Dynamic Shear Band Propagation Research Articles

High deformation capacity and dynamic shear band propagation of imprinted amorphous Cu50Zr50/crystalline Cu multilayered nanofilms

Molecular dynamics simulations are used to investigate the plastic deformation behavior of Cu50Zr50/Cu amorphous/crystalline (A/C) multilayered nanofilms with different interface directions, number of layers and temperatures under the imprinting process. The results show that the loading force of multilayered nanofilms grows with increasing thickness of layers and decreasing temperature. Specifically, the maximum stress is larger with increasing layer thickness, this shows an inverse Hall-Petch relationship between the stress and the layer thickness. The local stress is focused around the stamp and rises up as increasing imprinting depth. The plastic deformation behaviors are realized by dislocations or stacking faults in crystalline layers and shear transformation zones (STZs) or shear bands in amorphous layers. Additionally, the propagation of shear bands or spatial correlation of STZs in an amorphous layer can be disrupted at the amorphous/crystalline interfaces (ACIs). The centrosymmetry parameter (CSP) and dislocation analysis (DXA) reveal the nucleation of stacking faults at the ACIs and the Shockley partial dislocation account for 80% in the total dislocations in all cases. The affected region extends and displacement of atoms becomes more irregular as increasing temperature. Besides, the radial distribution function (RDF) reveals the structure of the material is more stable at low temperature. The transverse interface specimens and thinner layer thickness, and high temperature are facilitated to material formability.

Effects of shear band propagation on early waves generated by initial breakoff of tsunamigenic landslides

The landslide mechanism based on the phenomenon of the dynamic shear band propagation suggests that landslide run-out begins with a non-zero initial velocity. The initial uplift of the passive failure block during the landslide breakoff is similar to the reverse-fault, and therefore can generate an early tsunami wave that will be one of the first waves to start propagating and could be potentially detected by the deep ocean warning systems. This paper attempts to quantify this phenomenon using the Coupled Eulerian–Lagrangian (CEL) approach within ABAQUS and modeling both soil and water as Eulerian materials. This approach allows for capturing the entire process of failure initiation, propagation and breakoff, taking into account large landslide deformations, soil plasticity and interaction of the sliding layer with the seawater. The study shows that the highest breakoff velocities are achieved for the shortest lengths of the initial weak zones initiating the shear band propagation. It also shows that both interaction of the sliding layer with water and appearance of the large plastic/failure zones during the shear band propagation cause slower uplift velocities and accelerations. For the computed example of a typical mid-size landslide in normally consolidated clays, with the initial weak zone length of several hundred meters and the final landslide length of about 1km, the breakoff velocities can reach up to 4m/s within 15s, leading to an early wave of 0.5–1.5m height, caused purely by this breakoff. Such waves can be easily detected by existing Deep-ocean Assessment and Reporting of Tsunamis (DART) systems, potentially allowing for some early tsunami warning.

Dynamic analysis of strain localization in water‐saturated clay using a cyclic elasto‐viscoplastic model

SUMMARYA computational framework is presented for dynamic strain localization and deformation analyses of water‐saturated clay by using a cyclic elasto‐viscoplastic constitutive model. In the model, the nonlinear kinematic hardening rule and softening due to the structural degradation of soil particles are considered. In order to appropriately simulate the large deformation phenomenon in strain localization analysis, the dynamic finite element formulation for a two‐phase mixture is derived in the updated Lagrangian framework. The shear band development is shown through the distributions of viscoplastic shear strain, the axial strain, the mean effective stress, and the pore water pressure in a normally consolidated clay specimen. From the local stress–strain relations, more brittleness is found inside the shear bands than outside of them. The effects of partially drained conditions and mesh‐size dependency on the shear banding are also investigated. The effect of a partially drained boundary is found to be insignificant on the dynamic shear band propagation because of the rapid rate of applied loading and low permeability of the clay. Using the finer mesh results in slightly narrower shear bands; nonetheless, the results manifest convergency through the mesh refinement in terms of the overall shape of shear banding and stress–strain relations. Copyright © 2013 John Wiley & Sons, Ltd.

Overlapping and non-overlapping domain decomposition methods for large-scale meshless EFG simulations

Meshless methods have a number of virtues in problems concerning crack growth and propagation, strain localization, dynamic shear band propagation, projectile penetration, among others. The main drawback of such methods is that the resulting matrices are more densely populated and the computational cost for the formulation and solution of the problem is much higher than the conventional FEM. This is the reason for their limited application to academic and small-scale problems until now. In this paper, we introduce a novel approach for reducing the computational cost of meshless element free Galerkin (EFG) methods by employing domain decomposition techniques on the physical as well as on the algebraic domains. Specifically, we propose the implementation of the dual domain decomposition FETI family of methods that have been successfully used with FEM in many problems in computational mechanics. We address the issues of dividing an overlapping meshfree EFG domain to several overlapping subdomains, following the algebraic decomposition of the FETI method and alternatively on non-overlapping subdomains by modifying the required displacement compatibility conditions and by improving the accuracy of the solution through a stress update procedure.

