arXiv subjects

Falk K. Wittel

Publications and source records attributed to Falk K. Wittel.

At least 19 recordsLinked to original sources

arXiv2025

Orthotropic Viscoelastic Creep in Cellular Scaffolds

Recent measurements of Norway spruce have revealed stress-state-dependent normalized creep behavior, highlighting a gap in our fundamental understanding. This study examines whether the anisotropic response originates from the micro-structural, cellular nature of composite cell walls with varying tracheid types. Cell wall creep parameters are identified via surrogate-based inverse parameter identification, applied to hierarchical micro-mechanical and FEM models of increasing topological complexity up to the growth ring scale. Despite microstructural disorder, simulated creep curves converge toward a universal set of proportionality factors. The results indicate that directional creep behavior cannot be attributed solely to tissue-scale topology, and that realistic predictions require the inclusion of non-linear material responses at stress concentration sites.

q-bio.CB↗

arXiv2025

Resolving Discrepancies in Wood Micromechanics: Strain-Mapped Compression of Tracheid Wall Micropillars

Wood's increasing role as a structural resource in sustainable materials selection demands accurate characterization of its mechanical behavior. Its performance arises from a hierarchical structure, where the dominant load-bearing component is the S2 layer of tracheid cell walls-a thick, fiber-reinforced composite of cellulose microfibrils embedded in hemicelluloses and lignin. Due to the small dimensions and anisotropic nature of the S2 layer, mechanical testing presents significant challenges, particularly in producing homogeneous stress and strain fields. In this study, we apply micropillar compression (MPC) combined with digital image correlation (DIC) to Norway spruce tracheids, enabling direct and model-free strain measurements at the cell wall scale. Micropillars were oriented at different microfibril angles (MFAs), confirming the expected dependence of stiffness and yield stress on ultrastructural alignment, with higher stiffness and yield stress at low MFAs. For these under compression fibril-aligned kink bands occurred, while shear related failure was observed at higher angles. A parameter study on the acceleration voltage of the Scanning Electron Microscope revealed that electron beam exposure significantly degrades pillar integrity, which could explain data scatter and mechanical underestimation in earlier MPC studies. By controlling imaging protocols and using DIC-based strain measurements, we report the highest direct measurements of wood cell wall stiffness to date-up to 42 GPa for MFA=0°-closer matching micromechanical model predictions compared to previous results. Findings are compared with Finite Element Method-based displacement corrections to establish a robust protocol for probing soft, anisotropic biological composites' mechanical behavior while clarifying longstanding inconsistencies in reported results of wood MPC measurements.

physics.bio-ph↗

arXiv2025

Morphologic Evolution in Simulated Wood Densification

This study investigates the radial densification of spruce wood using explicit Finite Element Method simulations, focusing on the effects of various densification protocols. These protocols include quasi-static compression, oscillatory excitation, and self-densification through shrinking hydrogel fillings and their impact on the morphogenesis of folding patterns across different tissue types. The simulations incorporate the an isotropic mechanical behavior of wood tracheid walls and account for moisture and delignification effects using a hierarchical approach. Our results reveal the technological potential of targeted densification in creating tailored density profiles that enhance stiffness and strength. These insights offer valuable guidance for optimizing densification processes in practical applications.

physics.bio-ph↗

arXiv2024

Comprehensive creep compliance characterization of orthotropic materials using a cost-effective automated system

Determining the creep compliances of orthotropic composite materials requires experiments in at least three different uniaxial and biaxial loading directions. Up to date, data respecting multiple climates and all anatomical directions are sparse for hygro-responsive materials like Norway spruce. Consequently, simulation models of wood frequently over-simplify creep, e.g., by proportionally scaling missing components or neglecting climatic influences. To overcome such simplifications, an automated computer-controlled climatized creep rack was developed, that experimentally assesses moisture-dependent viscoelasticity and mechanosorption in all anatomical directions. The device simultaneously measures the creep strains of three dogbone tension samples, three flat compression samples, and six Arcan shear samples via Digital Image Correlation. This allows for ascertaining the complete orthotropic compliance tensors while accounting for loading direction asymmetries. This paper explains the creep rack's structure and demonstrates its use by determining all nine independent creep compliance components of Norway spruce at 65% relative humidity. The data shows that loading asymmetry effects amount up to 16%. Furthermore, the found creep compliance tensor is not proportional to the elastic compliance tensor. By clustering the compliance components, we identify four necessary components to represent the full orthotropy of the compliance tensor, obtainable from not less than two experiments.

