277 research outputs found
LCF assessment on heat shield components of nuclear fusion experiment “Wendelstein 7-X” by critical plane criteria
No full textThe Wendelstein 7-X modular advanced stellarator has started operations at the Max Planck Institute for Plasma Physics in
Greifswald, Germany, in 2016. In the first phase, the machine operated restricting the plasma pulses to low power and short
lengths. Plans to achieve actively cooled components are scheduled to start in 2020 when the machine will operate in steady-state
at full power. FEM simulations for steady-state operations revealed high plastic strains at several locations, for most of all the
rigidly supported Plasma Facing Components; therefore, there is the risk of a premature fatigue failure before the end of the
scheduled operations of the machine. The aim of this study is to analyse, by means of the commercial code ABAQUS, the
behavior of such critical components estimating, eventually, their fatigue life by means of the commercial code fe-safe
Nonlinear fatigue crack propagation in a baffle module of Wendelstein 7‐X under cyclic bending loads
Get PDFSimulation of the fatigue crack propagation in a Wendelstein 7‐X baffle module is performed in this study using both a finite element method‐based software and the UniGrow nonlinear model for small‐scale yielding (SSY) conditions. Some experimental fatigue tests of several cracked baffle modules have been performed through a servo‐hydraulic machine. One of these experimental tests
has been considered to simulate fatigue crack propagation in the baffle module. Before starting the experimental test, a first crack partly contained in the welding seam and partly in the steel pipe is found. Subsequently, owing to the applied load, the crack propagated both into the welding seam and into the steel pipe until the plastic zone in the near field attains SSY conditions.
Finally, owing to the increase in the extension of the plastic zone, SSY conditions are not more valid, and the breakage of the steel pipe is produced by plastic collapse
Shear and Anchorage Behaviour of Fire Exposed Hollow Core Slabs
No full textHollow core (HC) slabs are made of precast concrete with pretensioned strands. These slabs are popular as floor structures in offices and housing. At ambient conditions, the load bearing capacity can be dominated by four different failure modes, i.e. flexure, anchorage, shear compression and shear tension. As the economic production process does not allow for the inclusion of mild reinforcement, the slabs rely on the tensile strength of concrete for the shear and anchorage capacity. When exposed to a fire, the HC slabs have to maintain their load bearing and separating function for a certain time in order to facilitate the fire fighting actions and to provide sufficient time for the users of the building to escape and for rescue teams to search the building. Current design codes consider only flexural failure, while fire tests carried out in the past showed that the other failure modes can dominate the fire behaviour as well. As a result, design codes might overestimate the actual performance of fire exposed HC slabs. However, the experiments might represent a worst case compared to the practice. At least, fatalities caused by a premature collapse of fire exposed HC slabs, have never been reported up to the author's knowledge. Because there is a lack of fundamental understanding of the shear and anchorage behaviour, an optimum design between safety and economics can yet not be achieved. The objective of the research presented in this thesis is to gain a basic understanding of the shear and anchorage behaviour of fire exposed HC slabs and to develop FE models to predict this behaviour. With the models, design measures to improve the behaviour can be evaluated. The field of application is limited to HC slabs in accordance with the European product standard prEN 1168 [1197], exposed to standard fire conditions and simply supported on rigid supports like walls. The results are on the safe side for HC slabs with restraining support conditions.Civil Engineering and Geoscience
FEM-DBEM approach to analyse crack scenarios in a baffle cooling pipe undergoing heat flux from the plasma
No full textWendelstein 7-X is the world’s largest nuclear fusion experiment of stellarator type, in which a hydrogen plasma is confined by a magnet field generated with external superconducting coils, allowing the plasma to be heated up to the fusion temperature. The water-cooled Plasma Facing Components (PFC) protect the Plasma Vessel (PV) against radiative and convective heat from the plasma. After the assembly process of heat shields and baffles, several cracks were found in the braze and cooling pipes. Due to heat load cycles occurring during each Operational Phase (OP), thermal stresses are generated in the heat sinks, braze root and cooling pipes, capable to drive fatigue crack-growth and, possibly, a water leak through the pipe thickness. The aim of this study is to assess the most dangerous initial crack configurations in one of the most critical baffles by using numerical models based on a FEM-DBEM approach
Enzyme activities in liver and muscle biopsy specimens from thyrotoxic and hypothyroid patiens
Get PDFData files for ab initio calculations of the lattice parameter and elastic stiffness coefficients of bcc Fe with solutes
No full textAbstractWe present computed datasets on changes in the lattice parameter and elastic stiffness coefficients of bcc Fe due to substitutional Al, B, Cu, Mn, and Si solutes, and octahedral interstitial C and N solutes. The data is calculated using the methodology based on density functional theory (DFT) presented in Ref. (M.R. Fellinger, L.G. Hector Jr., D.R. Trinkle, 2017) [1]. All the DFT calculations were performed using the Vienna Ab initio Simulations Package (VASP) (G. Kresse, J. Furthmüller, 1996) [2]. The data is stored in the NIST dSpace repository (http://hdl.handle.net/11256/671)
Crack modelling in baffle modules of nuclear fusion experiment "wendelstein 7-X"
