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Polymer bearings are increasingly used due to their simple mass production and good tribological properties. The current contribution presents a macroscopic study of the effects of surface roughness and normal force on the sliding and... more
Polymer bearings are increasingly used due to their simple mass production and good tribological properties. The current contribution presents a macroscopic study of the effects of surface roughness and normal force on the sliding and rolling behavior of POM-H rolls. The counter-face roughness Ra was varied from 0.01 to 0.5 µm and the normal force from 150 up to 350 N. A polynomial regression model was used to analyze the effects of the two parameters and their interaction. The experiments showed that the coefficient of sliding friction depends strongly on the roughness with a distinct minimum at around 0.1 µm and only slightly on the normal force. In contrast, the coefficient of rolling resistance increases strongly with higher normal forces. From 150 to 350 N the coefficient of rolling resistance doubles in value. The roughness has a minor influence in rolling. The difference between sliding and rolling expresses also in the opposite interaction effects of the roughness and normal...
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The concept of cohesive zones has received revived interest and the cohesive zone modeling (CZM) approach has emerged as a powerful analytical tool for nonlinear fracture processes. This model considers the relation between the traction... more
The concept of cohesive zones has received revived interest and the cohesive zone modeling (CZM) approach has emerged as a powerful analytical tool for nonlinear fracture processes. This model considers the relation between the traction and separation that are normal to the fracture surfaces, and the unphysical stress singularity at the crack tip in the traditional linear elastic fracture mechanics is removed. The need of determining cohesive parameters is crucial in the understanding of the critical and post-critical behaviour of the sandwich composites, as their weakest part is the interface between the joining materials. Modern numerical methods provide solutions of predicting the behaviour of the sandwich materials’ interface zone using the cohesive zone model. In this paper we calibrate the constitutive parameters of a cohesive zone model to be used in larger scale models of structures containing an epoxy-bonded interface. The calibration is performed by comparing the local fie...
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We are working out a method for simulating cerebral aneurysm and spinal column surgery using three-dimensional (3D) biomodels reconstructed from 3D rotational angiography (RA), computer tomography (CT) and magnetic resonance (MR) images.... more
We are working out a method for simulating cerebral aneurysm and spinal column surgery using three-dimensional (3D) biomodels reconstructed from 3D rotational angiography (RA), computer tomography (CT) and magnetic resonance (MR) images. Subject-specific models of cerebral aneurysms have been reconstructed by Mimics (Materialise, Leuven, B) from 2D image data RA or CT (Fig.1) to construct 3D models (Fig.2).
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The current paper is describing the implementation of a multiscale numerical model for prediction of stiffness and strength in braided composites. The model is validated by experimental testing of single-layer braided tubes under... more
The current paper is describing the implementation of a multiscale numerical model for prediction of stiffness and strength in braided composites. The model is validated by experimental testing of single-layer braided tubes under torsional loading utilising digital image correlation (DIC). For the numerical model the entire braided structure is modelled at yarn detail level, taking into account the yarn behaviour as well as individual yarn-to-yarn interactions by using cohesive contact definitions. By means of Hashin’s failure criteria and cohesive contact damage, failure of the yarns and failure of the yarn-to-yarn interface is being accounted for. Thereby the material failure behaviour can be predicted. For validation of the model, torsion tests of biaxially braided single-layer composite tubes were performed. The strain distribution at the specimen surface was studied using the DIC system ARAMIS in 3D mode.
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Carbon fiber sheet moulding compounds (CF-SMC) are a promising class of materials with the potential to replace aluminium and steel in many structural automotive applications. In this paper, we investigate the use of CF-SMC materials for... more
Carbon fiber sheet moulding compounds (CF-SMC) are a promising class of materials with the potential to replace aluminium and steel in many structural automotive applications. In this paper, we investigate the use of CF-SMC materials for the realization of a lightweight battery case for electric cars. A limiting factor for a wider structural adoption of CF-SMC has been a difficulty in modelling its mechanical behaviour with a computational effective methodology. In this paper, a novel simulation methodology has been developed, with the aim of enabling the use of FE methods based on shell elements. This is practical for the car industry since they can retain a good fidelity and can also represent damage phenomena. A hybrid material modelling approach has been implemented using phenomenological and simulation-based principles. Data from computer tomography scans were used for micro mechanical simulations to determine stiffness and failure behaviour of the material. Data from static th...
