共查询到20条相似文献,搜索用时 14 毫秒
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NTM Zeitschrift für Geschichte der Wissenschaften, Technik und Medizin - „Zeit“ ist seit einigen Jahren ein intensiv debattiertes Thema in der Geschichtswissenschaft. Auch in der... 相似文献
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Strength and Toughness, Criteria of Safety for Construction of Apparata. In the chemical research technique often high requirements are being set on the materials of apparata due to pressure, temperature and corrosive medias. While the strength necessary to avoid macroscopical deformation can be guaranteed today without difficulties, this does not go similarly for the toughness. The toughness of steel being a condition in the strength calculation is the real safety criterion for welded structures. This is especially applicable in the case of local peak stress due to construction or of micro cracks caused during manufacturing or service. If – as mostly in practice – local inhomogeneities and unknown residual stresses are existing, the linear fracture mechanics can only be applied with empirical factors. The best safety even with unforeseen overloadings can be seen in such a toughness, which avoids brittle fracture at all. The possible test methods will in any case have to reliably consider also the heat affected zones of weldings, since brittle cracks also in tough matrix can only be absorbed under limited stresses. 相似文献
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Dr. G. H. Schneider 《Acta Mechanica》1981,41(3-4):157-184
Zusammenfassung Die Zustandsänderung eines, in einem Zylinder eingeschlossenen Gases konstanter Masse, dessen Volumen durch einen bewegten Kolben verändert werden kann, wird untersucht. Die Kolbengeschwindigkeit wird als klein im Vergleich zur Schallgeschwindigkeit im Gas, der Kolbenhub als von der Größenordnung der Zylinderlänge angenommen. Die Fragestellung ist durch das gleichzeitige Auftreten zweier charakteristischer Zeiten verschiedener Größenordnung bestimmt; ein kumulativer Effekt, der durch die Störung der lokalen Schallgeschwindigkeit zustande kommt, muß berücksichtigt werden. Darum wurde das Problem durch simultane Anwendung der Methode der Mehrfachen Variablen und des analytischen Charakteristikenverfahrens gelöst. Dabei wurde die quasistatische Näherung aus der Bedingung bestimmt, daß in der zweiten Näherung die Ergebnisse der Zustandsgrößen keine Säkularterme enthalten dürfen. Die Lösungen zweiter Ordnung sind dann ohne Kenntnis der dritten Ordnung festgelegt. Zur Ermittlung mehrfach reflektierter Verdichtungsstöße wurde die Pfriemsche Formel verwendet.
Mit 14 Abbildungen 相似文献
Compression and Expansion of a gas in a cylinder formulated as a perturbation problem
Summary The variation of state in a gas with constant mass inside a cylinder closed by a moving piston is considered. The piston velocity is assumed to be very small compared with the sound velocity in the gas; the piston stroke may be of the same order of magnitude as the length of the cylinder. The problem is governed by two time scales with different orders of magnitude; a cumulative effect due to local perturbations of the sound velocity has to be taken into account. Therefore, the problem has been solved by a simultaneous application of the method of multiple scaling and the analytical characteristics method. The quasistatic approximation is recovered from the condition that secular terms in the results for the second order flow quantities have to vanish. Then the second order solutions are uniquely determined without recourse to the third order equations. Multiply reflected shock waves are found by using Pfriem's formula.
