relation to the outer sample surface direction. A tomogram from the scanned sample can then be reconstructed in case the interactions of electromagnetic pulses with the existing in-herent interfaces are detectable and a model is assumed which describes the device under the test as multilayer structure composed of thin layers with different dielec-tric properties.
A short description of both the SAFT – and Optical Layer algorithm for the recon-struction of the inherent structure is initially given. Measurements on representative samples with a variety of artificially produced small and large scale. Reconstructed tomograms are presented to discuss and evaluate the benefits and limits of the two different reconstruction approaches.

THz-ToF techniques for the detection of inherent discontinuities in dielectric materials based on a SAFT – and an optical layer reconstruction algorithm

Holger Spranger

Papers

THz-ToF techniques for the detection of inherent discontinuities in dielectric materials based on a SAFT – and an optical layer reconstruction algorithm