http://hdl.handle.net/10993/89 Search this channel search https://orbilu.uni.lu/simple-search http://hdl.handle.net/10993/43890 Title: Laser joining of titanium alloy to polyamide: influence of process parameters on the joint strength and quality <br/> <br/>Author, co-author: Alsayyad, Adham Ayman Amin; Lama, Prashant; Bardon, Julien; Hirchenhahn, Pierre; Houssiau, Laurent; Plapper, Peter <br/> <br/>Abstract: Laser-assisted metal–polymer joining (LAMP) is a novel assembly process for the development ofminiaturized joints in hybrid lightweight products. This work adopts a design of experiments (DoE) approach to investigate the influence of several laser welding parameters on the strength and quality of titanium alloy (Ti-6Al-4V)–polyamide (PA6.6) assembly. Significant param- eters were highlighted using the Plackett Burmann design, and process window was outlined using the Response Surface Method (RSM). A statistically reliable mathematical model was generated to describe the relation between highlighted welding param- eters and joint strength. The analysis ofvariance (ANOVA) method was implemented to identify significant parametric interac- tions. Results show the prominence offocal position and laser power, as well as significant interaction between laser power and beam speed, on the joint strength. The evolution ofweld defects (bubbles, excessive penetration, flashing, titaniumcoloring, weld pool cavities, and welding-induced deflection) along the process windowwas investigated using optical microscopy. The resulted deflection in titaniumwas quantified, and its relationship with welding parameters was mathematically modeled. Robust process window was outlined to maintain insignificant deflection in the welded joints. Results showed that the growth ofweld defects correlates with a decline in joint strength. Optimal parameters demonstrated a defect-free joint, maximizing joint strength. http://hdl.handle.net/10993/43889 Title: Highlighting chemical bonding between nylon-6.6 and the native oxide from an aluminum sheet assembled by laser welding <br/> <br/>Author, co-author: Hirchenhahn,, Pierre; Alsayyad, Adham Ayman Amin; Bardon, Julien; Felten, Alexandre; Plapper, Peter; Houssiau, Laurent <br/> <br/>Abstract: Polymer/metal hybrid assemblies are well suited for automotive and biomedical applications because of their ability to create lightweight structures with a wide range of design possibilities. Laser welding is a promising technique for joining dissimilar materials thanks to its quickness, freedom of design and absence of adhesives. Still, the fundamental causes of adhesion in hybrid laser welding remain not well understood. Therefore the present work aims at highlighting a chemical bonding between a polymer, nylon-6.6 and a metal, aluminum. To access the interface information, the samples were first broken, leaving a residue on the surface, which was dissolved afterwards. The chemical reactive sites of nylon molecule able to react with aluminum surface were suggested and the feasibility of these reactions was analyzed in the light of the results obtained by means of X-ray photoelectron spectroscopy (XPS) and Time of flight secondary electron microscopy (ToF-SIMS). http://hdl.handle.net/10993/43469 Title: Finite deformations govern the anisotropic shear-induced area reduction of soft elastic contacts <br/> <br/>Author, co-author: Lengiewicz, Jakub; de Souza, Mariana; Lahmar, Mohamed A.; Courbon, Cédric; Dalmas, Davy; Stupkiewicz, Stanislaw; Scheibert, Julien <br/> <br/>Abstract: Solid contacts involving soft materials are important in mechanical engineering or biomechanics. Experimentally, such contacts have been shown to shrink significantly under shear, an effect which is usually explained using adhesion models. Here we show that quantitative agreement with recent high-load experiments can be obtained, with no adjustable parameter, using a non-adhesive model, provided that finite deformations are taken into account. Analysis of the model uncovers the basic mechanisms underlying anisotropic shear-induced area reduction, local contact lifting being the dominant one. We confirm experimentally the relevance of all those mechanisms, by tracking the shear-induced evolution of tracers inserted close to the surface of a smooth elastomer sphere in contact with a smooth glass plate. Our results suggest that finite deformations are an alternative to adhesion, when interpreting a variety of sheared contact experiments involving soft materials. http://hdl.handle.net/10993/43082 Title: Beam-inside-beam contact: Mechanical simulations of slender medical instruments inside the human body <br/> <br/>Author, co-author: