Influence of Slip Localization on Surface Relief Formation and Grain Boundary Microcrack Nucleation

2011 ◽  
Vol 465 ◽  
pp. 35-40 ◽  
Author(s):  
Maxime Sauzay ◽  
Pierre Evrard ◽  
Karine Bavard
Keyword(s):  
Aspect Ratio ◽  
Stress Field ◽  
Grain Boundary ◽  
Slip Band ◽  
Surface Relief ◽  
Tensile Deformation ◽  
Equivalent Stress ◽  
Slip Bands ◽  
Relief Formation ◽  
Analytical Formulae

Slip localization is often observed in metallic polycrystals after cyclic deformation (persistent slip bands) or pre-irradiation followed by tensile deformation (channels). To evaluate its influence on surface relief formation and grain boundary microcrack nucleation, crystalline finite element (FE) computations are carried out using microstructure inputs (slip band aspect ratio/spacing). Slip bands (low critical resolved shear stress (CRSS)) are embedded in small elastic aggregates. Slip band aspect ratio and neighboring grain orientations influence strongly the surface slips. But only a weak effect of slip band CRSS, spacing and grain boundary orientation is observed. Analytical formulae are deduced which allow an easy prediction of the surface and bulk slips. The computed slips are in agreement with experimental measures (AFM/TEM measures on pre-irradiated austenitic stainless steels and nickel, copper and precipitate-strengthened alloy subjected to cyclic loading). Grain boundary normal stresses are computed for various materials and loading conditions. A square root dependence with respect to the distance to the slip band corner is found similarly to the pile-up stress field. But the equivalent stress intensity factor is considerably lower. Analytical formulae are proposed for predicting the grain boundary normal stress field depending on the microstructure lengths. Finally, an energy balance criterion is applied using the equivalent elastic energy release rate and the surface/grain boundary energies. The predicted macroscopic stresses for microcrack nucleation are compared to the experimental ones.

Simulation of Microstructural Evolution during Superplastic Deformation

1999 ◽  
Vol 601 ◽  
Author(s):  
B.-N. Kim ◽  
K. Hiraga
Keyword(s):  
Aspect Ratio ◽  
Grain Boundary ◽  
Grain Growth ◽  
Boundary Diffusion ◽  
Tensile Deformation ◽  
Grain Boundary Diffusion ◽  
Initial Value ◽  
The Matrix ◽  
Dynamic Growth ◽  
Equiaxed Grains

AbstractSuperplastic tensile deformation is simulated in 2 dimensions by incorporating grain boundary diffusion and concurrent grain growth derived from static and dynamic growth mechanisms. The following relationship is found between microstructural changes and deformation behavior for constant stress conditions. Grain boundary diffusion produces an increase in the aspect ratio of the matrix grains during deformation and the increased aspect ratio causes a change in creep rate parameters: the stress exponent is decreased from the initial value of 1.0 for equiaxed grains and the grain size exponent is increased from the initial value of 3.0. Accelerated grain growth is also found by the present simulation.

Comparison between Pile-Up Singularities and Stress Fields Induced by Thin Slip Bands. Application to the Prediction of Grain Boundary Microcrack Nucleation

2013 ◽  
Vol 592-593 ◽  
pp. 61-66
Author(s):  
Maxime Sauzay ◽  
Mohamed Ould Moussa
Keyword(s):  
Slip Band ◽  
Tensile Deformation ◽  
Finite Thickness ◽  
Local Stress ◽  
Stress Fields ◽  
Stress Concentrations ◽  
Griffith Criterion ◽  
Slip Bands ◽  
Pile Up ◽  
Microcrack Initiation