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In this research, the mechanical properties of epoxy composite filled with nano-silica particles with different crosslinking densities were experimentally studied to clarify the interaction effects between nano-particles and the network... more
In this research, the mechanical properties of epoxy composite filled with nano-silica particles with different crosslinking densities were experimentally studied to clarify the interaction effects between nano-particles and the network structure in matrix resins. The composite materials were prepared by adding 240-nm silica particles to the bisphenol A diglycidyl ether with a volume fraction of 0.2. The bending elastic moduli of the composites were dependent on only the volume fraction of the particles regardless of the particle size and network structures. Filling the nano-silica particles was clarified to improve the bending strength and fracture toughness of the composites with a fine network structure. However the particles acted as defects, reducing the mechanical properties of composites with rough network structures.
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The fracture mechanics of random discontinuous Carbon Fiber Sheet Molding Compound (C-SMC) materials compared to traditional carbon fiber composites are not well understood. An experimental study was carried out to characterize the... more
The fracture mechanics of random discontinuous Carbon Fiber Sheet Molding Compound (C-SMC) materials compared to traditional carbon fiber composites are not well understood. An experimental study was carried out to characterize the fracture behavior of such C-SMC materials. Mode I tests, using double cantilever beam specimens, and mode II tests, adopting the four-point bend, end-notched flexure configuration, were performed. Results show high variations in the forcedeflection responses and scatter in the fracture toughness properties GIc and GIIc, due to the complex mesostructure defined by random oriented carbon fiber chips. To investigate the influence of the mesostructure, tensile tests with varying specimen width and thickness were assessed by stochastic measures to find the representative specimen size.
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Polymeric components are increasingly used in safety relevant applications and are exposed to severe loading conditions. Hence, the reliable prediction of the deformation and failure behaviour of these components is getting more and more... more
Polymeric components are increasingly used in safety relevant applications and are exposed to severe loading conditions. Hence, the reliable prediction of the deformation and failure behaviour of these components is getting more and more important. Although, the loading rate dependence of the deformation and failure behaviour of polymeric parts is widely acknowledged, the selection of adequate material models and the determination of proper material parameters is a challenging task. As the quality of the simulation is highly influenced by the material model quality, the computational engineers would like to have the best data for these simulations. From practical, experimental technique point of view, however, there are a number of problems associated with high rate testing of polymers. The objective of this paper is the comparison of the loading rate dependent mechanical behaviour of various standardized specimen configurations in terms of true stress-strain curves s and the determ...
For this study Polydimethylsiloxane (PDMS) was selected as matrix-material. Iron particles of various shapes (aspect ratios) and surface treatments were added as fillers in various amounts. Isotropic specimens were prepared and subjected... more
For this study Polydimethylsiloxane (PDMS) was selected as matrix-material. Iron particles of various shapes (aspect ratios) and surface treatments were added as fillers in various amounts. Isotropic specimens were prepared and subjected to dynamic mechanical measurements in presence or absence of homogeneous magnetic fields. In order to achieve the necessary field homogenity in the region of the test specimen, novel set-ups were developed using magnetostatic simulation software and rapid prototyping tools.
The applicability of novel rapid prototyping methods and techniques for improving various stages of rapid product development is described in this study. In addition to the conventional prototyping, functional prototypes with various... more
The applicability of novel rapid prototyping methods and techniques for improving various stages of rapid product development is described in this study. In addition to the conventional prototyping, functional prototypes with various properties may be generated. Finite element simulations and novel experimental techniques are successfully used for improve the prototyping process both on a macroscopic and on a microscopic scale. The 2 component prototyping offers new options for 2k industrial component design and for biomodelling.
