Spannungsrißbildung und Spannungsrißkorrosion bei Kunststoffen

Environmental stress cracking and stress-corrosion cracking in plastics Plastics are being used to an increasing extent in fields which have hitherto been the reserve of other, usually metallic, materials. Owing to their good processability and chemical resistance, plastics engineering materials are for example employed for vessels, pipes, valves and fittings, blowers, pumps and gear wheels. In these applications they are not only exposed to liquid media but, are also subjected to occasional or continuous mechanical stress. The trouble is that there are media to which plastics in the stress-free state are absolutely immune, but that, when tensile stresses are present, cracking may occur; this phenomenon is called environmental stress cracking. It is therefore necessary for the design engineer both to know whether the medium to be handled causes environmental stress cracking, and to be familiar with the other properties of the material. The following article describes the most important methods of detecting environmental stress cracking and the most important media which cause this form of cracking in common plastics materials.     

Ermüdung und Mikrostruktur von koronaren Stents

Fatigue and Microstructure of Coronary Artery Stents During implantation the structure of coronary artery stents is subjected to distinct plastic deformation. Subsequently the implant has to sustain up to 700 million cycles induced by the cyclic diameter change of coronary arteries. Regions of high deformation show characteristic deformation structures like slip traces or extrusions and intrusions of grains on the surface without failure of the material. In order to gain information about the microstructural and microtextural evolution during deformation microstructure characterisation by means of scanning electron microscopy as well as transmission electron microscopy and single grain orientation determination were carried out. The analysis of cyclic deformation with different frequencies is of special interest. Several correlations between microstructure and deformation state are observed. Few deformation characteristics occur in the solution annealed stents and increase after dilation. Inhomogeneous deformation behaviour depending on the deformation of individual grains is observed. After cyclic deformation further changes in the microstructure can be observed. These changes caused by cyclic deformation are sensitive to load frequency and generate the formation of a completely different microstructure. Knowledge of microstructure evolution caused by distinct plastic deformation and fatigue in so called oligocrystalline structures is basis for ongoing development in stent optimization.     

Einfluss von Randschichtzustand und Haltezeit auf die thermische Ermüdung der Magnesiumbasislegierung AZ31

In this paper thermal fatigue of magnesium base alloy AZ31 in the temperature range between +50°C and +290°C is investigated. Experiments were carried out under constant total deformation (out‐of‐phase loading) and the resulting stress amplitudes as well as the plastic strain amplitudes were recorded as a function of the number of thermal loading cycles. In particular the consequences of mechanical surface treatments (deep rolling) and of hold‐times were investigated. In both cases no particular influence compared with untreated specimens loaded without hold‐times was observed, which is due to the interaction of deformation and recrystallization processes during thermal fatigue.     

Zwillingsbildung bei der thermischen Ermüdung der Mg‐Basislegierung AZ31

Twinning at thermal fatigue of magnesium alloy AZ31 In this paper results of thermal fatigue tests of the magnesium base alloy AZ31 carried out in a temperature range between ‐50 °C and +290 °C are presented. Specimens were loaded under constant total strain and uniaxial homogeneous stresses. The resulting materials behaviour is described by stress amplitudes, plastic strain amplitudes and mean stresses as a function of the number of thermal loading cycles. It is well known that AZ31 shows different stress‐strain behaviour during tensile and compressive loading resp. at lower temperatures due to the fact that mechanical twinning depends on the loading direction. However untwinning processes may occur during unloading and reloading in the opposite direction. As a consequence, during the first thermal loading cycles, typical consequences of the formation and the dissolution of twins are observed. The interaction of deformation, recovery and recrystallization processes, characteristic for individual temperature ranges are discussed in detail to analyze the damage progress during thermal fatigue.     

Ermüdung von Keramik unter Schwingbeanspruchung

Fatigue of Ceramics under Cyclic Loading Fatigue of ceramics attracts more attention due to the expected availability of high-performance ceramic components e.g. for engines, However, the knowledge in fatigue of brittle materials is still inadequate, the actual standpoint as taken from literature is shortly reviewed. In this study two experimental-analytical ways are presented which manifest the chances and difficulties in this part of the fatigue research. First, the probabilistic approach to identify a true cyclic fatigue effect in brittle ceramics is envisaged. The second way applies direct observation of crack extension in static and cyclic loading mode in order to define the conditions and characteristic differences of both types of crack propagation. Results are presented from both test methods for several Si₃N₄, SiC and ZrO₂ materials and possible mechanisms are discussed. It is concluded, that toughened ceramics are more prone to cyclic fatigue effects than conventional and pure brittle ceramics (e.g. glass).     

