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.     

Zyklisches Werkstoffverhalten von Magnesium‐Druckguss‐ und ‐Knetlegierungen

Cyclic material behaviour of magnesium die castings and extrusions The cyclic deformation behaviour of the die castings AZ91 and AE42 and the extrusions AZ31 and AZ80 is investigated in strain controlled tension compression tests. The main topic of this investigation is the verification of the Masing and Memory behaviour. These two material properties are necessary for the component design by the local strain concept. The two die castings show approximately Masing behaviour, but the hysteresis of magnesium is different from the hysteresis of steel. For extrusions a totally different behaviour in tension and in compression direction can be observed. The hysteresis in experiment differs from the calculated hysteresis from the cyclic stress strain curve. The Masing behaviour is not observed because of the strong anisotropic behaviour in tension and in compression direction. All investigated magnesium alloys show the material memory M2.     

Werkstoffverhalten und Mikrostrukturentwicklung hochfester Mn‐Al‐Si‐Leichtbaustähle unter Zugbelastung

Material behaviour and microstructure evolution of high‐strength Mn‐Al‐Si‐light‐weight steels under tensile loading Because of their extraordinary combination of high strength and superior ductility high‐strength high Mn‐steels with reduced density and additions of aluminium and silicon are interesting candidates for structural applications. The initial microstructure consisting of stable austenite or austenite and ε‐ and α'‐martensite was achieved by alloying. During plastic deformation intense strain induced martensitic transformation and / or mechanical twinning was observed. These deformation mechanisms are used to extend the limited forming capability and contribute to a high energy absorption (in impact tests) up to very high strain rates. Tensile tests reveal that the properties are maintained up to strain rates of about 1000 1/s. The flow stress behaviour is strongly influenced by the initial microstructure and their evolution during deformation processes is determined by the rates of martensite and twin formation, respectively.     

Thermoschockbeständigkeit und das Verhalten von SiC-faserverstärkten Glas-Matrix-Verbundwerkstoffen bei einer thermischen Wechselbeanspruchung

The damage evolution of commercially available SiC-Nicalon? fiber-reinforced glass matrix composites under thermal shock and thermal cycling conditions in oxidizing atmospheres was investigated. The thermal shock tests involved quenching the samples from high temperatures (590–710°C) to room temperature in a water bath. For the thermal cycling tests the samples were quickly alternated between high temperature (T=700°C) and room temperature air for different number of cycles. Both destructive and non-destructive techniques were employed to characterize the samples and to detect differences in behavior for the various thermal loading conditions. In thermally shocked samples, damage in the form of matrix microcracks was induced by quenching from intermediate temperatures, e.g. 660°C. The extent of damage increased with the number of thermal shock cycles, as detected by a decrease in the Young’s modulus and a simultaneous increase in the internal friction measured non-destructively be a mechanical force resonance technique. In thermally cycled samples, material degradation was ascribed to porosity formation in the matrix as a consequence of the extended exposures at high temperatures. With increasing number of cycles, also interfacial oxidation was detected. An attempt was made also to explore the possibility of healing the induced microcracks in thermally shocked samples by an optimized post-thermal shock heat-treatment (annealing) schedule, exploiting the viscous flow of the glass matrix.     

Kohlenstoffbasierte Beschichtungen für automotive‐Komponenten

Carbon‐based coatings for automotive components The outstanding properties of carbon‐based coatings are the low friction against steel and the wear resistance, interconnected with the high hardness. In lubricated systems coatings only work in the mixed and boundary friction regime; this means as long as primary body and counter body are still in contact. The properties of coatings can be adjusted in wide ranges. A purposeful adaptation requires that the tribological system is sufficiently well understood. It has been shown that solutions based on coatings are only sub‐optimal if the coating is only introduced at the end of the designphase. A greater benefit is obtained, when the scope for development as given by a coating is utilized already in the early design phase. The coating is accepted as a design element. To test the suitability of the selected coating quickly and inexpensively in advance, the usual standard laboratory tests are not always appropriate. A test set‐up has to be close enough to the application to generate useful results. These considerations are illustrated using the example of a piston pin coating. The roughness of the primary body and of the counter body is of particular importance for the adjustment of the coating system, when high loads are applied. The example presented here shows that the usual indicators for surface roughness like R_a and R_z are not always sufficient.     

Zusammenwirken von Spannungsrißkorrosion und Ermüdung bei der Schwingungsrißkorrosion eines hochfesten Stahles

Conjoint Action of Stress Corrosion Cracking and Fatigue on Corrosion Fatigue of a High Strength Steel The corrosion fatigue characteristics of a high strength, martensitic steel in 0.5 n NaCl solution is investigated with regard to the fatigue and stress corrosion cracking behaviour of the material. Test parameters are stress ratio and frequency, testing is carried out with fracture mechanics methods, the crack surfaces are examined fractographically. An analysis of the results reveals that corrosion fatigue in high strength steel is caused by fatigue or by stress corrosion cracking, depending on the kinetics of the two processes. Fatigue and stress corrosion cracking do not act cumulative or additive. Instead, the kinetically faster process causes crack advance. The crack growth characteristics are interpreted with respect to the fractographic appearance of the crack surfaces. Corrosion fatigue cracks propagate either intergranular relative to the prior austenite grain boundaries as stress corrosion cracks do or transgranular like fatigue cracks, depending on the crack growth rates of the two processes. Fatigue and stress corrosion cracking do not interact, at least in a measurable degree, because of the different crack path of the two fracture processes. Results can be assessed quantitatively with the “process competition model”.     

