Ermüdungsrißwachstum in Kerben

Fatigue Crack Growth in Notches Nowadays it is wellknown that an important part of the fatigue life time, usually differenciated in crack initiation and crack growth, is often controlled by fatigue crack growth of cracks in notches. An elastic-plastic on the J-integral based crack growth model considering the crack opening and closure phenomenon will be described to determine crack growth of cracks in notches between crack initiation and failure. Experimental results and finite element analysis were used to verify the developed model.     

Langzeitiges Kriechrißverhalten kennzeichnender Kraftwerksstähle

Long Term Creep Crack Behaviour of Typical Power Plant Steels The creep crack behaviour of the steels was investigated in a wide loading range up to a test duration of 40 000 h and down to a creep crack growth rate of 2 · 10^?5 mm/h with specimens of different shape and size. For steels of type l%Cr-l%Mo-0.6%Ni-0.3%V, 1%Cr-0.9%Mo-0.7%Ni-03.%V, 12%Cr-1%Mo-0.3%V-0.22%C and 12%Cr-l%Mo-0.3%V-0.20%C tested at 550°C, the creep crack growth rate could be described by the parameter C₂* with significantly smaller scatter bands than by the parameter C₁* or the stress intensity factor K_I. For steel 12%Cr-2%Ni-1%Mo tested at 450°C, parameter K_I leads to the smallest scatter band. The creep crack initiation can be described in a two-criteria diagram based on nominal stress and stress intensity factor. However the method is assumed to be over-conservative in case of increasing specimen size. As a result of several aperiodic creep fatigue crack tests, precracking under fatigue conditions gave a weak increase of the creep crack growth rate whereas by precracking under creep conditions the fatigue crack rate was strongly decreased.     

Theoretisch-numerische Beanspruchungsanalyse und Versagensbeurteilung von Druckbehältern

Theoretic-numerical Stress Analysis and Failure Assessment of Pressure Vessels An extensive research program has been conducted to technically evaluate the burst behaviour of model vessels with nozzles as a function of the dimensions and material toughness. As a supplement of this program, the stresses in the vicinity of the nozzle under internal pressure were analysed using the Finite Element Method. The ratio of the wall thickness of the vessel and nozzle were varied. By applying different failure hypotheses, the trend of the experimental burst pressure with temperature could be established.     

