Constitutive laws for deformation and dynamic recrystallization in cubic metals

We describe two cases in which constitutive laws for deformation kinetics are available that are both physically well founded and experimentally well obeyed. New experiments on Al-Mg alloys, in the regime of viscous deformation, fit the solute drift equation very well, with n-3 and Q{sub D} unadjustable; they do not fit the solute drag model. The high-stress regime, as well as all data for pure copper, fit the model of hardening and dynamic recovery, at least up to temperatures of 0.6 T{sub m}. In both cases, dynamic recrystallization occurs at high temperatures. It seems to follow rather than determine the deformation kinetics

Single cell mechanics: stress stiffening and kinematic hardening

Cell mechanical properties are fundamental to the organism but remain poorly understood. We report a comprehensive phenomenological framework for the nonlinear rheology of single fibroblast cells: a superposition of elastic stiffening and viscoplastic kinematic hardening. Our results show, that in spite of cell complexity its mechanical properties can be cast into simple, well-defined rules, which provide mechanical cell strength and robustness via control of crosslink slippage.Comment: 4 pages, 6 figure

Structure and Strength of Dislocation Junctions: An Atomic Level Analysis

The quasicontinuum method is used to simulate three-dimensional Lomer-Cottrell junctions both in the absence and in the presence of an applied stress. The simulations show that this type of junction is destroyed by an unzipping mechanism in which the dislocations that form the junction are gradually pulled apart along the junction segment. The calculated critical stress needed for breaking the junction is comparable to that predicted by line tension models. The simulations also demonstrate a strong influence of the initial dislocation line directions on the breaking mechanism, an effect that is neglected in the macroscopic treatment of the hardening effect of junctions.Comment: 4 pages, 3 figure

Finite Sized Atomistic Simulations of Screw Dislocations

The interaction of screw dislocations with an applied stress is studied using atomistic simulations in conjunction with a continuum treatment of the role played by the far field boundary condition. A finite cell of atoms is used to consider the response of dislocations to an applied stress and this introduces an additional force on the dislocation due to the presence of the boundary. Continuum mechanics is used to calculate the boundary force which is subsequently accounted for in the equilibrium condition for the dislocation. Using this formulation, the lattice resistance curve and the associated Peierls stress are calculated for screw dislocations in several close packed metals. As a concrete example of the boundary force method, we compute the bow out of a pinned screw dislocation; the line-tension of the dislocation is calculated from the results of the atomistic simulations using a variational principle that explicitly accounts for the boundary force.Comment: LaTex, 20 pages, 11 figure

Constitutive modeling of a 5182 aluminum as a function of strain rate and temperature

The authors have measured the stress/strain response of a 5182 aluminum alloy as a function of strain rate and temperature. As expected at room temperature and quasi-static strain rate this alloy exhibits dynamic strain aging with negative strain-rate sensitivity. At higher temperature, they have separated the response into two categories, when the material displays a yield drop and when it does not. The yield drop was only observed if the yield stress was below 70 MPa. In this case the work-hardening curve was for practical purposes flat. Within this regime the deformation has been labeled Class A behavior. It occurs by continuous motion of dislocations accompanied by diffusion of solute. It is further shown that a constitutive relation such as {dot {var_epsilon}} = A({sigma}/{mu}){sup n} {center_dot} {mu}b{sup 3}/kT {center_dot} exp({minus}Q{sub D}/kT) is appropriate to describe deformation in this temperature/strain-rate regime where the solute drag mechanism dominates. In this expression Q{sub D} is the activation enthalpy for self diffusion of Mg in aluminum, which is 131 kJ/mol. In the high-stress regime, where the yield stress is above 80MPa, there is positive work hardening associated with flow stress behavior of the 5182 alloy. The yield stress was nearly constant; however, the hardening and saturation flow stress increases with decreasing temperature and increasing strain rate. In this regime the deformation is dominated by dislocation accumulation and dynamic recovery. The authors have found that the Mechanical Threshold Strength (MTS) model accurately describes the constitutive response as a function of temperature and strain rate

