Das Verhalten metallischer Werkstoffe unter mechanischer Beanspruchung

The Deformation Behaviour of Metallic Materials Under Mechanical Loading Simple loading conditions are reviewed with load, reaction-diagrams and characteristic mechanical data as well as typical surface appearances. The uniaxial tensile test, the uniaxial creep test, the fracture toughness test and the single-stage fatigue test are dealt with in detail. The basis of plastic deformation is discussed and the reasons for the formation of definite dislocation structures and the type of glide bands are considered. The influence of different parameters of structure on the yield stress is explained in examples together with different hardening mechanisms. The influence of temperature and strain rate on the yield stress is described quantitatively. The deformation processes which are represented in deformation mechanism maps. Finally the influence of environments on the mechanical behavior of materials is considered. In examples, an insight is given into the activity of electrolytes, gaseous environments, organic liquids and liquid metals on the deformation behaviour at typical loading conditions.     

Einfluß der Temperatur und der Umgebungsbedingungen auf das Ermüdungsverhalten metallischer Werkstoffe

Influence of Temperature and Environmental Conditions on Fatigue Behaviour of Metallic Materials The corrosion fatigue behaviour of metallic materials is influenced by environmental conditions. With increasing temperature a similar diminishing of fatigue strength can be observed as in the usual tensile properties. Air and water vapour decrease the number of cycles to fracture because they accelerate the crack propagation by adsorptive and reactive processes at the crack tip. By the effect of pure hydrogen or hydrogen sulfide and mixtures from both appears a marked increasing of crack propagation rate in fracture-mechanical test pieces alternating loaded at low frequency (1 Hz). Corrosive mediums influence also the formation of cracks. A mechanical electrochemical failure mechanism leads to a total loss of fatigue strength. Depending on performance of materials in an aggressive solution active or passive corrosion fatigue occurs. With regard to the form of appearance the two kinds differ in characteristic manner.     

Zur Beanspruchung metallischer Werkstoffe im Bereich der Elastizitätsgrenze

Zusammenfassung Durch Modifikation wird das Prandtl-Reuss-Modell im elastisch-plastischen übergangsbereich zur Beanspruchungshypothese erweitert. Die Theorie wird versuchstechnisch verifiziert. Herrn Professor Dr.Ulrich Gonser zum 65. Geburtstag gewidmet Der Stiftung Volkswagenwerk danken die Autoren für die Bereitstellung von Sachmitteln.     

Zur Beanspruchung metallischer Werkstoffe im Bereich der Elastizit?tsgrenze

Ohne Zusammenfassung The online version of the original article can be found at     

Ermüdungslebensdauer der Magnesiumlegierung AZ91: Experiment und Modellierung

Fatigue Life of the Die‐Cast Magnesium Alloy AZ91: Experiments and Modelling The cyclic deformation behaviour of the die‐cast magnesium alloy AZ91HP was investigated under total strain control at constant total strain amplitudes between 1.4 × 10⁻³ to 2 × 10⁻² at room temperature and at 130°C. Microstructural investigations in combination with the determination of crack‐growth behaviour using the replica technique and measurements of changes of the stiffness (compliance) of the specimen during a fatigue experiment led to a detailed understanding of the evolution of damage and the main damage mechanisms. Based on these findings, a microstructurally based life‐prediction concept was formulated.     

Einfluß von Druckvorspannungen auf die Dauerfestigkeit metallischer Werkstoffe bei ein‐ und mehrachsiger Beanspruchung

Effect of compression stresses on the fatigue strength of metals at uniaxial and multiaxial state of loading The calculation of the effect of multiaxial mean stresses on the fatigue strength required to consider the relative direction between the constant and cyclic parts of stresses. For this a new concept for the assessment of stresses has been presented for any fracture behaviour of materials. Experimental investigations on the effect of compression stresses on the fatigue strength of hollow cylinder samples from the steel 20MnCr5 has been used to support the parameter determination of the strength model     

Ermüdung des Stahles 42CrMo4 unter mehrachsiger,phasenverschobener Beanspruchung

Fatigue of the Quenched and Hardened Steel 42CrMo4 under Multiaxial In- and Out-of-Phase Loading Two kinds of specimens out of the quenched and hardened steel 42CrMo4 (similar SAE 4140) were tested by combined tension-torsion in- and out-of-phase loading. The specimens batches differed in the residual stress state. Under cyclic, stress controlled loading an elastic behaviour is registrated until 50% of the life time. Then a rapid softening occurs, which correlates with the von-Mises equivalent stress in the case of in-phase loading. At high stress amplitudes, the residual stresses may be neglected. The lifetime is best described by the fatigue criterion of Zenner which considers the integral average of the stress state in every plane. This stress state is described by a linear combination of the shear stress amplitude and the normal stress amplitude. Below the cyclic yield strength, the residual stresses must be taken into account as static stresses. The comparison of the local residual stress distributions is possible by using the weakest link model of Heckel, which is described in detail.     