Numerical investigation of dynamic shear bands in inelastic solids as a problem of mesomechanics

The main objective of the present paper is to discuss very efficient procedure of the numerical investigation of the propagation of shear band in inelastic solids generated by impact-loaded adiabatic processes. This procedure of investigation is based on utilization the finite element method and ABAQUS system for regularized thermo-elasto-viscoplastic constitutive model of damaged material. A general constitutive model of thermo-elasto-viscoplastic polycrystalline solids with a finite set of internal state variables is used. The set of internal state variables is restricted to only one scalar, namely equivalent inelastic deformation. The equivalent inelastic deformation can describe the dissipation effects generated by viscoplastic flow phenomena. As a numerical example we consider dynamic shear band propagation in an asymmetrically impact-loaded prenotched thin plate. The impact loading is simulated by a velocity boundary condition, which are the results of dynamic contact problem. The separation of the projectile from the specimen, resulting from wave reflections within the projectile and the specimen, occurs in the phenomenon. A thin shear band region of finite width which undergoes significant deformation and temperature rise has been determined. Shear band advance, shear band velocity and the development of the temperature field as a function of time have been determined. Qualitative comparison of numerical results with experimental observation data has been presented. The numerical results obtained have proven the usefulness of the thermo-elasto-viscoplastic theory in the investigation of dynamic shear band propagations.

On criteria for dynamic adiabatic shear band propagation

In this work, we postulate the physical criterion for dynamic shear band propagation, and based on this assumption, we implement a numerical algorithm and a computation criterion to simulate initiation and propagation of dynamic adiabatic shear bands (ASBs). The physical criterion is based on the hypothesis that material inside the shear band region undergoes a dynamic recrystallization process during deformation under high temperature and high strain-rate conditions. In addition to providing a new perspective to the physics of the adiabatic shearbanding process and identifying material properties that play a crucial role in defining the material's susceptibility to ASBs, the proposed criterion is instrumental in numerical simulations of the propagation of ASBs when multi-physics models are adopted to describe and predict the complex constitutive behavior of ASBs in ductile materials. Systematic and large scale meshfree simulations have been conducted to test and validate the proposed criterion by examining the formation, propagation, and post-bifurcation behaviors of ASBs in two materials, 4340 steel and OFHC copper. The effects of heat conduction, in particular the length scale introduced by heat conduction, are also studied. The results of the numerical simulations are compared with experimental observations and a close agreement is found for various characteristic features of ASBs, such as the shear band width, speed of propagation, and maximum temperature.

Dynamic shear band propagation and failure in AISI 4340 steel

Failure of metals and their alloys at high strain-rates is usually preceded by formation of adiabatic shear bands, along which cracks can easily nucleate and rapidly propagate causing fracture. In this study, adiabatic shearing and its effect on fracture behavior of AISI 4340 steel at high compressive strain-rate of about 1.9 × 10 3 s −1 was investigated. Cracking was initiated by formation of micropores during adiabatic heating and thermal softening along the shear bands. As adiabatic shearing proceeded, the pores coalesced to form clusters of pores. Microcracks that eventually grow and cause shear failure along the adiabatic shear bands are initiated from these clusters of pores.

Dynamic shear band propagation and micro-structure of adiabatic shear band

Meshfree Galerkin approximations in both two and three dimensions have been used in simulations of dynamic shear band propagation in an asymmetrically impact-loaded prenotched plate. Failure mode switching and failure mode transitions, which have been reported experimentally, are replicated in numerical computations. For intermediate impact speed ( 25 m/s<V⩽30 m/s ), the numerical results show that a cleavage crack initiates from the tip of the dynamic shear band, indicating a dominance of brittle failure mode, and a failure mode switch (ductile-to-brittle: shearband-to-crack). For high impact velocities ( V>30 m/s ), the numerical results show that a dynamic shear band penetrates through the specimen without trace of cleavage-type fracture, which is a ductile failure mode. Overall, with the increase of impact speed, the final failure mode of the impacted plate transits from brittle failure to ductile failure. By introducing a multi-physics model to describe the stress collapse state of the shear band, it has been found that there is a non-uniform temperature distribution inside the adiabatic shear band. Strong evidences indicate that temperature distribution inside the shear band has periodic patterns in both space and time, confirming the latest experimental results of P. Guduru et al. [Mech. Mater. (2000), submitted]. This suggests that there may exist a thermal–mechanical instability within the adiabatic shear band, reminiscent of hydrodynamic instability due to viscous heating.

Mesh-free Galerkin simulations of dynamic shear band propagation and failure mode transition

A mesh-free Galerkin simulation of dynamic shear band propagation in an impact-loaded pre-notched plate is carried out in both two and three dimensions.The related experimental work was initially reported by Kalthoff and Winkler (1987), and later re-examined by Zhou et al.(1996a,b), and others. The main contributions of this numerical simulation are as follows: (1) The ductile-to-brittle failure mode transition is observed in numerical simulations for the first time; (2) the experimentally observed dynamic shear band, whose character changes with an increase of impact velocity, propagating along curved paths is replicated; (3) the simulation is able to capture the details of the adiabatic shear band to a point where the periodic temperature profile inside shear band at lm scale can clearly be seen; (4) an intense, high strain rate region is observed in front of the shear band tip, which, we believe, is caused by wave trapping at the shear band tip; it in turn causes damage and stress collapse inside the shear band and provides a key link for self-sustained instability. 2002 Elsevier Science Ltd.All rights reserved.