No full textStructural analyses are performed on subassemblies of so-called baffle modules, critical plasma facing components of the "Wendelstein 7-X" stellarator, where the presence of detected large cracks can drastically reduce the expected fatigue life. Such baffle modules are designed to remove heat coming from the hot plasma, heated up to nuclear fusion temperatures, and are made of graphite tiles bolted onto a structure of CuCrZr heat sinks that, in turn, are brazed on water-cooled stainless steel pipes. High plastic deformations occurred during the shaping process of the pipes were responsible for crack initiation at the roots of the braze. Such cracks, if propagating under fatigue thermomechanical loads, could penetrate into the cooling pipes and cause leakage of the coolant fluid, with consequent damages and interruption of operations. In this work, the geometry of one of these cracks has been reconstructed by CT scans and analysed using the Dual Boundary Element (DBEM) method, in order to forecast its potential impact on the component life
Coupled FEM–DBEM approach on multiple crack growth in cryogenic magnet system of nuclear fusion experiment ‘Wendelstein 7-X’
No full textAt the Max Planck Institute for plasma physics in Greifswald, Germany, the world's largest nuclear fusion experiment of modular stellarator type Wendelstein 7-X has started plasma operation. The hot hydrogen plasma is confined in a plasma vessel by an electromagnetic field generated by 50 non-planar and 20 planar superconducting coils. The superconducting coils are encased in cast stainless steel housings. The coils are bolted onto a central support ring and welded together by so called lateral support elements (LSEs). In this paper, a procedure, based on a global–local finite element method (FEM)–dual boundary element method (DBEM) approach, is developed to simulate the propagation of multiple cracks detected in LSEs and undergoing a fatigue load spectrum. The global stress analysis on the superconducting coils is performed by FEM whereas the sub-modelling approach is adopted to solve the crack propagation in the DBEM environment. The boundary conditions applied on the DBEM submodel are the displacements calculated by the FEM global analysis, in correspondence of the cut surfaces (there are no body forces nor external loads applied on the submodel volume). Two cracks are simultaneously introduced, and a linear elastic fracture mechanics analysis is performed. Results in terms of cracks growth rates and evolving crack shapes are provided, and the residual life of the component is forecast
Multi-scale lumped modeling of micro-channels cooling structure for W7-X divertor unit target module
Get PDFA novel concept for the cooling system of the W7-X divertor unit target module has been proposed, which in-
volves tiles equipped with parallel arrays connected by hundreds of sub-millimeter rectangular micro-channels
(MCs), obtained using Additive Manufacturing techniques. The structural material proposed for the heat sink
substrate is galvanized copper, while the plasma facing material is tungsten. To reduce the high computational
cost of thermal-hydraulic simulations of the tiles, a multi-scale lumped modeling approach has been built in
previous studies. This involves replacing a group of hydraulic parallel MCs with a porous strip (PS), suitably
calibrated to reproduce similar thermal-hydraulic behavior of the MCs. The aim of the current work is to verify
that the PS-model is also suitable for the thermo-mechanical stress evaluation, comparing the results from arrays
with MCs and PS. Since the heat sink and plasma-facing tiles are bonded, the interfacial delamination and shear
stresses are analyzed, being crucial for structural integrity. The analyses, carried out in the elastic regime, show
that the PS model returns conservative results when compared to the MCs model, with an overestimation of the
delamination and shear stresses at the free edge. Results from both models reveal nearly identical interfacial
stress predictions at the free surface edge. Moreover, it is found that both MCs and PS blocks, located near the
bond interface, contribute to high-stress fluctuations that could lead to the delamination of the bond interface,
suggesting that the distance of the microchannel from the interface should be increased. The PS model can
reliably be used to design and verify the most convenient series/parallel connection of the tiles in the divertor
unit target module, taking operational constraints into consideration
A multi-scale hybrid approach to the modelling and design of a novel micro-channel cooling structure for the W7X divertor
Get PDFThe second operating phase of the W7X stellarator, with an expanded set of plasma-facing components, includes the test of divertor tiles with a continuous heat load reaching 10 MW/ m2. The divertor tiles are cooled by subcooled water. Here a novel cooling concept, based on a network of parallel arrays of micro-channels (MC) with sub-millimetre dimensions, is investigated on a 0.1 m x 0.1 m tile, realizable by Additive Manufacturing. Detailed CFD simulations of the mock-up are performed to check the cooling uniformity using a multi-scale approach, aiming at limiting the dimension of the computational grid without a major loss of accuracy. First, the detailed hydraulic and thermal characterization on a sub-domain with of a small group of MC is performed. Then, the block of MC is substituted with an equivalent porous strip (PS), calibrating the hydraulic and thermal characteristics of the porous medium. The model is verified on an array of MCs or PSs connected to the same manifolds, showing the capability to reproduce the pressure drop and temperature increase with maximum errors of 1.05% and similar to 20% in nominal conditions, respectively. The numerical model of the entire tile equipped with PSs is then reliably adopted to evaluate the thermal-hydraulic performance of the cooling device
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