arXiv subjects

Umar Khayaz

Publications and source records attributed to Umar Khayaz.

3 recordsLinked to original sources

arXiv2026

Dynamic phase-field model for brittle fracture in grounded glaciers

Fracture and calving of glaciers are key contributors to ice-mass loss and sea-level rise, yet predictive modeling remains challenging. Fracture in grounded glaciers is driven by gravitational forces and is typically studied within the framework of quasi-static linear elastic fracture mechanics. In this work, we show that purely quasistatic brittle fracture simulations within the phase field fracture framework under fixed self-weight can become strongly overdriven after crevasse initiation, producing unphysical thickening of the diffusive crack band and diffuse damage patterns. This pathology arises because gravity drives a growing region ahead of the crack tip beyond the strength surface. To resolve this, we show that the post-nucleation propagation is fundamentally a dynamic instability rather than a quasistatic process and propose the use of dynamic formulations of fracture. We demonstrate that accounting for inertia results in sharp, localized cracks that propagate through the ice thickness. As a second objective, this paper introduces a new dynamic formulation of the phase-field fracture model of Kumar et al. (J. Mech. Phys. Solids 2018) in which elastic, inertial, and gravitational contributions are degraded consistently in fractured regions.

cond-mat.mtrl-sci↗

arXiv2026

EvoStruct: Bridging Evolutionary and Structural Priors for Antibody CDR Design via Protein Language Model Adaptation

Equivariant graph neural network (GNN) methods for antibody complementarity-determining region (CDR) design achieve the highest sequence recovery but suffer from severe vocabulary collapse. The current best GNN methods over-predict very few amino acids, such as tyrosine and glycine, while ignoring functionally important residues. We trace this failure to GNN encoders learning amino acid distributions de novo from limited structural data, discarding substitution patterns encoded in evolutionary databases. To resolve this, we propose EvoStruct, which bridges a frozen protein language model (PLM) with 3D structural context from an E(3)-equivariant GNN via a cross-attention adapter. Unlike prior PLM-structure adapters for general protein design, EvoStruct targets the vocabulary collapse problem specific to CDR design through progressive PLM unfreezing and R-Drop consistency regularization. On the CHIMERA-Bench dataset, EvoStruct achieves the highest amino acid recovery and lowest perplexity among several antibody design methods, improving sequence recovery by 16% and reducing perplexity by 43% relative to the best GNN baselines, while recovering 2.3x greater amino acid diversity and the highest binding-pair correlation with ground truth.

cs.LG↗

arXiv2025

A comparison of phase field models of brittle fracture incorporating strength: I -- Mixed-mode loading

The classical variational phase-field model for brittle fracture effectively predicts the growth of large pre-existing cracks. However, the modeling of crack nucleation continues to be a significant challenge. Crack nucleation under uniform stress depends on the material's strength surface whose description is fundamentally incompatible with the energy-based Griffith propagation criterion. To address this, three main phase-field approaches have emerged, each attempting to reconcile material strength and toughness. The first, known as the classical variational approach, preserves the variational structure but fails to accurately incorporate the strength surface. In contrast, the other two approaches -- the complete nucleation and hybrid cohesive zone models -- sacrifice variational consistency. Among these, only the complete nucleation approach precisely accounts for the strength surface. All three approaches, especially the second one, deviate from the sharp variational theory of brittle fracture, raising concerns about their reliability in predicting the growth of cracks under non-mode-I loading. This paper evaluates precisely this issue. It is the first in a series of studies comparing the three approaches, systematically investigating crack growth under mode II, mode III, and mixed-mode loadings. The results confirm that the complete nucleation approach effectively predicts crack growth across all investigated problems, and its predictions agree well with those from other two approaches for tension-dominated cases. Additionally, the findings highlight that inaccurate accounting of the strength surface in the classical variational approach can influence crack path predictions. Lastly, they reveal that modifying the crack driving force to incorporate the strength surface in the hybrid cohesive zone approach causes crack propagation at an incorrect fracture toughness.

cond-mat.mtrl-sci↗