arXiv subjects

Falk K. Wittel

Publications and source records attributed to Falk K. Wittel.

At least 37 records · Page 2Linked to original sources

arXiv2015

Full-field moisture induced deformation in Norway spruce: intra-ring variation of transverse swelling

The transverse hygro-expansion of the Norway spruce wood is studied on the growth ring level using digital image correlation. This non-destructive technique offers the possibility to contactless study deformation fields of relatively large areas. The measured full-field strains are segmented into individual growth rings. Whereas radial strains closely follow the density progression with the maximum in the dense latewood, tangential and shear strain remain constant except for positions around the edges of the sample. A simple FE three phase growth ring model is in good agreement with the experimental values. The selective activation of individual phases like earlywood, transitionwood and latewood demonstrates that the radial hygro-expansion is dominated by the earlywood deformation, whereas tangential deformation is a complex interplay of expansion and compression that needs all tissues to fully develop.

cond-mat.mtrl-sci↗

arXiv2015

Generic failure mechanisms in adhesive bonds

The failure of adhesive bondlines has been studied at the microscopic level via tensile tests. Stable crack propagation could be generated by means of samples with improved geometry, which made in-situ observations possible. The interaction of cracks with adhesive bondlines under various angles to the crack propagation was the focus of this study as well as the respective loading situations for the adhesives UF, PUR, and PVAc, which have distinctly different mechanical behaviors. It is shown how adhesive properties influence the occurrence of certain failure mechanisms and determine their appearance and order of magnitude. With the observed failure mechanisms, it becomes possible to predict the propagation path of a crack through the specimen.

cond-mat.mtrl-sci↗

arXiv2015

Pore space analysis of beech wood: the vessel network

Water transport in wood is vital for the survival of trees. With synchrotron radiation X-ray tomographic microscopy (SRXTM), it becomes possible to characterize and quantify the 3D network formed by vessels that are responsible for longitudinal transport. In the present paper, the spatial size dependence of vessels and the organization inside single growth rings in terms of vessel induced porosity was studied by SRXTM. Network characteristics, such as connectivity, were deduced by digital image analysis from the processed tomographic data and related to known complex network topologies.

cond-mat.mtrl-sci↗

arXiv2015

Single Particle Fragmentation in Ultrasound Assisted Impact Comminution

Impact fragmentation is the underlying principle of comminution milling of dry, bulk solids. Unfortunately the outcome of the fragmentation process is more or less determined by the dimensionality of the impactor and its impact velocity. Since fragmentation is dominated by interfering shock waves, manipulating traveling shock waves and adding energy to the system during its fragmentation could be a promising approach to manipulate fragment mass distributions and energy input. In a former study we explored mechanisms in impact fragmentation of spheres, using a three-dimensional Discrete Element Model (DEM). This work is focused on studying how single spheres fragment when impacted on a planar vibrating target.

cond-mat.mtrl-sci↗

arXiv2015

Invasion Percolation with a Hardening Interface under Gravity

We propose a modified Invasion Percolation (IP) model to simulate the infiltration of glue into a porous medium under gravity in 2D. Initially, the medium is saturated with air and then invaded by a fluid that has a hardening effect taking place from the interface towards the interior by contact with the air. To take into account that interfacial hardening, we use an IP model where capillary pressures of the growth sites are increased with time. In our model, if a site stays for a certain time at interface, it becomes a hard site and cannot be invaded anymore. That represents the glue interface becoming hard due to exposition with the air. Buoyancy forces are included in this system through the Bond number which represents the competition between the hydrostatic and capillary forces. We then compare our results with results from literature of non-hardening fluids in each regime of Bond number. We see that the invasion patterns change strongly with hardening while the non-hardening behavior remains basically not affected.

cond-mat.soft↗

arXiv2015

Hierarchical structures for a robustness-oriented capacity design

In this paper, we study the response of 2D framed structures made of rectangular cells, to the sudden removal of columns. We employ a simulation algorithm based on the Discrete Element Method, where the structural elements are represented by elasto-plastic Euler Bernoulli beams with elongation-rotation failure threshold. The effect of structural cell slenderness and of topological hierarchy on the dynamic residual strength after damage $\ROne$ is investigated. Topologically \textit{hierarchical} frames have a primary structure made of few massive elements, while \textit{homogeneous} frames are made of many thin elements. We also show how $\ROne$ depends on the activated collapse mechanisms, which are determined by the mechanical hierarchy between beams and columns, i.e. by their relative strength and stiffness. Finally, principles of robustness-oriented capacity design which seem to be in contrast to the conventional anti-seismic capacity design are addressed.

cond-mat.soft↗

arXiv2015

Progressive Collapse Mechanisms of Brittle and Ductile Framed Structures

In this paper, we study the progressive collapse of 3D framed structures made of reinforced concrete after the sudden loss of a column. The structures are represented by elasto-plastic Euler Bernoulli beams with elongation-rotation failure threshold. We performed simulations using the Discrete Element Method considering inelastic collisions between the structural elements. The results show what collapse initiation and impact-driven propagation mechanisms are activated in structures with different geometric and mechanical features. Namely, we investigate the influence of the cross sectional size and reinforcement $α$ and of the plastic capacity $β$ of the structural elements. We also study the final collapse extent and the fragment size distribution and their relation to $α$, $β$ and to the observed collapse mechanisms. Finally, we compare the damage response of structures with symmetric and asymmetric reinforcement in the beams.

