Stephan Schoenfelder | HTWK Leipzig (original) (raw)

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Papers by Stephan Schoenfelder

Scientific Reports

Threads of modern pedicle screws can vary greatly in design. It is difficult to assess which inte... more Threads of modern pedicle screws can vary greatly in design. It is difficult to assess which interplay of design features is particularly advantageous for screw anchorage. This study aims to increase the understanding of the anchorage behaviour between screw and cancellous bone. Pull-out tests of six pedicle screws in two sizes each were performed on three densities of biomechanical test material. More general screw characteristics were derived from the screw design and evaluated using the test data. Selected screws were tested on body donor material. Some screw characteristics, such as compacting, are well suited to compare the different thread designs of screws with tapered core. The combination of two characteristics, one representing bone compacting and one representing thread flank area, appears to be particularly advantageous for assessing anchorage behaviour. With an equation derived from these characteristics, the pull-out strength could be calculated very accurately (mean d...

The mechanical properties and the failure of silicon wafers are defined by surface defects, like ... more The mechanical properties and the failure of silicon wafers are defined by surface defects, like cracks. In the wire sawing process different wire types and particle sizes as well as the machine settings affects the quality of the wafer surface. In order to optimize wire sawing technologies the influence of the crystallographic orientation on the strength and crack distribution of silicon wafers is investigated in this work. By ball-on-ring-tests the fracture strength of different orientations was determined and compared to the crack distribution, which was investigated by bevel polished method. The results show that a rotation of the silicon crystal by 45° (<110> direction) leads to an increase of the crack density in the surface of the samples and a decrease in the fracture strength of the wafers in comparisons to the standard sawing orientation (<100> direction).

Current Directions in Biomedical Engineering

Introduction: We apply deep learning to emulate Finite Element simulations by exploiting neural n... more Introduction: We apply deep learning to emulate Finite Element simulations by exploiting neural networks as universal function approximators. A novel processing pipeline automates the incorporation of three-dimensional biomechanical simulation data into an image-based deep learning task where spatially resolved deformations are predicted using pixelwise regression. We show that a model trained in this way can quickly reproduce simulation results with reasonable preliminary performance. Methods: We trained a customized U-Net (32 input features) to predict displacement models based on the input force vectors. The pipeline was applied to a basic indentation process for an idealized reference geometry (puck) and a 3D liver model from the LiTS dataset. The generated 3D simulation data is normalized and projected onto 2D images using the midpoint circle algorithm such that x, y, and z coordinates are mapped to pixel values of RGB image channels. This way, 1500 simulations of the reference...

Medical & Biological Engineering & Computing, 2021

In biomechanics, large finite element models with macroscopic representation of several bones or ... more In biomechanics, large finite element models with macroscopic representation of several bones or joints are necessary to analyze implant failure mechanisms. In order to handle large simulation models of human bone, it is crucial to homogenize the trabecular structure regarding the mechanical behavior without losing information about the realistic material properties. Accordingly, morphology and fabric measurements of 60 vertebral cancellous bone samples from three osteoporotic lumbar spines were performed on the basis of X-ray microtomography (μCT) images to determine anisotropic elastic parameters as a function of bone density in the area of pedicle screw anchorage. The fabric tensor was mapped in cubic bone volumes by a 3D mean-intercept-length method. Fabric measurements resulted in a high degree of anisotropy (DA = 0.554). For the Young’s and shear moduli as a function of bone volume fraction (BV/TV, bone volume/total volume), an individually fit function was determined and high...

Unlike the efficiency of the solar cell, the mechanical strength is not a key parameter for solar... more Unlike the efficiency of the solar cell, the mechanical strength is not a key parameter for solar cell and module manufacture, yet. Though, it is known that the metallization reduces the mechanical strength after texturization. But no systematic comparison between different cell types was performed so far in literature. In this work, a mechanical benchmark test for 10 different solar cells from 6 different manufacturers with 1525 cells is presented. Significant differences in the probability of failure were found depending on the individual cell process. The presented test procedures are powerful tools to identify the most damaging process steps and to increase the yield during cell and module manufacturing.

