When do small fatigue cracks propagate and when are they arrested?

Mainly the role of the following parameters is treated: material (steels, Al-alloys, Cu – heat treatment), kind of loading (stress range, R-ratio, mode I, mixed mode loading, constant and variable amplitudes, 10 Hz, 20 kHz) environment (vacuum, humid air at 20 °C, dry air at 90 °C, aqueous solution with different Cl --concentration Please click Additional Files below to see the full abstract

Preferential cataclastic grain size reduction of feldspar in deformation bands in poorly consolidated arkosic sands

AbstractThis study presents microstructural as well as bulk and mineral chemical investigations of deformation bands in uncemented, friable arkosic sands of Miocene age (Vienna Basin, Austria). Our microstructural study indicates grain size reduction by grain flaking in deformation bands with small offsets (0.5–8 cm), and dominant intragranular fracturing and cataclasis of altered feldspar grains at larger displacements (up to 60 cm). Relative to quartz, the sericitized feldspar grains are preferably fractured and abraded, which additionally leads to an enrichment of mainly phyllosilicates by mechanical expulsion from feldspar. Both cataclasis of quartz and feldspar grains and enrichment of phyllosilicates result in grain size reduction within the deformation bands. The measured reduction in porosity of up to 20% is in some cases associated with a permeability reduction, reflected in the retention of iron-oxide rich fluids along deformation bands. These deformation bands formed at very shallow burial depths in unconsolidated sediments indicate that fault sealing may occur in the absence of chemical alteration of the deformation bands and lead to a compartmentalization of a groundwater or hydrocarbon reservoir

Micromechanics of creep and relaxation of wood. A review COST Action E35 2004-2008: Wood machining - micromechanics and fracture

Wood, like all polymeric materials, shows viscoelastic behaviour. The time dependent behaviour of wood depends on material anisotropy, temperature, moisture and stresses. To predict the behaviour of wood, numerous mathematical models have been developed largely relying on experimental results. In this paper, time dependent viscoelastic behaviour of wood is reviewed under constant and cyclic climatic conditions, separately. More emphasis is given on results obtained in recent years on the behaviour of thin wood tissues, single fibres, thermo-viscoelasticity of wood, influence of hemicelluloses and the modelling of the effect of transient moisture at the molecular level on the mechanical respons

Displacement–length scaling of brittle faults in ductile shear

AbstractWithin a low-grade ductile shear zone, we investigated exceptionally well exposed brittle faults, which accumulated antithetic slip and rotated into the shearing direction. The foliation planes of the mylonitic host rock intersect the faults approximately at their centre and exhibit ductile reverse drag. Three types of brittle faults can be distinguished: (i) Faults developing on pre-existing K-feldspar/mica veins that are oblique to the shear direction. These faults have triclinic flanking structures. (ii) Wing cracks opening as mode I fractures at the tips of the triclinic flanking structures, perpendicular to the shear direction. These cracks are reactivated as faults with antithetic shear, extend from the parent K-feldspar/mica veins and form a complex linked flanking structure system. (iii) Joints forming perpendicular to the shearing direction are deformed to form monoclinic flanking structures. Triclinic and monoclinic flanking structures record elliptical displacement–distance profiles with steep displacement gradients at the fault tips by ductile flow in the host rocks, resulting in reverse drag of the foliation planes. These structures record one of the greatest maximum displacement/length ratios reported from natural fault structures. These exceptionally high ratios can be explained by localized antithetic displacement along brittle slip surfaces, which did not propagate during their rotation during surrounding ductile flow

Fracture behaviour of wood and its composites. A review COST Action E35 2004-2008: Wood machining - micromechanics and fracture

Fracturing of wood and its composites is a process influenced by many parameters, on the one hand coming from the structure and properties of wood itself, and on the other from influences from outside, such as loading mode, velocity of deformation, moisture, temperature, etc. Both types of parameters may be investigated experimentally at different levels of magnification, which allows a better understanding of the mechanisms of fracturing. Fracture mechanical methods serve to quantify the fracture process of wood and wood composites with different deformation and fracturing features. Since wood machining is mainly dominated by the fracture properties of wood, knowledge of the different relevant mechanisms is essential. Parameters that influence the fracture process, such as wood density, orientation, loading mode, strain rate and moisture are discussed in the light of results obtained during recent years. Based on this, refined modelling of the different processes becomes possibl

