The formation of crack patterns in drying starch-water slurries is studied by means of in-situ radiography (measuring of the crack front velocity) and X-ray microtomography as an example of crack patterns driven by inhomogeneous shrinkage. The tomograms show the 3D crack networks forming columns with polygonal cross-sections. After crack initiation, the average crack spacing increases with growing depth, even if the crack front velocity is constant. A constant velocity is obtained by maintaining a constant evaporation rate using a feedback control. When the crack front has propagated at a constant evaporation rate over a distance of some millimeters, the average crack spacing approaches a stable value which depends on crack front velocity according to a power law. This relationship is compared to corresponding results of other authors and model predictions. The increase of crack spacing before stable values are achieved, is interpreted as a result of successive crack front instabilities.

As residual stresses can reduce the lifetime of diamond-cobalt composite cutting tools, the composite stress state needs to be understood very well. Thus, the effect of microstructure on the residual stresses was investigated here. Stress measurements were carried out in the cobalt matrix by X-ray diffraction (XRD) and Synchrotron-XRD (SXRD). In addition to global stress measurements, investigations of stresses in small cobalt areas near the diamonds were performed by high brilliant synchrotron radiation using different apertures.

Diamond-cobalt composites are used for cutting tools. Residual stress after manufacture can reduce the lifetime of such composite cutting tools and, hence, the stress state needs to be well understood. Within this study, stress measurements on a cobalt diamond composite were made by SXRD to deduce stress states in the cobalt matrix using the (222) reflection. The application of different apertures allowed the investigation of stress in small areas of cobalt surrounding a diamond and in areas at different distances to a diamond. In the areas adjacent to the diamond increased residual stresses were found in the cobalt matrix. Furthermore, approximations for radial and tangential residual stresses have been derived which show to be different.

A better understanding of micro deformation and damage processes that, the microstructure of particle reinforced metal matrix composites (MMCp) undergoes at, microscale before macroscopical failure gives the right direction for the microstructural design of these materials. To this end, a mu CT-based analysis was performed that combines mu CT-experiments and FE simulations: The gauge length of tiny tensile Specimens (cross-section A = 2 x 1 mm(2)) consisting of the MMCp systems Cobalt/Diamond and Al/B4C was imaged by tomography at; different stages of deformation. 3D strain tensor fields and displacement, vector fields were determined by digital image correlation of the reconstructed tomograms. Based on tomogrants of the analyzed volume at the undeformed state, FE meshes were generated that model the microstructure close to reality. Using these meshes and the displacement vector fields measured at the volume boundaries, FE simulations of the deformation and damage behavior were carried out. In both composites volume strains below 1% have been found experimentally. The spatial resolution of deformation fields is limited by the characteristic microstructural length which depends on the particle diameter and the particle spacing. The results of the experiments and the simulations are compared on the basis of 3D-strain fields sampled within the analyzed microstructural region. Additionally, the impact of microstructural features on the localization of strain, the initiation of localized damage and the successive failure of the composite materials is discussed.

A better understanding of micro deformation and damage processes that the microstructure of particle reinforced metal matrix composites (MMCp) undergoes at microscale before macroscopical failure gives the right direction for the microstructural design of these materials. To this end, a μCT-based analysis was performed that combines μCT-experiments and FE simulations: The gauge length of tiny tensile specimens (cross-section A = 2 x 1 mm2) consisting of the MMCp systems Cobalt/Diamond and Al/B4C was imaged by tomography at different stages of deformation. 3D strain tensor fields and displacement vector fields were determined by digital image correlation of the reconstructed tomograms. Based on tomograms of the analyzed volume at the undeformed state, FE meshes were generated that model the microstructure close to reality. Using these meshes and the displacement vector fields measured at the volume boundaries, FE simulations of the deformation and damage behavior were carried out. In both composites volume strains below 1% have been found experimentally. The spatial resolution of deformation fields is limited by the characteristic microstructural length which depends on the particle diameter and the particle spacing. The results of the experiments and the simulations are compared on the basis of 3D-strain fields sampled within the analyzed microstructural region. Additionally, the impact of microstructural features on the localization of strain, the initiation of localized damage and the successive failure of the composite materials is discussed.