cond-mat.soft↗

arXiv2023

Damage Preserving Transformation for Materials with Microstructure

The failure of heterogeneous materials with microstructures is a complex process of damage nucleation, growth and localisation. This process spans multiple length scales and is challenging to simulate numerically due to its high computational cost. One option is to use domain decomposed multi-scale methods with dynamical refinement. If needed, these methods refine coarse regions into a fine-scale representation to explicitly model the damage in the microstructure. However, damage evolution is commonly restricted to fine-scale regions only. Thus, they are unable to capture the full complexity and breath of the degradation process in the material. In this contribution, a generic procedure that allows to account for damage in all representations is proposed. The approach combines a specially designed orthotropic damage law, with a scheme to generate pre-damaged fine-scale microstructures. Results indicate that the damage approximation for the coarse representation works well. Furthermore, the generated fine-scale damage patterns are overall consistent with explicitly simulated damage patterns. Minor discrepancies occur in the generation but subsequently vanish when explicit damage evolution continuous; for instance under increased load. The presented approach provides a methodological basis for adaptive multi-scale simulation schemes with consistent damage evolution.

cond-mat.mtrl-sci↗

arXiv2019

Collapse of orthotropic spherical shells

We report on the buckling and subsequent collapse of orthotropic elastic spherical shells under volume and pressure control. Going far beyond what is known for isotropic shells, a rich morphological phase space with three distinct regimes emerges upon variation of shell slenderness and degree of orthotropy. Our extensive numerical simulations are in agreement with experiments using fabricated polymer shells. The shell buckling pathways and corresponding strain energy evolution are shown to depend strongly on material orthotropy. We find surprisingly robust orthotropic structures with strong similarities to stomatocytes and tricolpate pollen grains, suggesting that the shape of several of Nature's collapsed shells could be understood from the viewpoint of material orthotropy.

cond-mat.soft↗

arXiv2019

Behavior of confined granular beds under cyclic thermal loading

We investigate the mechanical behavior of a confined granular packing of irregular polyhedral particles under repeated heating and cooling cycles by means of numerical simulations with the Non-Smooth Contact Dynamics method. Assuming a homogeneous temperature distribution as well as constant temperature rate, we study the effect of the container shape, and coefficients of thermal expansions on the pressure buildup at the confining walls and the density evolution. We observe that small changes in the opening angle of the confinement can lead to a drastic peak pressure reduction. Furthermore, the displacement fields over several thermal cycles are obtained and we discover the formation of convection cells inside the granular material having the shape of a torus. The root mean square of the vorticity is then calculated from the displacement fields and a quadratic dependency on the ratio of thermal expansion coefficients is established.

cond-mat.soft↗

arXiv2019

Evolution of fragment size distributions from the crushing of granular materials

We study the fragment size distributions after crushing of single and many particles under uniaxial compression inside a cylindrical container by means of numerical simulations. Under the assumption that breaking goes through the bulk of the particle we obtain the size distributions of fragments for both cases after large displacements. For the single particle crushing, this fragmentation mechanism produces a log-normal size distribution, which deviates from the power-law distribution of fragment sizes for the packed bed. We show that as the breaking process evolves, a power-law dependency on the displacement is present for the single grain, while for the many grains system, the distribution converges to a steady state. We further investigate the force networks and the average coordination number as a function of the particle size, which gives inside about the origin of the power-law distributions for the granular assembly under uniaxial compression.

cond-mat.soft↗

arXiv2018

Fluid structure interaction with curved space lattice Boltzmann

We present a novel method for fluid structure interaction (FSI) simulations where an original 2nd-order curved space lattice Boltzmann fluid solver (LBM) is coupled to a finite element method (FEM) for thin shells. The LBM can work independently on a standard lattice in curved coordinates without the need for interpolation, re-meshing or an immersed boundary. The LBM distribution functions are transformed dynamically under coordinate change. In addition, force and momentum can be calculated on the nodes exactly in any geometry. Furthermore, the FEM shell is a complete numerical tool with implementations such as growth, self-contact and strong external forces. We show resolution convergent error for standard tests under metric deformation. Mass and volume conservation, momentum transfer, boundary-slip and pressure maintenance are verified through specific examples. Additionally, a brief deformation stability analysis is carried out. Next, we study the interaction of a square fluid flow channel to a deformable shell. Finally, we simulate a flag at moderate Reynolds number, air flow channel. The scheme is limited to small deformations of O(10%) relative to domain size, by improving its stability the method can be naturally extended to multiple applications without further implementations.

