PowderSim: Lagrangian Discrete and Mesh-Free Continuum Simulation Code for Cohesive Soils

PowderSim is a calculation tool that combines a discrete-element method (DEM) module, including calibrated interparticle-interaction relationships, with a mesh-free, continuum, SPH (smoothed-particle hydrodynamics) based module that utilizes enhanced, calibrated, constitutive models capable of mimicking both large deformations and the flow behavior of regolith simulants and lunar regolith under conditions anticipated during in situ resource utilization (ISRU) operations. The major innovation introduced in PowderSim is to use a mesh-free method (SPH-based) with a calibrated and slightly modified critical-state soil mechanics constitutive model to extend the ability of the simulation tool to also address full-scale engineering systems in the continuum sense. The PowderSim software maintains the ability to address particle-scale problems, like size segregation, in selected regions with a traditional DEM module, which has improved contact physics and electrostatic interaction models

Numerische Untersuchungen der Bruchfestigkeit und inelastischen Deformationen von offenzelligen keramischen Schaumstrukturen

Die im Rahmen des Sonderforschungsbereiches SFB 920 entstandene Arbeit beschäftigt sich mit bruchmechanischen Vorgängen und der makroskopischen Beschreibung von offenzelligen Keramikschäumen unter Berücksichtigung des Materialverhaltens des Kompaktmaterials mithilfe von numerischen Simulationen. Dabei steht die thermomechanische Belastung einer solchen Struktur während eines Gießprozesses im Vordergrund. Im Rahmen der bruchmechanischen Untersuchungen konnte der Einfluss von verschiedenen Strukturparametern aufgezeigt werden. Die Belastungen entlang der scharfen Kerben im Inneren der Stege ergaben sich dabei als weniger kritisch als entlang der Stegaußenseiten. Das Kriechverhalten des kohlenstoffgebundenen Aluminiumoxides bei Hochtemperatur konnte erfolgreich beschrieben und für Schaumstrukturen angewendet werden. Das vorgeschlagene Modell kann dabei sowohl für virtuell erzeugte Schaumstrukturen als auch für reale Schaumproben angepasst werden. Mithilfe von homogenisierten Materialmodellen basierend auf neuronalen Netzen ergab sich eine drastische Reduzierung der Rechenzeit für komplexe Filterstrukturen. Es ist dabei eine Berücksichtigung von Plastizität und Schädigung für das Kompaktmaterial möglich.This thesis developed within the collaborative research centre SFB 920 deals with fracture mechanical analyses and the macroscopic description of open-cell ceramic foams considering the material behaviour of the bulk material by means of numerical simulations. In the centre of interest is the thermomechanical loading of such a structure during a casting process. Within the framework of fracture mechanical investigations, the influence of various structural parameters is demonstrated. The loads along the sharp notches inside the struts turned out to be less critical than along the outer surfaces of the struts. The creep behaviour of the carbon-bonded alumina at high temperature were successfully described and the mathematical description is applied to foam structures. The proposed model can be adapted for virtually generated foam structures as well as for real foam samples. Using homogenized material models based on neuronal networks, a drastic reduction of the computing time for complex filter structures was achieved. Meanwhile, it is possible to consider plasticity and damage effects for the bulk material

Product Tests in Virtual Reality: Lessons Learned during Collision Avoidance Development for Drones

Virtual reality (VR) and real-world simulations have become an important tool for product development, product design, and product tests. Product tests in VR have many advantages, such as reproducibility and shortened development time. In this paper, we investigate the virtual testing of a collision avoidance system for drones in terms of economic benefits. Our results show that virtual tests had both positive and negative effects on the development, with the positive aspects clearly predominating. In summary, the tests in VR shorten the development time and reduce risks and therefore costs. Furthermore, they offer possibilities not available in real-world tests. Nevertheless, real-world tests are still important

Charakterisierung von Intraokularlinsen im Hinblick auf biomechanische Eigenschaften und Oberflächenqualität