Fracture characterisation of yew (Taxus baccata L.) and spruce (Picea abies [L.] Karst.) in the radial-tangential and tangential-radial crack propagation system by a micro wedge splitting test

Common yew (Taxus baccata L.) and Norway spruce (Picea abies [L.] Karst.) are gymnosperm species that differ in their microscopic structure and mechanical characteristics. Compared to spruce, the density of yew wood is high, but the modulus of elasticity is low when loaded parallel to the grain. Information about the transverse load direction is largely lacking. Therefore, the goal of this study was to assess the elastic and fracture mechanical behaviour of both wood species in the radial-tangential plane (crack opening mode I). For this purpose, micro wedge splitting tests were performed. Characteristic elastic and fracture parameters (initial slope, critical load, specific fracture energy) were determined. After the tests, the fracture surfaces were evaluated using microscopic methods. The results reveal clear differences between the species regarding microscopic fracture phenomena and prove that yew wood was significantly stiffer than spruce wood. We suggest that the density and the cell geometry are predominantly responsible for both elasticity and failure behaviour in the transverse directio

River terraces of the Vltava and Malše

Die Arbeit präsentiert Ergebnisse der Kartierung und Datierung von pleistozänen Flussterrassen der Vltava (Moldau) und Malše (Maltsch) im Budweiser Becken und angrenzenden Gebieten. Zusätzlich werden neue Ideen zur spätpleistozänen Landschaftsentwicklung und neotektonischer Geschichte des Untersuchungsgebiets vorgestellt. Die verwendeten Methoden umfassen die Analyse eines hochauflösenden digitalen Höhenmodells, geomorphologische Kartierung, Interpretation von Bohrungsdaten, sedimentologische Analysen und Lumineszenzdatierung. Die Ergebnisse werden in Form von vier Manuskripten vorgestellt. Im ersten Manuskript werden sediment-petrologische Untersuchungen an feinkörnigen Sedimenten unterschiedlicher stratigraphischer Einheiten des Untersuchungsgebietes beschrieben. Die Studie präsentiert eine neue Methode zur eindeutigen Unterscheidung zwischen quartären und kretazischen/miozänen Sedimenten unter Verwendung von Röntgendiffraktometrie und Schwermineralanalyse. Das zweite Manuskript beschreibt eine geomorphologische Analyse des Budweiser Beckens unter Verwendung quantitativer morphometrischer Parameter zur Beurteilung der neotektonischen Aktivität von Störungen, die das Becken von dem variszischen Untergrund trennen. Die Flussterrassen im Budweiser Becken und angrenzender Gebiete nördlich der Hluboká-Störung werden im dritten und vierten Manuskript vorgestellt. Auf beiden Seiten der Störung wurden jeweils 5 Terrassenstufen, genannt BB 1 und BB 5, bzw. CB 1 bis CB 5, kartiert. Die niedrigsten Terrassenkörper BB 1, bzw. CB 1, entsprechen morphologisch der rezenten Überflutungsebene der Moldau und stellen komplexe Sedimentkörper mit mehreren Phasen fluviatiler Aufschüttung dar. Numerische Alter von Sedimenten aus diesem Terrassenkörper reichen von ca. 90 ka bzw. ca. 60 ka bis ins Holozän. Sedimente von höheren Terrassen konnten aufgrund der Einschränkungen der Lumineszenzmethode nicht direkt datiert werden. Man kann einen Versatz von gleichaltrigen spätpleistozänen Moldausedimenten, sowie einen Versatz von Terrassenunterkanten älterer Terrassenstufen über der Moldau auf beiden Seiten der Hluboká-Störung beobachten, was auf unterschiedliche Hebungsraten von Teilen des kristallinen Untergrundes relativ zur Beckenfüllung zurückgeführt wird.The study presents results from mapping and dating of Pleistocene fluvial terraces of the Vltava and Malše rivers in the Budějovice Basin and adjacent areas. Additionally, new ideas on late Pleistocene landscape development and the neotectonic history of the study area are presented. The methods used include the analysis of a high-resolution digital elevation model, geomorphological mapping, interpretation of borehole data, sedimentological analyses and luminescence dating. The results are presented in four papers. In the first, sedimentological-petrological analyses of fine-grained sediments from different stratigraphic units of the study area are described. The study presents a new method that shows a clear differentiation between Quaternary and Cretaceous/Miocene sediments by using a combination of X-ray diffraction of bulk samples and heavy mineral analysis. The second article describes a geomorphological analysis of the Budějovice basin by using quantitative morphometric parameters for assessing the neotectonic activity of the faults that delimit the basin from its Variscan basement. Fluvial terraces in the Budějovice Basin and adjacent areas north of the Hluboká Fault are described in the third and fourth papers. On both sides of the fault, five terrace levels (BB 1 to BB 5 and CB 1 to CB 5, respectively) were mapped. The lowermost terrace levels (BB 1 and CB 1) morphologically correspond to the recent Vltava River floodplain and occur as complex sediments bodies with several packages of river aggradation. Numerical ages of sediments from these terrace bodies range from ca. 90 ka and ca. 60 ka, respectively, to the Holocene. Sediments from higher terrace levels could not be directly dated because of the limitations of the luminescence method in this area. Vertical displacement of late Pleistocene sediments of the same age as well as a mismatch of higher terraces above the recent floodplain on both sides of the Hluboká Fault are interpreted to be a result of higher uplift rates of parts of the crystalline basement relative to the sedimentary basin fill