Slip localization is widely observed in metallic polycrystals after tensile deformation, cyclic deformation or pre-irradiation followed by tensile deformation. Such strong deformation localized in thin slip bands induces local stress concentrations in the quasi-elastic matrix around, at the intersections between slip bands (SBs) and grain boundaries (GBs) where microcrack initiation is often observed. Since the work of Stroh, such stress fields have been mostly modeled using the dislocation pile-up theory which leads to stress singularities similar to the LEFM ones. The Griffith criterion has then been widely applied, leading usually to strong underestimations of the macroscopic stress to GB crack initiation. In fact, slip band thickness is finite: 20nm-1000nm depending on material, temperature and loading conditions. Then, many slip planes are plastically activated through the thickness, and not only one single atomic plane. To evaluate more realistic stress fields, numerous crystalline finite element (FE) computations have been carried out using microstructure inputs (slip band aspect ratio, crystal and GB orientation...). A strong influence of slip band thickness close to the slip band corner has been highlighted, which is not accounted for by the pile-up theory. But far away, the thickness has a negligible effect and the predicted stress fields are close to the one predicted by the pile-up theory. Closed-form expressions are deduced from the numerous FE computation results allowing a straightforward prediction of GB stress fields. Slip band plasticity parameters, such as length and thickness, as well as crystal orientation, GB plane and remote stress are taken into account. The dependence with respect to the various parameters can be understood in the framework of matching expansions usually applied to cracks with V notches of finite thickness. As the exponent of the GB stress close-field is only about one-half of the pile-up or LEFM crack one, the Griffith criterion may not be used for GB microcrack prediction in case of finite thickness. That is why finite crack fracture mechanics is used together with both energy and stress criteria. Taking into account SB finite thickness, t>0, leads to predicted remote stresses to GB microcrack initiation three to six times lower than the ones predicted using the to pile-up theory, in agreement with experimental data.

Modeling of Fatigue Slip Bands and Application to Notch Fatigue Problems

2008 ◽  
Author(s):  
Hiroshi Matsuno
Keyword(s):  
Fatigue Strength ◽  
Slip Band ◽  
Equivalent Stress ◽  
Cyclic Stress ◽  
Empirical Equations ◽  
Fundamental Idea ◽  
Slip Bands ◽  
Heat Treated ◽  
Fatigue Data ◽  
Stress Ratios

In the present paper, equivalent stress ratios (REQ-ratios), which have been proposed as parameters for correspondence between cyclic stress conditions of notched and unnotched specimens, are reviewed. The REQ-ratios are formulated based on a concept of plastic adaptation hypothesized for a fatigue slip band from a viewpoint of macro-mechanics. A method for diagramming fatigue strength of metals based on the parameter of the REQ-ratios is newly proposed. The method diagramming together the fatigue strength of notched and unnotched specimens is applied not only to fatigue problems of usually annealed, normalized and heat-treated materials but also to those of severely heat-treated and surface-treated ones. Fatigue strength diagrams are characterized with two types of fatigue strength: σw1 and σw2. The character of σw2 appears not only in specimens with sharp notches but also in unnotched specimens fatigue-tested at lower RN-ratios. Criteria on fatigue strength σw1 and σw2 are derived from the fatigue strength diagrams and formulated as empirical equations. Characteristics of fatigue slip bands are reviewed and two types of fatigue mechanisms are proposed related with fatigue strength σw1 and σw2 from a viewpoint of micro-mechanics. Consequently, it is found that the hypothesis of plastic adaptation is a very useful and fundamental idea for modeling a fatigue slip band and also for analyzing fatigue data of practical metals and alloys.

The stress-driven redistribution of point defects in the vicinity of crack-like singularities

1990 ◽  
Vol 427 (1872) ◽  
pp. 1-23 ◽  
Keyword(s):  
Steady State ◽  
Stress Field ◽  
Grain Boundary ◽  
Point Defect ◽  
Slip Band ◽  
Point Defects ◽  
Elastic Interaction ◽  
Mode Ii ◽  
Fracture Of Materials ◽  
Kinetics Of

The most important term in the energy of the elastic interaction between a crack and a point defect is presented and used to estimate the kinetics of redistribution of point defects in the stress field of an isolated crack under mode II load and a slip band impinging against a grain boundary sink. Our analyses show that the point defects should migrate only to the tip of the crack, whereas they should enter both into the slip band tip and along the adjacent boundary interface. Explicit results are obtained for the concentrations, the number and flux distributions as well as the total numbers segregated in the transient depletion and the steady-state irradiation situation and serve to reinforce previous conclusions regarding the importance of such stress-driven processes in the fracture of materials.

