Formation Of Localized Shear Bands Research Articles

217 FCC 結晶の局所せん断帯分岐および分岐後解析

217 FCC 結晶の局所せん断帯分岐および分岐後解析

Superplastic behavior of a Zr–10Al–5Ti-–17.9Cu–14.6Ni metallic glass in the supercooled liquid region

The mechanical behavior of metallic glasses is characterized by either inhomogeneous or homogeneous deformation. Inhomogeneous deformation usually occurs when a metallic glass is deformed at room temperature (i.e., low temperature) and is characterized by the formation of localized shear bands, followed by the rapid propagation of these bands, and sudden fracture. Consequently, when a metallic glass is deformed under tension it exhibits only limited macroscopic plasticity ( 0.70T{sub g}), and the material can often exhibit significant plasticity, especially in the supercooled liquid (SL) region. In summary, large tensile ductility can generally be obtained in the supercooled liquid region from a metallic glass with a large {Delta}T. The exact deformation mechanism, however, and in particular whether an alloy deforms by Newtonian viscous flow remains a controversial issue. The purpose of this paper is to report an example of non-Newtonian behavior in a cast amorphous Zr-10Al-5Ti-17.9Cu-14.6Ni alloy in the supercooled liquid region.

Possible mechanism for formation of localized shear bands in amorphous alloys

We discuss a model for the formation of localized shear bands in amorphous alloys. We suppose that the observed localized plastic flow is connected with the preceding spontaneous formation of a narrow “weakened band” region in which cohesion is reduced due to arising of inhomogeneous flows of alloy components or free volume in the strong stress field. To describe the process we consider a generalization of the conventional elasticity and plasticity equations to the highly inhomogeneous case. Investigation of these equations shows that at certain values of stress and temperature an autolocalized solution appears which may correspond to the “weakened band” formation in materials.

Crack-tip behaviors of stationary and growing cracks in Al-Fe-X alloys: Part I. Near-tip strain field

The crack-tip strain fields of monotonically loaded stationary and growing cracks in Al-Fe-X alloys have been examined using the stereoimaging technique. The experimental results confirm the dominance of the Hutchinson[10,11] and Rice and Rosengren[12] (HRR) field in a stationary crack and the logarithmic singularity in a nonstationary crack. Initiation of fracture is preceded by the formation of localized shear bands emitted from the crack tip. The dominance of the HRR or the logarithmic field ceases to exist within the fracture process zone, whose size is related to the crack-tip opening displacement and the limit effective strain prior to shear localization. In all cases, the value of the limit effective strain outside the shear bands approximately equals the fracture strain measured in a tensile specimen. These observations are used to establish the origin of stable crack growth and a local fracture criterion.

General Theory of Shear Bands Formation by a Non-Coaxial Cam-Clay Model

In order to simulate the formation of localized shear bands, which is commonly observed during large deformation of soils, we first present a systematic extension of the well known Cam-clay model developed for small strains to the model for finite strains/deformations and then incorporates a non-coaxial term in the model. Finally, confining the deformation to undrained plane strain conditions, we examine the effects of the non-coaxial term on the shear bands formation.

Micro and macroscopic aspects of shear band formation in internally nitrided single crystals of Fe-Ti-Mn alloys

The formation of localized shear bands in single crystals of internally nitrided alloys of Fe-Ti-Mn was studied experimentally and theoretically. The experimental studies included observation of the dislocation substructures that are formed at and near shear band/matrix interfaces along with documentation of the crystallography of the localized shearing process. Computational studies of tensile deformation of the crystals using the finite element method, that incorporated a large strain, strain rate dependent constitutive theory for crystalline slip, were carried out and the results were compared to the experimental observations. Mechanical tests, and in situ metallographic observations of shear band formation, showed that localized shearing occurs while the materials are continuously hardening and before “damage”, through microfracture, begins. Electron diffraction and imaging of the deformation substructures near shear band interfaces showed that an important part of the localization mechanism is nonuniform lattice reorientations that cause a “geometrical softening” of the lattice. The numerical studies are in good agreement with the experiments and demonstrate the role of material strain hardening and strain rate sensitivity.

Flow localization in elastic-plastic material developing from stress-free surface

Localized shear band formation developing from a stress-free surface in a highly strained elastic-plastic material under plane strain is studied, taking effects of both geometrical and material factors into consideration. Two kinds of treatments are adopted concerning representation of the constitutive property of the material, that is, (1) using a phenomenological constitutive equation formulated on the basis of J2-flow, J2-kinematic and J2-deformation theories and (2) simulating a mechanical behavior of a polycrystalline material introducing an aggregate composed of a single crystal whose property is formulated on the basis of a slip theory proposed by Asaro et al. in which effects of finite deformation and rotation of the lattice are fully considered. Numerical analyses are carried out by a finite element method concerning occurrence of bifurcation in a uniformly deformed state and trace of strain localization which is induced by geometrical and material imperfection. It is pointed out that there exist some relations between appearance of the bifurcation and development of localization. From the results for the polycrystalline model it is shown how unevenness of the free surface grows and the shear band develops from an initial random inhomogeneous deformation field.

Plastic strain localization developing from free surface (2nd report, formation and development of localized shear band)

Localized shear band formation developing from stress free surface in a highly strained elastoplastic material under the uniaxial plane strain tension is studied taking effects of both geometrical and material factors into consideration. Analyses of bifurcation and shear band formation which appear in a rectangular region are carried out by a finite element method in regard to J2-deformation, single crystal and poly-crystalline material models. Numerical results show that existence of surface and inner mode bifurcation with short wave length seem to be important for shear band formation. From the results of the poly-crystalline material model, it is pointed out that shear band which gets through the region appers just after the maximum load state and the growth of diffused necking seems to be promoted by successive generation and development of the shear bands.

A structural study of the influence of pressure on shear band formation

Experiments have been performed to show the influence of applied pressure on the formation of localized shear bands in a naturally aged Al-Zn-Mg alloy. The results indicate that the onset of localization is insensitive to applied pressure but that the development of the bands is pressure sensitive indicating that dilational damage occurs during the growth of bands. Detailed electron microscopy studies indicates that although the overall macroscopic orientation of the bands may be nonrational they consist of sequential regions of localized shear on crystallographic planes and narrow regions of multi-slip.