Mit 14 Abbildungen 相似文献
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J. Ziaja 《Materialwissenschaft und Werkstofftechnik》1991,22(11):422-426
Residual stresses as parameters of X-ray criterion of fracture and of fatigue life prediction Fatigue testing is notorious for considerable scatter in specimen life even if special measures are taken to precisely control parameters of stress cycling. The reason for that must be ascribed to variation in material microstructure existing prior to fatigue loading. Structural condition of material may be determined by the following function: Extensive fatigue tests on α-Ti and (α + β)-Ti alloy at temperatures 77 K and 295 K showed that failure did not occur until function V reached its critical value at a plane characteristic of each lattice type (failure plane). The condition V = K may be therefore regarded an X-Ray criterion and, once determined numerically, a sort of material constant. The life NB of fatigue tested specimen was found to be clearly depended on structural condition of virgin material exhibited natural variation of structural characteristics. The finite fatigue life was found to conform with the relationship The expression may be used for predicting life NB if prior to fatigue loading the material condition function V value is made known for a characteristic lattice plane. 相似文献
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Zusammenfassung Instabile Str?mungsvorg?nge in Axialverdichtern werden entsprechend ihrer periodischen Erscheinungsform in die rotierende
Abl?sung und das Pumpen klassifiziert. Sie verursachen kritische Betriebszust?nde mit extrem hohen dynamischen Belastungen
aller Baugruppen. Dies gilt insbesondere für Flugtriebwerksverdichter, deren stabiler Betrieb durch h?ufige Lastwechsel und
zus?tzlich auftretende Einlaufst?rung stark gef?hrdet ist. Gegenma?nahmen werden um so erfolgreicher sein, je besser die str?mungsmechanischen
Vorg?nge sowohl in der Entstehungsphase als auch w?hrend des instabilen Verdichterbetriebes verstanden werden. Experimentelle
Untersuchungen sind in vielen F?llen aufgrund des Risikos gro?er Materialsch?den nicht durchführbar, so da? theoretische Verfahren
zunehmende Bedeutung erlangen.
In der vorliegenden Arbeit wird ein Rechenmodell vorgestellt, mit dessen Hilfe die instation?ren str?mungsmechanischen Vorg?nge
simuliert werden k?nnen und das Aussagen über den Zusammenhang zwischen einigen Systemparametern und den instabilen Str?mungsph?nomenen
erm?glicht. Im Vordergrund steht hierbei die Darstellung der rotierenden Abl?sung und des Pumpvorgangs als eine gekoppelte
Instabilit?tsform. Ein einfaches Modell einer Verdichteranlage wird ausgew?hlt, bestehend aus dem Einlauf, einem einzelnen
Laufrad, dem Druckstutzen und einem Speicher mit Drosselvorrichtung, um diese Kopplung mit einem numerischen L?sungsverfahren
nachzuweisen. Es stellt die Str?mung als einen zweidimensionalen, instation?ren, kompressiblen Vorgang dar, wobei die Verlust-
und Umlenkkorrelationen des Laufrades sowie die übertragungsgleichungen des Speichers in den entsprechenden Randkoordinaten
berücksichtigt werden. Umfangsabh?ngige Totaldruckst?rungen werden in Form eines St?rgitterbeiwertes der Str?mung im Ansaugstutzen
überlagert. Die Entwicklung des periodisch instation?ren Str?mungsfeldes in Abh?ngigkeit vom Speichervolumen und der Drosselstellung
wird in ausgew?hlten Simulationsbeispielen diskutiert. Zur vollst?ndigen Auswertung der Stabilit?tsuntersuchung wird die Pumpfrequenz,
die Frequenz umlaufender Str?mungsinstabilit?ten und die Anzahl der Abl?sezellen herangezogen.
Herrn Professor Dr.-Ing.Wilhelm Dettmering zum 75. Geburtstag gewidmet 相似文献
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S. Yamaguchi 《Materials Chemistry and Physics》1983,9(5):523-528
The magnetic and electric fields found in zinc ferrite and yttrium iron garnet have been studied with the aid of electron diffraction. In these magnetic dielectrics, the magnetic field strength in e.m.u. proved to be comparable with the electric one in e.s.u.; i.e., both the values were estimated to be about 100. The wavelength of microwave to be absorbed by the materials concerned was evaluated to be about 1 mm in terms of the LC model. 相似文献
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Tim Hosenfeldt 《真空研究与实践》2021,33(1):22-27
CO2‐neutral Mobility as a Challenge and an Opportunity – From the powertrain to the electrolyzer: Plasma surface technology along the energy chain Three concepts are suitable for CO2‐neutral and sustainable mobility: First, the direct use of electrical energy for battery electric vehicles (BEV). Secondly, the conversion of regeneratively generated electricity into green hydrogen as an energy carrier for fuel cell electric vehicles (FCEV) and thirdly, the generation of synthetic fuels from green hydrogen. The technologies will complement each other in terms of vehicle weight, distance and required propulsion power. Regardless of the powertrain concept, plasma surface technology offers outstanding opportunities for the optimization of highly stressed tribological systems. For example, Triondur PVD and PACVD coating systems in the automotive industry, where they were initially used to prevent wear, have become an extremely valuable design element for increasing energy efficiency and CO2 savings through friction reduction. As a result, more than 150 million components coated with Triondur were delivered worldwide in 2018. Defossilization of the energy chain requires increased industrialization of components for electrolyzers and fuel cells, such as the metallic bipolar plates of galvanic cells. Here, as well, plasma surface technology will play a key role in meeting the high demands for the required electrochemical properties and quality standards. For CO2‐neutral and sustainable mobility, plasma surface technology will always be and remain an important key technology, regardless of the powertrain concept. 相似文献
