Shear Localization in Dynamic Deformation: Microstructural Evolution

Investigations made by the authors and collaborators into the microstructural aspects of adiabatic shear localization are critically reviewed. The materials analyzed are low-carbon steels, 304 stainless steel, monocrystalline Fe-Ni-Cr, Ti and its alloys, Al-Li alloys, Zircaloy, copper, and Al/SiC_p composites. The principal findings are the following: (a) there is a strain-rate-dependent critical strain for the development of shear bands; (b) deformed bands and white-etching bands correspond to different stages of deformation; (c) different slip activities occur in different stages of band development; (d) grain refinement and amorphization occur in shear bands; (e) loss of stress-carrying capability is more closely associated with microdefects rather than with localization of strain; (f) both crystalline rotation and slip play important roles; and (g) band development and band structures are material dependent. Additionally, avenues for new research directions are suggested. This article is based on a presentation made in the symposium entitled “Dynamic Behavior of Materials,” which occurred during the TMS Annual Meeting and Exhibition, February 25–March 1, 2007 in Orlando, Florida, under the auspices of The Minerals, Metals and Materials Society, TMS Structural Materials Division, and TMS/ASM Mechanical Behavior of Materials Committee.     

Influence of Microcracking on Shear Localization

This work examines the influence of microcracking on a material’s tendency to shear localize under compressive loading. A two-dimensional (2D) finite-element framework with explicit crack representation using cohesive-element methodologies is employed. The influence of microcracking is examined by taking the fracture toughness of the cohesive elements as a free parameter. The simulations suggest that an optimum fracture toughness exists for promoting shear localization. This value corresponds to the limiting mode ?I fracture toughness, below which microscopic material defects lead to brittle compressive failure, as opposed to shear localization. While in the presence of confinement, this value is shown to be close to zero; in the absence of confinement, it is computed to be 28% of the shear band toughness for the specific case of ultrafine-grained tungsten. More generally, it is found that the ratio of mode?I fracture toughness to shear band toughness provides a crude indicator for predicting whether material defects are likely to lead to brittle failure or enhanced shear localization.     

Shear Localization Due to Liquefaction-Induced Void Redistribution in a Layered Infinite Slope

A new approach for estimating the susceptibility of a layered, liquefiable, infinite slope to shear deformations associated with void redistribution is presented. The excess pore water pressures associated with liquefaction produce upward seepage within the slope. The lower portion of a liquefied layer expels a certain volume of pore water, Vcon, as it contracts (densifies). If the liquefied layer is overlain by a lower permeability soil, then the pore water expelled from the lower contracting zones can become trapped causing void ratio increase in a dilating sublayer near the interface, reducing its undrained shear strength. The volume of water that can be absorbed by the dilating sublayer prior to slope instability is termed the dilation capacity, Vdil. The ratio Vcon/Vdil is a measure of the potential for localization due to strength losses from void redistribution. The localization potential is shown to strongly depend on relative density, slope angle, and thickness of the liquefiable layer. The thickness of the dilating sublayer, a critical parameter, is significantly greater than the thickness of a shear band (which may be only tens of grain diameters). A detailed example is presented to show how the procedure can be applied. The results of the analysis are shown to be consistent with observed deformations and localizations in centrifuge model tests.     

Postshaking Shear Strain Localization in a Centrifuge Model of a Saturated Sand Slope

This paper presents analyses of a test conducted on a 9-m-radius centrifuge to study the redistribution of pore water during diffusion of earthquake-induced excess pore pressures in a sand slope with embedded silt layers. The centrifuge model developed large postshaking deformations associated with shear strain localization at the interface between the sand and silt layers. Dense arrays of pore pressure transducers provided detailed measurements of pore pressure variations in time and space within the slope. A new data analysis approach is presented in which measured pore-pressures are used to compute flow rates and volumetric strains as a function of time and position throughout the slope. Hydraulic gradients were calculated by numerical differentiation of measured pore-pressure distributions with respect to position. Flow rates that were based on Darcy’s law were then integrated with respect to time to obtain flow quantities, from which volumetric strains were computed. A second data analysis approach that computes volumetric strains on the basis of soil compressibility and changes in pore pressure provided an independent computation of strains in consolidating zones. Results using these data analysis procedures confirm that a dilating (loosening) zone of significant thickness developed in the sand immediately beneath an embedded silt layer that had impeded the drainage of high pore pressures. These results support the hypothesis that the dilating zone corresponds to regions where the mobilized friction angle exceeds the critical state friction angle and that the dilating zone can be initially relatively thick before its size diminishes to the thickness of a thin shear band after the peak friction angle is mobilized. Quantification of the evolution of the size of the dilating zone is a key to understanding the magnitude of deformations associated with void redistribution.     