physics.comp-ph↗

arXiv2017

Tomographic Study of Internal Erosion of Particle Flows in Porous Media

In particle-laden flows through porous media, porosity and permeability are significantly affected by the deposition and erosion of particles. Experiments show that the permeability evolution of a porous medium with respect to a particle suspension is not smooth, but rather exhibits significant jumps followed by longer periods of continuous permeability decrease. Their origin seems to be related to internal flow path reorganization by avalanches of deposited material due to erosion inside the porous medium. We apply neutron tomography to resolve the spatio-temporal evolution of the pore space during clogging and unclogging to prove the hypothesis of flow path reorganization behind the permeability jumps. This mechanistic understanding of clogging phenomena is relevant for a number of applications from oil production to filters or suffosion as the mechanisms behind sinkhole formation.

physics.flu-dyn↗

arXiv2016

Evolution of Strength and Failure of SCC during Early Hydration

The early strength evolution of self-consolidating concrete (SCC) is studied by a set of non-standard mechanical tests for compressive, tensile, shear and bending failure. The results are applicable in an industrial environment for process control, e.g. of slip casting with adaptive molds in robotic fabrication. A procedure for collapsing data to a master evolution curve is presented that allows to distinguish two regimes in the evolution. In the first, the material is capable of undergoing large localized plastic deformation, as expected from thixotropic yield stress fluids. This is followed by a transition to cohesive frictional material behavior dominated by crack growth. The typical differences in tensile and compressive strength of hardened concrete are observed to originate at the transition. Finally, the evolution of a limit surface in principal stress space is constructed and discussed.

cond-mat.soft↗

arXiv2016

Micro-mechanical Failure Analysis of Wet Granular Matter

We employ a novel fluid-particle model to study the shearing behavior of granular soils under different saturation levels, ranging from the dry material via the capillary bridge regime to higher saturation levels with percolating clusters. The full complexity of possible liquid morphologies is taken into account, implying the formation of isolated arbitrary-sized liquid clusters with individual Laplace pressures that evolve by liquid exchange via films on the grain surface. Liquid clusters can grow in size, shrink, merge and split, depending on local conditions, changes of accessible liquid and the pore space morphology determined by the granular phase. This phase is represented by a discrete particle model based on Contact Dynamics, where capillary forces exerted from a liquid phase add to the motion of spherical particles. We study the macroscopic response of the system due to an external compression force at various liquid contents with the help of triaxial shear tests. Additionally, the change in liquid cluster distributions during the compression due to the deformation of the pore space is evaluated close to the critical load.

cond-mat.soft↗

arXiv2015

Packing of elastic wires in flexible shells

The packing problem of long thin filaments that are injected into confined spaces is of fundamental interest for physicists and biologists alike. How linear threads pack and coil is well known only for the ideal case of rigid containers, though. Here, we force long elastic rods into flexible spatial confinement borne by an elastic shell to examine under which conditions recently acquired knowledge on wire packing in rigid spheres breaks down. We find that unlike in rigid cavities, friction plays a key role by giving rise to the emergence of two distinct packing patterns. At low friction, the wire densely coils into an ordered toroidal bundle with semi-ellipsoidal cross-section, while at high friction, it packs into a highly disordered, hierarchic structure. These two morphologies are shown to be separated by a continuous phase transition. Our findings demonstrate the dramatic impact of friction and confinement elasticity on filamentous packing and might drive future research on such systems in physics, biology and even medical technology toward including these mutually interacting effects.

cond-mat.soft↗

arXiv2015

Continuous Wire Reinforcement for Jammed Granular Architecture

The mechanical behavior of continuous fiber reinforced granular columns is simulated by means of a Discrete Element Model. Spherical particles are randomly deposited simultaneously with a wire, that is deployed following different patterns inside of a flexible cylinder for triaxial compression testing. We quantify the effect of three different fiber deployment patterns on the failure envelope, represented by Mohr-Coulomb cones, and derive suggestions for improved deployment strategies.

