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In grinding, the crystal grain size of the workpiece material is relatively same range compared to the removal depth. This raises a question if an anisotropic material model, which considers the effect of the crystal grain size and orientations, would better predict the process forces when compared to an isotropic material model. Initially, a simple micro-indentation process is chosen to compare the two models. In this work, a crystal plasticity model and an isotropic Johnson-Cooke plasticity model are employed to simulate micro-identation of a twinning induced plasticity (TWIP) steel. The results of the two models are compared using the force-displacement curves from the micro-indentation experiments. In the future, the study will be extended to describe the material removal process during a single grit scratch test.
Finishing processes result in changes of near-surface morphology, which strongly influences the fatigue behavior of components. Especially, roller bearings show a high dependency of the lifetime on surface roughness and the residual stress state in the subsurface volume. To analyze the influence of different finishing processes on the near-surface morphology, including the residual stress state, roller bearing rings made of AISI 52100 are finished in this work using hard turning, rough grinding, and fine grinding. In addition, fatigue specimens made of AISI 52100 and finished by cryogenic hard turning are investigated. For each condition, the residual stresses are determined at different distances from the surface, showing pronounced compressive stresses for all conditions. While the ground roller bearing rings show highest compressive residual stresses at the surface, the hard turned bearing ring and the cryogenic hard turned fatigue specimens reveal maximum compressive stresses in the subsurface volume. Moreover, cyclic indentation tests (CITs) are conducted in the different subsurface volumes, showing a higher cyclic plasticity in relation to the respective initial state, which is assumed to be caused by finishing-induced compressive residual stresses. Thus, the presented results indicate a high potential of CITs to efficiently characterize the residual stress state.
Nuclear inelastic scattering of synchrotron radiation is used to study the changes induced by external tensile strain on the
phonon density of states (pDOS) of polycrystalline Fe samples. The data are interpreted with the help of dedicated atomistic
simulations. The longitudinal phonon peak at around 37 meV and also the second transverse peak at 27 meV are decreased
under strain. This is caused by the production of defects under strain. Also the thermodynamic properties of the pDOS demonstrate
a weakening of the force constants and of the mean phonon energy under strain. Remaining differences between
experiment and simulation are discussed.
Defects change the phonon spectrum and also the magnetic properties of bcc-Fe. Using molecular dynamics simulation, the influence of defects – vacancies, dislocations, and grain boundaries – on the phonon spectra and magnetic properties of bcc-Fe is determined. It is found that the main influence of defects consists in a decrease of the amplitude of the longitudinal peak, PL, at around 37 meV. While the change in phonon spectra shows only little dependence on the defect type, the quantitative decrease of PL is proportional to the defect concentration. Local magnetic moments can be determined from the local atomic volumes. Again, the changes in the magnetic moments of a defective crystal are linear in the defect concentrations. In addition, the change of the phonon density of states and the magnetic moments under homogeneous uniaxial strain are investigated.
Indentation into a metastable austenite may induce the phase transformation to the bcc phase. We study this process using
atomistic simulation. At temperatures low compared to the equilibrium transformation temperature, the indentation triggers the
transformation of the entire crystallite: after starting the transformation, it rapidly proceeds throughout the simulation crystallite.
The microstructure of the transformed sample is characterized by twinned grains. At higher temperatures, around the equilibrium
transformation temperature, the crystal transforms only locally, in the vicinity of the indent pit. In addition, the indenter
produces dislocation plasticity in the remaining austenite. At intermediate temperatures, the crystal continuously transforms
throughout the indentation process.
Ultrasonic processes such as ultrasonic welding or ultrasonic fatigue testing use power
ultrasound to stimulate materials with amplitudes in the range of 1–100 µm. The ultrasonic welding
process is sensitive to any changes in the system or even the environment that may result in lower
joint quality. The welding tools, so called sonotrodes, have to be accurately designed to endure high
mechanical and thermal loads while inducing a sufficient amount of welding energy into the joining
zone by oscillation with the Eigenfrequency of the whole system. Such sonotrodes are often made of
thermally treated metals where the heat treatment is accompanied by microstructural changes. During
ultrasonic stimulation, the material may further change its properties and microstructure due to cyclic
loading. Both are expected to be recognized and identified by loss coefficients. Therefore, the loss
coefficient was determined by modal analysis of rods and fatigue specimen made of different materials
to correlate microstructural changes to attenuation. The determined loss coefficients indicated
microstructural changes in all materials investigated, confirming results from previous investigations
that showed an increasing attenuation due to cyclic loading for AISI 347. For the sonotrode materials
Z-M4 PM and Ferrotitanit WFN, the loss coefficients decreased due to thermal treatments. Technically
most relevant, changes in elastic modulus due to thermal treatments were quantitatively related to
frequency changes, which can significantly simplify future sonotrode development.