Magliulo, Marco; Lengiewicz, Jakub; Zilian, Andreas; Beex, Lars <br/> <br/>Abstract: Background and Objective. This contribution presents a rapid computational framework to mechanically simulate the insertion of a slender medical instrument in a tubular structure such as an artery, the cochlea or another slender instrument. Methods. Beams are employed to rapidly simulate the mechanical behaviour of the medical instrument and the tubular structure. However, the framework’s novelty is its capability to handle the mechanical contact between an inner beam (representing the medical instrument) embedded in a hollow outer beam (representing the tubular structure). This "beam-insidebeam" contact framework, which forces two beams to remain embedded, is the first of its kind since existing contact frameworks for beams are "beam-to-beam" approaches, i.e. they repel beams from each other. Furthermore, we propose contact kinematics such that not only instruments and tubes with circular cross-sections can be considered, but also those with elliptical cross-sections. This provides flexibility for the optimization of patient-specific instruments. Results. The results demonstrate that the framework’s robustness is substantial, because only a few increments per simulation and a few iterations per increment are required, even though large deformations, large rotations and large curvature changes of both the instrument and tubular structure occur. The stability of the framework remains high even if the modulus of the inner tube is thousand times larger than that of the outer tube. A mesh convergence study furthermore exposes that a relatively small number of elements are required to accurately approach the reference solution. Conclusions. The framework’s high simulation speed originates from the exploitation of the rigidity of the beams’ cross-sections to quantify the exclusion between the inner and the hollow outer beam. This rigidity limits the accuracy of the framework at the same time, but this is unavoidable since simulation accuracy and simulation speed are two competing interests. Hence, the framework is particularly attractive if simulation speed is preferred over accuracy. http://hdl.handle.net/10993/42993 Title: Non-localized contact between beams with circular and elliptical cross-sections <br/> <br/>Author, co-author: Magliulo, Marco <br/> <br/>Abstract: Numerous materials and structures are aggregates of slender bodies. We can, for example, refer to struts in metal foams, yarns in textiles, fibers in muscles or steel wires in wire ropes. To predict the mechanical performance of these materials and structures, it is important to understand how the mechanical load is distributed between the different bodies. If one can predict which slender body is the most likely to fail, changes in the design could be made to enhance its performance. As the aggregates of slender bodies are highly complex, simulations are required to numerically compute their mechanical behaviour. The most widely employed computational framework is the Finite Element Method in which each slender body is modeled as a series of beam elements. On top of an accurate mechanical representation of the individual slender bodies, the contact between the slender bodies must often be accurately modeled. In the past couple of decades, contact between beam elements has received wide-spread attention. However, the focus was mainly directed towards beams with circular cross-sections, whereas elliptical cross-sections are also relevant for numerous applications. Only two works have considered contact between beams with elliptical cross-sections, but they are limited to point-wise contact, which restricts their applicability. In this Ph.D. thesis, different frameworks for beams with elliptical cross-sections are proposed in case a point-wise contact treatment is insufficient. The thesis also reports a framework for contact scenarios where a beam is embedded inside another beam, which is in contrast to conventional contact frameworks for beams in which penetrating beams are actively repelled from each other. Finally, two of the three contact frameworks are enhanced with frictional sliding, where friction not only occurs due to sliding in the beams’ longitudinal directions but also in the transversal directions. http://hdl.handle.net/10993/42977 Title: UNTERSUCHUNG DER METALLURGISCHEN PHASENBILDUNG UND DEREN EINFLUSS AUF DIE VERBINDUNGSEIGENSCHAFTEN SOWIE AUF DIE VERSAGENSURSACHEN VON LASERGESCHWEIßTEN HARTMETALL-STAHL-VERBUNDEN <br/> <br/>Author, co-author: Schiry, Marc <br/> <br/>Abstract: Laser beam welding of hard metal to steel offers multiple advantages regarding resource saving, mechanical strength of the joint and automation capability. The present work focuses on the fundamental research and development of the laser based welding process of tungsten carbide-cobalt hard metals with a tempering steel. Metallurgical analysis of the welding process showed that the formation of intermetallic and/or intermediate phases has a significant influence on the properties and mechanical strength of the dissimilar joint. The amount of molten hard metal in the steel melt bath plays a key role for the formation of the different phases. Therefore, a new parameter dy was defined, which correlates with the hard metal content in the melt pool. It is shown that for hard metals with 12 wt.