Untersuchungen zu den Gesetzen des Fernrohrsehens

Es wird über weitere Sehschärfenmessungen mit fernrohrbewaffnetem Auge berichtet, deren Auswertung eine unmittelbare Aussage gestattet, in welcher Weise Fernrohrvergrösserung und Objektivöffnung in die Sehleistung im Dämmerungs-und Nachtsehen eingehen. Der alte Meinungsstreit, ob im Gebiet des Dämmerungssehens eine Fernrohrübervergrösserung vorteilhaft ist oder nicht, wird dahingehend entschieden, dass bei gleicher Objektivöffnung eine stärkere Vergrösserung eine Erhöhung der Sehschärfe fast im gesamten Anwendungsgebiet der Feldstecher bewirkt. Der Vorteil ist am grössten beim Tagessehen (über 10 asb), ist im Dämmerungssehen (10⁰-10^-2, Mondschein etwa 10^0,5 asb) immer noch beträchtlich. Bei etwa 10^-3 asb ist die Fernrohrleistung von der Vergrösserung unabhängig, erst unterhalb dieser Leuchtdichte wirkt sich eine Übervergrösserung nachteilig auf die Sehleistung aus, die Fernrohraustrittspupille sollte hier also die Grösse der Augenpupille haben, zumindest nicht wesentlich kleiner sein. Die Sehschärfenmessungen wurden unter entsprechenden Vorsichtsmassnahmen im Labor vorgenommen. Als Sehschärfenkriterium diente die Erkennung der Lücke in Landoltringen verschiedener grösse und verschiedenen Kontrastes. Die Lichtschwächung über fünf Zehnerpotenzen (10¹-10^-4 asb) erfolgte mittels neuartiger Grauschichtenfilter, als Fernrohre fanden handelsübliche Feldstecher Verwendung, deren Unterschiede in den Verlusten und der Gesichtsfeldgrösse unberücksichtigt blieben. An den recht umfangreichen Messungen beteiligten sich vier Beobachter. Die Leistung eines Fernrohrs kann angenähert in der Form L = Cγ^1-x D^x oder L = C γ P^x dargestellt werden. Darin bedeutet C eine sehr allgemeine, von der Umfeldleuchtdichte, Beobachter-und Fernrohreigenschaften abhängende Konstante, Γ die Fernrohrvergrösserung, D den Objektivdurchmesser und P den Durchmesser der Austrittspupille des Fernrohrs. Der Exponent x ist in erster Linie von der Umfeldleuchtdichte abhängig, in geringerem Masse vom Kontrast der Sehzeichen, der Fernrohraustrittspupille, dem Adaptationszustand des Beobachters und Fernrohrverlusten. Als Mittelwert von vier Beobachtern, für Fernrohraustrittspupillen zwischen 1,6 und 7,1 mm Durchmesser und einen Sehzeichenkontrast von etwa - 1 ergaben sich für x die folgenden Zahlenwerte: Umfeldleuchtdichte in asb : 10^-4,5 10^-4 10^-3,5 10^-3 10^-2,5 10^-2 10^-1,5 10^-1 10^-0,5 10⁰ 10^0,5 10¹ x : 1,5 1,2 0,9 0,7 0,52 0,48 0,46 0,45 0,42 0,28 0,12 0,0 Im Gebiet 10^-2,5 bis^{10 -1} asb, wo x etwa den Wert 0,5 annimmt, wird die alte Faust-Formel L = C √ F.D bestätigt. Unterhalb von 10^-2,5 asb steigt x an, der Einfluss der Vergrösserung auf die Fernrohrleistung nimmt ab, der Einfluss des Objektivdurchmesserszu. Unter 10^-3,5 asb wächst x über den Wert 1. In diesem Bereich bringt jede Vergrösserung, die die Austrittspupille des Fernrohrs wesentlich kleiner (< 6 mm) als die Augenpupille des Beobachters (etwa 7,4 mm) werden lässt, eine Verminderung der Sehleistung mit sich. Oberhalb von 10^-1 asb geht x allmählich gegen Null, dieser Wert gilt im reinen Tagessehen, d. h. der Objektivdurchmesser ist in weitem Masse ohne Einfluss auf die Fernrohrleistung. Diese Ergebnisse bestätigen weitgehend die Theorie der Fernrohrsehens von Kühl, die zunächst nur für 7⁰ -extrafoveale Beobachtung gilt. New measurements of visual acuity are described for the eye in conjunction with a telescope, and results given showing how visual acuity at low light intensities depends on the magnification and aperture of the instrument. It is shown that an increase of magnification under twilight conditions produces an increase in visual acuity in almost the whole range of field-glasses. The gain is greater for day vision (above 10 asb), but is significant in the region 10⁰ to 10^-2 asb (moonlight gives about 10^-0.5 asb). Towards 10^-3 asb the instrumental efficiency is independent of magnification and below this acuity decreases with magnification; the exit pupil should then be approximately equal in size to that of the eye. The criterion of visual acuity was the recognition of the gap in Landolt rings at different contrasts. Reduction of illumination over a range 10^-5 (10¹ to 10^-4 asb) was obtained by a new type of grey filter; normal field-glasses were used. The performance of a telescope can be approximately represented by L = C τ^1-x D^x or L = C τ P^x, where C is a general constant varying with illumination, observer and instrument, τ ils the magnification, D is the diameter of the objective and P that of the exit-pupil. x depends primarily on the illumination, and less strongly on the object contrast and instrumental