- Elastic shakedown and roughness evolution in repeated elastic-plastic contactLucas Frérot, and Lars PastewkaarXiv:2308.13060 (2023)
Surface roughness emerges naturally during mechanical removal of material, fracture, chemical deposition, plastic deformation, indentation, and other processes. Here, we use continuum simulations to show how roughness which is neither Gaussian nor self-affine emerges from repeated elastic-plastic contact of a rough and rigid surface on a flat elastic-plastic substrate. Roughness profiles change with each contact cycle, but appear to approach a steady-state long before the substrate stops deforming plastically and has hence “shaken-down” elastically. We propose a simple dynamic collapse for the emerging power-spectral density, which shows that the multi-scale nature of the roughness is encoded in the first few indentations. In contrast to macroscopic roughness parameters, roughness at small scales and the skewness of the height distribution of the resulting roughness do not show a steady-state, with the latter vanishing asymptotically with contact cycle.
- Sound waves, diffusive transport, and wall slip in nanoconfined compressible fluidsHannes Holey, Peter Gumbsch, and Lars PastewkaarXiv:2305.07501 (2023)
Although continuum theories have been proven quite robust to describe confined fluid flow at molecular length scales, molecular dynamics (MD) simulations reveal mechanistic insights into the interfacial dissipation processes. Most MD simulations of confined fluids have used setups in which the lateral box size is not much larger than the gap height, thus breaking thin-film assumptions usually employed in continuum simulations. Here, we explicitly probe the long wavelength hydrodynamic correlations in confined simple fluids with MD and compare to gap-averaged continuum theories as typically applied in e.g. lubrication. Relaxation times obtained from equilibrium fluctuations interpolate between the theoretical limits from bulk hydrodynamics and continuum formulations with increasing wavelength. We show how to exploit this characteristic transition to measure viscosity and slip length in confined systems simultaneously from equilibrium MD simulations. Moreover, the gap-averaged theory describes a geometry-induced dispersion relation that leads to overdamped sound relaxation at large wavelengths, which is confirmed by our MD simulations. Our results add to the understanding of transport processes under strong confinement and might be of technological relevance for the design of nanofluidic devices due to the recent progress in fabrication methods.
- Why soft contacts are stickier when breaking than when making themAntoine Sanner, Nityanshu Kumar, Ali Dhinojwala, Tevis D B Jacobs, and Lars PastewkaarXiv:2307.14233 (2023)
Insects, pick-and-place manufacturing, engineered adhesives, and soft robots employ soft materials to stick to surfaces even in the presence of roughness. Experiments show that the force required for making contact is lower than for releasing it, a phenomenon known as the adhesion hysteresis. The common explanation for this hysteresis is either contact aging or viscoelasticity. Here, we show that adhesion hysteresis emerges even for perfectly elastic contacts and in the absence of contact aging and viscoelasticity because of surface roughness. We present a crack-perturbation model and experimental observations that reveal discrete jumps of the contact perimeter. These stick-slip instabilities are triggered by local differences in fracture energy between roughness peaks and valleys. Pinning of the contact perimeter retards both its advancement when coming into contact and its retraction when pulling away. Our model quantitatively reproduces the hysteresis observed in experiments and allows us to derive analytical predictions for its magnitude, accounting for realistic rough geometries across orders of magnitude in length scale. Our results explain why adhesion hysteresis is ubiquitous and reveal why soft pads in nature and engineering are efficient in adhering even to surfaces with significant roughness.
- matscipy: materials science at the atomic scale with PythonPetr Grigorev, Lucas Frérot, Fraser Birks, Adrien Gola, Jacek Golebiowski, Jan Grießer, Johannes L Hörmann, Gianpietro Moras, Wolfram G Nöhring, Jonas A., Punit Patel, Thomas Reichenbach, Lakshmi Shenoy, Michael Walter, Simon Wengert, and James R Kermodesubmitted to J. Open Source Softw. (2023)
Behaviour of materials is governed by physical phenomena that occur at an extreme range of length and time scales. Computational modelling requires multiscale approaches. Simulation techniques operating on the atomic scale serve as a foundation for such approaches, providing necessary parameters for upper-scale models. The physical models employed for atomic simulations can vary from electronic structure calculations to empirical force fields. However, construction, manipulation and analysis of atomic systems are independent of the given physical model but dependent on the specific application. matscipy implements such tools for applications in materials science, including fracture, plasticity, tribology and electrochemistry.
- Crack-path selection in phase-field models for brittle fractureW Beck Andrews, and Lars PastewkaarXiv:2203.16467 (2022)
This work presents a critical overview of the effects of different aspects of model formulation on crack path selection in quasi-static phase field fracture. We consider different evolution methods, mechanics formulations, fracture dissipation energy formulations, and forms of the irreversibility condition. The different model variants are implemented with common numerical methods based on staggered solution of the phase-field and mechanics sub-problems via FFT-based solvers. These methods mix standard approaches with novel elements, such as the use of bound-constrained conjugate gradients for the phase field sub-problem and a heuristic method for near-equilibrium evolution. We examine differences in crack paths between model variants in simple model systems and microstructures with randomly heterogeneous Young’s modulus. Our results indicate that near-equilibrium evolution methods are preferable for quasi-static fracture of heterogeneous microstructures compared to minimization and time-dependent methods. In examining mechanics formulations, we find distinct effects of crack driving force and the model for contact implicit in phase field fracture. Our results favor the use of a strain-spectral decomposition for the crack driving force but not the contact model. Irreversibility condition and fracture dissipation energy formulation were also found to affect crack path selection, but systematic effects were difficult to deduce due to the overall sensitivity of crack selection within the heterogeneous microstructures. Our findings support the use of the AT1 model over the AT2 model and irreversibility of the phase field within a crack set rather than the entire domain. Sensitivity to these differences in formulation was reduced but not eliminated by reducing the crack width parameter \ell relative to the size scale of the random microstructures.
- Efficient topology optimization using compatibility projection in micromechanical homogenizationIndre Jödicke, Richard J. Leute, Till Junge, and Lars PastewkaarXiv:2107.04123 (2021)
The adjoint method allows efficient calculation of the gradient with respect to the design variables of a topology optimization problem. This method is almost exclusively used in combination with traditional Finite-Element-Analysis, whereas Fourier-based solvers have recently shown large efficiency gains for homogenization problems. In this paper, we derive the discrete adjoint method for Fourier-based solvers that employ compatibility projection. We demonstrate the method on the optimization of composite materials and auxetic metamaterials, where void regions are modelled with zero stiffness.