Ermittlung geschlossener probabilistischer Auslegungskennlinien für einstufige Schwingfestigkeitsbeanspruchungen vom LCF‐ bis VHCF‐Bereich

During the probabilistic evaluation of single‐stage life tests (SN‐tests) the approximation in the double logarithmic diagram is traditionally carried out by using piecewise linear functions. In many SN‐tests an exchange mechanism regarding failure is observed. Therefore a classification is carried out in accordance with this change in different SN‐curve types. The statistic parameters can be determined by using the Maximum‐Likelihood‐Method, wherein the consideration of runouts or test abortions can be cited as a major advantage. With the help of an additive approach it can be deduced for a single‐stage load from the concerning failure probabilities of each area to a closed probabilistic interpretation beginning from LCF to VHCF. This design curve can be determined for different quantiles and eliminates the implausibilities of traditionally prepared peacewise linear functions.     

Gewebeelastizität und biomechanische Interaktion vaskulärer Komponenten

Tissue Elasticity and Biomechanical Interaction of Vascular Components. Many contributions have been made on the mechanical behaviour of soft biological tissues. Relatively small, however, is the knowledge of the biomechanical interaction of their components. Vascular tissues show a highly nonlinear load-deformation response which in their passive state depends on the three following, major wall components: elastic and collagen fibres and groundsubstance, a nonfibrous matrix. With increasing pressure, namely, blood vessels lose their distensibility, collagen and elastic fibres becoming straight and form a strong wall reinforcement which keeps the vessel from being blown out at higher intraluminal pressures. In order to better understand the material properties of the vessel wall and the interaction of these vascular components a continuum mechanical model is developed. Based on thermodynamical considerations reasonably approaching the physiological situation, constitutive equations are derived by which the amounts of stresses contributed by these components as well as the deformation energy stored by them, can be determined. Of particular significance, thereby, are the distribution densities and distribution functions of the fibres becoming and having become aligned during the vascular dilatation. These functions were determined from uniaxial tensile tests performed with electronic tensile testing machines using quasi pure fibre structures histologically corresponding. As specimen human tendons were taken for the evaluation of the collagen and ligaments from the neck of cattle for the evaluation of the elastic fibres' mechanisms. When simulating the pressure-radii response of an aortic segment on a computer using these data the results definitely show that the contribution of elastic and collagen fibres to the vessel mechanics is considerable already at mean physiological pressures and by far prevailing at higher ones. The present investigations are a necessary step forward in the biomechanics and materials research of vessels and, hence finally, in the research of the circulatory system.     

Einfluß eines Konzentrationsgradienten bei röntgenographischen Spannungsmessungen

Influence of a Gradient of Composition on X-Ray Stress Analysis In the last years several factors were shown which lead to nonlinear sin²Ψ-distributions in x-ray stress analysis, like rotation of the principal stresses with respect to the normal to surface, steep stress gradients under the surface and a texture in the material under investigation. A fourth factor is revealed which becomes active, when gradients of the composition are present. In case of a chromized Fe-sample the influence on x-ray stress analysis is shown by measurement and theoretically as well. A method of correction is introduced, which is checked by use of a model sample. The x-ray residual stresses of the chromized Fe-sample are in accordance with such ones determined mechanically.     

Accelerated LCF‐creep experimental methodology for durability life evaluation of turbine blade

This paper proposes an accelerated low cycle fatigue (LCF)‐creep experimental methodology in laboratory to investigate the durability life of turbine blades. A typical mission profile of the turbine blade was obtained by means of rain flow counting method, considering both the actual flight condition and ground test data. Finite element analysis (FEA) was conducted to obtain the stress and temperature fields of turbine blade. A test system was constructed to conduct LCF‐creep experiments of turbine blades, simulating the stress and temperature distributions of critical section properly. LCF‐creep experiments of full‐scale turbine blades were performed under a trapezoidal loading spectrum. Experiment results showed that the durability life of turbine blade based on numerical method was longer than that based on this experimental methodology, even an order of magnitude. Furthermore, this experimental methodology helped to extend the service life of this blade safely, and its validity was verified in actual service condition.     

Rissbildung und ‐fortschritt bei Schwingbelastung von Schraubenverbindungen

Crack initiation and crack propagation of fasteners under fatigue loading In this investigation the fatique strength of fasteners under variation of the property class, the sequence of manufacturing, the coating and the kind of lubrication, was determined. Thereby the crack initiation and the crack propagation of the individual variations were observed by suitable measurement instrumentation. As a result of the investigation the S‐N curves for crack initiation and for fracture of the different variations of fasteners are presented.     

Zum Ermüdungsverhalten eines vakuum- und eines elektroschlackeumgeschmolzenen martensitaushärtenden (Maraging-) Stahles (Typ X2 NiCoMo 18 8 5) bei Umlaufbiegung und Rollreibung

The fatigue behaviour of a vacuum- and an electroslagremelted 18% nickel-cobald-molybdenum maraging steel under rotating bending and rolling contact conditions. The endurances of an eletroslag- and a vacuumremelted maraging steel have been compared under conditions of rotating bending as well as under rolling fatigue conditions; with the finishing conditions (grinding and scouring) as additional parameter. Under conditions of rotating bending no significant difference in endurance has been found between the vacuum- and the electroslagremelted material. The same applies for rolling contact experiments under conditions of lubricant starvation. – Under conditions of thick-film lubrucation, however, the vacuumremelted material shows a considerable longer life expectancy than the electroslagremelted material. – Under conditions of rotating bending a scouring finish gives a shorter life expectancy than a grinding finish. Under conditions of rolling contact fatigue the opposite is found. – The effects are discussed in terms of structure and surface topography, special attention has been paid to structural changes that occur in heavily strained areas.