losses. Taking the average for four observers, for exit pupils from 1.6 to 7.1 mm and object contrast about -1, the following values of x were obtained. Illumination (in asb) : 10^-4,5 10^-4 10^-3,5 10^-3 10^-2,5 10^-2 10^-1,5 10^-1 10^-0,5 10⁰ 10^0,5 10¹ x : 1,5 1,2 0,9 0,7 0,52 0,48 0,46 0,45 0,42 0,28 0,12 0,0 From 10^-2.5 to 10^-1 where x is about 0.5, Faust's formula L = C δ Γ D is confirmed. Below 10^-2.5 asb, x increases, and the influence of magnification falls, relative to that of objective diameter. Below 10^-3.5 asb, x exceeds unity and any increase of magnification which makes the exit pupil significantly less than that of the eye decreases visual acuity. Above 10^-1 asb x approaches zero corresponding to normal daylight vision so that the instrument performance is largely independent of objective diameter. These results give general confirmation to Kuhl's theory of telescopic vision, previously only confirmed for extra foveal vision at 7⁰. De nouvelles mesures d'acuité visuelle sont décrites, concernant l' -il muni d'un instrument télescopique; l'examen des résultats permet de dire directement de quelle façon le grossissement de l'instrument et l'ouverture de l'objectif interviennent dans l'acuité visuelle de la vision crépusculaire et nocturne. Existe-t-il oui ou non un avantage à augmenter le grossissement dans le domaine de la vision crépusculaire, c'est l'ancienne controverse qui se trouve réglée en ce sens qu'à égale ouverture d'objectif, un grossissement plus fort produit une amélioration de l'acuité visuelle dans la presque totalité du domaine d'emploi des jumelles. L'avantage est le plus important en vision diurne (audessus de 10 asb), mais reste encore notable en vision crépusculaire (10⁰-10^-2 asb, le clair de lune donne environ 10^-0,5 asb). Pour 10^-3 asb environ, la puissance de l'instrument est indépendante du grossissement, et au-dessous de cette luminance, un excès de grossissement commence à devenir défavorable pour l'acuité visuelle, dans ce cas la pupille de sortie de l'instrument devrait donc avoir la grandeur de la pupille de l' -il, ou tout au moins ne pas être sensiblement plus petite. Les mesures d'acuité visuelle ont été entreprises au laboratoire en prenant les précautions convenables. Pour ce critère d'acuité visuelle, on utilisait des anneaux de Landolt de dimensions et contrastes différents. L'affaiblissement de la lumière jusque dans un rapport 10 5 (de 10¹ à 10^-4 asb) était obtenu à l'aide de filtres gris d'un nouveau modèle. Comme instruments télescopiques, on utilisait des jumelles de type courant, dont les différences étaient négligeables en ce qui concerne les pertes et la grandeur du champ visuel. Quatre observateurs ont pris part à de nombreuses mesures. La puissance d'un instrument télescopique peut être représentée approximativement sous la forme: L = C τ^1-x D^x ou L = C τ P^x, où C désigne une constante très générale dépendant de la luminance ambiante et des propriétés de l'observateur et de l'instrument, τ le grossissement de l'instrument, D le diamètre de l'objectif et P le diamètre de la pupille de sortie de l'instrument. L'exposant x dépend en premier lieu de la luminance ambiante et, dans une plus faible mesure, du contraste de l'objet, de la pupille de sortie de l'instrument, de l'état d'adaptation de l'observateur et des pertes dans l'instrument. En moyenne, pour quatre observateurs, pour des pupilles de sortie de diamètre compris entre 1,6 et 7,1 mm, et pour un contraste de l'objet de l'ordre de—1, on a obtenu pour x les valeurs numériques suivantes: Luminance ambiante (en asb) : 10^-4,5 10^-4 10^-3,5 10^-3 10^-2,5 10^-2 10^-1,5 10^-1 10^-0,5 10⁰ 10^0,5 10¹ x : 1,5 1,2 0,9 0,7 0,52 0,48 0,46 0,45 0,42 0,28 0,12 0,0 Dans la région de 10 ^-2,5 à 10^-1 asb, où x prend la valeur 0,5, l'ancienne formule approximative est confirmée: L = C√ P.D. Au-dessous de 10^-2,5 asb, x croît, c'est-à-dire que l'influence du grossissement sur la puissance de l'instrument baisse tandis que l'influence du diamètre de l'objectif augmente. Au-dessous de 10^-3,5 asb, x croît jusqu'à dépasser l'unité. Dans ce domaine, chaque grossissement rendant la pupille de sortie de l'instrument nettement plus petite (6 mm) que la pupille de l' -il de l'observateur (7,4 mm) apporte une diminution de l'acuité visuelle. Au-dessus de 10⁰ asb, x se rapproche progressivement de zéro, valeur correspondant à la vision purement diurne, c'est-à-dire que le diamètre de l'objectif est, dans une large mesure, sans influence sur la puissance de l'instrument. Ces résultats confirment largement la théorie de la vision télescopique de Kühl, d'abord valable seulement pour l'observation extrafovéale à 7⁰.     

