Zur Rißausbreitung in Stahl unter schwingender Belastung. Der Einfluß einiger Versuchsparameter und des Gefüges auf die Rißwachstumsgeschwindigkeit

Crack Growth in Steel under Fatigue Conditions. The Influence of Research Parameters and Microstructure on Crack Growth Rates Crack growth rates in a plain carbon eutectoid steel were measured. The microstructure and the stress ratio were variied. The experiments were conducted in air and in a 5% NaCl-H₂O solution. The effect of the research parameters, atmosphere and microstructure on the constants m and C in the PARIS equation were determined. Possible mechanisms for the increase of crack growth rates due to a corrosive medium, an increase of stress ratio R, and/or the mechanical strength of the steel by specific heat treatment are presented.     

Experimentelle Ermittlung des Ver‐ und Entfestigungsverhaltens durch zyklische plastische Verformung auf Basis von Biegewechselversuchen

Für Biegeprozesse mit wechselnder Beanspruchung werden Materialgrößen benötigt, welche die Ver‐ bzw. Entfestigung durch den Bauschinger‐Effekt berücksichtigen. Hierzu werden Biegewechselversuche an Drahtproben durchgeführt. Durch Ermittlung der ?bezogenen Maximalkraftdifferenz“ wird die Ver‐ bzw. Entfestigung quantitativ angegeben. Für eine Auswahl an Eisenwerkstoffen und Nichteisenmetallen werden entsprechende bezogene Maximalkraftdifferenzen bereitgestellt und miteinander verglichen. Aufgezeigt wird, welche Werkstoffe tendenziell verfestigen und welche eher eine Entfestigung erfahren. Auch der Einfluss des Verformungsgrades wird erläutert.     

Untersuchungen zur Schallemission von Rißbildung und -ausbreitung in Punktschweißverbindungen unter Last

Crack Initiation and Propagation in external loaded Spot Weld Joints controlled by Acoustic Emission The crack initiation and propagation in spot welded joints is controlled by acoustic emission. A high strength steel with different mechanical properties after heat treatment was welded and the different stages of the fracture and their influences are studied by analyzing methods like acoustic emission and metallographic and scanning electron microscopic techniques.     

Berechnung der Dauerschwingfestigkeit unter Berücksichtigung der spannungsmechanischen und statistischen Stützziffer

Fatigue Limit and Geometrical and Statistical Size Effect The ratio of the fatigue limit of an unnotched specimen to that of a notched one, the fatigue notch factor K_f, is usually smaller than the theoretical stress concentration factor K_t. With the assumption of a plastic cyclic deformation ?_apW at the level of the fatigue limit the fatigue limit for a notched specimen can be calculated. According to formula (4) this fatigue limit σ_naD is a function of K_t, ?_apW, the notch stress amplitude σ_a, the cyclic strengthening exponent n′ and the fatigue limit of a smooth specimen σ_W. Moreover, taking into account the statistical size effect with the known “weakest link concept”, see flow chart Fig. 9, the calculation is in a good agreement with 77 test results for steel.     

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.     

Berechnung der Blitztemperaturen der Mikrokontakte in künstlichen Hüftgelenken unter Benutzung des Kontaktmodells von Greenwood – Williamson

Calculation of Flash Temperatures of Micro‐Contacts in Artificial Hip Joints by Using the Contact Model of Greenwood – Williamson In hip replacement the wear rate is reduced when using femoral heads and acetabular cups made of alumina ceramics. The use of zirconia ceramics (Y‐TZP) is discussed controversy. The process on the articulating surfaces head / cup are not totally understood, especially the temperature rise due to frictional heating. Alumina is a stable material, no problems are expected. When using Y‐TZP, friction temperature can be higher then the coagulate temperature of synovia or the phase transition is induced. The model proposed by Kuhlmann – Wilsdorf was used to calculate the flash temperature under in vivo conditions of micro‐contacts at the different articulating surfaces. The real contact conditions were calculated with the analytical contact model of Greenwood – Williamson. The surface temperature of the wear couple Y‐TZP/Y‐TZP was higher than the one for alumina/alumina.     

