Non-Schmid Phenomena in HCP Materials

2016 ◽  
Vol 258 ◽  
pp. 29-32
Author(s):  
Andriy Ostapovets ◽  
Olena Vatazhuk
Keyword(s):  
Plastic Deformation ◽  
Shear Stress ◽  
Computer Simulations ◽  
Slip Plane ◽  
Critical Value ◽  
Deformation Twinning ◽  
Twin Boundaries ◽  
Stress Components ◽  
Hcp Materials

The Schmid law says that yielding takes place when resolved shear stress on slip plane reaches the critical value. It is valid for wide variety of materials. However, it is well known that breaking of Schmid law takes place in bcc materials due to non-planar splitting of dislocation cores. The non-Schmid behavior is also possible for plastic deformation of fcc and hcp materials. Particularly, it is sometimes reported for deformation twinning. Present paper demonstrates the non-Schmid phenomena in hcp magnesium by means of computer simulations. We consider influence of non-glide stress components on motion of screw <a> dislocation as well as migration of twin boundaries.

scholarly journals INVESTIGATION OF THE INFLUENCE OF JUNCTION DISCLINATIONS ON PLASTIC FLOW STRESS OF POLYCRYSTALS

2021 ◽  
Vol 83 (3) ◽  
pp. 285-293
Author(s):  
Yu.V. Svirina ◽  
S.V. Kirikov ◽  
V.N. Perevezentsev
Keyword(s):  
Plastic Deformation ◽  
Shear Stress ◽  
Flow Stress ◽  
Plastic Flow ◽  
Slip Plane ◽  
Critical Shear Stress ◽  
Elastic Field ◽  
Pile Up ◽  
Plastic Flow Stress ◽  
Junction Disclinations

Plastic deformation of polycrystalline solids is accompanied by the appearance of linear rotational-type mesodefects at grain boundary ledges and triple junction of grains, such as starin induced junction disclinations. Junction disclinations generate long-range spatially inhomogeneous fields of elastic stresses, which significantly influence on the structure formation, strain hardening and fracture of materials. In present work a comparative analysis of the contributions of junction disclinations of different sign and strength to the plastic flow stress of a polycrystal is performed. The results of calculationsshow, that when a pile-up of lattice dislocations passes through the elastic field of disclinations, general regularities are observed.Regardless of the sign of disclination, it has a retarding effect on the plastic shear. The equilibrium distributions of the linear density and the density of the Burgers vector of dislocations pile-upretarded by the elastic field of disclination are calculated.It is shown that the largest number of dislocations is concentrated not in the pile-up head, as in classical dislocation pile-upsstoped near impenetrable barriers, but in its central part. The dependences of the critical stress of the passage of the head dislocation of the pile-up through the force barrier of disclination are calculated depending on the strength and sign of disclination, the number of dislocations in the pile-up, and the distance between the disclination and the slip plane of lattice dislocations.It is shown that the change in the sign of disclination significantly influences on the form of the equilibrium distribution of dislocations along the length of the pile-up, but practically does not affect the value of the critical shear stress. It is shown that for a fixed number of dislocations in the pile-up, the critical shear stress increases with the distance between the slip plane and disclination. Thus, when plastic deformation is localized, the greatest strengthening effect from the elastic field of junction disclination is achieved not near the boundary, but far from it.

scholarly journals Modelling the nucleation and propagation of cracks at twin boundaries

2021 ◽  
Author(s):  
Nicolò Grilli ◽  
Alan C. F. Cocks ◽  
Edmund Tarleton
Keyword(s):  
Plastic Deformation ◽  
Shear Stress ◽  
Cohesive Zone Model ◽  
Bulk Material ◽  
Zone Model ◽  
Fracture Path ◽  
Twin Boundaries ◽  
Interface Elements ◽  
Local Term ◽  
Good Agreement

AbstractFracture arising from cracks nucleating and propagating along twin boundaries is commonly observed in metals that exhibit twinning as a plastic deformation mechanism. This phenomenon affects the failure of macroscopic mechanical components, but it is not fully understood. We present simulations in which a continuum model for discrete twins and a cohesive zone model are coupled to aid the understanding of fracture at twin boundaries. The interaction between different twin systems is modelled using a local term that depends on the continuum twin variables. Simulations reveal that the resolved shear stress necessary for an incident twin to propagate through a barrier twin can be up to eight times the resolved shear stress for twin nucleation. Interface elements are used at the interfaces between all bulk elements to simulate arbitrary intragranular cracks. An algorithm to detect twin interfaces is developed and their strength has been calibrated to give good agreement with the experimentally observed fracture path. The elasto-plastic deformation induced by discrete twins is modelled using the crystal plasticity finite element method and the stress induced by twin tips is captured. The tensile stress caused by the tip of an incident twin on a barrier twin is sufficient to nucleate a crack. A typical staircase fracture path, with cracks propagating along the twin interfaces, is reproduced only if the strength of the twin interfaces is decreased to about one-third of the strength of the bulk material. This model can be used to help understand fracture caused by the activation of multiple twin systems in different materials.

