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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., European Journal of Mechanics. A, Solids, 100, ISSN:0997-7538, ISSN:1873-7285

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., Comment: 18 pages, 19 figures

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.

A multi-scale scheme for the invasion percolation of rock fracture networks with heterogeneous fracture aperture fields is proposed. Inside fractures, fluid transport is calculated on the finest scale and found to be localized in channels as a consequence of the aperture field. The channel network is characterized and reduced to a vectorized artificial channel network (ACN). Different realizations of ACNs are used to systematically calculate efficient apertures for fluid transport inside differently sized fractures as well as fracture intersection and entry properties. Typical situations in fracture networks are parameterized by fracture inclination, flow path length along the fracture and intersection lengths in the entrance and outlet zones of fractures. Using these scaling relations obtained from the finer scales, we simulate the invasion process of immiscible fluids into saturated discrete fracture networks, which were studied in previous works., Comment: 21 pages

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.

The main purpose of this work is to simulate two-phase flow in the form of immiscible displacement through anisotropic, three-dimensional (3D) discrete fracture networks (DFN). The considered DFNs are artificially generated, based on a general distribution function or are conditioned on measured data from deep geological investigations. We introduce several modifications to the invasion percolation (MIP) to incorporate fracture inclinations, intersection lines, as well as the hydraulic path length inside the fractures. Additionally a trapping algorithm is implemented that forbids any advance of the invading fluid into a region, where the defending fluid is completely encircled by the invader and has no escape route. We study invasion, saturation, and flow through artificial fracture networks, with varying anisotropy and size and finally compare our findings to well studied, conditioned fracture networks., 18 pages, 10 figures

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.

The description of progressive failure of unidirectional fibre reinforced composites using, e.g., fibre bundle models depends on realistic assumptions for load redistributions from broken to unbroken fibres. Therefore, gaining qualitative and quantitative insight in real fibre–matrix and fibre–fibre interactions is essential, however, available techniques have serious drawbacks in applicability or in significance of results. This paper presents a non-destructive method for investigating stress conditions in model samples using combined experimental and simulated photoelasticity. The approach provides a powerful basis of comparison between straight experiments and comprehensive finite element analysis (FEA). Its result is an experimentally based fully tensorial 3D stress field in the matrix and fibres. Hence the method is a reliable tool to study fibre, matrix and interface properties including shear transmission and debonding. The presented application on polymer model specimens containing up to five parallel glass fibres loaded at angles from 0° to 15° proves the capability of the approach. It provides deeper insights into angle-dependent successive failure events.

Class-1 myosins play key roles in the dynamic organization of the cytoskeleton and membrane shape as they act as divalent cross-linkers interconnecting and generating force between both actin and membrane systems. Previously we have shown that the long-tailed Dictyostelium myosin-1C not only is involved in membrane remodeling during endocytic processes, but also plays a role in spindle dynamics during mitosis through direct microtubule interactions. Here, we characterized myosin-1D as the second myosin-1 member with roles in mitosis. Our studies reveal that myosin-1D binds spindle pole microtubules and associates with the nuclear envelope of dividing cells. Myosin-1D targets microtubules and nuclear structures early during mitosis and decorates the membrane embedded chromatin masses until the end of cell division. This suggests a role of the motor in nuclear membrane fission by mediating microtubule-membrane interactions. Since the nuclear envelope in Dictyostelium remains intact throughout mitosis, except for regions of perforations that are found near the spindle poles, myosin-1D appears to act as a dynamic linker connecting spindle pole microtubules to the nuclear membrane important to hold the envelope together as it gets stretched upon spindle elongation. To investigate the mechanisms that govern myosin-1s to distinct mitotic structures, we have generated a set of motor and tail domain constructs and characterized binding affinities and kinetics of intermolecular interactions with lipid membranes, actin, and microtubules. Our results show that differences in strength and specificity of lipid and microtubule binding and regulatory roles of the tails account for the specialized functions of the motors in mitosis.