Medical Engineering & Physics, 2022

Insertion and pull-out tests of synthetic test material are well established for the initial labo... more Insertion and pull-out tests of synthetic test material are well established for the initial laboratory evaluation of screws. However, not all test parameters are sufficiently described. The influence of small density deviations of the test material, of tapping or of manual or machine insertion has not been fully examined. The aim of the present study was to examine the influence of these specimen preparation parameters on the measurement results in order to increase the reproducibility and reliability of screw pull-out tests. For this purpose, a commercial polyurethane foam and a clinically used type of screw are evaluated with insertion and pull-out tests. Within a foam apparent density grade, small deviations in apparent density led to significant and relevant differences in the measured values of insertion torque and pull-out strength. Furthermore, an influence on the measurement results was found during tapping and during manual or machine insertion of screws. For these reasons, specimens with the same apparent density should be used as far as possible and evenly distributed among the test groups. In addition, the reproducibility of the results can be increased by machine insertion of the screws.

Lecture Notes in Mechanical Engineering, 2021

In this work, inclusions of silicon carbide and silicon nitride in multicrystalline silicon are i... more In this work, inclusions of silicon carbide and silicon nitride in multicrystalline silicon are investigated experimentally by photoelasticity and theoretically by numerical simulation in a finite-element model. Large tensile stresses were observed in experiment at the interface from silicon carbide to silicon while silicon nitride induced much lower stresses. The results of the finite-element model indicate that the stresses are induced while cooling during crystallization, by the mismatch of coefficients of thermal expansion.

Wear, 2020

Abstract Diamond wire sawing (DWS) is the dominant manufacturing process for thin silicon substra... more Abstract Diamond wire sawing (DWS) is the dominant manufacturing process for thin silicon substrates. The understanding of micromechanical damaging and material removal mechanisms is essential for improving the reliability of the processes and the quality of the products. Due to the lack of models that are capable to predict subsurface damage during diamond wire sawing of silicon, process analyses are so far only possible on the basis of models originally developed for loose abrasive sawing (LAS). However, the application of LAS models to the DWS process does not seem appropriate, because of the different underlying deformation mechanisms. Within this work a three-dimensional Finite-Element analysis is conducted in order to investigate the cracking mechanisms during the sawing process. Experimental as well as numerical studies concerning median and lateral cracking are carried out using a Vickers indenter scratching on silicon. The results show that the numerically predicted dimensions of damage zones are in good agreement with the experimental results and that the new model is suitable for the analysis of subsurface damage during diamond wire sawing of silicon.

Medical Engineering & Physics, 2020

Pedicle screw loosening is still observed in clinical practice. Therefore, the understanding of t... more Pedicle screw loosening is still observed in clinical practice. Therefore, the understanding of the interaction between screw and bone is evermore subject of research. However, complex relationships between screw design and anchorage are rarely examined. For this reason, it is investigated whether screw fixation is predictable by a parameter that maps the thread. Two types of pedicle screws, 6.5 mm and 7.5 mm in major diameter, respectively, are pulled out from synthetic polyurethane foam. Representative design parameters were evaluated with regard to their influence on the insertion torque and pull-out strength. A correlation between the insertion torque and the degree of compacting of the bone was observed. In addition, the pull-out strength was found to be related to the contact area of the bone-screw interface, the bone volume displaced during screwing, and the displaced diameter. The relationship between insertion torque and pull-out strength was also linear. In addition, a relational equation was derived for the pull-out strength versus the insertion torque with the major diameter as scaling factor. Thus, the pull-out strength from cancellous bone is predictable by a representative design parameter. Furthermore, the derived relation equation opens a new perspective for the evaluation of screw fixation.