Schadensakkumulationshypothesen zur Lebensdauervorhersage bei schwingender Beanspruchung. Teil 1. – Ein kritischer Überblick—

Cumulative Damage Theories for the Prediction of Fatigue Life . Most fatigue data are determined in constant stress amplitude tests. Therefore they are not applicable directly for the prediction of fatigue life under service loads: A ?cumulative damage theory”? is necessary. For about 350 program test series (blocked 8 stress level tests) the cumulative damage sum Σ n_i/N_i at failure is calculated. The mean value of this ratio is near 1,0 and thus agrees with Miner's rule; however the scatter is extremely high. Tests in bending give significantly lower damage sums than tests under axial loads. Furthermore about 130 random and flight by flight tests are analysed. Next, modified linear damage theories are discussed and it shows that only theories which take residual stresses into account will improve the accuracy. A relative fatigue life estimation is proposed, where one test under service conditions is the basis and Miner's rule is used as a transfer function.     

Verwendung von statistischen Grundlagen für die Lebensdauervorhersage bei schwingender Beanspruchung

A concept to describe the fatigue lifetime by using statistical methods In order to spend time and money, cyclic loading tests have been usually undertaken with a relative small number of specimens. On the other hand, it is well established that fatigue life data, i.e. the cycle number of crack initiation N_i as well as the cycle number of total failure N_f show a scatter for a given load level. Therefore, fatigue live predictions must be undertaken using statistical methods. In this case it has to be taken into account that the quantity N_f consists of two parts, i.e. crack initiation and propagation. The first one has to be calculated in a statistical manner, whereas the second one with deterministic methods. Therefore, a superposition of statistical and deterministic parts has to be taken into account. Up to now it was estimated that in all cases the failure results from the same damage mechanism. In this case a scatter of fatigue life may be described by the Weibull distribution and characterised by the location parameter and the Weibull exponent. By using the correct density function, all fatigue life data N_i can be plotted in a failure probability P_v(N_i) vs. N_i plot. In case of some Al-alloys no linear dependence between P_v and N_i has been found, indicating that more than one type of defect spectrum may initiate the failure. Indeed, in AlMgSi cold worked specimens two types of cracks, e.g. surface as well as corner cracks, have been found that initiate the failure of the specimens. If was possible to correlate each type of crack with an individual Weibull distribution. By using this procedure, fatigue life can be estimated better than before.     

Berechnung der Dauerschwingfestigkeit bei mehrachsiger Beanspruchung — Teil 1

Estimation of Endurance Limit under Multiaxial Loading Multiaxial criteria for endurance limit prediction of metallic materials will be discussed in this paper. The Shear Stress Intensity Hypothesis SIH is improved under consideration of the variable fatigue limit ratio τ_w/σ_w and mean stress effects. Fatigue behaviour under synchronous and nonsynchronous stresses and influences such as mean stresses, out-of-phase stresses, different frequencies, nonsinusoidal stress time function are analysed theoretically and verified with test results. The prediction according to SIH shows good agreement with the test results. Based on an extensive statistical investigation it can be recommended to use the SIH for ?crack free materials”? and the normal stress hypothesis NSH for ?materials with cracks”?. For components the hypothesis SIH and NSH are developed under consideration of the notch effect. In this study a concept for the prediction of endurance limit under multiaxial loads has been developed.     

Werkstoffverhalten unter mehrachsiger schwingender Beanspruchung. Teil 2: Experimentelle Untersuchungen

Behaviour of Materials under Multiaxial Cyclic Loading . A great many structures and structural components are subject to multiaxial cyclic loading, e. g. the shafts. In the first part of the investigations several possible methods of calculation were discussed, by which the strength behaviour of metallic materials under multiaxial cyclic stress conditions can be described. They are, in the second part, critically compared with the results of a series of experimental investigations.     

Übertragbarkeit – ein zentrales Problem der Lebensdauervorhersage schwingbelasteter Bauteile

Transferability – a fundamental problem for fatigue life prediction Frequently specimen fatigue data have to be applied for fatigue life assessment of components and structures. It has to be assured that such data are relevant for the component under consideration. Equally it has to be assured that constant amplitude S-N data can be used for fatigue assessment in the case of spectrum loading. It is shown that application of transfer functions which are often of empirical nature requires an understanding of basic principles of material and component behaviour. Life prediction concepts should acknowledge the existence and relative portion of the crack initiation and crack propagation phases which are dominated by different mechanisms.     

Pervaporation – Ein Membranverfahren zur Trennung von Flüssigkeitsgemischen –

Pervaporation is an energy saving membrane process for difficult to separate and heat sensitive liquid mixtures. The article describes the process and the separation principle. Components required for a pervaporation installation are identified. The importance of low pressure and vacuum generation is emphasized. Examples of applications and development trends illustrate the application range of pervaporation for the separation of liquid mixtures.     

