arXiv subjects

O. U. Salman

Publications and source records attributed to O. U. Salman.

11 recordsLinked to original sources

arXiv2025

Connecting strain rate dependence of fcc metals to dislocation avalanche signatures

Strain rate sensitivity is a key feature of material deformation, whose importance is growing both because miniaturized components experience higher effective rates and because small scale simulations increasingly probe such conditions. As a dynamical characteristic, strain rate dependence is shown to be intimately connected to dislocation avalanches, which are a fundamental mechanism of dislocation dynamics. Using carefully designed, state of the art dislocation dynamics simulations in the intermediate range strain rate from 5 to 1000, we show that increasing strain rate promotes the activation of a growing number of stronger sites. The dislocation microstructure progressively rearranges into configurations with shorter segments. Dislocation avalanches become larger through the superposition of simultaneous events and because stronger obstacles are required to arrest them. As a result, the avalanche statistics are strongly affected by strain rate, with a reduced power law regime and an increasing power law exponent. Larger avalanches, in turn, lead to an enhanced dislocation storage rate. Contribution from collinear systems to avalanches and cross slip activity decreases, altering the fraction of screw dislocations and the resulting microstructure. These results provide an original mesoscopic picture of rate sensitivity in this strain rate range and offer a mechanistic interpretation of existing observations and findings from experiments and simulations.

cond-mat.mtrl-sci↗

arXiv2025

Navigating with Stability: Local Minima, Patterns, and Evolution in a Gradient Damage Fracture Model

We investigate the computation of stable fracture paths in brittle thin films using one-dimensional damage models with an elastic foundation. The underlying variational formulation is non-convex, making the evolution path sensitive to algorithmic choices. In this paper, we inquire into the effectiveness of quasi-Newton algorithms as an alternative to conventional Newton-Raphson solvers. These algorithms improve convergence by constructing a positive definite approximation of the Hessian, trading improved convergence for the risk of missing bifurcation points and stability thresholds. In the absence of irreversibility constraints, we construct an equilibrium map that represents all stable and unstable equilibrium states as a function of the external load, using well-known branch-following bifurcation techniques. Our main finding is that quasi-Newton algorithms fail to select stable evolution paths without exact second variation information. To overcome this, we introduce a spectral stability criterion based on the full Hessian, which enables the identification of optimal perturbations and improves path-following accuracy. We then extend our analysis to the irreversible case, where admissible perturbations are constrained to a cone. We develop a nonlinear constrained eigenvalue solver to compute the minimal eigenmode within this restricted space and show that it plays a key role in distinguishing physical instabilities from numerical artefacts. Our results provide practical guidance for robust computation of fracture paths in irreversible, non-convex settings.

nlin.PS↗

arXiv2023

Inelastic rotations and pseudo-turbulent plastic avalanches in crystals

Plastic deformations in crystals often produce textures in the form of randomly oriented patches of the unstressed lattice. We use a novel mesoscopic Landau-type model of crystal plasticity to show that in such textures large crystallographic lattice rotations can originate from a highly coordinated inelastic slip at the microscale. Our numerical experiments show that dislocation avalanches, which lead to the formation of such rotations, involve pseudo-turbulent motions with power-law distributed spatial correlations.

cond-mat.mes-hall↗

arXiv2022

Homogeneous nucleation of dislocations as a pattern formation phenomenon

Dislocation nucleation in homogeneous crystals initially unfolds as a linear symmetry-breaking elastic instability. In the absence of explicit nucleation centers, such instability develops simultaneously all over the crystal and due to the dominance of long range elastic interactions it advances into the nonlinear stage as a collective phenomenon through pattern formation. In this paper we use a novel mesoscopic tensorial model (MTM) of crystal plasticity to study the delicate role of crystallographic symmetry in the development of the dislocation nucleation patterns in defect free crystals loaded in a hard device. The model is formulated in 2D and we systematically compare lattices with square and triangular symmetry. To avoid the prevalence of the conventional plastic mechanisms, we consider the loading paths represented by pure shears applied on the boundary of the otherwise unloaded body. These loading protocols can be qualified as exploiting the 'softest' and the 'hardest' directions and we show that the associated dislocation patterns are strikingly different.

cond-mat.mtrl-sci↗

arXiv2021

Atomistic simulation of martensite microstructural evolution during temperature driven $β\rightarrow α$ transition in pure titanium

Titanium and its alloys undergo temperature-driven martensitic phase transformation leading to the development of complex microstructures at mesoscale. Optimizing the mechanical properties of these materials requires an understanding of the correlations between the processing parameters and the mechanisms involved in the microstructure formation and evolution. In this work, we study the temperature-induced phase transition from BCC to HCP in pure titanium by atomistic modeling and investigate the influence of local stress conditions on the final martensite morphology. We simulate the transition under different stress conditions and carry a detailed analysis of the microstructural evolution during transition using a deformation gradient map that characterizes the local lattice distortion. The analysis of final martensite morphologies shows how mechanical constraints influence the number of selected variants and the number/type of defects in the final microstructure. We give insight on the origin and structure of different interfaces experimentally observed, such as inter-variant boundaries and antiphase defects. In particular, we show how antiphase defects originate from the two-fold degeneracy shuffling displacement arriving during the transition and how the triple junction formation drives the texture evolution when local stresses prevent a free shape change of the matrix surrounding the growing martensite nuclei.

