Showing papers on "Shear band published in 1986"

Effect of Yield Surface Curvature on Necking and Failure in Porous Plastic Solids

Abstract: The effect of material path dependent hardening on neck development and the onset of ductile failure is analyzed numerically. The calculations are carried out using an elastic-viscoplastic constitutive relation that has isotropic hardening and kinematic hardening behaviors as limiting cases and that accounts for the weakening due to the growth of micro-voids. Final material failure is incorporated into the constitutive model by the dependence of the plastic potential on void volume fraction. Results are obtained for both axisymmetric and plane strain tension. Failure is found to initiate by void coalescence at the neck center in axisymmetric tension and within a shear band in plane strain tension. The increased curvature of flow potential surfaces associated with the kinematic hardening solid leads to somewhat more rapid diffuse neck development than occurs for the isotropic hardening solid. However, a much greater difference between the predictions of the two constitutive models is found for the onset of ductile failure.

Deformation and fracture in Al–Li-base alloys

Abstract: Aluminium–lithium-base alloys are of considerable interest because of their low density and high modulus. However, they have been shown to have low ductility and poor fracture toughness. This has been attributed to a variety of factors, including intense shear band formation, segregation to grain boundaries, and weakened grain boundaries due to precipitation and precipitate-free zones. The authors have investigated the deformation structures observed in binary and more complex commercial alloys. As would be expected, considering the microstructure of the alloys, extensive strain localization and shear band formation occurs in these alloys. However, it is shown that the commercial alloys are less sensitive to strain localization than the model binary alloy systems investigated. The stresss–train behaviour has been investigated. The alloys exhibit jerky flow, which is indicative of negative strain rate sensitivity, and strain rate change tests showed this to be the case. This is consistent with the ...

Flow localization in cold–rolled α-brass

Abstract: A detailed microstructural study of heavily rolled α-brass has revealed that in addition to straight shear bands a ‘wavy’ structure is also present. The cold-rolled material is considered to exhibit laminar structures and properties that are similar in some respects to the behaviour of laminar composite materials. This is examined in detail and conditions for shear band formation and propagation are investigated in terms of flow localization in laminar structures.MST/250

On evolution of thermo-plastic shear band

Abstract: The present paper briefly reviews analytical studies of the evolution of thermoplastic shear band, i.e. emergence from uniform deformation, post-instability growth and late stage behaviour. The case studied is the simple shear of temperature and rate-dependent materials with heat transfer. Uniform mode exists before a critical state, if no heat flows out of testpiece. Upon reaching the critical state, bifurcation appears as a result of disturbances, which leads to instability and the formation of narrow shear band. Initially, the band, due to temperature disturbance, can shrink with increasing temperature and strain rate owing to unsteady flow. Then heat conduction dominates and causes the shear band to expand. The postmortem appearance of thermo-plastic shear band manifests itself as balance of plastic work rate and heat diffusion. Melting may also take place within the band.

Plastic shearing of materials exhibiting strain hardening or strain softening

Abstract: We consider the plastic shearing of a strain-rate dependent material exhibiting strain hardening or strain softening, subjected to steady shearing. We establish the existence of classical solutions and study the stability of uniform shearing. For materials exhibiting strain hardening or a moderate degree of strain softening we show that, as t → ∞, every solution approaches, at specific rates of convergence, uniform shearing; thus shear bands do not form.

High temperature failure of ceramics

Abstract: Microstructural heterogeneities are observed to have a dual influence on the creep rupture behavior of a polycrystalline alumina. The duality is related to a transition in rupture mechanism. The heterogeneous regions in the microstructure provide the cracks which cause creep crack propagation controlled failures at small strains. Additionally, shear bands nucleate at the heterogeneities, and thereby contribute to damage controlled failure at large strains. The transition in rupture mechanism is associated with a high incidence of creep blunting of cracks at a threshold stress intensity factor.