% of cobalt binder, the phase transformation in the weld seam starts with a relative hard metal content of 10 vol.%. This threshold is dependent on the relative cobalt concentration in the hard metal. The tungsten carbide grain size has a low influence on the phase transformation in the weld seam. Steel melt pools with hard metal content lower than 10 vol.% show under metallographic observation a martensitic/bainitic microstructure. Simulation of the stress formation in the joint showed that due to the volume expansion of martensite during the transformation, tensile stress in the hard metal part was formed. Under shear load, these tensile stresses compensate with the induced compressive stresses and results an almost stress free interface. High shear strengths of the dissimilar joints are possible. A higher percentage of hard metal melting during the welding process increases the carbon and tungsten content in the melt bath. Consequently, the martensite start temperature decreases significantly. When the initiating temperature for martensite transformation falls under room temperature, the weld seam transforms into an austenitic microstructure. Because of the missing volume expansion during cooling of the weld seam volume, low stresses in the hard metal are generated. Under shear load of the joint area, high tensile stresses appear in the sintered part. These stress concentration decreases the shear strength of the weld and lead to premature failure. For the industrial use case, high mechanical strength and a robust manufacturing process is needed. Therefore, the laser welding process of hard metal to steel was optimized. The joint properties strongly depend on the weld bead geometry. Weld seams with x- or v-shaped profiles enable local concentrated metallurgical bonding of the sintered part to the steel sheet. Reduction of the horizontal focal distance of the laser beam to the interface increases the bonding ratio, but also intensifies the melting of the hard metal part and lead to the metallurgical transformation. By tilting a v-shape weld seam, it was possible to optimize the bonding behavior and to minimize the amount of liquefied hard metal in the melt bath. Hard metal with low amounts of binder showed a high temperature sensitivity. After laser welding of these grades, hot cracks were found in the sinter material. These cracks were formed due to the high stresses, which are generate during cooling of the dissimilar joint. Therefore, a laser based heat treatment process was developed and applied. With a defined pre- and post-heating of the joint area, the cooling rate was reduced significantly and the stresses in the hard metal part minimized. High shear strengths were the result. http://hdl.handle.net/10993/42821 Title: A new variant of the Burgers model describing the flow of uncured styrene-butadiene rubber <br/> <br/>Author, co-author: Řehoř, Martin; Gansen, Alex; Sill, Clemens; Polińska, Patrycja; Westermann, Stephan; Dheur, Jean; Baller, Jörg; Hale, Jack <br/> <br/>Abstract: Uncured styrene-butadiene rubber (SBR) can be modelled as a viscoelastic material with at least two different relaxation mechanisms. In this paper we compare the classical Burgers model with additional dissipation, a generalised Burgers model originally developed to describe the response of asphalt binders, and a newly derived Burgers model combining the strengths of the two existing models. We select the model that best fits the experimental data obtained from a modified stress relaxation experiment in the torsional configuration of the plate-plate rheometer. The optimisation of the five model parameters for each model is achieved by minimising the weighted least-squares distance between experimental observations and the computer model output using a tree-structured Parzen estimator algorithm to find an initial guess, followed by further optimisation using the Nelder-Mead simplex algorithm. The results show that our new model is the most suitable variant to describe the observed behavior