scholarly journals Interaction of Migrating Twin Boundaries with Obstacles in Magnesium

Metals ◽  
2021 ◽  
Vol 11 (1) ◽  
pp. 154
Author(s):  
Andriy Ostapovets ◽  
Konstantin Kushnir ◽  
Kristián Máthis ◽  
Filip Šiška
Keyword(s):  
Shear Stress ◽  
Finite Elements ◽  
Twin Boundary ◽  
Computer Simulations ◽  
Bulk Material ◽  
Twin Boundaries ◽  
Twin Growth ◽  
Critical Resolved Shear Stress

Interaction of migrating {101¯2} twin boundary with obstacles was analyzed by atomistic and finite elements computer simulations of magnesium. Two types of obstacles were considered: one is a non-shearable obstacle and another one is the void inside bulk material. It is shown that both types of obstacles inhibit twin growth and increased stress is necessary to engulf the obstacle in both cases. However, the increase of critical resolved shear stress is higher for the passage of the twin boundary through raw of voids than for interaction with non-shearable obstacles.

Efficiency prediction for storage chambers using computational fluid dynamics

1996 ◽  
Vol 33 (9) ◽  
pp. 163-170 ◽  
Author(s):  
Virginia R. Stovin ◽  
Adrian J. Saul
Keyword(s):  
Fluid Dynamics ◽  
Computational Fluid Dynamics ◽  
Shear Stress ◽  
Particle Tracking ◽  
Critical Value ◽  
Complex Geometries ◽  
Bed Shear Stress ◽  
Breadth Ratio ◽  
Computational Fluid Dynamics Cfd ◽  
Simulated Flow

Research was undertaken in order to identify possible methodologies for the prediction of sedimentation in storage chambers based on computational fluid dynamics (CFD). The Fluent CFD software was used to establish a numerical model of the flow field, on which further analysis was undertaken. Sedimentation was estimated from the simulated flow fields by two different methods. The first approach used the simulation to predict the bed shear stress distribution, with deposition being assumed for areas where the bed shear stress fell below a critical value (τcd). The value of τcd had previously been determined in the laboratory. Efficiency was then calculated as a function of the proportion of the chamber bed for which deposition had been predicted. The second method used the particle tracking facility in Fluent and efficiency was calculated from the proportion of particles that remained within the chamber. The results from the two techniques for efficiency are compared to data collected in a laboratory chamber. Three further simulations were then undertaken in order to investigate the influence of length to breadth ratio on chamber performance. The methodology presented here could be applied to complex geometries and full scale installations.

scholarly journals Structural Transformations and Mechanical Properties of Metastable Austenitic Steel under High Temperature Thermomechanical Treatment

Metals ◽  
2021 ◽  
Vol 11 (4) ◽  
pp. 645
Author(s):  
Igor Litovchenko ◽  
Sergey Akkuzin ◽  
Nadezhda Polekhina ◽  
Kseniya Almaeva ◽  
Evgeny Moskvichev
Keyword(s):  
Plastic Deformation ◽  
Grain Size ◽  
High Temperature ◽  
Austenitic Steel ◽  
Structural Transformations ◽  
Initial State ◽  
Twin Boundaries ◽  
Metastable Austenitic Steel ◽  
Average Grain Size ◽  
Heating Up

The effect of high-temperature thermomechanical treatment on the structural transformations and mechanical properties of metastable austenitic steel of the AISI 321 type is investigated. The features of the grain and defect microstructure of steel were studied by scanning electron microscopy with electron back-scatter diffraction (SEM EBSD) and transmission electron microscopy (TEM). It is shown that in the initial state after solution treatment the average grain size is 18 μm. A high (≈50%) fraction of twin boundaries (annealing twins) was found. In the course of hot (with heating up to 1100 °C) plastic deformation by rolling to moderate strain (e = 1.6, where e is true strain) the grain structure undergoes fragmentation, which gives rise to grain refining (the average grain size is 8 μm). Partial recovery and recrystallization also occur. The fraction of low-angle misorientation boundaries increases up to ≈46%, and that of twin boundaries decreases to ≈25%, compared to the initial state. The yield strength after this treatment reaches up to 477 MPa with elongation-to-failure of 26%. The combination of plastic deformation with heating up to 1100 °C (e = 0.8) and subsequent deformation with heating up to 600 °C (e = 0.7) reduces the average grain size to 1.4 μm and forms submicrocrystalline fragments. The fraction of low-angle misorientation boundaries is ≈60%, and that of twin boundaries is ≈3%. The structural states formed after this treatment provide an increase in the strength properties of steel (yield strength reaches up to 677 MPa) with ductility values of 12%. The mechanisms of plastic deformation and strengthening of metastable austenitic steel under the above high-temperature thermomechanical treatments are discussed.