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.     

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.     

Erfassung von Eigenspannungen und Rißbildungsmechanismen bei Ermüdungsbeanspruchung

Measurement of Internal Stresses and Crack Mechanisms under Fatigue Conditions The necessity is being examined to correlate materials reaction of pieces undergoing fatigue conditions with possible crack initiating mechanisms. For the understanding of the development of methods of early recording fatigue damages it is important to understand the mechanisms under fatigue conditions at the materials' surface and within the material. Owing to the particular importance to the surface, damages due to varying stress can often be recorded there earlier than on the interfaces. Therefore early recording methods often start off with the recording of changes at the surface. Under the necessary circumstances X-ray measurement of stress at the surface can be an appropriate means to make life-time predictions. Likewise the coherent light of a laser beam makes it possible to record changes at the surface. Other methods which can be employed under certain specific circumstances are the analysis of sound emission and the measurement of electro-potentials during varying stresses. Finally the knowledge of the mechanisms of fracture formation and fracture expansion enables us to use the fracture surface as a sort of data store. By means of the informations gained by this method on damage mechanisms it is possible to devise effective measures to prevent damage.     

Schädigungsentwicklung bei Ermüdung verschiedener CFK-Laminate

Fatigue Damage Development of Various CFRP-Laminates The behaviour of a woven carbon-fibre reinforced laminate in a balanced eight-shaft satin weave style was compared to non-woven laminates with an equivalent cross-ply lay-up (50% of the fibres in the 0° and 50% in the 90° direction) Two types of non-woven laminates were investigated consisting of continuous fibres and aligned discontinuous fibres, both produced from carbon fibre prepregs. The static strength and the fatigue behaviour was measured. Stiffness reduction was monitored during fatigue loading as a damage analogue to which the mechanisms of damage could be associated. Similarities and differences in the fatigue behaviour and damage development of the three laminates will be discussed.     

Beitrag zum Nachweis von Rissen infolge Spannungsrißkorrosion mit Ultraschall

Contribution concerning the ultrasonic detection of cracks due to stress corrosion . Topic of the present report: Experimental examinations concerning ultrasonic crack detection, carried out on compressedgas bottles showing different stress-corrosion damages. Following the discussion of the test results hints are given with respect to the practical test performance.     

Ermüdungsverhalten von Schraubendruckfedern bei konstanten und variablen Beanspruchungsamplituden bei sehr hohen Schwingspielzahlen

A test rig for simultaneous testing of up to 88 compression springs under constant as well as variable amplitude loading is presented in this paper. The test rig utilizes a servo‐hydraulic testing machine. The results of long‐term fatigue tests of compression springs under constant and variable amplitude loading up to 5 ? 10⁸ and 1.4 ? 10⁷ cycles are presented. Experimental Woehler‐ and Gassner‐curves are obtained using the maximum likelihood method. Theoretical Gassner‐curves are generated using Miner's rule and experimental Woehler‐curves. The theoretical Gassner‐curves are compared to the experimental ones. The results of the constant amplitude loading tests are compared to literature data. The possibility to increase the testing frequency in variable amplitude loading tests is discussed. Thereto, the comparability of results from fatigue tests of material specimens using torsional ultrasonic fatigue testing equipment to results from fatigue tests on compression springs is addressed.     

Zur thermischen Ermüdung der Magnesium‐Basislegierung AZ91

Thermal fatigue of magnesium‐base alloy AZ91 Thermal fatigue tests of the magnesium‐base alloy AZ91 were carried out under total strain control and out‐of‐phase‐loading conditions in a temperature range between ‐50°C and +190°C. Specimens produced by a vacuum die casting process were loaded under constant total strain and uniaxial homogeneous stress. To simulate the influence of different mean stresses, experiments were started at different temperature levels, e.g. the lower, mean or upper temperature of the thermal cycle. The thermal fatigue behavior is described by the resulting stress amplitudes, plastic strain amplitudes and mean stresses as a function of the number of thermal loading cycles. Depending on the maximum temperature and the number of loading cycles, cyclic softening as well as cyclic hardening behavior is observed. Due to the complex interaction of deformation, recovery and recrystallization processes and as a consequence of the individual temperature and deformation history, thermal fatigue processes of the material investigated cannot be assessed using results of isothermal experiments alone. The upper temperatures or the resp. temperature amplitudes determine the total fatigue lifetime.