Simulation des Mikrorisswachstums unter Schwingbeanspruchung. Teil 2: Simulationsergebnisse – Einfluss des Spannungszustandes und Reihenfolgeeffekte

Simulation of fatigue micro crack growth. Part 2: Results of simulation – influence of stress state and sequence effects In part one the modelling of micro crack growth due to alternating loading has been presented. Simulation results for tension/compression, torsion and proportional multiaxial loading, the scatter of the simulated lifetimes to a macroscopic crack length of 500 μm as well as the influence of the density of crack seeds and the grain size have been presented. In part two the influence of the stress state under proportional and non‐proportional loading is examined. Additionally the sequence effect of High‐Low and Low‐High as well as consecutive load sequences will be discussed. The comparison of the simulation results to experimental results shows that the influences of multiaxial loading and sequences can be simulated qualitatively correctly. The simplifications of the modelling have been to be considered. If the length of the maximum crack is interpreted as a measure of the damage, it can be concluded that the damage accumulation is non linear and non continuous. The main proportion of lifetime from the crack seed to the macro crack is contributed in the phase in which the length of the maximum micro crack comes close to the size of the grain size.     

Schallemissionsmessungen beim Rißfortschritt in Stahl unter statischer Belastung

Acoustic emission evaluation of the crack growth in a steel under static loading. A method for evaluation of the crack growth as a function of stress intensity factor K_I by the acoustic emission signals (AE) of statically loaded high-strength steels has been developed and verified. Prismatic 38HN3MFA steel specimens have been used under three-point bending with simultaneous record of the AE signals. According to the AE data the crack start, its parameters and propagation stages have been defined. It has been shown that the maximum increment in the crack area is observed as the stress intensity factor approaches its maximum value.     

Berechnung von Bauteilwöhlerlinien unter Berücksichtigung der statistischen und spannungsmechanischen Stützziffer

Estimation of S-N Curves under Consideration of Geometrical and Statistical Size Effects In this paper a concept for the estimation of fatigue limit in [11] will be extended to the calculation of S-N curves. The statistical size effect is taken into account with the well known weakest link theory. The geometrical size effect is considered according to the cyclic plastic deformation in the notch root. This model gives good correlation with test results. According to the present model the slope of the S-N curve depends on the specimen size and strength of materials, but does not depend on the stress concentration factor. The S-N curve becomes flatter with larger notch size and higher material strength.     

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     

Experimentelle und analytische Untersuchung des out‐of‐plane‐Verhaltens von unbewehrten Mauerwerkswänden unter Erdbebenbelastung

Experimental and analytical investigation of the seismic out‐of‐plane behavior of unreinforced masonry walls In addition to the vertical and horizontal load‐bearing in‐plane, masonry must also withstand out‐of‐plane loads that occur in earthquake scenarios. The out‐of‐plane behavior of unreinforced masonry walls depends on a variety of parameters and is very complex due to the strong non‐linearity. Current design methods in German codes and various international codes have not been explicitly developed for out‐of‐plane behavior and contain considerable conservatism. In the present work, shaking‐table experiments with heat‐insulating masonry walls have been conducted to investigate the out‐of‐plane behavior of vertical spanning unreinforced masonry walls. As shown in previous numerical investigations, important parameters are neglected in existing design and analysis models and the out‐of‐plane capacity is underestimated significantly. In the conducted experiments the results of these numerical investigations are verified. Furthermore, the development of an analytical design model to determine the force‐displacement relationship and the out‐of‐plane load‐bearing capacity considering all significant parameters is presented.     

Werkstücktemperaturen bei der Zerspanung – Messung und Berechnung

Workpiece Temperatures in Machining - Measurement and Calculation Workpiece temperatures in machining of an austenitic steel, an aluminum alloy and copper have been both determined experimentally using infrared pyrometry and calculated numerically with a finite-difference-model. The influence of the cutting parameters such as cutting speed, cutting depth and rake angle on the workpiece temperature were investigated. Depending on the thermal diffusivity of the material an increase in cutting speed or cutting depth can either increase or decrease the maximum workpiece temperature. Increasing the rake angle always leads to decreasing workpiece temperatures.     

Lebensdauerberechnung festgewalzter Kerbproben unter Axialbelastung auf der Basis eines Rißfortschrittskonzepts

Axial fatigue life calculation of fillet rolled specimens by means of a crack growth model Fillet rolling is a method which significantly improves the fatigue strength of members. Residual compressive stresses induced in the surface layer during the fillet rolling process are able to retard or prevent crack propagation. An elastic‐plastic on the J‐integral based crack growth model considering the crack opening and closure phenomenon in nonhomogeneous plastic stress fields is described. Experimentally determined crack growth curves and fracture fatigue life curves at constant amplitude loading were used to verify the developed model.