cond-mat.mtrl-sci↗

arXiv2021

De-localizing brittle fracture

Extreme localization of damage in conventional brittle materials is the source of a host of undesirable effects. We show how artificially engineered metamaterials with all brittle constituents can be designed to ensure that every breakable sub-element fails independently. The main role in the proposed design is played by high contrast composite sub-structure with zero-stiffness, furnishing nonlocal stress redistribution. The ability to de-localize cracking in such nominally brittle systems is revealed by the fact that their continuum description is dominated by the gradient (bending) rather than classical (stretching) elasticity. By engineering a crossover from brittle to effectively ductile (quasi-brittle) behavior in prototypical systems of this type, we reveal the structural underpinning behind the difference between fracture and damage.

cond-mat.mtrl-sci↗

arXiv2021

Discontinuous yielding of pristine micro-crystals

We study the mechanical response of a dislocation-free 2D crystal under homogenous shear using a new mesoscopic approach to crystal plasticity, a Landau-type theory, accounting for the global invariance of the energy in the space of strain tensors while operating with an infinite number of equivalent energy wells. The advantage of this approach is that it eliminates arbitrariness in dealing with topological transitions involved, for instance, in nucleation and annihilation of dislocations. We use discontinuous yielding of pristine micro-crystals as a benchmark problem for the new theory and show that the nature of the catastrophic instability, which in this setting inevitably follows the standard affine response, depends not only on lattice symmetry but also on the orientation of the crystal in the loading device. The ensuing dislocation avalanche involves cooperative dislocation nucleation, resulting in the formation of complex microstructures controlled by a nontrivial self-induced coupling between different plastic mechanisms.

cond-mat.mtrl-sci↗

arXiv2020

Variety of scaling behaviors in nanocrystalline plasticity

We address the question of why larger, high symmetry crystals are mostly weak, ductile and statistically sub-critical, while smaller crystals with the same symmetry are strong, brittle and super-critical. We link it to another question of why intermittent elasto-plastic deformation of sub-micron crystals features highly unusual size sensitivity of scaling exponents. We use a minimal integer-valued automaton model of crystal plasticity to show that with growing variance of quenched disorder, which can serve in this case as a proxy for increasing size, sub-micron crystals undergo a crossover from spin-glass marginality to criticality characterizing the second order brittle-to-ductile (BD) transition. We argue that this crossover is behind the non-universality of scaling exponents observed in physical and numerical experiments. The non-universality emerges only if the quenched disorder is elastically incompatible and it disappears if the disorder is compatible.

cond-mat.soft↗

arXiv2019

Landau theory of bending-to-stretching transition

Transition from bending-dominated to stretching-dominated elastic response in semi-flexible fibrous networks plays an important role in the mechanical behavior of cells and tissues. It is induced by changes in network connectivity and relies on construction of new cross-links. We propose a simple continuum model of this transition with macroscopic strain playing the role of order parameter. An unusual feature of this Landau-type theory is that it is based on a single-well potential. We predict that bending-to-stretching transition proceeds through propagation of the localized fronts separating domains with affine and non-affine elastic response.

cond-mat.soft↗

arXiv2019

Landau theory of crystal plasticity

We show that nonlinear continuum elasticity can be effective in modeling plastic flows in crystals if it is viewed as Landau theory with an infinite number of equivalent energy wells whose configuration is dictated by the symmetry group GL(3,Z). Quasi-static loading can be then handled by athermal dynamics, while lattice based discretization can play the role of regularization. As a proof of principle, we study in this Letter dislocation nucleation in a homogeneously sheared 2D crystal and show that the global tensorial invariance of the elastic energy foments the development of complexity in the configuration of collectively nucleating defects. A crucial role in this process is played by the unstable higher symmetry crystallographic phases, traditionally thought to be unrelated to plastic flow in lower symmetry lattices.

cond-mat.mtrl-sci↗

arXiv2011

Origin of stabilization of macrotwin boundaries in martensites

The origin of stabilization of complex microstructures along macrotwin boundaries in martensites is explained by comparing two models based on Ginzburg-Landau theory. The first model incorporates a geometrically nonlinear strain tensor to ensure that the Landau energy is invariant under rigid body rotations, while the second model uses a linearized strain tensor under the assumption that deformations and rotations are small. We show that the approximation in the second model does not always hold for martensites and that the experimental observations along macrotwin boundaries can only be reproduced by the geometrically nonlinear (exact) theory.

cond-mat.mtrl-sci↗