Ermüdung von Stahl × 20 Cr 13. Rißwachstum bei Lastwechselfrequenzen von 10−3 Hz bis zu 21 KHz,Bestimmung des Grenzwertes ΔK0 mit Ultraschall

Fatigue Crack Growth in a 13% Chromium Steel at Frequencies from 10^?3 to 10⁴ Hertz. Determination of the Threshold Stress Intensity ΔK₀ by Ultrasonic Fatigue Fracture mechanics fatigue crack growth tests were run with loading frequencies from 10^?3 to 10⁴ Hertz. The results show that the environmental effects on fatigue crack growth decrease strongly at higher frequencies. Ultrasonic fatigue tests in water yield the same crack growth rates as low frequency fatigue tests in vacuum. Ultrasonic fatigue tests allow the measurement of the fatigue threshold stress intensity ΔK₀ within reasonable testing times.     

Untersuchungen zu Zusammenhängen zwischen Schichtmorphologie und Herstellungsbedingungen an amorphen Siliziumnitridschichten

Electron microscopic investigations of magnetron sputtered amorphous SiN_x-coatings showed, that morphology changes with variation of working gas pressure and substrate position corresponding to the Structure Zone Model by Thornton. Increasing working gas pressure and displacement of substrate to larger radial distances influence columnar structures. Energy of particles and adatom mobility decrease with increasing pressure. In positions far from the centre of deposition there are more particles with oblique incidence. This leads to a shadowing mechanism. Columnar diameter increases and packing density decreases. The influence of working gas pressure and angle of incidence bears also an effect on the behaviour of mechanical stress. Coatings with coarse columnar structures have lower mechanical stress than coatings with fine columnar structures.     

Strahlverschleiß von Stahl mit niedrigem Kohlenstoffgehalt: Untersuchungen zur Verschleißpartikelgeometrie

Erosion of low‐carbon steel by solid particle impingement: aspects of wear debris geometry Image analysis software was used to analyse the geometry of debris formed during the erosion of low‐carbon steel by impinging solid particles. Depending on the two‐dimensional aspect ratio (ratio between debris height and debris width), three different debris types could be distinguished. The most frequent type observed was a platelet‐type debris as suggested by the Bellman‐Levy (1981) model. This wear debris shape type covered about 60 % of all acquired debris. Plain micro‐machining according to Finnie’s (1959) suggestion played a negligible role only, but other processes, namely ploughing as suggested by Winter and Hutchings (1974), were more important. The statistically estimated mean debris size was about 14 μm. About 92 % of all wear debris had sizes smaller than the target material grain size. This result supports the figure that multi‐step removal modes – the formation and detachment of lip or platelet from crater rims ‐ were responsible for material removal.     

Bruchzähigkeit und Ermüdungs-Rißwachstum von Gußeisen

Fracture Toughness and Fatigue Crack Growth of Cast Irons Fracture toughness, elastic moduli and fatigue crack growth rates in air and in vacuum were measured for 17 different cast irons. The graphite shape in the cast irons varied from flakes to nodules, the matrix varied from ferrite to pearlite to martensite. In the fatigue crack growth rate tests, using fracture mechanics methods, it was observed that the fatigue crack growth rate increases significantly as the cyclic stress intensity range approaches the critical value for stable crack growth. This phenomenon was used to determine the fracture toughness of the cast irons. Such toughness data agree well with literature data on the fracture toughness of cast irons. An extensive review of the effects of strength on the fracture toughness of commercial cast irons is presented. In cast irons with flake graphite, cyclic loading results in a reduced modulus of elasticity. This is attributed to the rupture of the graphite flakes under cyclic loading.     