Based on lightweight design concepts, lattices are increasingly considered as internal structures. This work deals with the simulation of periodically constructed lattices to characterize their behaviour under different loadings... more
Based on lightweight design concepts, lattices are increasingly considered as internal structures. This work deals with the simulation of periodically constructed lattices to characterize their behaviour under different loadings considering various material models. A thermo-mechanical analysis was done, which is resulting in negative CLTE-values (Coefficient of Linear Thermal Expansion). Simulations with linear-elastic behaviour were evaluated regarding the tensile, compression and shear modulus and the Poisson’s ratio. Some of the investigated structures behave auxetic. Beside the linear elastic behaviour, also the hyper-elastic and visco-elastic behaviour of some structures were investigated. Furthermore, elasto-plastic simulations were performed where the applied loading was biaxial. As a result the initial yield surfaces were presented. The individual RVEs (Representative Volume Elements) can be utilized for different areas of application dependent on the used materials.
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Nowadays short fibre reinforced polymers are often used in load carrying structural parts. Compared to continuous fibre reinforced polymers they exhibit a more complex morphology. Hence the determination of the strength is a difficult but... more
Nowadays short fibre reinforced polymers are often used in load carrying structural parts. Compared to continuous fibre reinforced polymers they exhibit a more complex morphology. Hence the determination of the strength is a difficult but important task. Therefore this was the objective of this research. The strength of short fibre reinforced polymers was numerically determined for low-speed to high-speed strain rates for specimens with different fibre orientations. For the failure modelling the micromechanical approach “First pseudo grain failure” in Digimat was used. The parameters for the material and failure description were determined with the reverse engineering method. Integrated finite element simulations<br />were performed to validate the material and failure models by tensile and bending tests with different specimens. The comparison of the results of the experiments and simulations showed low deviation.
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Extracting material parameters from test specimens is very intensive in terms of cost and time, especially for viscoelastic material models, where the parameters are dependent of time (frequency), temperature and environmental conditions.... more
Extracting material parameters from test specimens is very intensive in terms of cost and time, especially for viscoelastic material models, where the parameters are dependent of time (frequency), temperature and environmental conditions. Therefore, three different methods for extracting these parameters were tested. Firstly, digital image correlation combined with virtual fields method, secondly, a parallel network material model and thirdly, finite element updating. These three methods are shown and the results are compared in terms of accuracy and experimental effort.
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The effect of joining by extrusion welding on the tensional stiffness and strength of a Polypropylene copolymer was analysed. Short-term and creep tests with laboratory specimens were conducted. Welded joint sub-components were simulated... more
The effect of joining by extrusion welding on the tensional stiffness and strength of a Polypropylene copolymer was analysed. Short-term and creep tests with laboratory specimens were conducted. Welded joint sub-components were simulated with the finite element method and the results were validated by experiments.
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The rapid and efficient development of soft active materials requires readily available, compact testing equipment. We propose a desktop-sized, cost-efficient, and open source radial stretching system as an alternative to commercially... more
The rapid and efficient development of soft active materials requires readily available, compact testing equipment. We propose a desktop-sized, cost-efficient, and open source radial stretching system as an alternative to commercially available biaxial and uniaxial stretching devices. It allows for doubling the diameter of an elastomer membrane while measuring the applied force. Our development enables significant cost reduction ( €) and increase the availability of equibiaxial deformation measurements for scientific material analysis. Construction plans, source code, and electronic circuit diagrams are freely available under a creative commons license.
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Joining experiments using different adhesives were carried out. In addition to the adhesive, the specimens were also treated with ultrasonic waves to improve the load carrying capacity of the joined parts. Lap joint shear tests have been... more
Joining experiments using different adhesives were carried out. In addition to the adhesive, the specimens were also treated with ultrasonic waves to improve the load carrying capacity of the joined parts. Lap joint shear tests have been conducted to quantify this improvement.
