LEM Group

Geneva, Switzerland

LEM Group

Geneva, Switzerland
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Mukherjee K.,Technical University of Denmark | Faester S.,Technical University of Denmark | Hansen N.,Technical University of Denmark | Dahl A.B.,Technical University of Denmark | And 3 more authors.
Materials Characterization | Year: 2017

Thick-walled ductile iron casts have been studied by applying (i) cooling rate calculations by FVM, (ii) microstructural characterization by 2D SEM and 3D X-ray tomography techniques and (iii) fatigue testing of samples drawn from components cast in sand molds and metal molds. An analysis has shown correlations between cooling rate, structure and fatigue strengths demonstrating the benefit of 3D structural characterization to identify possible causes of premature fatigue failure of ductile cast iron. © 2017 Elsevier Inc.

Madsen S.B.,University of Aalborg | Ibsen C.H.,LEM Group | Gervang B.,University of Aalborg | Kristensen A.S.,University of Aalborg
American Society of Mechanical Engineers, Pressure Vessels and Piping Division (Publication) PVP | Year: 2015

The focus of this paper is the validation and comparison of simplified numerical models of the mechanical rolling process used in tube to tubesheet joints. The investigated models is an axisymmetric model and planar models with plane strain and stress. There are different pros and cons for the different simplified models and therefore they are compared to an experiment where all geometries are measured before and after expansion. This comparison gives an insight into when the different models are valid. The models investigated is an axisymmetric model, a simple 2D expansion model with both plane strain and stress assumptions. Therefore, it is desirable to investigate how close these simplified models can predict the geometry changes after expansion measured in the experiment. The conclusion of the paper is that a planer model with plane strain is the best model at predicting the actual deformation after expansion. Copyright © 2015 by ASME.

Madsen S.B.,University of Aalborg | Gervang B.,University of Aarhus | Ibsen C.H.,LEM Group | Kristensen A.S.,University of Aalborg
ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE) | Year: 2015

The focus of this paper is a numerical investigation of tubesheet deformation in tube-tubesheet joints used in tube bank heat exchangers. To increase the thermal performance of a cooler the tubes can be moved closer together to increase turbulence and the heat transfer coefficient. Reducing the tube spacing is an alternative to using finned tubes to increase thermal performance. Finned tubes is more susceptible to fouling and therefore the small tube spacing can be an attractive alternative in some cases. From a manufacturing point of view, there are some problems with small tube spacing. When the tube spacing is reduced the deformation of the tube sheet increases. The relation between the tube spacing and tubesheet deformation is of great interest to the thermal designers of heat exchangers since it sets a limit for the tube spacing. The limit depends on how much tube sheet deformation is acceptable in the design. In most cases, the tubes are joined to the tube sheet by an expansive cold forming process. The expansion process that is investigated in this paper is the mechanical rolling expansion process. The tube is plastically deformed by a series of hardened steel rollers that roll across the inner tube surface while being pushed outwards. This results in a residual contact pressure between the tube and tube sheet after relaxation/springback. The expansion process is modeled using finite elements, the model used is a 2D planestrain model that is capable of capturing the effect of adjacent holes. This approach is new compared to previous research done in the field of mechanical roll expansion where the most commonly used model is an axisymmetric model. It is however not possible to investigate tube sheet deformation versus tube spacing with the axisymmetric model since it doesn't include the effect of adjacent holes. The material used in the analysis is a typical stainless steel, the material is modeled using a bilinear isotropic hardening model. At some point, the deformation of the tube sheet will exponentially increase with reduced tube spacing. This effect can only be investigated using numerical tools. Too much tube sheet deformation is not desirable since it will cause the tube sheet to permanently change shape and in some cases, adjacent tube-tubesheet joints will be compromised due to the nearby expansion. The results of this paper is the relation between the tube spacing and the amount of tubesheet deformation for a given amount of apparent tube wall reduction. This relation is used to set up limits for the amount of expansion and the minimum tube spacing for the given material combination. Copyright © 2015 by ASME.