Soft Crosslinkable Azo Polymer for Rapid Surface Relief Formation and Persistent Fixation

2001 ◽  
Vol 13 (22) ◽  
pp. 1693-1697 ◽  
Author(s):  
N. Zettsu ◽  
T. Ubukata ◽  
T. Seki ◽  
K. Ichimura
Keyword(s):  
Surface Relief ◽  
Azo Polymer ◽  
Relief Formation

Surface Relief Formation in Relation to the Underlying Dislocation Arrangement

2016 ◽  
Vol 258 ◽  
pp. 526-529 ◽  
Author(s):  
Veronika Mazánová ◽  
Milan Heczko ◽  
Ivo Kuběna ◽  
Jaroslav Polák
Keyword(s):  
Electron Microscopy ◽  
Dislocation Structure ◽  
Fatigue Crack Initiation ◽  
Surface Profile ◽  
Specimen Surface ◽  
Slip Bands ◽  
Transmission Electron ◽  
Persistent Slip Bands ◽  
Relief Formation ◽  
Cyclic Slip

Two fatigued materials with f.c.c. lattice, i.e. pure polycrystalline copper and austenitic Sanicro 25 stainless steel, were subjected to the study of the persistent slip markings (PSMs) developed on the surface of the suitably oriented grains. They were observed using scanning electron microscopy (SEM) and thin surface FIB lamellae were prepared and studied by transmission electron microscopy (TEM). The aim was to correlate the specimen surface profile with the underlying internal dislocation structure. The localization of the intensive cyclic slip into persistent slip bands (PSBs) of the material was observed and associated with the PSMs on the specimen surface. Extrusions, intrusions and the dislocation structure appertaining to them were analysed, documented and discussed in relation to the models of fatigue crack initiation.

A Numerical and Experimental Study of Cavitation in a Hot Tensile Axisymmetric Testpiece

2005 ◽  
Vol 40 (6) ◽  
pp. 571-586 ◽  
Author(s):  
Y Liu ◽  
J Lin ◽  
T. A Dean ◽  
D. C. J Farrugia
Keyword(s):  
Experimental Study ◽  
Flow Stress ◽  
Grain Boundary ◽  
Thermal Gradient ◽  
Tensile Testing ◽  
Tensile Deformation ◽  
Strain Rates ◽  
Test Results ◽  
Integrated Technique ◽  
Hot Tensile Deformation

During axisymmetric hot tensile testing, necking normally takes place due to the thermal gradient and the accumulation of microdamage. This paper introduces an integrated technique to predict the damage and necking evolution behaviour. Firstly, a set of multiaxial mechanism-based unified viscoplastic-damage constitutive equations is presented. This equation set, which models the evolution of grain boundary (intragranular) and plasticity-induced (intergranular) damage, is determined for a free-cutting steel tested over a range of temperatures and strain rates on a Gleeble thermomechanical simulator. This model has been implemented using the CREEP subroutine of the commercial finite element (FE) solver ABAQUS. Numerical procedures to simulate axisymmetric hot tensile deformation are developed with consideration of the thermal gradient along the axis of the tensile testpiece. FE simulations are carried out to reproduce the necking phenomenon and the evolution of plasticity-induced and grain boundary damage. The simulated results have been validated with experimental tensile test results. The effects of necking and its associated stress state on flow stress and ductility are investigated. The flow stress and ductility data obtained from a Gleeble material simulator under various hot deformation conditions have also been numerically studied.

scholarly journals Photo-embossed Surface Relief Structures with an Increased Aspect Ratio by Addition of Kinetic Interfering Compounds

2009 ◽  
Vol 22 (5) ◽  
pp. 667-670 ◽  
Author(s):  
Jolke Perelaer ◽  
Ko Hermans ◽  
Cees W. M. Bastiaansen ◽  
Dirk J. Broer ◽  
Ulrich Schubert
Keyword(s):  
Aspect Ratio ◽  
Surface Relief

The mechanism of work-hardening and slip-band formation

1957 ◽  
Vol 242 (1229) ◽  
pp. 147-159 ◽  
Keyword(s):  
Single Crystals ◽  
Slip Band ◽  
Work Hardening ◽  
Stage I ◽  
Polycrystalline Materials ◽  
Temperature Dependent ◽  
Band Formation ◽  
Slip Bands ◽  
Three Stages ◽  
The Difference

A summary is given of some present ideas on the mechanism of work-hardening of single crystals and polycrystalline materials. In particular, the difference is stressed between the three stages of hardening: stage I, or easy glide; stage II, the region of rapid hardening accompanied by short slip lines; and stage III, the region of slow or parabolic hardening which is temperature-dependent and in which long slip bands are formed.

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