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Der Ersatz von Röntgenfilmen durch digitale Detektoren in der Zerstörungsfreien Prüfung im Anlagenbau wird seit 2013 durch neue ISO-Normen unterstützt. Nach der Erfolgsgeschichte der digitalen Photographie und der fast voll-ständigen Ablösung der Filmtechnik in der optischen Bilderfassung wird erwartet, dass sich der Übergang von der Filmradiographie zur digitalen Radiographie schnell vollzieht. Dieser Trend ist derzeit so noch nicht zu beobachten. Zur mobilen Prüfung wird in Europa immer noch vorrangig die Filmradiographie und nur teilweise die Computerra-diographie mit Speicherfolien eingesetzt. Im Bereich der stationären Prüftechnik haben sich die Matrixdetektoren (DDA) weitgehend durchgesetzt, wobei es auch hier noch alternative Detektoren wie Bildverstärker und Fluorosko-pe gibt. Mit Einführung der DIN EN ISO 17636-2 zur Schweißnahtprüfung wird seit 2013 erstmalig geregelt, wie man einen digitalen Detektor für die technische Radiographie auswählen soll, um im Vergleich zur Filmradiographie eine äquivalente Bildqualität zu erhalten. Essentielle Parameter bestimmen, ob die geforderte Bildgüte (Fehlernachweiß-empfindlichkeit) erreicht wird. Ein Überblick zeigt, welche Parameter für den praktischen Einsatz wichtig sind und wie sie ermittelt werden. Praktische Einsatzbeispiele im Anlagenbau werden diskutiert.
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 or deformed 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.
We present our research on the development of a Brillouin optical frequency-domain analysis (BOFDA) using a perfluorinated graded-index polymer optical fiber (PFGI-POF) as a sensing fiber. The described works include investigations both on the selection of the setup components with respect to mode coupling effects and on the impact of the humidity cross-sensitivity. The suitability of the developed POF-based BOFDA is proved by a distributed measurement on an 86 m long PFGI-POF recorded with the spatial resolution of 6 m.
The distributed measurement of relative humidity is a sought-after capability for a wide range of applications in civil engineering and structural health monitoring. We show that polymethyl methacrylate (PMMA) optical fi-bers can be employed as a sensor medium to conduct distributed humidity measurement by analyzing Rayleigh backscattering traces obtained by OTDR. We make use of the effect that water penetrates the fiber core and directly influences the local fiber attenuation and Rayleigh backscatter coefficient. We conducted distributed backscattering analysis for two different pulse wavelengths: 500 nm and 650 nm. The 650 nm results are susceptible to both, attenuation changes and backscatter changes, whereas backscatter results at 500 nm are not affected by humidity-induced attenuation and only exhibit a change of Rayleigh backscattered power as a function of humidity. The combined measurement and analysis of both parameters at these two wavelengths has the advantage that cross-sensitivities on backscatter change and attenuation, such as strain and tempera-ture changes, could be separated from the humidity response of the fiber. We present laboratory results for a humidity range between 30% and 90% for both pulse wavelengths: including step responses, humidity cycles and hysteresis analysis. In addition to the attenuation and backscatter coefficient dependence, we also analyze optical runtime changes as a function of humidity. POFs have the advantage that they can be directly embed-ded into materials such as concrete or soil to measure water content or localize water ingress. Standard step-index PMMA POFs can be used as a distributed relative humidity sensor up to 200 m distance.
The lecture gives an overview of the core competencies of the department 8.5 Micro NDT (CT equipment, X-Ray Refraction, WAXS - Texture analysis, Data correction, Reconstruction algorithms, Image analysis, Dimensional control, Multi-Source Data Fusion and Combination, Micromechanical Properties by Neutron Diffraction) with regard to the characterization of lightweight materials (fibre composites, aluminum and titanium MMCs) used in aviation. The focus is on characterization by synchrotron and X-ray refraction techniques as well as computed tomography.
Residual stresses count among the most limiting factors in the application of additively manufactured materials in safety relevant components subject to cyclic loading. The source of such stresses is inherent in the manufacturing Laser Beam Melted (LBM) process due to rapid cooling and solidification and their distribution in the specimen or component is not homogeneous, but it is usually characterized by high gradients. Moreover, the magnitude of the residual stress field depends very much on the orientation, being higher in the build direction.
Many works in the literature advise to carry out a post-LBM treatment to relieve residual stresses, even though particular attention should be paid in choosing the parameters for the thermal treatment in order to avoid microstructural/phase transformations, which would affect greatly the mechanical properties of the material.
Particularly in the case of the austenitic stainless steel 316L, it has been shown that annealing at high temperatures (above 900°C), besides relieving the residual stresses, may cause recrystallization, grain growth and even phase transformation. In contrast, if a too low annealing temperature is chosen in order to preserve the microstructure, the residual stresses cannot be completely relieved.
This work aims to address the effect of residual stresses on short and long crack propagation for SEN(B) specimens made of 316L fabricated by LBM, in which notches have been machined by electro-discharge machining (EDM) on the mid-plane, perpendicular to the build direction. The specimens underwent different annealing treatments in inert atmosphere, in which the maximum temperature has been varied up to 900°C. The amount of residual stresses and their distribution has been measured by X-ray and neutron diffraction and the specimens have been subject to cyclic loading in a resonant testing machine. The tests show a massive influence of the residual stresses in the build direction on the resistance to fatigue crack propagation of additively manufactured 316L. Finally, a comparison with conventionally manufactured 316L is presented.