Anwendung des instrumentierten Kerbschlagbiegeversuchs zur Abschätzung der Rissauffangzähigkeit

Application of Charpy V‐notch testing to estimate the crack‐arrest toughness Modern structural integrity assessment relies upon fracture mechanics, thus utilizing fracture mechanical parameters describing the material fracture resistance against crack initiation and crack propagation as well as the material crack‐arrest behaviour. However, crack‐arrest fracture toughness values are usually difficult and expensive to determine. In this paper correlations are proposed for estimating the nil‐ductility temperature (T_NDT) and the crack‐arrest fracture toughness (K_Ia) from a transition temperature, based on instrumented Charpy‐V crack‐arrest load information. The transition criteria used are the 4 kN crack‐arrest force and the mean crack‐arrest fracture toughness of 100 MPa√m according to the master curve approach. Correlations between transition temperatures, T(F_a = 4 kN), T(K_Ia), and T_NDT, which were proposed for various structural steels, work very well for the 18Ch2MFA material.     

Beitrag zur Abschätzung der Dauerfestigkeit nitrierter bauteilähnlicher Proben

Estimation of the fatigue limit of nitrided component-type specimens The “weakest link” model of Kogaev and Serensen comprehend the statistical and the stress mechanical size effect on the fatigue limit. In combination with the concept of the local fatigue limit on the basis of the damage parameter according to Smith, Watson and Topper, it allows the consideration of component specific effects on the fatigue limit of surface strengthened materials. The applicability of the concept has been proved by experiments at nitrided specimens with different geometries and type of loads.     

Mechanische Prüfung metallischer koronarer Stents unter überlagerter chemischer Beanspruchung

Examination of the mechanical behaviour of coronary stents in physiological environment Since the introduction in 1986 today hundreds of thousands of stent implantations are carried out worldwide. Nevertheless, only few examinations of the mechanical behavior of coronary stents have been carried out. It becomes obvious, that the burst open pressure and the recoil of a stent expansion curve depend on the design and the pre‐deformation of the stent. Additionally the recoil depends also on the maximum dilation pressure. The environmental electrolyte does not influence the mechanical behavior. Despite the high amount of plastic deformation during the expansion process, no strain hardening can be observed. During dilation some metallic ions are released into the solution. After the plastic deformation a repassivation of the material occurs. Over a period of three months after the dilation, no metallic ions can be detected in the solution without further mechanical deformation.     

Abschätzung von Bruchzähigkeitskennwerten aus der Bruch‐ oder Kerbschlagarbeit

Estimation of Fracture Toughness from the Charpy Fracture Energy By means of a simplistic mechanical model a mathematical relation between the total fracture energy of an edge‐cracked beam under bending and the fracture toughness was derived. Inserting the parameters of the standard Charpy specimens and accounting for the finite root radius and the shallowness of the notch, an equation to calculate fracture toughness from the Charpy fracture energy was obtained. Unlike the commonly used empirical correlation formulas, the presented equation is applicable to any elastic‐plastic material. From its theoretical basis and the underlying assumptions it is expected to be conservative in any case. Although the derivation only holds for the upper‐shelf regime, it also is applicable to the brittle‐to‐ductile transition regime as a lower bound. In this range, the degree of conservatism can be reduced by accounting for the well‐known shift of the transition temperature. Comparison with experimental data confirm these features of the presented formula.     