Automation in strain and temperature control on VHCF with an ultrasonic testing facility.

Increased safety and reliability in mechanical components has become a subject of prime importance in recent years. Therefore, a proper understanding of damage and fracture mechanics in materials and components designed to withstand very high cycle fatigue (VHCF) loadings is extremely important nowadays. However, the use of conventional machines for fatigue testing is very time consuming and costly for VHCF tests. Ultrasonic machines have been introduced as a way to increase the number of cycles in fatigue testing up to IE8 to IE10 cycles within a considerably reduced amount of time. Nevertheless, the accurate measurement of the parameters that influence fatigue life at ultrasonic frequencies (e.g., stress, displacement, strain rate, temperature, and frequency) is still a matter of concern and ongoing development. Because of the high frequencies involved in VHCF testing, a huge amount of heat is generated over the specimen, which greatly affects the variables determining the fatigue behavior. This paper describes the design and instrumentation of an ultrasonic fatigue testing machine that operates at a working frequency of 20 kHz. Among other features, it incorporates automated strain and temperature control. In order to run automated tests, a closed-loop monitoring and control system was developed based on the measured temperature and displacement amplitudes. Temperature readings are made with a pyrometer and thermography camera, and displacement is monitored at the free end of the specimen with a high-resolution laser. The machine's power output is continuously adjusted from the displacement readings, so that the stress variations within the specimen are as flat as possible. When the temperature increases above a certain set value, a cooling function is triggered and the test is interrupted until the specimen is cooled down. Data are acquired, managed, and processed with a data acquisition device working at a 400 kHz sampling frequency. The advantages and limitations of metal fatigue testing at very high frequencies are discussed in this paper, with special emphasis on strain and temperature-control issues. Comparisons are made of tests carried out with and without both displacement and temperature control on two metallic alloys, copper 99 % and carbon steel, with the determination of strength-life (S-N) curves

Potential of a geoscientific multi-analytical approach to investigate technology and provenance of ceramic materials