Conditioning: A Historical Note.

In the controversy over the Estes Park Report contradictory assertions have been made about Pavlov's involvement in the phenomenon variously labeled as "psychic reflex" and "conditioned reflex." A reconciliation between the apparently contradictory views may be brought about through recognition that the labels are not interchangeable, even though they refer to the same phenomenon. The difference is in the implied theoretical interpretations. It may also be of value to recall that in the early observations of the phenomenon Pavlov's theoretical position had not been consistent. The following quotations are presented as supporting evidence in the interest of accuracy in the historical record. The source is F. P. Maiorov's authoritative monograph Istoriia Ucheniia ob Uslovnykh Refleksakh (see record 1952-01235-000). (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Study of Plastic Shear Localization via the Flow Theory of Mechanism-Based Strain Gradient Plasticity

The plastic shear localization is studied via the flow theory of mechanism-based strain gradient plasticity. The shear strain rate displays a rather sharp decrease from the center to the boundary of shear band, and gradually approaches the uniform remote shear strain rate outside the shear band. The shear band thickness obtained analytically is linearly proportional to the (length of) Burgers vector, and also depends on the softening modulus and mesoscale cell size. The maximum shear strain rate in the shear band, however, is sensitive to other material properties such as the ultimate tensile strength and intrinsic material length in strain gradient plasticity.     

Gas-Enhanced Ultra-High Shear Mixing: A Concept and Applications

Metallurgical and Materials Transactions B - The processes of mixing, homogenizing, and deagglomeration are of paramount importance in many industries for modifying properties of liquids or...     

Adiabatic Shear Localization for Steels Based on Johnson-Cook Model and Second- and Fourth-Order Gradient Plasticity Models

At moderate or high strain rates , steels fre-quently show narrow zone of highly localized de-formation, referred to as adiabatic shear band(ASB) .It is believed that ASBs are formed by athermal-mechanical instability processin whichther-mal softening can overcome the effect of strain hard-eningin a deformed region[1 -3]. ASBs can be generally observed in various ap-plications ,such as metal forming, perforation,i m-pact on structures ,ballistic i mpact , machining,tor-sion,explosive fragme…     

Microstructure and Microtexture Evolution of Shear Localization in Dynamic Deformation with Different Strains in Annealed Copper

Shear localizations with different strains in annealed copper were obtained by a modified split Hopkinson pressure bar. Microstructure and microtexture evolution of the shear localization regions were examined using optical microscopy, electron backscatter diffraction technique, and transmission electron microscopy. The results show that both the mechanical response and deformation behavior are correlated closely to the shear strains. The elongated dislocation cells, stretched subgrains, and the refinement of subgrains are observed within shear localizations during dynamic deformation. Ultrafine grains of 100 to 300 nm with high-angle-boundaries are produced within the shear band with the shear strain of 5.8. Microtexture characterization reveals that a stable orientation, in which 〈110〉 directions of the crystals tend to align with the shear direction, develops both in the deformation and recrystallization areas. The {111} planes of the crystals tend to parallel to the shear plane in the deformation area, whereas the aggregated extent of this orientation becomes weak in the recrystallization area. In addition, some grains exist with the {100} planes parallel to the shear plane in the deformation and recrystallization areas. The rotational dynamic recrystallization is a reasonable mechanism for the microstructure evolution. The effects of cooling stage on the growth of grains and the change of dislocation density are estimated as a complementarity to this mechanism.     