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Development of FricRiveting as a new joining technique for polymer and lightweight alloys The increasing demand on environmental consciousness, cost savings and high performance end products has been guiding scientists and engineers to a constant development of new materials and technologies. This class of lightweight structures are specially used in industrial fields such as transportation and modern civil engineering. Currently available joining methods for polymer‐metal structures (adhesive bonding and mechanical fastening) are usually application‐specific, presenting high operational costs, limited mechanical performance or are not environmental friendly. A new Friction Riveting technique for polymeric‐metallic joints was developed, demonstrated and characterized in this work, as an alternative, reliable, environmental compatible and economically viable spot joining process. In the simplest process variant a rotating cylindrical metallic rivet is inserted in one or more thermoplastic base plates. The high rotation speed and pressure increase friction and heat is generated. When the preset time is achieved the temperature highly increases and the rivet tip plasticizes. At this point rotation is decelerated and the axial pressure increased, so the plasticized rivet tip becomes deformed; after cooling it becomes anchored in the polymeric base plate. In this work case‐study joints on commercially available polyetherimide (PEI) and aluminium 2024‐T351 (Al‐Cu‐Mg alloy) were chosen for demonstrating proposed theories and mechanisms of FricRiveting. Sound friction riveted point‐on‐plate and single‐rivet overlap joints with elevated joint efficiencies in terms of base materials strength were obtained (tensile joint efficiencies of about 97 % of the rivet strength and shear joint efficiencies of about 70 % of the polymer strength) through tensile and lap shear testing at room temperature. Finally, the microstructural changes and properties were described for this case‐study joint. The feasibility of FricRiveting was demonstrated in this work by the presented technical and scientific results. From this work it can be suggested that FricRiveting has the potential to be established as a reliable, simple, cost effective and environmental friendly joining technique for polymer‐metal components. 相似文献
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Load-Displacement Diagrams – a Basis for the Understanding of Notched Bar-Impact Tests . The absorbed energy as well as the effect of temperature on absorbed energy during the recording of load – displacement diagrams of notched bar – impact tests of mild steels, Ck 10, Ck 35, Ck 45, Ck 60 and C 85W 2 in normalized state were analysed. The considerable values of relation between discontinuity in nature of characteristic load and displacement of load displacement diagrams and irregularity of fracture toughness temperature curves were obtained there. From these investigations one can define characteristic temperatures (transition temperatures), which are not arbitrary, but physically founded and which describe the behaviour of materials in different temperature-ranges. These characteristic temperatures, above all, the crack arrest temperature could also be defined after fractographical and microfractographical investigations. The absorbed energy temperature curves were also determined arithmetically with the application of simple methode of area computation of load displacement diagrams. Therefore, it can be made sure that the available curves are correct and determined on principle of physics. 相似文献
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Dr.-Ing. Thomas Carolus 《Forschung im Ingenieurwesen》1986,52(6):191-197
Zusammenfassung Instation?re Drucksignale werden durch die übertragungseigenschaften des in der Praxis h?ufig eingesetzten Me?systems, elektromechanischer
Druckaufnehmer—Kunststoffverbindungsleitung’ beeinflu?t und u.U. erheblich verf?lscht. In diesem Beitrag wird ein solches
Me?system mit verschiedenen Kunststoffleitungen für den Einsatz in Luft und Wasser untersucht und dessen Einsatzgrenzen bestimmt.
Von besonderem Interesse sind dabei die Parameter der Verbindungsleitung sowie des Str?mungsmediums. Sehr h?ufig werden Verbindungsleitungen
aus Polymer-Werkstoffen eingesetzt. Au?er den geometrischen Abmessungen bestimmen in unterschiedlicher Gewichtung die viskoelastischen
und damit frequenzabh?ngigen Verformungseigenschaften des Leitungsmaterials sowie die frequenzabh?ngige Dissipation im Str?mungsmedium
die übertragungseigenschaften dieses Me?systems. 相似文献