Korrosion durch Biokraftstoffe – Schutz durch Beschichtungen auch bei zyklischer Beanspruchung

Corrosion by biofuels ‐ Protection by coatings also under cyclic loadings The influence of corrosion on fatigue design must be considered in numerous component parts in the automotive industry. By using appropriate coatings the negative influence of corrosion under static and cyclic loading can be suppressed. Using the aluminum cast alloy AlSi7Mg0,3 T6 as an example, selected material‐coating‐systems were to be characterized under mechanical‐thermal‐corrosive complex loadings and characteristic properties for the fatigue design under cyclic loadings were to be determined. Conditioning tests were carried out with uncoated, anodized and chemical‐nickel‐plated specimen at different temperatures in the media fuel E5 (fuel with 5 % ethanol added) and E10 (fuel with 10 % ethanol added) and followed by SEM examined. The cyclic fatigue tests with chemically‐nickel‐plated and tempered aluminum in the fuel E5 at 100 °C no reduced influence on the fatigue strength, within the scattering, was discerned. This result agrees with the conditioning tests in the fuel E5 at 100 °C, where no corrosion was observed. Despite the coating of the specimens salt spray fog leads to a clear reduction in the fatigue strength. Using a damage accumulation calculation, it could be shown, that the real damage sum of the callipers and specimens, despite different types of coating, are comparable.     

Vakuumwaagen – Teil 2: Mikrowaagen

For sorption measurements and for thermogravimetric investigations vacuum beam balances were developed with resolution down to the nanogram region. In corrosive atmosphere or under very clean conditions the magnetic suspension balance should be favoured. The sample tube is separated from the balance and can be hermetically sealed from the environment. It contains only the pan with the sample connected to a permanent magnet, suspended at a stable position in the field of a controlled electromagnet. The spring balance is an inexpensive alternative, if a minor relative sensitivity can be accepted. Quartz resonators are used to control evaporating and sputtering processes. Resonator systems of any type need no gravitational field and can be applied, therefore, in space technology.     

Der Pendelfurcher – ein Prüfgerät zur Untersuchung des Formänderungsverhaltens duktiler Werkstoffe bei dynamischer Beanspruchung

The pendulum furrower – a test device for the investigation of the deformation behaviour of dilatable materials at dynamical loading Within this contribution a new test device is presented, allowing the investigation of the deformation behaviour of dilatable materials at shock loading. The base equipment is a commercial pendulum striking apparatus, equiped with a Brinell ball at the end of the pendulum. From the adjustable feeding of the rigid fixed flat specimen and the pendulum path through the material of the specimen follows automatically an arc-shaped wearing track. By means of the energy of friction, registered during the oscillation of the pendulum and the following measuring of the volums of the furrows new special charakteristic values of the materials can be find out.     

Mikrorissentstehung und Mikrorisswachstum in Aluminiumlegierungen bei zyklischer Beanspruchung – Werkstoffliche Untersuchung und Simulation

Microcrack Initiation and Microcrack Growth in Aluminium Alloys Subjected to Cyclic Loading – Material Analysis and Simulation In this paper a model designed to simulate the growth of microcracks under the influence of cyclic loading is presented. Considering fatigue crack growth microstructural barriers as well as the state of stress play an essential role. The polycristalline metal was modelled as an aggregate of hexagonal grains with each of the grains showing a different crystallographic orientation. The crack growth is initially dominated by shear stresses leading to microstructurally short cracks (stage I). As the tip of the microcrack approaches a grain boundary the crack growth rate decreases. The transition from stage I to stage II crack growth is also considered in the model as the crack reaches a specific length and continues to grow under the influence of normal stresses (physically short cracks). The model is applied to tubular specimens of the aluminum alloy AlMgSi1 which are subjected to tension and torsion as well as to combined tension‐torsion loading cycles. In terms of the microcrack distribution as a function of their orientation the simulated crack growth behaviour reveals a close match with the experimental results.     

Über Korrosionserscheinungen,ausgelöst durch duroplastische Formstoffe (Teil 2) – Elektrolytische Korrosion

Investigations on corrosion caused by therm osetting materials (part 2) Electrolytic corrosion . Electrolytic corrosion on metallic materials is caused by plastics under the influence of a direct voltage. This second part of the investigations on plastic corrosion deals with the characteristic qualities of thermosetting materials, responsible for such type of corrosion. These qualities may be determined by the composition of the plastics and by the curring conditions. Measuring the insulation current in the plastic specimen gives an indication to the corrosive effect. It is shown by measuring the insulation current in longtime tests with varying materials and test conditions that short-time tests may give a conclusive statement on the corrosiveness of plastics.     

Hochtemperaturprüfung – Ein Beitrag zur Werkstoffentwicklung und ‐qualifizierung sowie Simulation der Bauteilbeanspruchung

High temperature testing – A contribution to alloy development, alloy qualification and simulation of component loading In parallel to continued developments of steam and gas turbines as well as traffic engineering machines on the one hand, and marginal conditions like low specific fuel consumption and sufficient environment‐friendliness on the other hand, the aim of improving the degree of efficiency by augmenting process parameters such as temperature and pressure is being followed. These efforts impact especially components of thermic machines and facilities subject to high thermal and mechanic exposure. Still largely unexplored is the interaction between microstructure characteristics determined through chemical composition, production processes and heat treatment, changes in the microstructure due to multiaxial load and the time‐dependent deformation and stability resulting hereof. With regard to this background, improved methods of material properties determination, their modelling and transfer on the component enable to optimize wall thicknesses and degrees of efficiency. In the course of evaluation of static and cyclic material properties carried out also on faulty specimens, uncertainties occur which can originate from the testing process and analysis, as well as being influenced by the material itself and its process of production. Altogether, the demand for reliable determination of material properties and methods of scatterband treatment and their mathematical‐statistical evaluation is in business. For simulation, consistent material datasets that describe the complex interaction between temperature, period of exposure and type of exposure are needed. Summarizing, the tasks dealt with qualify the entire process from production to the operational behaviour of components.