Bruchmechanische Untersuchungen an niedriglegierten Baustählen mit Hilfe des COD-Konzepts

COD Investigations on Low Alloy Steels Fracture toughness tests were performed at low alloy steels using the COD concept. Especially have been investigated the effect of evaluation methods, determination of stable crack initation and the effect of specimen geometry on the critical crack tip opening displacement: The evaluation methods in use today yield different δ_c-values, therefore a standard procedure method may be desirable. The potential method seems to give the best results for characterizing stable crack growth. A control by unloading and marking the crack extension by additional fatigue loading or other methods might be necessary. An effect of specimen thickness on the critical crack opening displacement was only found at thin specimens, in which no plane strain appeared or only arised in a small region within the center of the specimen. The crack opening displacement at the moment of first crack propagation (stable or instable) proved to be independent of specimen thickness if the amount of plane strain exceeded 50%. For this condition also was found no influence of crack length within the range for a/W = 0,2 to 0,65.     

Untersuchungen zur lokalen Zusammensetzung von Glasfasern mit Hilfe der Röntgenmikroanalyse

Investigations of the Local Composition of Glass Fibers with the Help of the Scanning Electron Microanalysis Various glass-fiber types show different chemical resistance in the surrounding of aggressive media. Through the influence of inorganic acid or alkali media certain elementary glass fibres can be leached. This effect can be seen optically in some kinds of glass-fibers. Other glass-fiber types show no optical change in the surface, although there is an aggressive attack Now it is possible to understand the leaching process in glass-fibers with the help of the scanning electron microanalysis. In that way corrosion procedures of various glass-fiber types can be better described.     

Untersuchungen zur Volumenabhängigkeit von KIC-Daten mit Hilfe einer modifizierten Weibullstatistik

A Modified Weibull Statistics of Ceramic K_IC Data An explanation is given for the standard deviation of K_IC-data and for the dependence of K_IC values measured with ceramic materials on the specimen volume (specimen thickness). The probability distribution of measured K_IC-data is described by a modified Weibull theory. The theory is verified by experimental data using specimens with different thickness and a through the thickness notch. The experimental results are in good agreement with the theory.     

Versagensverhalten keramischer Faserverbundwerkstoffe mit poröser Matrix – experimentelle Untersuchungen und Modellierung

Failure Behavior of Fiber Reinforced Ceramics with Porous Matrix – Experiments and Modeling A 2D reinforced carbon/carbon‐composite was investigated in tensile tests with variable fiber orientation relative to the loading direction. The experimentel results from the 0° ‐ and 45° ‐ tests allow the derivation of material parameters to be implemented into a macroscopic model describing the materials response under quasi‐static loading conditions. The model is based on the damaging concept and plasticity theory and is used to predict strength and elastic constants of the composite under variable angles between fiber orientation and loading direction. The predicted data are confirmed by experimental results.     

Exergetische Untersuchungen von CO2-Laseranlagen mit Entspannungsturbinen

Forschung im Ingenieurwesen - Für einen CO2-Hochleistungslaser, werden auf der Grundlage exergetische Untersuchungen für einen CO2-Laser-Kreisprozeß, bestehend aus Verdichter V,...     

Rißwachstum bei metallischen Legierungen. Anwendung und Grenzen des Potentialsondenverfahrens

Applicability of the Potential Drop Technique for Crack Growth Investigations A potential drop method, based on the compensation concept is described; technical parameters and capability are discussed. Selected examples with metallic alloys for high temperature and heat resistant application demonstrate the applicability of the method for fatigue and creep crack growth investigations in the temperature range up to 1173 K and 1223 K, respectively. At room temperature and at 1173 K the results of the potential drop method are verified by optical means.