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Jordens C.,TU Braunschweig | Scheller M.,TU Braunschweig | Wietzke S.,TU Braunschweig | Romeike D.,TU Braunschweig | And 5 more authors.
Composites Science and Technology | Year: 2010

We employ terahertz time-domain spectroscopy (THz TDS), a novel, non-destructive testing method, to study the fibre orientation and fibre content in reinforced plastics. The birefringent properties of plastics filled with differing amounts of short glass fibres are measured at frequencies from 100 GHz up to 1 THz. To predict the permittivity of the experimentally examined composite materials, we use an effective medium theory first introduced by Polder and van Santen. On the basis of the measured data and this model, we deduce the additive content ξ, the preferential orientation of the fibres φ and the fraction of orientated fibres a. Our findings agree well with corresponding mold flow simulations performed with commercially available software. © 2009 Elsevier Ltd. All rights reserved. Source


Berer T.,Christian Doppler Laboratory | Berer T.,Research Center for Non Destructive Testing Gmb | Hochreiner A.,Christian Doppler Laboratory | Zamiri S.,Christian Doppler Laboratory | And 2 more authors.
Optics Letters | Year: 2010

We report on remote and contactless photoacoustic imaging (PAI) for the inspection of solid materials using a twowave mixing interferometer. In this Letter, a semitransparent sample was excited with picosecond laser pulses. The local absorption of the electromagnetic radiation led to generation of broadband ultrasonic waves inside the sample. Ultrasonic waves arriving at the sample surface were detected utilizing a two-wave mixing interferometer. After data acquisition, the initial pressure distribution was reconstructed using a Fourier space synthetic aperture technique algorithm. We show the potential of PAI for the inspection of semitransparent solid materials. © 2010 Optical Society of America. Source


Veres I.A.,Research Center for Non Destructive Testing Gmb | Berer T.,Research Center for Non Destructive Testing Gmb | Berer T.,Christian Doppler Laboratory | Burgholzer P.,Research Center for Non Destructive Testing Gmb | Burgholzer P.,Christian Doppler Laboratory
Ultrasonics | Year: 2013

Laser-generation of ultrasound is investigated in the coupled dynamical thermoelasticity in the presented paper. The coupled heat conduction and wave equations are solved using finite differences. It is shown that the application of staggered grids in combination with explicit integration of the wave equation facilitates the decoupling of the solution and enables the application of a combination of implicit and explicit numerical integration techniques. The presented solution is applied to model the generation of ultrasound by a laser source in isotropic and transversely isotropic materials. The influence of the coupling of the generalized thermoelasticity is investigated and it will be shown, that for ultra high frequency waves (i.e. 100 GHz) generated by laser pulses with duration in the picosecond range, the thermal feedback becomes considerable leading to a strong attenuation of the longitudinal bulk wave. Moreover, the coupling leads to dispersion influencing the wave velocities at low frequencies. The numerical simulations are compared to theoretical results available in the literature. Wave fields generated by a line focused laser source are presented by the numerical model for isotropic and for transversely isotropic materials. © 2012 Elsevier B.V. All rights reserved. Source


Veres I.A.,Research Center for Non Destructive Testing Gmb | Berer T.,Research Center for Non Destructive Testing Gmb | Berer T.,Christian Doppler Laboratory | Grunsteidl C.,Research Center for Non Destructive Testing Gmb | And 2 more authors.
Ultrasonics | Year: 2014

This article elaborates on the crossing points of the frequency-wavenumber branches for the symmetric and anti-symmetric Lamb modes in a homogeneous plate. It is shown both theoretically as well as experimentally that at these crossing points either the normal or the longitudinal components of modal displacement attain an extreme value, i.e. a maximum or it vanishes. This behavior is assessed herein using a method due to Mindlin, who showed that the dispersion curves for a plate with mixed boundary conditions - which are associated with uncoupled shear and dilatational modes - provide bounds to the spectral lines of the free plate. Therefore, a subset of the crossing points of the symmetric and antisymmetric Lamb modes for a free plate coincide with the crossing points for a plate with mixed boundary conditions. © 2013 The Authors. Published by Elsevier B.V. All rights reserved. Source


Berer T.,Research Center for Non Destructive Testing Gmb | Berer T.,Christian Doppler Laboratory | Veres I.A.,Research Center for Non Destructive Testing Gmb | Grun H.,Research Center for Non Destructive Testing Gmb | And 4 more authors.
Journal of Biophotonics | Year: 2012

The frequency response of fiber optic line detectors is investigated in the presented paper. An analytical model based on oblique scattering of elastic waves is used to calculate the frequency dependent acousto-optical transfer functions of bare glass optical and polymer optical fibers. From the transfer functions the transient response of fibers detectors to photoacoustically excited spherical sources is derived. Photoacoustic tomography is simulated by calculating the temporal response of arrays of fiber optic line detectors and subsequent image reconstruction. The results show that the choice of the fiber material is of significant importance and influences the quality of imaging. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Source

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