cond-mat.soft↗

arXiv2015

Computational up-scaling of anisotropic swelling and mechanical behavior of hierarchical cellular material

The hygro-mechanical behavior of a hierarchical cellular material, i.e. growth rings of softwood is investigated using a two-scale micro-mechanics model based on a computational homogenization technique. The lower scale considers the individual wood cells of varying geometry and dimensions. Honeycomb unit cells with periodic boundary conditions are utilized to calculate the mechanical properties and swelling coefficients of wood cells. Using the cellular scale results, the anisotropy in mechanical and swelling behavior of a growth ring in transverse directions is investigated. Predicted results are found to be comparable to experimental data. It is found that the orthotropic swelling properties of the cell wall in thin-walled earlywood cells produce anisotropic swelling behavior while, in thick latewood cells, this anisotropy vanishes. The proposed approach provides the ability to consider the complex microstructure when predicting the effective mechanical and swelling properties of softwood.

cond-mat.mtrl-sci↗

arXiv2015

Inverse determination of effective mechanical properties of adhesive bondlines

A new approach for determining effective mechanical bondline properties using a combined experimental-numerical modal analysis technique is proposed. After characterizing clear spruce wood boards, an adhesive layer is applied to the boards surfaces. The shift of the eigenfrequencies resulting from the adhesive layer together with information on the bondline geometry can then be used to inversely determine the mechanical properties of the adhesive layer using Finite Element Models. The calculated values for clear wood as well as for the adhesive layer lie within reasonable ranges, thus demonstrating the methods potential.

cond-mat.mtrl-sci↗

arXiv2015

Discrete Element Simulation of Transverse Cracking During the Pyrolysis of Carbon Fibre Reinforced Plastics to Carbon/Carbon Composites

The fracture behavior of fiber-ceramics like C/C-SiC strongly depends on the initial damage arising during the production process. We study the transverse cracking of the 90° ply in [0/90]S cross-ply laminates due to the thermochemical degradation of the matrix material during the carbonization process by means of a discrete element method. The crack morphology strongly depends on the fiber-matrix interface properties, the transverse ply thickness as well as on the carbonization process itself. To model the 90° ply a two-dimensional triangular lattice of springs is constructed where nodes of the lattice represent fibers. Springs with random breaking thresholds model the disordered matrix material and interfaces. The spring-lattice is coupled by interface springs to two rigid bars which capture the two 0° plies or adjacent sublaminates in the model. Molecular dynamics simulation is used to follow the time evolution of the model system. It was found that under gradual heating of the specimen, after some distributed cracking, segmentation cracks occur in the 90° ply which then develop into a saturated state where the ply cannot support additional load. The dependence of the micro-structure of damage on the ply thickness and on the disorder in spring properties is also studied. Crack density and porosity of the system are monitored as a function of the temperature and compared to an analytic approach and experiments.

cond-mat.mtrl-sci↗

arXiv2015

Mechanisms in Impact Fragmentation

The brittle fragmentation of spheres is studied numerically by a 3D Discrete Element Model. Large scale computer simulations are performed with models that consist of agglomerates of many spherical particles, interconnected by beam-truss elements. We focus on a detailed description of the fragmentation process and study several fragmentation mechanisms involved. The evolution of meridional cracks is studied in detail. These cracks are found to initiate in the inside of the specimen with quasi-periodic angular distribution and give a broad peak in the fragment mass distribution for large fragments that can be fitted by a two-parameter Weibull distribution. The results prove to be independent of the degree of disorder in the model, but mean fragment sizes scale with velocity. Our results reproduce many experimental observations of fragment shapes, impact energy dependence or mass distribution, and significantly improve the understanding of the fragmentation process for impact fracture since we have full access to the failure conditions and evolution.

cond-mat.mtrl-sci↗

arXiv2015

Lattice-Boltzmann Method for Geophysical Plastic Flows

We explore possible applications of the Lattice-Boltzmann Method for the simulation of geophysical flows. This fluid solver, while successful in other fields, is still rarely used for geotechnical applications. We show how the standard method can be modified to represent free-surface realization of mudflows, debris flows, and in general any plastic flow, through the implementation of a Bingham constitutive model. The chapter is completed by an example of a full-scale simulation of a plastic fluid flowing down an inclined channel and depositing on a flat surface. An application is given, where the fluid interacts with a vertical obstacle in the channel.