Manufacturing diamond-cobalt composites by sintering results in residual stresses due to the mismatch of thermal expansion coefficients mainly. To understand the influence of manufacturing process parameters on residual stresses of sintered diamond-cobalt composites samples are produced by different process parameters. The investigated diamond composites are pressureless sintered as well as pressureless sintered combined with hot isostatically pressing. Here the influence of powder characteristics and process parameters like compaction pressure and sintering temperature on the residual stresses and microstructure was analysed by X-ray diffraction, microscopy and tomography, The aim of this study is to correlate residual stresses with manufacturing parameters and to give hints for optimising the residual stress state and for improving the lifetime of diamond-cobalt composites.

For many reasons, coated components are steadily increasing in importance in e.g. the aerogas and automobile industries. Because of the stringent safety requirements in these industries, quality control of these products is an essential part of their fabrication. Conventional non-destructive testing (NDT) techniques, however, often have serious shortcoming as regards testing, e.g. for thin metallic or ceramic coatings. Some NDT techniques have been modified to be suitable for such cases, and new techniques have already been developed and applied in those cases where the modified techniques fail. The new technique of thermal wave analysis, which can be used for the evaluation of coating thickness, is presented. For the description and detection of coating flaws other techniques, such as the eddy current pulse technique and the method of holographic soundfield visualization, can be employed. The theoretical bases of these techniques are presented and their practical application to various testing problems discussed.

An improved acoustic-holographic reconstruction-algorithm is presented which allows a three-dimensional image-reconstruction, together with - for the first time - an axial resolution much below one wavelength. This high resolution makes it possible not only to evaluate exactly the contours of defects in the axial direction but also to measure its deformation under thermal or mechanical load. Experimental measurements correspond to the theoretical model, being limited primarily by the dynamic range of the experimental setup.

AbstractDie Entwicklung moderner technischer Bauteile und Bauteilkomponenten erfordert den Einsatz hochbeanspruchbarer Leichtbauwerkstoffe, die zunehmend aus kohlenstoffaserverstärkten Epoxidharzen hergestellt werden. Der zuverlässige Einsatz derartiger Werkstoffe setzt neben der Kenntnis des Versagensablaufes auch die Prüfbarkeit der Verbunde auf Fertigungsfehler und Schädigungen infolge Betriebsbeanspruchung voraus. Die Zuverlässigkeit von Faserverbunden ist daher entscheidend von der Qualitätssicherung in der Fertigung und im späteren Betrieb abhängig. Dabei ist nicht das Auffinden jeglicher Fehlstellen von Bedeutung, sondern lediglich solcher Ungänzen, durch die die Einsatzfähigkeit des Bauteils in Frage gestellt wird. Als geeignetes Prüfverfahren hierzu hat sich die Schallemissionsanalyse (SEA) erwiesen, da sich mit ihrer Hilfe bereits in einem frühen Stadium Werkstoffveränderungen nachweisen und interpretieren lassen.Es wurde der Einfluß von fertigungsbedingten Fehlern auf den Versagensablauf und die mechanisch‐technologischen Eigenschaften von CFK‐Laminaten untersucht. Bei den Fehlern handelt es sich um quasinatürliche Fehler (durchtrennte Faserlage, poröse Matrix, sowie fehlende Haftung der Fasern).Der Einfluß dieser Fehler auf den Beginn und Ablauf der Schädigungen wurde mit Hilfe der SEA abhängig vom Laminataufbau (0, ± 45, 90°) beobachtet. Durch Schliffbilduntersuchungen sowie mit „InSitu”︁ Belastungen im REM wird die Zuordnung zwischen den Schallemissionsanalysen und den Schädigungen des Laminates ermöglicht. In einem weiteren Schritt wird die Übertragbarkeit der an Laborproben gewonnenen Erkenntnisse auf innendruckbelastete Rohre überprüft.