of SBR in the given regime. The predictive capabilities of the three models are further examined in changed experimental and numerical configurations. Full data and code to produce the figures in this article are included as supplementary material. http://hdl.handle.net/10993/42616 Title: Metallographic Studies of Dissimilar Al-Cu Laser-Welded Joints Using Various Etchants <br/> <br/>Author, co-author: Schmalen, Pascal Guy; Mathivanan, Karthik; Plapper, Peter <br/> <br/>Abstract: The welding of Al and Cu is considered as difficult due to the formation of intermetallic compounds, which cause a brittle joint with increased electrical resistance. This paper investigates etching techniques that were used to contrast the intermetallic compounds for optical microscope analysis. A 0.5 mm AA-1050 sheet was welded to a 0.5 mm SF-Cu sheet in overlap configuration. The cross sections were etched by using 17 different reagents, including common Al-grade 2xxx etchants, Al-bronze etchants, and specific IMC etchants. A complete microstructural characterization, including the formation of intermetallic compounds, is presented. The experimental result showed that a clear distinction of metallurgic structures is possible, thus enabling a more detailed analysis of Al-Cu welds. It was found that etchants #09, #14, and #16 revealed best the four different intermetallic compounds θ-Al2Cu, η-AlCu, ζ-Al3Cu4, and γ-Al4Cu9. http://hdl.handle.net/10993/42549 Title: New opportunities by laser joining of dissimilar materials <br/> <br/>Author, co-author: Plapper, Peter <br/> <br/>Abstract: The presentation gives an overview about research projects at UL related to laser welding of dissimilar materials. Literature reference at the end enables the reader to dig deeper in case some specific projects are of interest. http://hdl.handle.net/10993/42365 Title: Systematic derivation of Generalized Langevin Equations for coarse-graining and bridge-scaling procedures <br/> <br/>Author, co-author: Meyer, Hugues <br/> <br/>Abstract: In many branches of physics, one must often deal with processes involving a huge number of degrees of freedom. Instead of describing the dynamics of each individual of them, one rather wants to characterize the process of interest via a small set of observ- ables that capture its main features of the process. Even if the microscopic dynamics can be resolved using Newton’s equations of motion, it quickly becomes a computation- ally very expensive calculation to make. It is however much more convenient to come up with a self-consistent equation of motion for the ’global’ observable of interest itself in order to reduce the complexity of the problem. The development of the Mori-Zwanzig formalism in the 1960’s allowed to systematically derive such equations for arbitrary observables in stationary processes. This framework, derived from first principles by means of projection operator techniques, proves the structure of what is now known as the Generalized Langevin Equation, i.e. a stochastic equation of motion which a priori exhibits memory effects in the form on non-localities in time. We propose to extend the formalism and its corollaries to a broad class of out-of- equilibrium processes. We show that the structure of the Generalized Langevin Equa- tion is overall robust but must be adapted to account for the non-stationary dynamics [1,2]. The function that controls memory effects the stochastic term are related through a relation that can be associated to fluctuation-dissipation theorems. This formalism is very convenient to study two-time auto-correlation functions for which we can write a self-consistent differential equation as well. We finally show a new method to evaluate the memory function from numerical or experimental data [3]. http://hdl.handle.net/10993/42364 Title: Equation-free multiscale modeling of metallic lattices with geometrical and material nonlinearity and variability <br/> <br/>Author, co-author: Chen, Li; Berke, Peter; Beex, Lars; Massart, Thierry; Bordas, Stéphane <br/> <br/>Abstract: An nonlinear equation-free concurrent multiscale numerical framework, being the generalization of the quasicontinuum method [2] is proposed in this contribution to model 3D metallic lattice structures. The proposed equation-free multiscale method (EFMM) relies on the use of fully-resolved domains (FRD) in which all of the details of the lattice micro-structure are captured, and of coarse-grained domains (CGD) in which a model reduction is performed by interpolation and summation steps. The particularity of the lattice geometry description is that cross section variations along the lattice struts (that are experimentally observed as a result