Felix Weber

Structures and components in a wide variety of industries must withstand cyclic loads over an extended period of time. If failure occurs due to material fatigue, far-reaching consequences may entail. Accordingly, knowledge of the material’s fatigue life is essential. Since determining the relevant material properties in terms of Wöhler or strain-life curves requires significant experimental effort, numerous studies in the literature focus on accelerated lifetime prediction using so-called short-time evaluation procedures. In this article, such short-time evaluation procedures are adapted for the first time to the numerical framework of microstructure-sensitive fatigue modeling. Micromechanically motivated fatigue indicator parameters calculated by a crystal plasticity motivated material model serve as the numerical surrogate of the material’s response, which is otherwise derived from experiments. By applying boundary conditions of a strain increase test to multiple statistically representative volume elements of the underlying microstructure, the incremental cyclic damage at each load level can be calculated from the fatigue indicator parameters. Using these damage values, virtual lifetimes can then be calculated based on the damage accumulation in analogy to Palmgren-Miner. The numerically determined fatigue lives, based on the two fatigue indicator parameters – accumulated plastic strain and strain energy density – show good agreement with experimental results. The material under investigation in this study is an austenitic stainless steel of grade AISI 347, often used in nuclear power plant components.

As part of the energy transition, fluctuations in the energy supply occur repeatedly. For this reason, power plants are increasingly operated in load-following mode, which entails additional cyclic loads, often not considered in initial power plant design. Consequently, understanding the cyclic material properties of the components used – particularly their fatigue life – is of particular importance. Determining the latter with conventional constant amplitude tests involves significant experimental effort, which is associated with considerable time and cost. Modern lifetime prediction methods, however, enable the determination of fatigue life using a single load increase test in the HCF regime by calculating the incremental cyclic damage based on Palmgren-Miner. This study adapts these methods for use in the LCF regime based on the results of total strain-controlled strain increase tests, determining the incremental cyclic damage based on the dissipated strain energy at each strain level of the strain increase test. The newly proposed MiDAcStrain approach shows good agreement with conventional constant amplitude tests and outperforms the already established lifetime prediction method StrainLife, which requires at least two additional constant amplitude tests in conjunction with a single strain increase test. The material examined in this study is a metastable austenitic stainless steel of type AISI 347, which is frequently used in pressure retaining components of nuclear power plants

Gear hobbing is one of the most established manufacturing processes for manufacturing external gears. Typically, gear hobbing tools are coated by means of physical vapor deposition (PVD). In this study the influence of chemical vapor deposition (CVD) hob coatings on tool wear under dry cutting conditions was investigated and compared to the wear behavior of typical PVD coatings. 20MnCr5 steel and C45 tempered steel were machined using the fly-cutting trial as an analogy process for gear hobbing. The gear module was mn = 2.557 mm, which represents a common gear size for a passenger car manual transmission. Sintered tungsten carbide-cobalt WC-Co K30 tools with a PVD AlCrN coating and with a CVD TiCN/α-Al2O3 were used and macro- and microscopically analyzed with regard to the result-ing tool life and wear behavior. To investigate different wear behaviors the cutting speed vc and axial feed rate fa were varied in the range of vc = 100-600 m/min and fa = 0.4-3 mm for the individual trials. In total, eight different trials were analyzed. For the CVD-coated tools, no stable cutting conditions could be found and the tools failed by breakout during machining the first part at each parameter combination. Characteristic chip values were calculated using the manufacturing simulation SPARTAPRO. Based on the simulation results the observed wear phenomena were inter-preted and analyzed with regard to their cause of occurrence. The trials showed a significant influence of the coating on the tool life. The mean tool life of the PVD-coated tools was 113 times longer than that of the CVD-coated tools.

Gear hobbing is one of the most established manufacturing processes for manufacturing external gears. Typically, gear hobbing tools are coated by means of physical vapor deposition (PVD). In this study the influence of chemical vapor deposition (CVD) hob coatings on tool wear under dry cutting conditions was investigated and compared to the wear behavior of typical PVD coatings. 20MnCr5 steel and C45 tempered steel were machined using the fly-cutting trial as an analogy process for gear hobbing. The gear module was mn = 2.557 mm, which represents a common gear size for a passenger car manual transmission. Sintered tungsten carbide-cobalt WC-Co K30 tools with a PVD AlCrN coating and with a CVD TiCN/α-Al2O3 were used and macro- and microscopically analyzed with regard to the result-ing tool life and wear behavior. To investigate different wear behaviors the cutting speed vc and axial feed rate fa were varied in the range of vc = 100-600 m/min and fa = 0.4-3 mm for the individual trials. In total, eight different trials were analyzed. For the CVD-coated tools, no stable cutting conditions could be found and the tools failed by breakout during machining the first part at each parameter combination. Characteristic chip values were calculated using the manufacturing simulation SPARTAPRO. Based on the simulation results the observed wear phenomena were inter-preted and analyzed with regard to their cause of occurrence. The trials showed a significant influence of the coating on the tool life. The mean tool life of the PVD-coated tools was 113 times longer than that of the CVD-coated tools.

LiNixCoyMn1–x–yO2 (NMC) is one of the most relevant lithium-ion battery cathode materials. In this study, micromechanical tests were used to examine the fracture behavior of LiNi0.8Co0.1Mn0.1O2 (NMC811) secondary particles. Furthermore, the grain boundary strength was measured and the grain boundary stiffness was estimated to 521GPaµm−1. Using an optimized push-to-pull sample geometry produced by focused ion beam (FIB) technology and measuring the displacement through digital image correlation (DIC), micro tensile tests were performed and validated against nanoindentation. A Weibull distribution characteristic tensile strength of 745MPa followed by a brittle fracture behavior was measured. The fracture surface was examined and showed an intercrystalline fracture. Through electron backscatter diffraction (EBSD) analysis, less frequently occurring transcrystalline fracture was detected. Scanning transmission electron microscopy (STEM) showed a slight increase in dislocation density within the testing section in comparison to reference positions. The results can be used for a more detailed understanding of the mechanical degradation during the cycling of batteries as well as for building accurate simulation models.

Heavy-section ductile cast iron components, such as main shafts or planet carriers in modern wind turbines, are produced under complex solidification and cooling conditions that lead to pronounced local variations in microstructure. These variations, governed by the casting process, significantly influence the fatigue strength and are currently compensated for by high safety factors in design. Conventional fatigue testing to determine microstructure-specific S-N curves requires a large number of specimens, which is impractical for heavy castings. This study presents the applicability of MiDAcLife, an accelerated fatigue testing approach that combines load increase tests (LIT) with thermometric monitoring to derive microstructure-specific S-N curves from only a few specimens and, thus, for only a very specific solidification and cooling condition. By analyzing the characteristic temperature response during LIT and applying a Palmgren–Miner-based evaluation, both fatigue strength and finite life regime are estimated. The method is demonstrated on industrially produced austempered ductile cast iron grade 2, heat-treated after sand casting, with casting simulation data used to select specimens of defined solidification modulus. The resulting virtual fatigue data show good agreement with constant amplitude tests. Moreover, it shows reproducibility for similar solidification and cooling conditions and, thus, is proven to be applicable for quality control. Therefore, the presented approach enables targeted, location-specific material characterization directly linked to casting parameters, supporting both component design optimization and quality control in foundry practice.