Lupker M.,French National Center for Scientific Research | France-Lanord C.,French National Center for Scientific Research | Lave J.,French National Center for Scientific Research | Bouchez J.,IPG Inc | And 6 more authors.
Journal of Geophysical Research: Earth Surface | Year: 2011

The Ganga River is one of the main conveyors of sediments produced by Himalayan erosion. Determining the flux of elements transported through the system is essential to understand the dynamics of the basin. This is hampered by the chemical heterogeneity of sediments observed both in the water column and under variable hydrodynamic conditions. Using Acoustic Doppler Current Profiler (ADCP) acquisitions with sediment depth profile sampling of the Ganga in Bangladesh we build a simple model to derive the annual flux and grain size distributions of the sediments. The model shows that ca. 390 (30) Mt of sediments are transported on average each year through the Ganga at Haring Bridge (Bangladesh). Modeled average sediment grain size parameters D 50 and D84 are 27 (±4) and 123 (±9) μm, respectively. Grain size parameters are used to infer average chemical compositions of the sediments owing to a strong grain size chemical composition relation. The integrated sediment flux is characterized by low Al/Si and Fe/Si ratios that are close to those inferred for the Himalayan crust. This implies that only limited sequestration occurs in the Gangetic floodplain. The stored sediment flux is estimated to c.a. 10% of the initial Himalayan sediment flux by geochemical mass balance. The associated, globally averaged sedimentation rates in the floodplain are found to be ca. 0.08 mm/yr and yield average Himalayan erosion rate of ca. 0.9 mm/yr. This study stresses the need to carefully address the average composition of river sediments before solving large-scale geochemical budgets. Copyright 2011 by the American Geophysical Union.

Amaral P.,University of Évora | Correia A.,University of Évora | Lopes L.,University of Évora | Rebola P.,LEM Group | And 3 more authors.
Key Engineering Materials | Year: 2013

The use of dimension stones in architecture and civil engineering implies the knowledge of several mechanical, physical, and chemical properties. Even though it has been usual practice to measure physical and mechanical properties of dimension stones the same is not true for thermal properties such as thermal conductivity, thermal diffusivity, specific heat capacity, and heat production. These properties are particularly important when processes related with heating and cooling of buildings must be considered. Thermal conductivity, thermal diffusivity, and specific heat capacity are related with the way thermal energy is transmitted and accumulated in stones; heat production has to do with the amount of radioactive elements in the rocks and so with the environmental impact of radioactivity and public health problems. It is important to start to measure on a routine basis those four thermal properties in rocks and, in particular, in dimension rocks so that their application can be improved and optimized. With this is mind three sets of different rock types (granites, limestones, and marbles) were collected to measure the thermal conductivity, the thermal diffusivity, and the specific heat capacity with the objective of characterizing them in terms of those properties. Since the same set of rocks has also been studied for other physical properties, a correlation amongst all the measured properties is attempted. For each rock type several samples were used to measure the thermal conductivity, the thermal diffusivity, and the specific heat capacity, and average values were obtained and are presented. As an example, for granites the thermal conductivity varies between 2.87 and 3.75 W/mK; for limestones varies between 2.82 and 3.17 W/mK; and for marbles varies between 2.86 and 3.02 W/mK. It is hoped that measuring thermal properties on dimension stones will help to better adequate them to their use in civil engineering as well as to adequate their use in terms of a CE product. © (2013) Trans Tech Publications.

Haydar C.M.,Lebanese University | Haydar C.M.,LEM Group | Nehme N.,Lebanese University | Nehme N.,LEM Group | And 8 more authors.
Physics Procedia | Year: 2014

Water pollution in the Lake of Qaraaoun became a common criterion and it has been exaggerated in the last few decades as a result of population growth and the changing climatic conditions. The Qaraaoun Lake is the largest artificial reservoir in Lebanon, which is located along the Litani River. It represents the connecting point between the Upper and Lower Litani River Basins. The study aimed at assessing the levels of heavy metals concentration of and their sources in the sediment of the Qaraaoun Lake at three representative sites during dry season of the year 2012. © 2014 Elsevier B.V.

Teppan W.,LEM Group | Schlafli D.,LEM Group
PCIM Europe Conference Proceedings | Year: 2012

A high temperature current transducer with enhanced rejection of external magnetic fields is presented. Working principle, design, and application (aircraft actuators) are explained. The main features are small size, enhanced rejection of external magnetic fields and - most important for the application - an operating temperature in the range between -65 °C and 225 °C. Results of FEM simulations are compared with measurement results. The performance at static and dynamic operating conditions over the temperature range is presented. The research lead in this project has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement ACP8-GA-2009-243119 "CREAM". © VDE Verlag GMBH - Berlin.