In this study, the dependence of the cyclic deformation behavior on the surface morphology of metastable austenitic HSD® 600 TWinning Induced Plasticity (TWIP) steel was investigated. This steel—with the alloying concept Mn-Al-Si—shows a fully austenitic microstructure with deformation-induced twinning at ambient temperature. Four different surface morphologies were analyzed: as-received with a so-called rolling skin, after up milling, after down milling, and a reference morphology achieved by polishing. The morphologies were characterized by X-Ray Diffraction (XRD), Focused Ion Beam (FIB), Scanning Electron Microscopy (SEM) as well as confocal microscopy methods and show significant differences in initial residual stresses, phase fractions, topographies and microstructures. For specimens with all variants of the morphologies, fatigue tests were performed in the Low Cycle Fatigue (LCF) and High Cycle Fatigue (HCF) regime to characterize the cyclic deformation behavior and fatigue life. Moreover, this study focused on the frequency-dependent self-heating of the specimens caused by cyclic plasticity in the HCF regime. The results show that both surface morphology and specimen temperature have a significant influence on the cyclic deformation behavior of HSD® 600 TWIP steel in the HCF regime.
Metastable austenitic CrNi steels undergo phase transformation when loaded or deformed plastically. In the current work a macroscopic and phenomenological constitutive model is presented to model the strain induced transformation of austenite to martensite. The approach is based on the previous works of Olsen and Cohen [1] & Stringfellow et al. [2]. The kinetics of the phase transformation is modelled based on the assumption that the intersections of the shear bands in the austenitic phase, act as potential martensite nucleation locations. Evolution of the shear band density and their intersections are modelled using the plastic strain in the austenitic phase. The probability of the intersection creating martensite is given by a Gaussian cumulative distribution, which in turn depends on the temperature and stress triaxiality. The resulting stress- strain behavior considers the volume fraction, plastic strains and the strain hardening parameters of the individual phases as internal variables. An explicit formulation of the material model is implemented as a user subroutine in a bi-linear element formulation of FEM. Some of the required material parameters are estimated by fitting experimental stress-strain and martensite volume evolution curves. For the purpose of illustrating the model's behavior, boundary value problems of components with structured surfaces are presented.
In this paper, the effect of shot peening and cryogenic turning on the surface morphologyof the metastable austenitic stainless steel AISI 347 was investigated. In the shot peeningprocess, the coverage and the Almen intensity, which is related to the kinetic energy of thebeads, were varied. During cryogenic turning, the feed rate and the cutting edge radiuswere varied. The manufactured workpieces were characterized by X-ray diffractionregarding the phase fractions, the residual stresses and the full width at half maximum.The microhardness in the hardened surface layer was measured to compare the hardeningeffect of the processes. Furthermore, the surface topography was also characterized. Thenovelty of the research is the direct comparison of the two methods with identical work-pieces (same batch) and identical analytics. It was found that shot peening generally leadsto a more pronounced surface layer hardening, while cryogenic turning allows the hard-ening to be realized in a shorter process chain and also leads to a better surface topog-raphy. For both hardening processes it was demonstrated how the surface morphology canbe modified by adjusting the process parameter.
Cyclic indentation is a technique used to characterize materials by indenting repeatedly on the same location. This technique allows information to be obtained on how the plastic material response changes under repeated loading. We explore the processes underlying this technique using a combined experimental and simulative approach. We focus on the loading–unloading hysteresis and the dependence of the hysteresis width ha,p on the cycle number. In both approaches, we obtain a power-law demonstrating ha,p with respect to the hardening exponent e. A detailed analysis of the atomistic simulation results shows that changes in the dislocation network under repeated indentation are responsible for this behavior.