AbstractEs werden Untersuchungsergebnisse vorgestellt, die belegen, daß Verfahren der Korrelationsanalyse (matched filter) in der Lage sind, die Nachweisempfindlichkeit von Reflektoren in schallstreuenden Werkstoffen zu verbessern.Es wird ein neues Konzept zur praktischen Durchführung von Korrelationsanalysen erarbeitet, bei dem die Korrelationsfunktion bereits in CS‐Sendeimpulsen enthalten ist. Die Messungen ergeben, daß sich die Ergebnisse praktisch nicht von der wesentlich aufwendigeren empfangsseitigen Korrelationsanalyse unterscheiden. Als wesentlicher Vorteil ergibt sich, daß die Prüfgeschwindigkeit gegenüber der üblichen Ultraschallprüfung nicht verringert wird.

Thermally sprayed coating are usually tested destructively, i.e. metallo-graphically or by applying various strength tests. In most cases the results of non-destructive testing NDT procedures, which are of major importance for the inspection and eventual repair of structural components, do not allow satisfactory decisions to be made on component safety.

AbstractMerkmale, die mit Hilfe verschiedener Analyseverfahren aus Schallemissionssignalen gewonnen werden, können durch den Einsatz von Rechenverfahren zur Mustererkennung bewertet werden. Durch Anwendung dieser Programme kann festgestellt werden, ob die verwendeten Merkmale – maximale Amplitude, Signalanstiegszeit, Schallsumme, Impulsfläche, Impulsenergie und mittlere Amplitude sowie einfache Kenngrößen aus Frequenzspektren – die Schallemissionssignale trennen können oder ob für den Fall, daß ein einzelnes Merkmal diese Aufgabe nicht löst, eine Kombination von Merkmalen zu einem besseren Ergebnis führt.Die Möglichkeiten der verschiedenen Analysen sowie der Mustererkennungsprogramme werden anhand von vier simulierten Signalarten überprüft. Je nach Analyseverfahren ergeben sich dabei deutliche Unterschiede hinsichtlich der Trennung der Schallemissionen. Jedoch kann selbst bei Signalgruppen, die mit einem Analyseverfahren nicht getrennt werden können, durch die Anwendung von Merkmalskombinationen die Unterscheidung verbessert werden.

AbstractInvestigations performed to study the strength behaviour supply informations about the failure pattern under loading. Beside metallographic and fractographic investigations acoustic emission measurements are used for interpretation. The results obtained for brazed joints – Ni Cr 20 Ti Al/BNi‐5‐ show by means of the analyzed acoustic signals the possibility to evaluate the quality of a joint after a short loading period. Furthermore, hints were given to reduce the testing expenditure being necessary until now by the application of the acoustic emission analysis remarkably.

AbstractDie Anwendung der Schallemissionsanalyse zur zerstörungsfreien Prüfung von Bauteilen leidet im Betrieb stark unter ungünstigen Signal‐Rauschverhältnissen, so daß eine Auffindung der Signale und eine Ermittlung des Schallquellenortes nur mit hohen Ungenauigkeiten möglich ist.Für diesen Fall werden die Einsatzmöglichkeiten von Rechen‐ und Analysenverfahren untersucht, die eine Trennung von Nutzsignal und Störgeräuschen zum Ziel haben.

AbstractBei der Messung und Auswertung von Schallemissionen sind Einflußgrößen vorhanden, deren Nichtbeachtung zu Fehldeutungen und nichtreproduzierbaren Meßergebnissen führen kann. Anhand simulierter Schallsignale werden die Einflüsse an dem praktischen Beispiel einer Stahlplatte mit ihrer Auswirkung auf Frequenz‐ und Amplitudengehalt des Schallsignales sowie auf die Genauigkeit einer Schallquellenlokalisierung aufgezeigt.