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Direct Laser Writing (DLW) and X-ray computed tomography (XCT) both offer unique possibilities in their respective fields. DLW produces full three-dimensional (3D) polymer structures on the microscale with resolutions below 100 nm. The fabricated structures can be analysed by XCT or X-ray microscopy (XRM), which incorporates additional X-ray lenses, in three dimensions down to a minimal basic spatial resolution of about 500 nm or 50 nm, respectively. In this work, two different DLW structures are analysed via XCT. Internal defects are detected and analysed for the purpose of quality control. Defects and structures with sizes down to 1.5 µm are successfully analysed. A 3D reconstruction and internal, hidden features of the fabricated structures are shown and discussed. In a first-of-its-kind study, we demonstrate the detectability of a single-voxel line inside a fabricated structure that would not be detectable with SEM or light microscopy. Furthermore, the direct fabrication on a PET substrate is shown to overcome the high X-ray absorbance of commonly used glass substrates. Attenuation spectra of SZ2080 and glass substrates are compared to a fabrication route direct on a 170 µm PET foil. The practical aspects of XCT measurements for DLW structures on different substrates will be discussed.
A composite material is a combination of two or more materials with very different mechanical, thermal and electrical properties. The various forms of composite materials, due to their high material properties, are widely used as structural materials in the aviation, space, marine, automobile, and sports industries. However, some defects like voids, delamination, or inhomogeneous fiber distribution that form during the fabricating processes of composites can seriously affect the mechanical properties of the composite material. In this study, several imaging NDE techniques such as: thermography, high frequency eddy current, ultrasonic, x-ray radiography, x-ray laminography, and high resolution x-ray CT were conducted to characterize the microstructure of carbon fiber composites. Then, a 3D analysis was implemented by the destructive technique of serial sectioning for the same sample tested by the NDE methods. To better analyze the results of this work and extract a clear volume image for all features and defects contained in the composite material, an intensive comparison was conducted among hundreds of 3D-NDE and multi serial sections’ scan images showing the microstructure variation.
Ultrasonic testing is a widely applied measurement method in materials research and medicine. Commonly, a transducer is coupled to the specimen directly or via a liquid coupling agent. While reducing acoustic transmission losses significantly, this procedure is time-consuming and cannot be used for sensitive specimens. Air-coupled ultrasound is a viable alternative in such cases, although suffering from very high acoustic transmission losses between transducer, air and specimen. The recently introduced fluidic transducer (FT) generates ultrasound by utilizing the instability of a supersonic air jet switched inside a fluidic amplifier. Since only air is used as the working medium and no vibrating surfaces are used for ultrasound generation, the transducer is able to efficiently generate large acoustic pressure amplitudes. The resulting acoustic field shares its directivity with the ejected high-velocity air jet. Thus, the acoustic energy needs to be redirected from the jet axis in order to make the fluidic transducer applicable to sensitive specimens. In this study, the effectivity of using sonic crystals (SCs) for this redirection is investigated using acoustic and flow measurements. SCs are air-permeable while being reflective to large acoustic frequency bands. It was shown that both a defect waveguide and a mirroring strategy successfully redirected the acoustic field from the air jet. Furthermore, the interaction of flow and SC showed strong acoustic quenching if the SC was placed too close to the FT outlet. Blockage of the jet entrainment due to the SC may result in slightly higher off-axis flow velocities locally, which should be considered in sensitive applications.
Die Bewertung des Zustandes und der Haftung von Putz und Fassadenelementen auf den darunterliegenden Bauwerksstrukturen ist eine häufig auftretende Fragestellung bei der Erhaltung und Instandsetzung von Bauwerken und Baudenkmälern. Hier müssen nicht nur wirtschaftliche und denkmalpflegerische, sondern auch sicherheitsrelevante Aspekte berücksichtigt werden. Fehlstellen und Ablösungen können, auch wiederholend, mit hoher Nachweisempfindlichkeit mit aktiven Thermografieverfahren geortet und unter bestimmten Voraussetzungen quantitativ bewertet werden. Dazu wird der zu untersuchende Bereich zunächst kurzzeitig (mehrere Minuten lang) erwärmt und anschließend die Abkühlung mit einer Infrarotkamera aufgezeichnet. Systematische Untersuchungen an Probekörpern mit einer Vielzahl unterschiedlicher Fehlstellen unterhalb von Kalkputz, Fliesen und Klinkerriemchen zeigen, dass die Fehlstellen sowohl mittels künstlicher Erwärmung durch einen Infrarotstrahler als auch mittels natürlicher Erwärmung durch die Sonne detektiert und charakterisiert werden können. ---------------------------------------------------------------------------------------------------------------------------------------
The evaluation of condition and adhesion of plaster and façade elements to the underlying building structure is a common issue in the maintenance and repair of buildings and monuments. Here not only economic and preservation, but also safety relevant aspects have to be considered. Voids and delaminations can be detected repeatedly with high sensitivity using active thermography methods. Under certain conditions, also a quantitative evaluation is possible. The area under investigation has to be heated up for a short time (up to several minutes). Subsequently, the cooling down is recorded using an infrared camera. Systematic studies on test specimens with a variety of defects below lime plaster, tiles and brick tiles show that the defects can be detected and even characterized both by means of artificial heating using an infrared heater, as well as by natural solar heating.
