We present an approach for estimating E-N curves and their scatter. The scatter of a number of load cycles to failure at an arbitrary amplitude-strain level is modelled using a two-parametric Weibull distribution with the constant shape parameter and the scale parameter dependent on the strain amplitude by the Coffin-Manson equation. In this way the E-N curve and its scatter can be described using five parameters: the four parameters of the Coffin-Manson equation for the scale parameter of the Weibull distribution and the shape parameter of the Weibull distribution. The objective was to estimate these five parameters, which are generally unknown (since the data from the literature are manly known only for the median E-N curves), on the basis of the known fatigue-life data to obtain not only the trend of the E-N curve, but also its scatter. In order to estimate these parameters on the basis of the fatigue-life data, two evolutionary algorithms were applied: a real-valued genetic algorithm (GA) and the differential ant-stigmergy algorithm (DASA). In the article a mathematical background of the approach is presented and applied to 27 test cases of simulated fatigue-life data and one real case of experimentally obtained fatigue-life data. The results are analysed and discussed.
COBISS.SI-ID: 13042971
This study focuses on modelling the fatigue life reduction of an AlSi9Cu3 aluminium alloy caused by macro-porosity. The objective was to develop an engineering approach to fatigue life prediction for structural elements with macro-porosity defects. By following this approach a finite element analysis is first applied to assess the dependence of the strain-concentration factor, caused by a failure-dominant pore, on the nominal strain, the pore size and the proximity to the specimen surface. This dependence is then modelled by a nonlinear equation with three independent variables, i.e. the nominal strain, the pore size and the pore proximity to the specimen surface. The parameters of this equation are determined according to the results of the finite element analysis using a real-valued genetic algorithm. The proposed numerical approach was validated using experimental results. For this purpose, cylindrical specimens were manufactured by a high-pressure die casting and three levels of porosity were deliberately introduced into the specimens. The specimens were then tested at several strain levels and the corresponding fatigue life curves were estimated. The statistical significance of the fatigue life reduction due to the porosity level was statistically analysed and the experimental results were compared to the fatigue lives that were calculated with the presented strain-concentration model. The comparison showed a good agreement between the calculated and the experimentally obtained fatigue lives.
COBISS.SI-ID: 13241627
The paper presents an approach to the energy dissipation calculation under arbitrary multiaxial thermomechanical fatigue (TMF) loading. In such an approach the total area of plastic hysteresis loops is taken as a measure of dissipated energy. The calculation is based on the concept of the developed temperature dependent Prandtl type operator. Energy dissipation is associated to irreversible dislocation movements represented by slider shifts of three independent operators. The dissipated energy is then obtained continuously at any time by collecting dissipated energy increments of each operator. It is shown that the multiaxial operator approach gives us the same total energy of plastic deformation as compared to the classical integration approach. Furthermore, the presented approach enables to automatically split the obtained dissipated energy between the "true" dissipated energy and the elastically "stored" energy. In order to satisfy the request for a minimum number of dedicated material tests, the approach assumes fixed principal directions. Therefore, the proportional as well as non-proportional loading conditions are addressed in the same manner, which is currently the main deficiency of the approach.
COBISS.SI-ID: 12541467
Fatigue loading appears frequently at many machine parts and it can, under certain conditions, cause a failure. Many parts of an internal combustion engine are subjected to a raised temperature where the fatigue properties are different than they are at room temperature. The exhaust system is one of the parts that are loaded with fatigue at different temperatures, known as thermo-mechanical fatigue (TMF). Welds are used frequently in the exhaust system and present critical locations for crack initiation. During loading initial crack can propagate through a section to a final failure, so if the lifetime of the exhaust system is determined by the results of a base material, it can be misjudged. In the paper it is presented that the fatigue degradation regards to the weld influence. Test sample preparation, material and special high cycle fatigue testing manner are also presented. Test specimens without weld, with weld and additionally with aging influence are used. The material exposure to increased temperature over time affects the stress-strain response and other mechanical properties. This effect is known as aging and the results of this influence on the fatigue of welded parts are presented and discussed.
COBISS.SI-ID: 12456731
The paper presents the calculation of the coefficient of friction between the cutting tool and the chip in case of linear orthogonal cutting in the direction of 90-0°. Beech wood was cut with HSS cutting-tool at the cutting speed of 0.03 m.s-1. Rake angles were 16, 22, 31 and 42°, and the depths of cut were 0.3, 0.25, 0.2, 0.15 and 0.1 mm. A type II chip according to Franz was formed at the angles of 16 and 22°, and a type I chip was formed at the angles of 31 and 42°. The force resulting from the ploughing effect due to cutting tool tip bluntness was subtracted from the forces measured perpendicularly to the direction of cutting Fy, while the force resulting from the ploughing effect and the fracture force for creation of new surfaces were subtracted from the forces measured in the direction of cutting Fx. The coefficient of friction at all rake angles and depths of cut, calculated from the difference of forces, was 0.46 on average with a standard deviation of 0.009. The model with subtracted forces resulting from the ploughing effect has proved to be correct; the calculated coefficient of friction at all rake angles and chip thicknesses is constant since the coefficient of friction has to be independent of the force magnitude, while in the literature found, the coefficient of friction varies with the chip thickness and rake angle.
COBISS.SI-ID: 2127241