Mikromechanisches stochastisches Versagensmodell uniaxial faserverstärkter Verbundwerkstoffe

Micromechanical stochastic failure model of uniaxial fibre-reinforced composites A theoretical model of stress transfer between a transversal isotropic fibre and the surrounding matrix material in a uniaxially fibre-reinforced composite near a single matrix flaw is discussed including friction controlled fibre-matrix interface debonding. The rise of fracture toughness due to frictional fibre sliding is studied accounting for Weibull strength distribution of fibres. The total dissipative work may be used as figure of merit regarding the damage tolerance. A critical evaluation is presented concerning some previous models of local failure probabilities. Numerical results are demonstrated. Conditions for an optimized C/Al-composite are presented.     

Lokales kumuliertes Schadensmaß für Komposite - Rißspitzen ohne Singularität

Local cumulative damage measure-crack tips without singularity A pointwise probability density for failure initiation is proposed regarding the evaluation of damaged composite structures. The local cumulative damage measure accounts for both the accumulation of local stochastic microdamaging and the load redistribution to undestroyed subregions. It bases on the global failure rate of heterogenous composites. Firstly the damage behaviour of beams under load is studied. Secondly it is proved that stress redistribution by a crack tip damage zone yields to the disappearence of singularity dominated stress fields.     

Ein verbessertes Verfahren für die Auswertung von Treppenstufenversuchen

An Improved Method for the Evaluation of Staircase Tests The assessment of the endurance limit by a time- and cost-saving methode - taking into account the mean, the scatter and the confidence limits - is an important engineering challenge. Among the many methods used up to now, only the staircase method developed by Dixon and Mood in 1948 specifies the confidence limits of the mean and standard deviation. Based on certain considerations it has to be doubted, however, wether the method of Dixon and Mood evaluating the “less frequent occurrence: failures or runouts” gives reliable estimates and confidence limits. Using a large numer of staircase tests, randomly computer-generated by way of the Monte Carlo-method, the evaluation method of Dixon and Mood was examined. An important result was that the estimate of the standard deviation is always too small and the use of the “less frequent occurrence” does not result in optimum confidence limits. The method was therefore improved considering all accurrences: failures and runouts in the evaluation. The new method for the evaluation of the estimate of means, standard deviations and their confidence limits is outlined. Moreover, an optimum way to plan staircase tests is explained. Finally a practical application of the new method is presented.     

Plasmapolymerisation — Ein Verfahren für viele Anwendungen

Plasma Polymerization is a new technology that allows to produce thin coatings with manifold properties. It enables especially the coating of plastics because of its low thermal heating effect. The technology will be described in this paper by explanation of microwave Plasma Polymerization. Plasma, process and equipment are described. A survey of some applications shows, how many different technical domains are affected. Details concerning industrial use, like coated area, process time, and economy, are discussed.     

Ein neues Wärmedämmsystem für Architekturglass

Vacuum coated glass plays an outstanding role for the energy saving of buildings. The basis of such low emissivity coatings is a 10 nm thick silver layer causing an effective reflexion of room temperature radiation. With conventional coatings k‐values of 1,3 W/m²K are realized. In the frame of a research project funded by the BMBF reactive mid frequency sputtering (Leybold TwinMag®) has been introduced for manufacturing of low e glass. The new developed system glass — TiO₂ — ZnO — Ag — NiCrO_x — Si₃N₄ enables a k‐value of 1,1 W/m²K and an improvement of optical properties.     

Ein einfacher Festigkeitsversuch für Scheiben aus spröden Werkstoffen

A simple strength test for brittle discs The Ball‐on‐three‐balls test (B3B) is a simple method for strength tests of brittle discs. A disc shaped sample is supported on three points at the top surface and centrally loaded from the opposite site. Both the support and the application of load is realised with spherical parts. This test assembly shows a threefold symmetry. In this work the B3B test assembly is used for tests on samples made of alumina. The size of the samples, the ratio of disc thickness to disc diameter and the support situation is varied systematically. It was possible to confirm the numerical results for the maximum stress in the disc with fracture experiments. Therefore the Weibull behaviour of the ceramic has to be taken into account. Advantages of the ball‐on‐three‐balls test are especially the great tolerance for small deviations of the real from an idealised geometry, the ability to test slightly bended discs, i.e. as sintered samples, the small influence of frictional effects and the great potential for testing very small samples. A disadvantage is the more complicated test evaluation compared to the common bending test.     