28 Intraokularlinsentypen werden untersucht und verglichen. Zur Charakterisierung der Optikoberfläche werden die Parameter Rauhigkeit und Benetzbarkeit bestimmt. Die mechanischen Untersuchungen umfassen die Messung der Rückstellkraft der Haptiken, den Anstellwinkel der Haptiken zum Kapselsack, die axiale Verschiebung und Verkippung der Optik und die Verschiebung der Optik in der Frontalebene nach Deformation der Haptiken auf unterschiedliche Durchmesser. Anhand der Messergebnisse ist es möglich, die Intraokularlinsentypen nach positiver und negativer Ausprägung der Parameter einzuteilen.In total 28 artifical intraocular lenses (IOL) are investigated and compared. For characterization of optical surface the parameters of roughness and wettability were determined. Mechanical examination includes the measurement of haptic compression forces; contact angle between haptic and capsular bag; axial displacement, tilt and decentration of IOL optics after haptic deformation to different compression diameters. Using the measured parameters, the different IOL types can be divided with respect to positive and negative occurrence of the parameters

Hybrid mimetic finite-difference and virtual element formulation for coupled poromechanics

We present a hybrid mimetic finite-difference and virtual element formulation for coupled single-phase poromechanics on unstructured meshes. The key advantage of the scheme is that it is convergent on complex meshes containing highly distorted cells with arbitrary shapes. We use a local pressure-jump stabilization method based on unstructured macro-elements to prevent the development of spurious pressure modes in incompressible problems approaching undrained conditions. A scalable linear solution strategy is obtained using a block-triangular preconditioner designed specifically for the saddle-point systems arising from the proposed discretization. The accuracy and efficiency of our approach are demonstrated numerically on two-dimensional benchmark problems.Comment: 25 pages, 17 figure

A phase-field model for hydraulic fracture nucleation and propagation in porous media

Many geo-engineering applications, e.g., enhanced geothermal systems, rely on hydraulic fracturing to enhance the permeability of natural formations and allow for sufficient fluid circulation. Over the past few decades, the phase-field method has grown in popularity as a valid approach to modeling hydraulic fracturing because of the ease of handling complex fracture propagation geometries. However, existing phase-field methods cannot appropriately capture nucleation of hydraulic fractures because their formulations are solely energy-based and do not explicitly take into account the strength of the material. Thus, in this work, we propose a novel phase-field formulation for hydraulic fracturing with the main goal of modeling fracture nucleation in porous media, e.g., rocks. Built on the variational formulation of previous phase-field methods, the proposed model incorporates the material strength envelope for hydraulic fracture nucleation through two important steps: (i) an external driving force term, included in the damage evolution equation, that accounts for the material strength; (ii) a properly designed damage function that defines the fluid pressure contribution on the crack driving force. The comparison of numerical results for two-dimensional (2D) test cases with existing analytical solutions demonstrates that the proposed phase-field model can accurately model both nucleation and propagation of hydraulic fractures. Additionally, we present the simulation of hydraulic fracturing in a three-dimensional (3D) domain with various stress conditions to demonstrate the applicability of the method to realistic scenarios

A Hybrid Approach to Describe the Elastic-Plastic Deformation Behavior of 2D Cellular Solids Including Damage Effects

The constitutive description of the inelastic deformation behavior of porous media is a challenging task. The complex hardening behavior (simultaneous isotropic, kinematic and distortional hardening) and anisotropic yielding depend strongly on the micro-structure of the porous medium and the inelastic behavior of its bulk material.  In previous work, the authors presented a homogenized material model for an elastic-plastic material at the microscopic scale based on an adapted yield function to describe the elastic-plastic deformation behavior, including damage, of open-cell structures. In this approach, the shape of the yield function is not specified completely a priori. The proper shape is rather found by regression with results of cell model simulations using neural networks. The aim of this contribution is to demonstrate that this hybrid approach shows good agreement with direct simulations. The necessary size of the neural network, the number of training data and the computational efficiency are also discussed. It can be concluded that this model can be used to analyze the deformation behavior of porous structures while considering the coupling of plastic deformations and damage of the bulk material