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

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

On the Counter-Intuitiveness of Quantum Entanglement

Es gibt die verbreitete Aussage, dass Quantenverschränkung, ein Schlüsselkonzept in der Quantenmechanik, kontraintuitiv sei. Diese Behauptung kann aber nicht nur Laien zugeschrieben werden, sondern findet sich auch bei Wissenschaftlern der Quantenmechanik wieder. Wie können wir uns das verständlich machen? Ich bearbeite dieses Thema auf Basis von vier miteinander verwobenen Gesichtspunkten: Erstens, auf einer theoretischen Ebene lege ich die hier relevanten Unterschiede zwischen der klassischen Physik und der Quantenmechanik vor. Dies geschieht hauptsächlich anhand des EPR Gedankenexperiment und den entsprechenden Standpunkten von Einstein et al. und Schrödinger. Zweitens, auf einer experimentellen Ebene veranschauliche ich die Kontraintuitivität der Quantenverschränkung im Zusammenhang mit einem spezifischen Forschungsfeld. Im Konkreten erfolgt dies anhand der Quanten-Optomechanik, genauer der Experimente der Forschungsgruppe „Quantum Foundations and Quantum Information on the Nano- and Microscale“ der Universität Wien. Drittens, auf einer persönlichen Ebene werte ich meine Interviews mit den Mitgliedern ebendieser Forschungsgruppe aus. Viertens, auf einer philosophischen Ebene mache ich mir Schlüsselkonzepte von Gaston Bachelards Epistemologie zunutze, wie zum Beispiel das epistemologische Profil oder einer durch Erfahrung belehrten Vernunft. Alle diese vier Aspekte werden zusammengenommen um zwei Aufgaben zu erfüllen: Einerseits die Konkretisierung der Besonderheiten und Merkmale der Kontraintuitivität im Kontext der Quantenverschränkung. Andererseits die Betrachtung der Frage ob und wie das Experimentieren mit Quantenverschränkung den Umgang mit dessen Kontraintuitivität erleichtert, zum Beispiel durch Gewöhnung. Im Großen und Ganzen stellt die Diplomarbeit keine tiefgreifende Analyse dieser beiden Themen zur Verfügung, sondern liefert eine plausible Annäherung an das Problem.There is a rather common dictum that quantum entanglement, a key feature of quantum mechanics, is counterintuitive. This assertion can be assigned not only to laypeople, but also to scientists engaged in quantum mechanics. How can we make sense of this? I elaborate the issue based on four intertwined aspects: First, on a theoretical level by introducing the fundamental differences between classical physics and quantum mechanics. This is done along the lines of the EPR thought experiment and the corresponding perspectives of Einstein and Schrödinger. Second, on an experimental level by illustrating the counter-intuitiveness of quantum entanglement in a specific field of research. Namely quantum opto-mechanics, and more particularly the experiments of the research group 'Quantum Foundations and Quantum Information on the Nano- and Microscale' at the University of Vienna. Third, on a personal level by utilizing my interviews conducted with members of this very research group. Fourth, on a philosophical level by exploiting key concepts of Gaston Bachelard’s epistemology, such as epistemological profile or scientific reason being instructed by fabricated experience. These four aspects are brought into contact to accomplish two tasks: On the one hand to flesh out the specifics and characteristics of counter-intuitiveness in the context of quantum entanglement. On the other hand to consider the question whether and how experimenting with quantum entanglement could assist in coping with its counter-intuitiveness, for example by familiarization. Overall, this thesis does not provide an in-depth analysis, but a plausible approach to and depiction of the subject matter

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

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

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

On specimen design for size effect evaluation in ultrasonic gigacycle fatigue testing

Literature datasets showed that gigacycle fatigue properties of materials may be affected by the specimen risk-volume, i.e., the part of the specimen subjected to applied stress amplitudes above a prescribed percentage of the maximum applied stress amplitude. The paper proposes a Gaussian specimen shape able to attain large risk-volumes for gigacycle fatigue tests, together with a general procedure for its design: wave propagation equations are analytically solved in order to obtain a specimen shape characterised by a uniform stress distribution on an extended length and, as a consequence, by a larger risk-volume. The uniformity of the stress distribution in the Gaussian specimen is numerically verified through a finite element analysis and experimentally validated by means of strain gauge measurement