Volume 51, Issue 5

Mechanical stress-strain hysteresis, temperature, and electrical resistance measurements were performed for the microstructurerelated characterisation of the cyclic deformation behaviour and for the fatigue life calculation of metals. The electrical resistance is strongly influenced by the defect density, and allows the detection of a proceeding fatigue damage under cyclic loading and during load-free inspection intervals as well. On the basis of comprehensive mechanical, thermal, and electrical fatigue data the physically based fatigue life calculation “Phybal” was developed at the Institute of Materials Science and Engineering at the University of Kaiserslautern. This method requires only one load increase test and two constant amplitude tests for a fast and nevertheless precise calculation of S-N (Woehler) curves, leading to a significant reduction in experimental time and costs.

Computer-aided fatigue design supposes many ingredients among which a damage indicator is considered. Since many years, energy based approaches have been considered as good candidates and many propositions have been done. However, as energy can be split in several parts which conduct to the definition of elastic, plastic, dissipated or stored energies, a discussion on the role of each of them is necessary. This is the objective of this paper in order to synthesize the different propositions which have been made during the last decades.

Kurzfassung Zyklische Beanspruchung bei konstant steigender oder variabler Temperatur führt bei metallischen Bauteilen zu einer komplexen Schädigungsentwicklung, die nur schwer mit einfachen Mitteln zu beschreiben ist. Während des Betriebes tritt in vielen Bauteilen thermomechanische Ermüdung (TMF) auf, z.B. in Kokillen in der Stahlproduktion, Turbinenscheiben, Zylinderköpfen. Daher ist es wichtig, die thermomechanischen Ermüdungseigenschaften dieser Materialien zu untersuchen und zu charakterisieren. TMF-Versuchsserien wurden durchgeführt, um eine Basis für Schädigungsparameter und energiebasierende Ansätze zu finden. In diesem Beitrag werden die TMF-Ergebnisse von galvanisch beschichtetem Nickel, kaltgewalztem wärmebehandeltem Nickel (Ni200/201) und einer geschmiedeten Nickelbasislegierung für Turbinenscheiben (IN718) dargestellt und verglichen.

The paper includes the analysis of influence of cyclic stress-strain curve parameters on the fatigue life estimation. The parameters of cyclic stress-strain curve, described by the Ramberg-Osgood relation, were determined by a new method from the variable-amplitude loading histories. The variable-amplitude stresses were used to calculate strain courses based on different values of cyclic stress-strain curve parameters. The calculated and experimental strain courses were applied to calculate fatigue life with use of damage accumulation hypothesis by Serensen-Kogayev.

Low cycle fatigue of the single-crystal nickel-base superalloy CMSX-4 was investigated in the temperature range 700—950°C under strain controlled cyclic loading. The points of interest were the crystallographic anisotropy of low cycle fatigue behaviour and the effect of creep damage on low cycle fatigue life. For the testing conditions used the low cycle fatigue life of CMSX-4 is strongly orientation dependent as well as very sensitive to the formation of rafted γ/γ’-microstructure.

Thermal fatigue tests of the magnesium-base alloy AZ91 were carried out under total strain control and out-of-phase-Ioading conditions in a temperature range between 50°C and +190°C. Specimen 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, i. e. the lower, mean or upper temperature of the thermal cycle. The thermal fatigue behaviour 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 behaviour is observed. The upper temperatures or the respective temperature amplitudes determine the total fatigue lifetime.

The influence on the Charpy V-notch impact behaviour in the transition region is investigated taking into consideration the residual stresses, work hardening and the surface topology at the notch root caused by manufacturing. Three materials were made available for the investigation, an unalloyed structural steel in the normalised and in the partially austenitised microstructural state, a water quenched hardened and tempered high strength, fine grained structural steel, and a heat treated steel. Initially, the effects of the major geometric parameters are identified with constant manufacturing conditions at the notch root (part I). As shown in the present contribution, the required almost constant notch root manufacturing conditions are obtained by means of a defined notch grinding process. Following notch machining of two steels, a stress relieving heat treatment was carried out. This heat treatment relieves virtually all the macroscopic residual stresses and the work hardening in the surface layer. For constant notch manufacturing conditions, the notch root radius is the most effective geometrically influencing factor at low temperature. The specimen width has the least influence. Increasing level of notch impact energy reduce the effect of the geometric parameters. This is the case for increasing test temperature just as for tougher material states following partial austenitising of the unalloyed structural steel as well as for the heat treated steel with the finest microstructure. The present results provide a solid basis for further investigations (part II) which are to detect specifically varied notch manufacturing conditions on the Charpy V-notch impact behaviour.

A common project of Temsa and LMS covered the complete CAE- based durability process for the development of a new bus type, starting with the evaluation of the customer usage profile (CUP) for the target market and finishing with the CAE-based fatigue life prediction of the complete bus structure. This paper describes the load identification process for a CAE-based fatigue life prediction for a new bus type. The challenge in this project was to setup a process for load prediction which finds a good compromise between effort, accuracy, and sustainability. With the LMS Hybrid Road method the effective road profile will be back-calculated based on an initial measurement with one vehicle specimen. This approach enables a load prediction for vehicle variants without repeating time and cost consuming measurements for each new vehicle type.

The effect of a MoS 2 additive on the microstructural evolution and properties of copper based brake linings has been investigated in this study. It was found that the quantity of additive as well as sintering cycle has significant effect on the density and hardness values of the produced material. The microstructure of designed brake lining materials showed that copper based matrix was surrounded by MoS 2 particles. The microstructural investigation also proved that the lower boiling point elements such as Pb in the as-supplied powder vaporise during sintering from the structure, and this result in an increase in the porosity amount by lowering the overall density of brake linings. The Vickers hardness of brake linings decreased with the addition of molybdenum disulphide particles. MoS 2 addition reduces wear rate of samples due to the solid lubrication effect.

Doloma (sintered dolomite) having a 0.5 wt.-% optimum mill scale content were used to form bricks with addition of resin and graphite, so that the effects of graphite was evaluated. Dolomites were first sintered at a temperature of 1750°C for four hours, and then mixed with various additives to prepare mixtures. These mixtures were pressed under 1100 kg/cm 2 pressure. Graphite free and 10 wt.-% graphite containing bricks were produced. The bricks were cured at a temperature of 300°C for three hours, and this was followed by inspection of their bulk density, apparent porosity, and cold crushing strength and thermal expansions. Then, the bricks were baked at 1000°C for two hours, and the same tests were applied. The properties of brick samples were compared, and SEM and X-Ray techniques were performed to examine the structural features.