Die vorliegende Arbeit beschäftigt sich mit materialbezogenen archäologischen Fragestellungen, die mit Hilfe von modernen geowissenschaftlichen analytischen Methoden wie etwa XRF, ICP-MS, XRD, EPMA, SEM und Raman Spektroskopie bearbeitet wurden. Antike Keramiken verschiedener kulturhistorischer Gegebenheiten wurden hinsichtlich ihrer Herkunft sowie ihrer spezifischen Herstellungsverfahren untersucht. Die auf naturwissenschaftlicher Basis gewonnenen Informationen sind erforderlich für weitere archäologische Forschungen und ermöglichen überhaupt erst einen fundierten Einblick in die Entwicklung von frühen Gesellschaftssystemen, deren Gewohnheiten sowie deren Interaktion mit anderen Kulturen. Im Mittelpunkt dieser Arbeit stehen bronzezeitliche Artefakte, die sowohl aus Ausgrabungen in Zypern als auch Ägypten und anderen Ländern des Mittelmeerraumes stammen. Die „multi-analytische“ Herangehensweise ermöglichte es nicht nur, die Herkunftsorte der einzelnen Keramiken zu bestimmen, sondern darüber hinaus auch viele wichtige Aussagen über die physiko-chemischen Eigenschaften von Keramiken, ihre Herstellungstechnik, über Phasen-Transformationen während des keramischen Brandes und Veränderungen durch jahrtausendelange Bodenlagerung zu treffen. Gesamt- und Mineral-Chemismus sowie mikrotexturelle, mikromorphologische, petrologische, geologische und paläozoologische Informationen wurden herangezogen, um die verwendeten Ausgangsmaterialien sowie die Art der Aufbereitung dieser, naturwissenschaftlich einordnen zu können. Neben dieser Zuordnung von Keramiken zu bestimmten Rohmaterialien, aus denen sie gefertigt wurden sowie deren regionale Einschränkung, war es durch Spezifizierung der Herstellungstechnologien möglich, reichlich neue Erkenntnisse über die Fertigkeiten von Töpfern der späten Bronzezeit zu erfahren. Auch wenig untersuchte mineralogische, geochemische und mikrotexturelle Veränderung, die vor allem während des keramischen Brandes stattfinden und deren Auswirkungen entscheidend das keramische Gefüge prägen, werden in dieser Arbeit aufgezeigt.The present PhD thesis concerns material-related archaeological interests that were approached using combined, state of the art, geoscientific analytical methods like XRF, ICP-MS, XRD, EPMA, SEM and Raman Spectroscopy. Ancient ceramics from different cultural backgrounds were investigated in order to reveal their origin as well as their specific manufacturing techniques. This provides necessary base information for further archaeological contemplations and allows getting new insights into ancient societies, their development, their habits and their interaction with other cultures. The main issue was the detailed analysis of Bronze Age ceramics excavated in Cyprus as well as Egypt and other Mediterranean countries. With the use of an integrated methodological approach, it was possible to gain sufficient evidence for revealing the provenance of the study materials and additionally to examine physical and chemical characteristics of the ceramics, their manufacturing procedures, changes during ceramic firing and alterations during the post-depositional stage. Evidence from bulk- and mineral chemistry as well as from microstructural, micromorphological, petrological, geological and palaeozoological observations provided the required information to identify the used source materials and the way these were prepared and conditioned for pottery production. Besides these assignments of ceramic products to distinct source materials and certain production centres respectively, it was possible to get new insights into the knowledge and skills of Bronze Age potters by elucidating ceramic manufacturing technologies. Moreover, new aspects of mineralogical, geochemical and microtextural processes and transformations that take place during the ceramic manufacturing procedure (mainly during the firing) and their effects to the ceramic fabric are presented in the thesis

Ultra-slow Fatigue Crack Propagation in Metallic Alloys

AbstractThe influence of frequency (20kHz ultrasonic tests and conventional 20 to 35Hz tests) and environment (air and vacuum) on near-threshold fatigue crack propagation of three metallic alloys, Ti-6Al-4V, 2024-T351 and 12% Cr stainless steel is compared experimentally. The effective stress-intensity factor which is considered as the propagation driving force is determined from closure measurements or tests run at high R-ratio. Based on microfractographic observations, the results are discussed in terms of a preexisting model for intrinsic and environmentally assisted fatigue crack propagation