Zorlu O.,Middle East Technical University | Zorlu O.,Ecole Polytechnique Federale de Lausanne | Kejik P.,Ecole Polytechnique Federale de Lausanne | Teppan W.,LEM Group
Sensors and Actuators, A: Physical | Year: 2010

In this paper, we present a new microfabricated fluxgate sensor structure using cascaded planar rings as the ferromagnetic core. The planar ring structures provide closed magnetic excitation loops enabling uniform core saturation with relatively small excitation magnetic field. The magnetic excitation is provided with a rod passing through the ring cores. Planar coils placed under the edges of the core are used as sensing elements. By using this structure, fluxgate sensors having a closed core configuration are realized with a developed microfabrication process in a very small area (<0.1 mm 2) with reduced number of via connections. The ferromagnetic rings are realized with a standard FeNi (iron-nickel) electroplating process. The main advantage of this structure is the possibility to arrange the linear operation range of the sensor by only changing the number of ring cores, without affecting the excitation mechanism. This is demonstrated by simulations and microfabricated prototypes having 18 and 12 ring cores with ±300 μT and ±550 μT linear operation ranges, respectively. A maximum linear operation range of ±2 mT is achieved with a larger size, 4-ring core. This is the widest linear operation range achieved with the microfabricated fluxgate type sensors, without using a feedback loop, to the best of our knowledge. © 2010 Elsevier B.V. All rights reserved.

Molinari A.,LEM Group | Molinari A.,Charles III University of Madrid | Cheriguene R.,LEM Group | Cheriguene R.,Charles III University of Madrid | Miguelez H.,Charles III University of Madrid
International Journal of Solids and Structures | Year: 2012

This paper is aimed at providing a comprehensive analysis of the contact problem in orthogonal cutting based on the simple assumption that contact is governed by a Coulomb law. Effects of the sliding friction coefficient and of the cutting conditions are analyzed in details. The problem is analyzed numerically by using an Arbitrary Lagrangian Eulerian Finite Element technique. In parallel, analytical models are developed allowing us to interpret the numerical data and to make them more meaningful. Distributions along the tool-chip interface are analyzed for stresses, temperature and sliding velocities. The shear stress exerted along the sticking zone is found to be equal to the shear flow stress of the work-material. Of particular significance is the investigation of the interface heating as the chip temperature appears to be a key factor governing the contact regime. The increasing of the chip temperature along the tool rake face appears to be mainly controlled by the mean value of the shear stress on the rake face and the Péclet number taking into account the phenomena of advection and heat diffusion. At low values of the friction coefficient the contact is governed by chip-tool sliding for the whole range of cutting speeds considered here (1ms -1≤V≤50ms -1). For larger values of the sliding friction coefficient, a transition to a contact dominated by sticking is found when the cutting speed is increased. Then, contact variables appear to be mostly determined by the value of the flow stress of the work-material with a negligible effect of the sliding friction coefficient. Thermal softening of the flow stress of the work-material governs the relationship between cutting variables and cutting conditions. An asymptotic regime occurs at relatively high cutting velocities (larger than 10 m/s) and for values of the sliding friction coefficient larger than 0.4. The analysis of the effects of cutting conditions on the morphology of the secondary shear zone reveals the existence of a boundary layer regime in the range of high cutting velocities. This is in keeping with the occurrence of the asymptotic regime mentioned above. Despite the simplicity of the contact model used, reasonable agreement is obtained with respect to experimental trends. The interplay between numerical and analytical approaches happens to be fruitful for understanding which are the main parameters influencing the contact variables, for checking the consistency of the numerical approach and for offering a route for future improvements of machining models. © 2012 Elsevier Ltd. All rights reserved.

Diab W.,Lebanese University | Diab W.,LEM Group | Toufaily J.,Lebanese University | Villieras F.,LEM Group | And 4 more authors.
Physics Procedia | Year: 2014

In this paper, we were interested in studying some of interesting physicochemical properties of colloidal sediments of Litani River in Lebanon. More than 15 geographical sites were selected in different zones composing the Litani River from Baalbek region located in the north-east of Lebanon to Kasmeyeh in the south of Lebanon where the River meets the Mediterranean sea. The effect of the seasonal variations in the sediment quality and the relationship of this variability with human activities were also studied including the physical and chemical properties of sediments of Litani river in Lebanon, and especially the zeta potential of the sediments selected from the different sites on the Litani River. © 2014 Elsevier B.V.

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