of the manufacturing process) are explicitly represented by their discretization in several beam finite elements, both in the FRDs and CGDs. The interpolation step of the EFMM refers to a kinematic approximation of the lattice deformation within CGDs based on the movement of a reduced number of material points at the CGD corners. One of the originalities of this work is the consideration of a separate interpolation of each type of degrees of freedom within the CGDs, as a function of the connectivity of the lattice beam nodes (i.e. taking the location of different cross sections into account) and their kinematical pattern. This, together with accounting for plasticity, by the development and implementation of a 3D co-rotational beam finite element [1] with embedded plastic hinges [3], are unprecedented and original contributions. The EFMM is applied to metallic BCC lattices with various sizes and loading conditions. By comparing to direct numerical simulation (DNS), it is shown that both material and geometrical non-linearities can be captured at a fraction of the DNS cost (the computational time is reduced by 97.27% while introducing an error of only 3.76%). http://hdl.handle.net/10993/42362 Title: Adaptive equation-free multiscale modeling of metallic lattices with geometrical nonlinearity and variability <br/> <br/>Author, co-author: Chen, Li; Berke, Peter; Beex, Lars; Massart, Thierry; Bordas, Stéphane <br/> <br/>Abstract: An equation-free concurrent multiscale framework is proposed to model 3D metallic lattice structures. The proposed equation-free multiscale method (EFMM) is effectively a generalization of the quasicontinuum method [2] and relies on the use of fully-resolved domains (FRD) in which all details of the lattice micro-structure are captured, and of coarse-grained domains (CGD) in which a model reduction is performed by interpolation and summation steps. The particularity of the lattice geometrical description is that cross section variations along the lattice struts (caused by the manufacturing process) are explicitly represented by their discretization in several beam finite elements, both in the FRDs and CGDs. The interpolation step of the EFMM refers to a kinematic approximation of the lattice deformation within CGDs based on the displacement of a reduced number of material points. One of the originalities of this work is the consideration of a separate interpolation of each type of kinematic variables within the CGDs, as a function of the connectivity of the lattice beam nodes (i.e. taking the location of different cross sections into account) and their kinematical pattern. This, together with accounting for geometric nonlinearity, by the development and implementation of a 3D co-rotational beam finite element [1], are innovative contributions. Choosing the appropriate sizes of the FRDs and the CGDs for a lattice to be simulated is a trade-off because larger FRDs prevail the accuracy but compromise the efficiency while larger CGDs do the opposite. Since the required sizes of the FRDs and CGDs are generally not known a priori for specific applications, an adaptive coarse-graining strategy is developed. To be specific, the whole lattice is initially considered as a CGD. Two kinds of error indicator are proposed (e.g. the Zienkiewicz-Zhu error indicator [4, 3] and the error indicator based on the discrepancy of strain energy). The error indicator guides on: 1) introducing more material points and rearranging the interpolation for the CGDs; 2) changing the localization-prone parts of the lattice into FRDs. The adaptive EFMM is applied to metallic BCC lattices with various sizes and loading conditions. By comparing to the results of those of the direct numerical simulation (DNS), it is shown that geometrical non-linearities can be captured at a fraction of the DNS cost. http://hdl.handle.net/10993/42250 Title: PANAS- Project: Heat Transfer Model Development for Passive Safety Systems, <br/> <br/>Author, co-author: Leyer, Stephan; Hampel; Schuster, C; Lippmann, W; Walther, M; Kosowski, K http://hdl.handle.net/10993/42248 Title: Detailed Characterization of the Effect of Application of Commercially Available Surface Treatment Agents on Textile Wetting Behavior <br/> <br/>Author, co-author: Staudt, J; Leyer, Stephan; Duchowski, J.K http://hdl.handle.net/10993/41924 Title: Continuous solution of poroelastic problems using Artificial Neural Networks <br/> <br/>Author, co-author: Dehghani, Hamidreza; Zilian, Andreas http://hdl.handle.net/10993/41912 Title: Poroelastic material characterisation by means of Artificial Neural Network <br/> <br/>Author, co-author: Dehghani, Hamidreza; Zilian, Andreas <br/> <br/>Abstract: Poroelastic problems require multiscale and multiphysics techniques that are expensive and time-consuming, which result in either several simplifications or costly experimental tests. The latter motivates us to develop a more efficient approach to address more complex problems with an acceptable computational cost. In this manuscript, first, the necessary equations derived from Asymptotic homogenisation for poroelastic media are mentioned. Then, the variational formulation of the cell problems is carried out and solved by the open-source FE package FEniCS. This is followed by presenting the advantages and downsides of macroscale properties identification via asymptotic homogenisation and the application of Artificial Neural Network (ANN) to solve the issues stated as its downsides by means of bypassing the process of solving the cell problems. Finally, we study a practical example, namely, spatial dependent porosity (in macroscale) to demonstrate the feasibility of using the provided framework to include more details. Further applications, including growth and remodelling, are subjects of future articles. http://hdl.handle.net/10993/41886 Title: Adhesive and Reinforcing Properties of Soluble Cellulose: A Repulpable Adhesive for Wet and Dry Cellulosic Substrates <br/> <br/>Author, co-author: Ferreira, Elisa; Martin Lanzoni, Evandro; Costa, Carlos A. R.; Deneke, Christoph; Bernardes, Juliana S.; Galembeck, Fernando http://hdl.handle.net/10993/41880 Title: Separating the influence of electric charges in magnetic force microscopy images of inhomogeneous metal samples <br/> <br/>Author, co-author: Arenas, Mónica P.; Martin Lanzoni, Evandro; Pacheco, Clara J.; Costa, Carlos A. R.; Eckstein, Carlos B.; de Almeida, Luiz H.; Rebello, João M. A.; Deneke, Christoph F.; Pereira, Gabriela R. <br/> <br/>Abstract: In this study, we investigate artifacts arising from electric charges present in magnetic force microscopy images. Therefore, we use two austenitic steel samples with different microstructural conditions. Furthermore, we examine the influence of the surface preparation, like etching, in magnetic force images. Using Kelvin probe force microscopy we can quantify the charges present on the surface. Our results show that electrical charges give rise to a signature in the magnetic force microscopy, which is indistinguishable from a magnetic signal. Our results on two differently aged steel samples demonstrate that the magnetic force microscopy images need to be interpreted with care and must be corrected due to the influence of electrical charges present. We discuss three approaches, how to identify these artifacts – parallel acquisition of magnetic force and electric force images on the same position, sample surface preparation to decrease the presence of charges and inversion of the magnetic polarization in two succeeding measurement. http://hdl.handle.net/10993/41877 Title: Unraveling the Role of Sn Segregation in the Electronic Transport of Polycrystalline Hematite: Raising the Electronic Conductivity by Lowering the Grain-Boundary Blocking Effect <br/> <br/>Author, co-author: Soares, Mario R. S.; Costa, Carlos A. R.; Martin Lanzoni, Evandro; Bettini, Jefferson; Ramirez, Carlos A. O.; Souza, Flavio L.; Longo, Elson; Leite, Edson R. <br/> <br/>Abstract: This paper describes the role of SnO2 in the electronic transport of polycrystalline hematite (α-Fe2O3). The proper sintering process allows for freezing of a state of electronic defects, in which the electrical properties of hematite are controlled by the grain boundary and Sn segregation. Impedance spectroscopy and dc conductivity measurements show that current flows through preferential pathways associated with Sn segregation that occurs at the grain boundary, leading to a decrease in grain-boundary resistance. Atomic force microscopy and electric force microscopy measurements confirm the results of the impedance analysis. The identification of preferential grain boundaries for electrical conductivity may have a direct influence on the light-induced water-splitting performance of the hematite photoanode. http://hdl.handle.net/10993/41872 Title: Wearable binary cooperative polypyrrole nanofilms for chemical mapping on skin <br/> <br/>Author, co-author: Morais, Vitória Brito De; Corrêa, Cátia Crispilho; Martin Lanzoni, Evandro; Costa, Carlos Alberto Rodrigues; Bufon, Carlos César Bof; Santhiago, Murilo <br/> <br/>Abstract: Wearable polypyrrole nanofilm arrays have been developed to extend the natural capabilities of the skin. Chemical mapping of acids in different states of matter were accurately detected on skin using binary cooperative polypyrrole structures on cellulose-based substrates.