Attempts to model the generation of new grain boundaries during the deformation of polycrystals

We explore ways by which new orientations (``nuclei``) can arise during essentially homogeneous deformation in polycrystals. All of these can be categorized as generating new large-angle boundaries: one is due to excessive storage of dislocations of one sign parallel to existing grain boundaries; another is due to reactions of cell walls with grain edges; and in the last, new boundaries are created surrounding a domain of different slip system distribution

Kink pair production and dislocation motion

The motion of extended defects called dislocations controls the mechanical properties of crystalline materials such as strength and ductility. Under moderate applied loads, this motion proceeds via the thermal nucleation of kink pairs. The nucleation rate is known to be a highly nonlinear function of the applied load, and its calculation has long been a theoretical challenge. In this article, a stochastic path integral approach is used to derive a simple, general, and exact formula for the rate. The predictions are in excellent agreement with experimental and computational investigations, and unambiguously explain the origin of the observed extreme nonlinearity. The results can also be applied to other systems modelled by an elastic string interacting with a periodic potential, such as Josephson junctions in superconductors

Dislocation Creep of Olivine: Backstress Evolution Controls Transient Creep at High Temperatures

Transient creep occurs during geodynamic processes that impose stress changes on rocks at high temperatures. The transient is manifested as evolution in the viscosity of the rocks until steady-state flow is achieved. Although several phenomenological models of transient creep in rocks have been proposed, the dominant microphysical processes that control such behavior remain poorly constrained. To identify the intragranular processes that contribute to transient creep of olivine, we performed stress-reduction tests on single crystals of olivine at temperatures of 1250–1300°C. In these experiments, samples undergo time‐dependent reverse strain after the stress reduction. The magnitude of reverse strain is ~10-3 and increases with increasing magnitude of the stress reduction. High-angular resolution electron backscatter diffraction analyses of deformed material reveal lattice curvature and heterogeneous stresses associated with the dominant slip system. The mechanical and microstructural data are consistent with transient creep of the single crystals arising from accumulation and release of backstresses among dislocations. These results allow the dislocation‐glide component of creep at high temperatures to be isolated, and we use these data to calibrate a flow law for olivine to describe the glide component of creep over a wide temperature range. We argue that this flow law can be used to estimate both transient creep and steady‐state viscosities of olivine, with the transient evolution controlled by the evolution of the backstress. This model is able to predict variability in the style of transient (normal versus inverse) and the load-relaxation response observed in previous work.LH and DW acknowledge support from the Natural Environment Research Council, grant NE/M000966/1, LH and CT acknowledge support from the Natural Environment Research Council, grant 1710DG008/JC4, and DW acknowledges support from the Netherlands Organisation for Scientific Research, User Support Programme Space Research, grant ALWGO.2018.038, and startup funds from Utrecht University. LH recognizes funds used to develop the uniaxial apparatus from the John Fell Fund at the University of Oxford

Pathogen Entrapment by Transglutaminase—A Conserved Early Innate Immune Mechanism

Clotting systems are required in almost all animals to prevent loss of body fluids after injury. Here, we show that despite the risks associated with its systemic activation, clotting is a hitherto little appreciated branch of the immune system. We compared clotting of human blood and insect hemolymph to study the best-conserved component of clotting systems, namely the Drosophila enzyme transglutaminase and its vertebrate homologue Factor XIIIa. Using labelled artificial substrates we observe that transglutaminase activity from both Drosophila hemolymph and human blood accumulates on microbial surfaces, leading to their sequestration into the clot. Using both a human and a natural insect pathogen we provide functional proof for an immune function for transglutaminase (TG). Drosophila larvae with reduced TG levels show increased mortality after septic injury. The same larvae are also more susceptible to a natural infection involving entomopathogenic nematodes and their symbiotic bacteria while neither phagocytosis, phenoloxidase or—as previously shown—the Toll or imd pathway contribute to immunity. These results firmly establish the hemolymph/blood clot as an important effector of early innate immunity, which helps to prevent septic infections. These findings will help to guide further strategies to reduce the damaging effects of clotting and enhance its beneficial contribution to immune reactions