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Damage and fiber misalignment of woven fabrics during discontinuous polymer processing remain challenging. To overcome these obstacles, a promising switchable elastomeric adherence gripper is introduced here. The inherent surface... more
Damage and fiber misalignment of woven fabrics during discontinuous polymer processing remain challenging. To overcome these obstacles, a promising switchable elastomeric adherence gripper is introduced here. The inherent surface tackiness is utilized for picking and placing large sheets. Due to the elastomer’s viscoelastic material behavior, the surface properties depend on loading speed and temperature. Different peeling speeds result in different adherence strength of an interface between the gripper and the substrate. This feature was studied in a carefully designed experimental test set-up including dynamic thermomechanical, as well as dynamic mechanical compression analyses, and adherence tests. Special emphases were given to the analyses of the applicability as well as the limitation of the viscoelastic gripper and the empirically modeling of the gripper’s pulling speed-dependent adherence characteristic. Two formulations of poly(dimethylsiloxane) (PDMS) with different hardne...
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The paper describes the possibilities of additive manufacturing with multiphoton lithography. The basis of this technology is that a laser beam (with a certain wavelength) is fired into the mixture of a monomer and a photo-initiator. When... more
The paper describes the possibilities of additive manufacturing with multiphoton lithography. The basis of this technology is that a laser beam (with a certain wavelength) is fired into the mixture of a monomer and a photo-initiator. When the energy of the laser is high enough, the latter acts as a catalyser for the polymerization of the monomer compound. This study focuses on the influences of certain parameters of the multiphoton lithography process. One of the important aspects is the choice of the solvent for the post processing. In sequence to the solvent problem, the influence of the layer height is examined. Furthermore the limits and possibilities of the setup in use are investigated. As an example the differences in fabrication with the laser firing with "constant frequency" and "constant density" were subject of this investigation. The second goal of the study was to compare three different structures consisting of periodically repeating elements, scaled in s...
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Mouthsticks are quite an old kind of assistive technology (AT) but nevertheless they are up to now the Swiss army knives among AT. Unfortunately the popularity of mouthsticks massively decreased during the 1990s with the result that... more
Mouthsticks are quite an old kind of assistive technology (AT) but nevertheless they are up to now the Swiss army knives among AT. Unfortunately the popularity of mouthsticks massively decreased during the 1990s with the result that knowledge about how to produce good mouthsticks got lost and that there are hardly any adaptable mouthsticks available on the market. This paper discusses the development of a personalized mouthstick with the involvement of end users - people with severe physical disabilities - and occupational therapists as experts of everyday use. A participatory approach was chosen. The results of the analysis of a standardized questionnaire, group discussions and a collaborative workshop with IT-designers, polymer engineers, end users, occupational therapists and gender and diversity researchers are presented and discussed. This proved the necessity of the development of a personalized mouthstick.
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To increase the independence of people with reduced hand/arm functionality, a process to generate personalizable mouth sticks was developed based on the participatory design principle. In a web tool, anybody can choose the geometry and... more
To increase the independence of people with reduced hand/arm functionality, a process to generate personalizable mouth sticks was developed based on the participatory design principle. In a web tool, anybody can choose the geometry and the materials of their mouth piece, stick and tip. Manufacturing techniques (e.g. 3D printing) and materials used in the process are discussed and evaluated.
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The characterization of the loading rate dependence of the fracture behavior of polymers is of prime theoretical and practical interest for supporting demanding engineering applications. To gain more insight into the high rate fracture... more
The characterization of the loading rate dependence of the fracture behavior of polymers is of prime theoretical and practical interest for supporting demanding engineering applications. To gain more insight into the high rate fracture behavior of polymers, fracture tests were performed under tensile loading conditions up to 12 m/s loading rate using a neat model polymer (PVC grey) in this study. A conventional single actuator test set-up for compact tension C(T) specimens was developed based on the previous experience of the authors and implemented on a new high rate servohydraulic testing machine. In addition, a novel double action test set-up was developed by applying two twin actuators and implemented in a rigid horizontal test frame. The conventional load and force measurement was extended by instrumented test specimens and by a high speed optical strain analysis system for both set-ups. Force based fracture toughness values using the peak load values, KIcPL and displacement ba...