21The design, realization and operation of a prototype or "demonstrator" version of an active target and time projection chamber (ACTAR TPC) for experiments in nuclear physics is presented in detail. The heart of the detection system features a micromegas gas amplifier coupled to a high-density pixelated pad plane with square pad sizes of 2×2 mm 2 . The detector has been thoroughly tested with several different gas mixtures over a wide range of pressures and using a variety of sources of ionizing radiation including laser light, an α-particle source and heavy-ion beams of 24 Mg and 58 Ni accelerated to energies of 4.0 MeV/u. Results from these tests and characterization of the detector response over a wide range of operating conditions will be described. These developments have served as the basis for the design of a larger detection system that is presently under construction.1

A novel approach is presented for the ultrasonic determination of the elastic constants in plate-like structures of an orthotropic material (e.g. composites) using a time-of-flight version of the pulsed ultrasonic polar scan (TOF P-UPS). A forward numerical model of the TOF P-UPS is coupled to an inversion algorithm, based on the genetic optimization principle, targeting the determination of the orthotropic elastic parameters, and the quality of the inversion is demonstrated for synthetic data representative for composites. The advantage of the new approach is that the presented TOF P-UPS inversion method does not require a priori knowledge about the symmetry class of the material, nor about the orientation of the main axes of symmetry. Furthermore, the TOF P-UPS inversion method yields an accurate characterization of the orthotropic elasticity tensor, even when applied to composite plates with small frequency-thickness ratios in which the traditional bulk wave approaches no longer hold. Finally, the robustness of the TOF P-UPS inversion method is demonstrated for noisy data by evaluating the results for a range of signal-to-noise ratios.

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An enhanced two-stage inversion scheme for the determination of the viscoelastic properties of orthotropic plates (e.g. fiber reinforced polymers) based on the pulsed ultrasonic polar scan (P-UPS) is presented. A forward numerical model for the P-UPS is combined with an efficient optimizer routine (particle swarm optimizer, PSO) to inversely extract the complex orthotropic stiffness tensor, as well as the orientation of the main axes of orthotropy. The developed inversion technique is validated on synthetic data, yielding accurate results with small deviations: elasticity tensor < 0.5%, viscosity tensor < 2% and main axes of orthotropy < 0.03°. The inversion procedure has been further applied to analyze actual experimental P-UPS recordings on an aluminum plate as well as a unidirectional [0] 8 carbon/epoxy laminate. The obtained inversion results yield complex stiffness parameters which are in close agreement to reported values in literature, and confirms the potential of the UPS technique and its inversion optimization routine as an enhanced NDT technique to quickly and accurately obtain characteristic material properties of composites.

Numerical finite elements simulations and postprocessing analysis methods are presented for Ultrasonic Polar Scan (UPS) measurements involving a Circular Phased Array (C-PA) to determine the plane wave reflection coefficient of plates. Apodization weights for the C-PA elements are determined in order to assure the generation of a quasi-plane wave upon excitation at the plate surface and to mitigate bounded beam effects on the assessed reflection coefficient. In addition, postprocessing of the reflection signals is performed via the synthetic plane wave technique to further filter out any bounded beam effects. Reflection coefficients for three test cases are presented, namely for an aluminum, a unidirectional Carbon Epoxy and a cross-ply Carbon Epoxy plate. For all three cases, the comparison with analytical plane wave theory shows an excellent agreement with the reflection coefficients obtained by the C-PA and the additional post-processing steps for both pulsed and harmonic signals. It is also shown that the agreement becomes considerably worse if the non-specular reflection field is disregarded in the post-processing treatment, thus enforcing the need to capture the full reflected field via the Phased Array whenever plane wave reflection coefficients are needed.