This paper presents the progress of successful location of grouting faults in tendon ducts with ultrasonic imaging. The examples were obtained in the research group FOR 384 funded by DFG (German Research Foundation). The co-operation of experimental research and modeling allowed imaging and identification of grouted and ungrouted areas of tendon ducts (including strands) in a large test specimen (40 m²). In addition to the criteria for indicating grouting faults in post-tensioned ducts known until now the phase evaluation of reflected ultrasonic pulses is described. Experiments and modeling of wave propagation are presented for reflections at metal plates in concrete (thickness range 0.5 mm to 40 mm) and for tendon ducts including strands.
The main part of the progress was achieved by automated measurements using dry contact transducers, 3D-SAFT reconstruction including phase evaluation and modeling considering wave propagation for typical elastic parameters and exact experimental site conditions. The results for shear waves as well as for pressure waves are compared in the frequency range from 50 kHz to 120 kHz.
Ultrasound propagation in inhomogeneous anisotropic materials is difficult to examine because of the directional dependency of elastic properties. Simulation tools play an important role in developing advanced reliable ultrasonic non destructive testing techniques for the inspection of anisotropic materials particularly austenitic cladded materials, austenitic welds and dissimilar welds. In this contribution we present an adapted 2D ray tracing model for evaluating ultrasonic wave fields quantitatively in inhomogeneous anisotropic materials. Inhomogeneity in the anisotropic material is represented by discretizing into several homogeneous layers. According to ray tracing model, ultrasonic ray paths are traced during its energy propagation through various discretized layers of the material and at each interface the problem of reflection and transmission is solved. The presented algorithm evaluates the transducer excited ultrasonic fields accurately by taking into account the directivity of the transducer, divergence of the ray bundle, density of rays and phase relations as well as transmission coefficients. The ray tracing model is able to calculate the ultrasonic wave fields generated by a point source as well as a finite dimension transducer. The ray tracing model results are validated quantitatively with the results obtained from 2D Elastodynamic Finite Integration Technique (EFIT) on several configurations generally occurring in the ultrasonic non destructive testing of anisotropic materials. Finally, the quantitative comparison of ray tracing model results with experiments on 32 mm thick austenitic weld material and 62 mm thick austenitic cladded material is discussed.
One of the great strengths of X-ray computed tomography over conventional inspection methods (ultrasound, thermography, radiography) is that it can image damage in 3D. However for curved ordeformed composite panels it can be difficult to automatically ascribe the damage to specific plies or inter-ply interfaces. An X-ray computed tomography (CT) data processing methodology is developed to extract the through-thickness distribution of damage in curved or deformed composite panels. The method is applied to [(0°/90°)2]s carbon fibre reinforced polymer (CFRP) panels subjected low velocity impact damage (5 J up to 20 J) providing 3D ply-by-ply damage visualisation and analysis. Our distance transform approach allows slices to be taken that approximately follow the composite curvature allowing the impact damage to be separated, visualised and quantified in 3D on a ply-by-ply basis. In this way the interply delaminations have been mapped, showing characteristic peanut shaped delaminations with the major axis oriented with the fibres in the ply below the interface. This registry to the profile of the panel constitutes a significant improvement in our ability to characterise impact damage in composite laminates and extract relevant measurements from X-ray CT datasets.
Fe K-edge XANES measurements were performed to determine the oxidation state of iron in iron gall inks. These data are used to investigate the relationship between the Fe oxidation state and ink corrosion in historical manuscripts. The oxidation state is determined using the pre-edge based on a general variation trend that was derived from model compounds and validated here independently on a suite of samples with known oxidation state and Fe in octahedral coordination (i.e. similar to Fe in gall ink). As long as suitable references for the pure ferric and ferrous state of iron gall inks cannot be pointed out, the analysis of the pre-edge of the Fe K-edge represents the only way to acquire absolute values for the Fe oxidation state (Fe3+/Fetot) with spatial resolution for this usually non-crystalline compound. Fe3+/Fetot values for the ink samples vary between ca. 0.7 and 1.0. Spatially resolved analyses performed across ink strokes of various historical manuscripts as well as newly made samples did not reveal any systematic spatial dependence. On average, historical samples that were exposed to conservation treatments show a more oxidized state. However, the measurements demonstrate explicitly that the exposure to X-rays during the XANES acquisition affects the oxidation state of Fe in most iron gall inks.