All myosins undergo the same ATP dependent biochemical cycle but with variations in the lifetime of the individual states of the cycle and the fraction of the total cycle time spent in each state. E.g., processive class-5 myosins are high duty ratio motors that stay most of the ATPase cycle time (> 0.5) attached to actin, whereas fast myosins such as muscle myosin-2 and some class-1 myosin members have a low duty ratio (< 0.1). The members of the respective groups are assumed to use different ways to couple conformational changes at the nucleotide binding regions to changes that occur at the actin binding sites. Conserved active-site elements termed switch-1 and switch-2 play a major role in the coupling mechanism; however, whether and to which extent small variations in the sequence of switch-1 and switch-2 affect the duty ratio of a given myosin, thus determining its ability for processive movement, fast contractility or tension bearing remains unresolved. Based on structural considerations and confirmed by mutational analyses, we identified key residues in the nucleotide binding pocket that are responsible for making ADP dissociation kinetics dependent on the concentration of free magnesium ions. The exchange of a single amino acid in switch-2 affected the motor properties of all myosins tested, but also transformed low duty ratio motors into high duty ratio and vice versa. In addition, x-ray structural analyses and molecular modeling allowed us to relate the observed changes to altered coupling between the active sites and actin binding regions. These results, together with our cell biological studies demonstrating for the first time that magnesium ions have a regulatory role on motor protein function in vivo reveal that, switch-2 can act as magnesium sensor critically determining the mechanochemical properties of myosins.

Central to the mechanism of myosins are two conserved sequence motifs, switch-1 and switch-2, which contact nucleotide and Mg2+ at the rear of the nucleotide-binding pocket. They act as gamma-phosphate sensors, thus controlling hydrolysis and product release. The movement of switch-2 from a ‘closed’ to an ‘open’ conformation is translated via the relay region into a rotation of the converter domain, which drives the force generating power stroke. To investigate the molecular details of this coupling mechanism, we introduced mutation S456Y in switch-2 of the myosin-2 motor domain from Dictyostelium discoideum and analyzed the structural and functional consequences. Our kinetic results showed that the S456Y mutant lost the ability to bind effectively ATP, displayed a strongly decreased actin affinity in the ADP-bound state, and moved actin filaments with highly reduced velocities. It has been proposed that the exchange of S456 by a large amino acid would prevent the complete closing of switch-2 and therefore the full generation of the power stroke providing an explanation for the observed kinetic and mechanical defects of the mutant myosin. We have solved the crystal structure of the myosin motor domain with S456Y mutation in complex with ADP-VO4. The overall crystal structure and conformation of the nucleotide binding region resembles that of the wild-type revealing that switch-2 indeed can adopt the ‘closed’ conformation. We therefore conclude that not the complete ‘closing’ but the complete ‘opening’ of switch-2 is required for the full power stroke. Additional conformational changes in the crystal structure of S456Y, e.g. the actin binding loop-2 and loop-4, explain the disturbed actin binding properties of the mutant construct.

The construction of biohybrid devices in which motor proteins are integrated as biomechanical components for powering nano- to microscale movement holds great potential for a wide range of nanotechnological applications, ranging from basic research to diagnostic lab-on-a-chip technologies. However, to operate motor proteins efficiently in regard to cargo transport, sorting, and assembly processes, it is important to control parameters such as velocity, motile activity, force production, directionality, and processivity of movement tightly. Long-term stability of the motor protein on synthetic environments is an additional prerequisite for their successful integration in biohybrid devices. Here we describe a structure-based molecular engineering approach leading to the design and generation of two myosin constructs that maintain their motile activity when immobilized on glass surfaces over greatly extended periods. Direct functional assays and single molecule experiments show that important motor properties of the engineered nanomotors can be modulated by controlled changes in buffer conditions. We show that the motile behavior of the dimeric M5P construct can be switched between processive and non-processive modes of movement and the motor activity of the monomeric M5S construct can be turned on and off in a controlled, continuous, and reversible manner by coordinated changes in the concentration of MgCl2 and KCl. The parametric control is achieved accurately and with great ease. The resulting effects on motor function can be used for applications ranging from organising directed transport with targeted accumulation of cargo as well as assembly and sensing functions on the nano- to micro-scale levels.