Ermüdungsverhalten ausgewählter Werkstoffe und Bauteile bei sehr hohen Schwingspielzahlen

Very High‐Cycle Fatigue of Selected Materials and Components Results of several fatigue tests using a servohydraulic testing machine (VHF 50 D) and a spring testing machine are presented. Investigations were carried out at room temperature with. – smooth and notched specimens made of aluminium wrought alloy EN AW 6082,. – screws M 8 made of EN AW 6056,. – smooth specimens made of die cast magnesium AZ91 hp (here also tests at 125 °C) and. – shot‐peened helical compression springs of four different high strength steels. Tests were done with a frequency of 20 Hz (spring testing machine) and between 250 Hz and 400 Hz (VHF50D) up to a maximum number of cycles of N = 1,5 x 10⁹. Crack initiation sites were investigated and could be found at high number of cycles below the surface for smooth magnesium and aluminium specimens and also for most of the shot‐peened helical compression springs, but not for screws and other notched specimens.     

Grenzflächenpressung von Eisenwerkstoffen und Leichtmetallen unter kontinuierlicher Beanspruchung

Limiting surface pressure under continued pressure of ferrous and lightweight metals In this article the results of tests examining the limiting surface pressure of ferrous and light weight metals are presented. The tests were carried out under consistent conditions using a standardised test procedure.     

Einfluss der Netztopologie der künstlich neuronalen Netze auf das Ergebnis der Abschätzung zyklischer Kennwerte

Several estimation methods have been developed to estimate the cyclic material parameters out of the static material properties. Most of these methods are based on empirical equations. Increasing numbers of input‐ and influencing parameters lead to an rising effort for determining these equations and the accuracy decreases. For this reason new suitable methods are sought to estimate the cyclic material behaviour. A very promising approach is the application of the artificial neural networks, which can derive self‐depended a relationship between in‐ and output parameters. Static parameters such as yield strength, tensile strength …? etc., which can rapidly be determined used as input parameters. The output parameters are the cyclic material parameters of the strain‐life curve and stress‐strain curve according to the Manson‐Coffin‐Basquin‐ and Ramberg‐Osgood curve. Many different artificial neural networks with different structures and complexity can be applied. In this paper the influence of the topology of an artificial neural network on the estimation accuracy will be investigated. Based on the results of a reference artificial neural network it will be shown, that more complex topologies in the network do not lead inevitably to better estimations.     

Das ANSLC Programm zur Abschätzung zyklischer Werkstoffkennwerte

Die betriebsfeste Auslegung von spaltprofilierten Blechbauteilen erfordert geeignete Methoden und Übertragbarkeitskriterien, die für verzweigte Strukturen noch nicht zu Verfügung stehen. Wesentlich hierfür sind die Kenntnisse über das zyklische Werkstoffverhalten. Aus diesem Grund werden die zyklischen Werkstoffkennwerte in Abhängigkeit der Produkteigenschaften (Umformgrad, Gefüge, Oberflächenrauhigkeit, Eigenspannungen) und unterschiedlichen Belastungsparametern ermittelt. Da jedoch die Ermittlung der zyklischen Kennwerte mit hohen Versuchaufwand und Kosten verbunden ist, wird nach Methoden gesucht, diese kostengünstig und schnell zu bestimmen. Ein viel versprechender Ansatz hierfür ist der Einsatz von künstlichen Neuronalen Netzen (KNN) [1, 2, 3, 4, 5], da sie in der Lage sind, die Einflüsse aus Fertigungs‐ und Umweltparametern auf die Schwingfestigkeit aus sinnvoll gewählten Eingangsparametern zu generieren. Sie bieten die Möglichkeit, auf erlerntes Wissen zurückzugreifen und somit, auf Basis weniger Versuche ein mehrdimensionales Kennfeld zu erstellen.     

Einfluss der Versuchsunterstützung auf die Abschätzungsgenauigkeit mittels künstlich neuronaler Netze

Suitable methods and transferability criteria and knowledge of the cyclic material behaviour is essential for a durable design of a component. For this reason, the cyclic material parameters are determined as a function of the product's properties (level of deformation, microstructure, surface finish, residual stresses) and different loading parameters. However, since the determination of the cyclic parameters is associated with considerable experimental effort and costs, a cost‐effective and easy method is sought to determine these parameters. A very promising approach for this is the application of artificial neural networks (ANN) since they have the ability to generate the influences on the fatigue strength from the manufacturing and environmental parameters using sensibly selected input parameters. They offer the possibility to access acquired knowledge and to thus construct a multidimensional map based on a few tests. By combining a few experimental tests with the ANN, the result of the estimation can be improved and the experimental effort can be reduced.