A Thermal Discrete Element Analysis of EU Solid Breeder Blanket subjected to Neutron Irradiation

Due to neutron irradiation, solid breeder blankets are subjected to complex thermo-mechanical conditions. Within one breeder unit, the ceramic breeder bed is composed of spherical-shaped lithium orthosilicate pebbles, and as a type of granular material, it exhibits strong coupling between temperature and stress fields. In this paper, we study these thermo-mechanical problems by developing a thermal discrete element method (Thermal-DEM). This proposed simulation tool models each individual ceramic pebble as one element and considers grain-scale thermo-mechanical interactions between elements. A small section of solid breeder pebble bed in HCPB is modelled using thousands of individual pebbles and subjected to volumetric heating profiles calculated from neutronics under ITER-relevant conditions. We consider heat transfer at the grain-scale between pebbles through both solid-to-solid contacts and the interstitial gas phase, and we calculate stresses arising from thermal expansion of pebbles. The overall effective conductivity of the bed depends on the resulting compressive stress state during the neutronic heating. The thermal-DEM method proposed in this study provides the access to the grain-scale information, which is beneficial for HCPB design and breeder material optimization, and a better understanding of overall thermo-mechanical responses of the breeder units under fusion-relevant conditions.Comment: 6 Pages, 3 Tables, 4 Figures, Fusion Science and Technology, 201

Buckling-driven self-formation of microchannels

Patroonvorming is alomtegenwoordig in de natuur: van menselijke huid tot bladeren, vruchten en groenten. Dergelijke patronen, met name netwerken van kanaaltjes, zijn ook interessant voor veel moderne technologische toepassingen, zoals flexibele elektronica, optische roosters en lab-on-a-chip systemen. De fabricage van deze structuren op zeer kleine schaal (orde nano- en micrometer) is echter niet eenvoudig. Ratna Kumar Annabattula verrichtte theoretisch onderzoek naar de vorming van micro- en nanokanaaltjes op basis van een recent ontwikkelde techniek, de ‘release and bondback’-methode. Deze methode bestaat uit drie stappen: (i) het groeien van een dunne film met voorspanning op een substraat (ii) het relaxeren van de film door het verminderen van de hechtsterkte (door chemisch etsen of elektrolyse) en tenslotte (iii) bondback, het terugvallen van de vrijgekomen film op het substraat als gevolg van de cohesiekracht tussen de film en het substraat. Annabattula ontwikkelde een nieuwe eindige-elementen-methode om het proces van delaminatie en bondback te bestuderen. De uiteindelijk gevormde kanaalafmetingen en structuren van verschillende systemen zijn onderzocht als functie van de verschillende systeemparameters, zoals laagdikte, laaggrootte, elastische eigenschappen en cohesie-energie tussen de dunne laag en het substraat. In het proefschrift is het gehele proces van kanaalvorming vastgelegd. Er zijn vier verschillende systemen onderzocht: twee- dimensionale lijnpatronen, lineaire kanalen, polygoon-vormige patronen en netwerken van nanokanaaltjes. De resultaten laten zeer goede overeenkomsten zien met experimentele kanaalstructuren, waardoor de resultaten gebruikt kunnen worden als designtool voor toekomstige kanaalsystemen

Phase field modelling of crack propagation in functionally graded materials

We present a phase field formulation for fracture in functionally graded materials (FGMs). The model builds upon homogenization theory and accounts for the spatial variation of elastic and fracture properties. Several paradigmatic case studies are addressed to demonstrate the potential of the proposed modelling framework. Specifically, we (i) gain insight into the crack growth resistance of FGMs by conducting numerical experiments over a wide range of material gradation profiles and orientations, (ii) accurately reproduce the crack trajectories observed in graded photodegradable copolymers and glass-filled epoxy FGMs, (iii) benchmark our predictions with results from alternative numerical methodologies, and (iv) model complex crack paths and failure in three dimensional functionally graded solids. The suitability of phase field fracture methods in capturing the crack deflections intrinsic to crack tip mode-mixity due to material gradients is demonstrated. Material gradient profiles that prevent unstable fracture and enhance crack growth resistance are identified: this provides the foundation for the design of fracture resistant FGMs. The finite element code developed can be downloaded from www.empaneda.com/codes

Photo-activated dynamic isomerization induced large density changes in liquid crystal polymers: A molecular dynamics study

Recent experimental results [Liu and Broer, Nat. Commun. 6 8334 (2015)] reveal that light-responsive azo-doped liquid crystal polymers under dual-wavelength illumination exhibit a significant reduction in density. This reduction in density was attributed to dynamic trans-cis-trans isomerization cycles. The light-induced isomerization kinetics suggest that the fraction of isomers undergoing dynamic isomerization increases with the light sources' intensity. However, experiments have shown that such an increase in intensity does not result in a monotonic decrease in density. Further, it was observed that there exists an optimal combination of the intensities of the dual-wavelength illumination that results in a maximum density reduction. The exact reason for the existence of such an optimal combination remains elusive. In this work, we have performed atomistic simulations to confirm the hypothesis that the density reduction is caused by the dynamic trans-cis-trans isomerization cycles. Subsequently, the atomistic simulations are used to decipher the underlying physics responsible for the counter-intuitive relation between density reduction and intensities. Intensity variations are simulated by varying the forward and backward isomerization probabilities. The simulations show that an optimal combination of these two probabilities will exhibit a maximum density reduction corroborating experimental observations. Consequently, we discovered that a specific frequency of the dynamic trans-cis-trans isomerization cycles would induce maximum distortion in the polymer network resulting in significant density reduction