Interface Shear Stress: A New Design Criterion for Plate Debonding

This paper critically assesses the applicability and reliability of existing analytical techniques to predict and∕or prevent brittle plate debonding failure that occurs in reinforced concrete (RC) beams strengthened with externally bonded steel or fiber-reinforced-polymer composite plates. The experimental results, available to date in literature, have been very carefully reviewed and analyzed for this purpose. A new approach, very different from existing methods, and based on the interface shear stress obtained form elastic analysis of RC beam cross section and the fundamentals of force transfer mechanism in a bonded joint, is presented to predict the premature plate debonding phenomenon. The paper identifies important structural, material, and force parameters that influence this critical interface shear stress value between the bonded plate and concrete. The relations between these parameters and interface shear stress value are also examined and found to be consistent and logical to predict plate debonding at the plate cutoff end. The validity of this new design-oriented approach and scope for further research are also discussed.     

The Jungians: A Comparative and Historical Perspective: Book review.

Review of book: Thomas B. Kirsch (Au.) The Jungians: A Comparative and Historical Perspective. London: Routledge, 2000, 276pp. Reviewed by Nathan G. Hale, Jr. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Overview: Section introduction.

Even for those whose professional lives are consumed with research, practice, or advocacy in this arena, keeping up with the latest developments on acquired immunodeficiency syndrome (AIDS) is a daunting challenge. The scientific literature and the public press churn out new information with dizzying rapidity, and new contradictions and disturbing new questions arise every day. Developing a framework for understanding either the larger context or where our own work fits in is difficult. One set of guideposts for understanding how the science and profession of psychology fits into worldwide work on the AIDS health crisis is provided in this opening section. The overview begins with an appraisal of what we psychologists have accomplished so far, by one of the pioneers in this young field, psychologist Stephen Morin. Morin's analysis draws our attention to how psychology interfaces with "three separate epidemics": (a) spread of HIV infection, (b) an epidemic of the diseases characteristic of AIDS, and (c) the social, cultural, economic, and political reactions to the HIV and AIDS epidemics. Psychologists have been involved intensively in responses to each of these three epidemics, as indicated by the other articles in this special issue. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

A New Shear Test for Sheet Metal Characterization

In order to predict plastic material behavior for sheet forming processes by finite element simulation, shear tests are useful to identify material parameters. Since the existing shear test setups have certain disadvantages, a new twin bridge torsion shear test is proposed. Stress and strain calculation is derived from the presented geometrical features. The clamping situation and the shear gauge dimensions are investigated to evaluate the quality of the obtained flow curves. It is shown that this test specimen is suitable to determine anisotropic yield behavior and to characterize prestrained specimen, for instance due to cold rolling, when the yield locus is shifted by a backstress tensor.     

Overview no. 44: The chemistry of fracture: A basis for analysis

We discuss the correlation between such apparently diverse properties as the superconducting transition temperature and Young's modulus. This correlation suggests that it is possible to pick a number of different bases to describe a property of interest. In this paper we introduce the concepts of bond directionality and charge polarizability and argue that these concepts are the most natural ones with which to describe elastic properties. Using these concepts the elemental trends observed in the shear modulus and Poisson's ratio are explained. The ease with which these trends are explained justifies the application of the same concepts toward an atomistic understanding of environmentally induced fracture.     

Peer Review: An Overview.

In the context of professional psychology, peer review refers to an organized system of assessing quality of services and requires analysis and judgment of professional practice by other practitioners within the profession. This special issue on peer review and quality assurance represents a report on the status of developments in quality assurance within the mental health professions and emphasizes the major role of groups within the American Psychological Association (APA). The first set of articles discusses general issues in quality assessment. The next set of articles is specifically relevant to the progress and development of the Defense Department's Civilian Health and Medical Program of the Uniformed Services (CHAMPUS) Project. The third section comprises a series of articles that describe the application of quality assurance methodology to a variety of service delivery settings. Other approaches to quality assessment and quality assurance are described in the final section. The editors hope that the articles in this special issue can assist the profession in achieving greater and more consistent effectiveness with the public it serves. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Industrial Crystallization Research: An Overview

Abstract

The paper outlines the key advances that have been made in the understanding of crystallization processes with due emphasis on crystal size distribution analysis and estimation of crystallization kinetics. Several concepts used in process simulation, design and identification analyses for crystallization configurations have been reported. The present state of knowledge in various related areas of industrial crystallization is assessed to help direct future trends in research.