Papers by Author: Andreas Öchsner

Abstract: In the present study, the extraordinary, i.e., not yet reproduced by anybody, data of Rodrigez, Baker, Gupta et al. on the “super” storage of hydrogen in activated graphite nanofibers (GNFs) were analyzed and interpreted in approximation of the first-order sorption processes. Our developed methodology was used for processing, analysis and interpretation of thermal desorption and thermogravimetric spectra of hydrogen in the GNF samples. The analysis has shown that there is a real possibility of reproducing of the modified extraordinary results, but only in the case of revealing the know-how technology of the activation treatment of the GNF samples.

Abstract: We present results of the constructive critical analysis and interpretation of some recent studies (Blavette, Sauvage, Wilde and others) at the atomic scale (using three-dimensional atom-probe field-ion microscopy) of impurity nanosegregation at dislocations, including “Cottrell atmospheres”, and grain boundaries in deformed intermetallics and metallic materials, and their relevance to mechanical properties and diffusion processes.

Abstract: In the current study, five cases of fiber distributions are considered in a fiber-reinforced composite: one random, three partitioned (one uniform and two biased cases), and one aligned case for benchmarking. The finite element method and the principal component analysis were used to interpret the results of orientation tensors and detect any possible clusterings of a representative volume element (RVE). The obtained effective conductivity values were extensively controlled by the fiber volume fraction. At the same time, the uniformity of the random distributions could be recognized. Cross-partition resistance was also detected for the partitioned cases which contributed to a reduced heat transfer capability. Finally, the clustering indexes did not show a direct correlation with the conductivity results, and thus a case-by-case investigation is recommended to consider the anisotropic aspects of a microstructure.

Abstract: Powder-bed based additive manufacturing techniques are of high interest for the medical sector and recent trial studies have shown their feasibility. Due to the rapid improvements made in the machinery and the related changes in the type and characteristics of the utilized power source, optimizations regarding the fabrication parameters tend to differ amongst various machines. In this study, a parameter optimization was undertaken for a biocompatible dental CoCrMo alloy on a SLM 280HL machine, featuring a 400 W fibre laser. It was shown that the availability of higher laser powers enables a more energy efficient fabrication. Moreover, parameter sets for fast and economic fabrication, as well as for high density and fine-grained microstructure, were defined.

Abstract: In the present research, imperfect graphene sheets were generated and their vibrational property was studied via finite element analysis. The effect of vacant sites in the arrangement of these nano-structures was examined. The fundamental frequency of the defect free and imperfect nano-sheets was acquired based on two different approaches. The first approach was a pure finite element simulation. The second approach for comparison purpose was a recently reported refined finite element simulation at which the vicinity of a defect was first evaluated according to the density functional theory (DFT) and then the refined geometry was implemented into a finite element model. The findings of this research show that the fundamental frequency of graphene sheets decreases by presenting microscopic imperfection to the formation of these nano-materials. In addition, it was pointed out that the geometry based on the more precise DFT simulations gives a higher decrease in the fundamental frequency of the sheets for all considered cases.

Abstract: Connected CNTs were simulated and the effect of impurities on the frequency of these specific nanostructures was analyzed. For this purpose, the three most likely microscopic imperfections, i.e., doping with Si atoms, vacancy and perturbations to the pure models were simulated. Lastly, the vibrational behavior of imperfect hybrids was evaluated and compared with the behavior of the pure ones. It was pointed out that deficiencies in the structure of hybrids reduces the frequency and as a result, lowers the vibrational stability of the CNTs.

Abstract: In the current study, two extreme cases are considered for the dispersion of carbon nanotubes(CNTs) in a polymeric matrix: randomly-oriented and randomly-aligned. The representative volume element (RVE) is used to represent the composite material consisting of epoxy resin matrix and CNT-reinforcement. The finite element method acts as the computational tool to conduct the simulations and investigate the effective parameters, i.e., the influence of the aspect ratio and the orientation, on the thermal conductivity of the matrix. A Fortran subroutine was used for both generation and analysis of the models by means of the MSC Marc finite element package and a Python script was used for the sensitivity analysis. The results indicate that optimum performance of the CNTs in terms of thermal conductivity can be reached by orienting them along the temperature gradient whereas a random distribution improves the conductivity by a smaller magnitude.

Abstract: Additive manufacturing processes offer the ability to manufacture highly complex geometries, but are limited in terms of the achievable surface quality. These limitations are based on physical restrictions, especially the need of support and the powder-bed environment, and economic decisions. In this study, the development of the morphology of surfaces with varying inclination angles was investigated on the example of 316L stainless steel. Surfaces with low inclination angles to the manufacturing plane suffered extensively from the process related staircase effect, whereas perpendicular side faces revealed high dependencies on the interaction with the powder-bed.

Abstract: In the current study, the representative volume element (RVE) is used to model randomly generated nanocomposite structures consisting of carbon nanotubes (CNTs) embedded in an epoxy resin matrix. The finite element Method is utilized for numerical simulations and investigation of the influential parameters on the generated RVEs. In order to automatize the whole procedure - fromgenerating the finite element models to conducting the analyses - a subroutine-based programming approach is adopted using the MSC Marc finite element package and Fortran programming language. The simulations can successfully predict the increase in thermal conductivity of CNT-reinforced nanocomposites by increasing the fiber volume fraction.

Abstract: Recent investigations revealed major fluctuations in the material properties of selective laser melted AlSi10Mg, which corresponded with the varying precipitation-hardening state of the microstructure, caused by the differing dwell times at elevated temperatures. It was indicated that a subsequent heat treatment balances the age-hardening and results in a homogenized material strength. In order to further investigate this statement selective laser melted AlSi10Mg samples were subject to multiple post-heat-treatments. Subsequently, the surface hardness and tensile strength was determined and compared with the as-built results. The post-heat-treatment led to an arbitrary occurrence of rupture, indicating a successful homogenization, coupled with a remarkable improvement in ductility, but to the costs of a lowered tensile strength, which was highly dependent on the chosen heat-treatment procedure.