scholarly journals Design and Optimization of a Shearing Machine Using FEA

2017 ◽  
Vol 7 (2 (S)) ◽  
pp. 181
Author(s):  
Satish Bahaley ◽  
Rasika Khairkar
Keyword(s):  
Plastic Deformation ◽  
Stainless Steel ◽  
Shear Stress ◽  
Cad Model ◽  
Design And Optimization ◽  
Steel Sheets

Shearing is the process to cut sheets using pair of blades, by applying shear stress along the thickness of the sheet. Shearing happens by extreme plastic deformation followed by breaking which propagates deeper into the thickness. The upper blade is fixed to the ram assembly that moves vertically and lower knife is fixed in the stationary table. This project is rooted on the necessity of industry to develop a shearing machine for cutting 5mm thick stainless steel sheets. In this project we will design a CAD model of shearing machine and analyze using FEA technique.

scholarly journals Mechanical Resonance Dispersion Revisited

2014 ◽  
Vol 1 (1) ◽  
pp. 1-14 ◽  
Author(s):  
Haskell V. Hart
Keyword(s):  
Plastic Deformation ◽  
Shear Stress ◽  
Chemical Analysis ◽  
Normal Modes ◽  
Theoretical Explanation ◽  
Mechanical Resonance ◽  
Complex Shear ◽  
High Degree ◽  
Shear Compliance ◽  
Strain Responses

Mechanical resonance dispersion is the inelastic response of a solid to a periodic shear stress. Instead of the elastic Young's Modulus, the phenomenon is described by both a real J', and an imaginary J'' component of complex shear compliance, corresponding to in phase and out of phase strain responses, respectively. The experimental results are plots of J' and J'' vs. frequency, which are typically in the audiofrequency range of 10 - 5600 Hz. Resonances are observed as maxima in J'' and inversions in J' at frequencies corresponding to modes of plastic deformation, which are much lower frequencies (audiofrequency range) than elastic normal modes. The theoretical explanation of Edwin R. Fitzgerald involves particle waves and momentum transfer and leads to a particle-in-a-box frequency formula for these inelastic modes. Unfortunately, most of his and other published raw data were never analyzed by this model. The purpose of this article is to apply this formula to previously uninterpreted resonance dispersion curves and to address some of the earlier criticism of Fitzgerald's work. Results of these calculations support the Fitzgerald Theory to a high degree, demonstrate the importance of impurities and chemical analysis, largely mollify previous criticisms, and suggest the possibility of a new particle wave mass spectroscopy at great distances.

deformation twinning in pure Ti during dynamic plastic deformation

2012 ◽  
Vol 541 ◽  
pp. 190-195 ◽  
Author(s):  
Feng Xu ◽  
Xiyan Zhang ◽  
Haitao Ni ◽  
Qing Liu
Keyword(s):  
Plastic Deformation ◽  
Deformation Twinning ◽  
Dynamic Plastic Deformation

Limits to elastic response. Yielding and failure

2020 ◽  
pp. 357-368
Author(s):  
Lallit Anand ◽  
Sanjay Govindjee
Keyword(s):  
Shear Stress ◽  
Tensile Stress ◽  
Critical Value ◽  
Maximum Principal Stress ◽  
The Body ◽  
Elastic Response ◽  
Elastic Behaviour ◽  
Pressure Independent ◽  
Yield Conditions ◽  
Is Failure

This chapter presents conditions for determining the limits of elastic behaviour for isotropic materials. The stress invariants of equivalent pressure, equivalent shear stress, and equivalent tensile stress are defined. These are then used to define common yield conditions, viz. the pressure-independent Mises and Tresca yield conditions, as well as the pressure-dependent Coulomb-Mohr and the Drucker-Prager yield conditions. Rankine’s failure criterion for brittle materials in tension, that is failure in a brittle material will initiate when the maximum principal stress at a point in the body reaches a critical value, is also discussed.

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