physics.flu-dyn↗

arXiv2015

Study on the fragmentation of shells

Fragmentation can be observed in nature and in everyday life on a wide range of length scales and for all kinds of technical applications. Most studies on dynamic failure focus on the behaviour of bulk systems in one, two and three dimensions under impact and explosive loading, showing universal power law behaviour of fragment size distribution. However, hardly any studies have been devoted to fragmentation of shells. We present a detailed experimental and theoretical study on the fragmentation of closed thin shells of various materials, due to an excess load inside the system and impact with a hard wall. Characteristic fragmentation mechanisms are identified by means of a high speed camera and fragment shapes and mass distributions are evaluated. Theoretical rationalisation is given by means of stochastic break-up models and large-scale discrete element simulations with spherical shell systems under different extreme loading situations. By this we ex-plain fragment shapes and distributions and prove a power law for the fragment mass distribution. Satisfactory agreement between experimental findings and nu-merical predictions of the exponents of the power laws for the fragment shapes is obtained.

cond-mat.soft↗

arXiv2015

From fracture to fragmentation: discrete element modeling -- Complexity of crackling noise and fragmentation phenomena revealed by discrete element simulations

Discrete element modelling (DEM) is one of the most efficient computational approaches to the fracture processes of heterogeneous materials on mesoscopic scales. From the dynamics of single crack propagation through the statistics of crack ensembles to the rapid fragmentation of materials DEM had a substantial contribution to our understanding over the past decades. Recently, the combination of DEM with other simulation techniques like Finite Element Modelling further extended the field of applicability. In this paper we briefly review the motivations and basic idea behind the DEM approach to cohesive particulate matter and then we give an overview of on-going developments and applications of the method focusing on two fields where recent success has been achieved. We discuss current challenges of this rapidly evolving field and outline possible future perspectives and debates.

cond-mat.mtrl-sci↗

arXiv2015

Orthotropic rotation-free thin shell elements

A method to simulate orthotropic behaviour in thin shell finite elements is proposed. The approach is based on the transformation of shape function derivatives, resulting in a new orthogonal basis aligned to a specified preferred direction for all elements. This transformation is carried out solely in the undeformed state leaving minimal additional impact on the computational effort expended to simulate orthotropic materials compared to isotropic, resulting in a straightforward and highly efficient implementation. This method is implemented for rotation-free triangular shells using the finite element framework built on the Kirchhoff--Love theory employing subdivision surfaces. The accuracy of this approach is demonstrated using the deformation of a pinched hemispherical shell (with a 18° hole) standard benchmark. To showcase the efficiency of this implementation, the wrinkling of orthotropic sheets under shear displacement is analyzed. It is found that orthotropic subdivision shells are able to capture the wrinkling behavior of sheets accurately for coarse meshes without the use of an additional wrinkling model.

math.NA↗

arXiv2015

Simulating Thin Sheets: Buckling, Wrinkling, Folding and Growth

Numerical simulations of thin sheets undergoing large deformations are computationally challenging. Depending on the scenario, they may spontaneously buckle, wrinkle, fold, or crumple. Nature's thin tissues often experience significant anisotropic growth, which can act as the driving force for such instabilities. We use a recently developed finite element model to simulate the rich variety of nonlinear responses of Kirchhoff-Love sheets. The model uses subdivision surface shape functions in order to guarantee convergence of the method, and to allow a finite element description of anisotropically growing sheets in the classical Rayleigh-Ritz formalism. We illustrate the great potential in this approach by simulating the inflation of airbags, the buckling of a stretched cylinder, as well as the formation and scaling of wrinkles at free boundaries of growing sheets. Finally, we compare the folding of spatially confined sheets subject to growth and shrinking confinement to find that the two processes are equivalent.

cond-mat.soft↗

arXiv2015

Grain Scale Modeling of Arbitrary Fluid Saturation in Random Packings

We propose a model for increasing liquid saturation in a granular packing which can account for liquid redistribution at saturation levels beyond the well-studied capillary bridge regime. The model is capable of resolving and combining capillary bridges, menisci and fully saturated pores to form local liquid clusters of any shape. They can exchange volume due to the local Laplace pressure gradient via a liquid film on the surfaces of grains. Local instabilities like Haines jumps trigger the discontinuous evolution of the liquid front. The applicability of the model is demonstrated and compared to benchmark experiments on the level of individual liquid structures as well as on larger systems.

cond-mat.soft↗

arXiv2015

Moisture-induced Damage Evolution in Laminated Beech

Structural elements made of laminated hardwood are increasingly used in timber engineering. In this combined numerical and experimental approach, damage onset and propagation in uni-directional and cross-laminated samples out of European beech due to climatic changes are studied. The inter- and intra-laminar damage evolution is characterized for various configurations adhesively bonded by three structural adhesive systems. Typical situations are simulated by means of a comprehensive moisture-dependent non-linear rheological finite element model for wood with the capability to capture delaminations. The simulations give insight into the role of different strain components such as visco-elastic, mechano-sorptive, plastic, and hygro-elastic deformations under changing moisture content in progressive damage and delamination. We show the stress buildup under cyclic hygric loading resulting in hygro-fatigue and modify an analytical micro-mechanics of damage model, originally developed for cross-ply laminates, to describe the problem of moisture-induced damage in beech lamellae.

cond-mat.mtrl-sci↗