cond-mat.mtrl-sci↗

arXiv2015

The role of water in the behavior of wood

Wood, due to its biological origin, has the capacity to interact with water. Sorption/desorption of moisture is accompanied with swelling/shrinkage and softening/hardening of its stiffness. The correct prediction of the behavior of wood components undergoing environmental loading requires that the moisture behavior and mechanical behavior of wood are considered in a coupled manner. We propose a comprehensive framework using a fully coupled poromechanical approach, where its multiscale implementation provides the capacity to take into account, directly, the exact geometry of the wood cellular structure, using computational homogenization. A hierarchical model is used to take into account the subcellular composite-like organization of the material. Such advanced modeling requires high resolution experimental data for the appropriate determination of inputs and for its validation. High-resolution x-ray tomography, digital image correlation, and neutron imaging are presented as valuable methods to provide the required information.

cond-mat.mtrl-sci↗

arXiv2015

Contraction gradient induced microcracking in hardened cement paste

Drying induced cracking of concrete surfaces and repair layers is a common problem. A principal cause for this type of cracking is the moisture and resulting contraction gradient that develops in the cement paste matrix upon drying. This phenomenon has been experimentally quantified in unconfined hardened cement paste samples using a fluorescent resin impregnation technique. The effects of sample thickness and drying method on surface crack density and crack penetration depth are reported and explained. Finite element modelling of moisture gradients indicate the important role of the film coefficient in desiccation cracking of unconfined samples. The critical thickness for samples to remain crack-free upon drying was in the range of 2-5 mm depending on drying method. In thicker samples a crack spacing doubling process was observed that is in agreement with theoretical predictions.

cond-mat.mtrl-sci↗

arXiv2015

Multiscale Modeling of Polymer Gels-Chemo-Electric Model versus Discrete Element Model

Polyelectrolyte gels are a very attractive class of actuation materials with remarkable electronic and mechanical properties with a great similarity to biological contractile tissues. They consist of a polymer network with ionizable groups and a liquid phase with mobile ions. Absorption and delivery of solvent lead to a large change of volume. This mechanism can be triggered by chemical (change of salt concentration or pH of solution surrounding the gel), electrical, thermal or optical stimuli. Due to this capability, these gels can be used as actuators for technical applications, where large swelling and shrinkage is desired. In the present work chemically stimulated polymer gels in a solution bath are investigated. To adequately describe the different complicated phenomena occurring in these gels, they can be modeled on different scales. Therefore, models based on the statistical theory and porous media theory, as well as a coupled multi-field model and a discrete element formulation are derived and employed. In this paper, the coupled multi-field model and the discrete element model for chemical stimulation of a polymer gel film with and without domain deformation are employed. Based on these results, the presented formulations are compared and conclusions on their applicability in engineering practice are finally drawn.

cond-mat.mtrl-sci↗

arXiv2015

Mechanical Properties of Norway Spruce: Intra-Ring Variation and Generic Behavior of Earlywood and Latewood until Failure

The alternating earlywood and latewood growth ring structure has a strong influence on the mechanical performance of Norway spruce. In the current study, tensile tests in the longitudinal and tangential directions were performed on a series of specimens representing one growth ring at varying relative humidities. All tested mechanical parameters, namely modulus of elasticity and ultimate tensile stress, followed the density distribution in the growth ring, with the minimum values in earlywood and the maximum values in latewood. The samples were conditioned at three the relative humidities 50%, 65% and 95%. With increasing relative humidity, the values of the mechanical parameters were found to decrease. However, due to the high local variability, this decrease was not statistically significant. The test in the tangential direction on a set of earlywood and latewood specimens at 65% relative humidity revealed a similar limit of linear elasticity for both early- and latewood. Where the strength of both tissues was equal, the strain at failure was significantly greater for earlywood. Furthermore, the portion of the non-linear stress/strain behavior for earlywood was significantly greater. A Weibull analysis on the ultimate tensile strength revealed a tissue-independent Weibull modulus, which indicates similar defect distributions. For both, the failure occurred in the middle lamella.

cond-mat.mtrl-sci↗