Ein neues Zeitstandgerät für Kriechversuche bei wechselnden Belastungen und Temperaturen

A New Instrument for Measuring Creep under Alternating Loading and Temperature Conditions. It is an established fact that plastics are particularly subject to creep, i. e. deformation over a period of time under load. Under practical conditions unchanged load and constant temperature are seldom realized. The deformations associated with changing mechanical loads cannot generally be derived from the results of ordinary creep tests. In order to be able to study creep behaviour of plastics under intermittent load conditions (i. e. a sequence of loading and unloading), a special instrument has been developed at the Austrian Plastics Institute. The period of time as well as the ratio of loading and unloading phase can be programmed within wide ranges. Creep tests at varying temperatures can also be carried out with this instrument. Under intermittent load very significant differentiation of deformation and fracturing behaviour depending on the polymeric structure has been observed. Comparison of results of ordinary creep with those under intermittent loading at constant temperature shows, that in the case of amorphous thermoplastics destruction by stress crazing and fracture is markedly accelerated. On the other hand semicrystalline thermoplastics have excellent resistance against intermittent load.     

WHIPOX®: Ein faserverstärkter oxidkeramischer Werkstoff für Hochtemperatur– Langzeitanwendungen

WHIPOX®: A fiber‐reinforced oxide‐ceramic matrix composite for long‐term high‐temperature applications In the last years the all oxide fiber reinforced ceramic matrix composite WHIPOX® ( W ound Hi ghly P orous Ox ide CMC) have been developed to fabricate mature components. Besides the applications in the field of aircraft and space technology, which are developed in interdisciplinary projects at DLR, the material was successfully tested for “spin‐off” applications. The specific and unique combination of physical and mechanical properties enables new and innovative solutions for problems with materials in industrial furnaces, metallurgical equipments, filter technologies, fire prevention, catalytic converters, soot filters, electro technology and medical applications. High speed burner injectors for industrial furnaces are produced in licence by an industrial ceramic manufacturer.     

Ventilwerkstoffe für Verbrennungsmotoren

Valve Materials for Combustion Engines An overview is being given on production numbers, requirements and properties of valve materials. Their development from the beginning till today's state of the art is reviewed with a focus on Chromium-Manganese-Nitrogen-alloys. Valve materials are grouped by the criteria of density – heavy, lightweight – and alloying elements. The multiple stresses of the material in the valve are being met with a few standardised grades and specific ways of manufacture. The path of the material from bar stock to the finished valve is being followed. Engine development in the past decades increased the load on the valves which could be met by continuously developing their structural strength. Also the strength of all valve materials could be raised to nominal strength above 1100 Mpa by applying specific methods. Higher strength at the surface is being effectuated by work hardening effects. The technology to increase reliability of hollow valves and new aspects of valve seat facing including residual stress is explicitly discussed. General aspects of alloy utilisation is followed by a discussion of lightweight valve materials as Titanium alloys, intermetallic Titaniumaluminide alloys and ceramic materials, spec. Silicon Nitride, which all have a potential as forthcoming valve materials capable of reducing fuel consumption of the engines.     

Membrantechnologien für Vakuumpumpen

Diaphragm Technologies for Vacuum Pumps Diaphragm vacuum pumps are used in chemistry and as roughing pumps for high vacuum systems. Both planar diaphragms and moulded (also as »structured«) diaphragms are common. Long term tests confirmed that after some hundred hours of operation the ultimate vacuum of pumps with moulded diaphragms deteriorates due to fatigue and abrasion of the elastomer. In addition the pumping speed curve of moulded diaphragms is unfavourable for practical applications. Planar diaphragm pumps show a superior pumping speed curve and — depending on manufacturing quality — a stable performance over the whole lifetime in excess of 10,000 hours.