Numerical modeling of the propagation of an adiabatic shear band

Abstract: The critical phenomena determining the propagation of an adiabatic shear band occur at its extremity. The stress and strain distributions at the tip of a shear band are calculated as a function of applied shear strain using the finite element method for an elasto-plastic material. Three assumptions simplify the calculations considerably: (a) the mechanical response of the material follows an adiabatic stress-strain curve; (b) the material within the shear band has zero shear strength; (c) the body is taken to be in equilibrium. The distribution of stresses and strains in the adiabatically-deformed material is compared to that of a quasi-statically deformed material. While the stress-strain curve for an isothermally deformed material is monotonic with continuous work-hardening, the adiabatic work-hardening curve reaches a plateau followed by work-softening (due to thermal softening). The stress and strain fields for both cases are nearly identical, except in the region directly in front of the shear band. In the adiabatically-deformed material a thin region (~5 μm) with large strains and lowered stresses is produced. This region, in which accelerated deformation takes place as the applied shear deformation increases, is absent in the isothermally-deformed material. The formation of this instability region, ahead of the shear band, is considered to be the mechanism for the propagation of an adiabatic shear band.

Recovery of thick shear bands in atactic polystyrene

Abstract: The DSC thermograms of the shear band material cut out by a diamond saw to include some undeformed material revealed two Tg's clearly separated by about 10°C. The first Tg was at the same temperature as the Tg of the undeformed material. The second Tg, which was at a higher temperature than the first Tg, appeared shortly after the shear strain recovery during the heating of the shear band material in the DSC. When the shear strain in the shear band was partially reversed by mechanical means before taking the DSC thermogram, the ΔT between the two Tg's decreased and when the shear strain was mechanically reversed to almost zero, the second Tg disappeared. The stored energy of shear band material was found to be similar to that of the bulk compressed material for large strains. Dimensional recovery during heating of specimens with thick and fine bands was similar, both taking place above Tg.

Toughening mechanisms in elastomer-modified epoxies. I - Mechanical studies. II - Microscopy studies

Abstract: Tensile dilatometry at constant displacement rates is presently used to ascertain whether voiding, crazing or shear banding are the deformation mechanisms at work in the toughening of brittle epoxies through the addition of an elastomeric phase. The results obtained for epoxies toughened by various levels of several types of carbonyl-terminated butadiene nitrile liquid rubber indicate that, at low strain rates, the rubber particles simply enhance shear deformation; at sufficiently high strain rates, the particles cavitate and promote further shear deformation. Examinations of these materials are conducted by means of SEM, TEM, and optical microscopy in the second part of this study. Toughening is hypothesized to involve cavitation's relieving of triaxial tension at the crack tip, as well as shear band formation, which creates a plastic zone.

Shear Band Formation During Sheet Forming

Abstract: Shear bands grow across the dimensions of deforming sheet during some sheet forming and most metal separation operations. Their formation accelerates failute and determines its location in the sheet. Examined is shear band development during plane strain extension using both experimental observations and geometric models. Many factors are reviewed which combined to create the specific conditions required for shear band formation.

Shear Band Formation in Al-2mass%Cu Single Crystals Containing ϑ Precipitates

Abstract: Influence des precipites CuAl 2 et de la deformation sur la formation de bandes de cisaillement dans des monocristaux lamines a froid. Etude basee sur l'observation des bandes de glissement et des structures de dislocations. Influence de la temperature. On montre une correlation entre la formation de bandes de cisaillement et les structures de dislocation

Corrosion fatigue crack initiation and growth in 18 Ni maraging steel

Abstract: Nucleation of fatigue cracks in air and 3.5 wt% NaCl solution has been studied in an 18 wt% Ni maraging steel. Specimens tested on reverse bending fatigue machine showed a marked decrease in fatigue strength of the steel in NaCl solution reducing the 107 cycles endurance limit from 410 MPa in air to 120 MPa. Microscopic studies revealed crack initiation to be predominantly associated with non-metallic silicate inclusions in both cases. In air, initiation is caused by decohesion of the inclusion/matrix interface, while in NaCl solution complete detachment of inclusions from the matrix results due to the dissolution of the interface. 70% more inclusions are quantitatively shown to be associated with cracks in NaCl solution than in air at the same stress levels. Experimental and theoreticalS-N curves and inclusion cracking sensitivity data are consistent with the mechanism suggested. The final fracture occurs by the main crack consuming the inclusions ahead of it by the “unzipping” of the shear band produced between the crack tip and the inclusion ahead.

Some Macroscopic Consequences of the Granular Structure of Sand

Abstract: The occurrence of shear bands in rate-independent, incrementally nonlinear solids submitted to multiaxial monotonic loading paths is investigated. The phenomenon is viewed as a bifurcation of the continuing equilibrium equations in terms of velocity. Essentially demonstrative examples show the influence of incremental nonlinearity on the ratio of the velocity gradients inside and outside the band, on the regime of the equilibrium equations, on the bifurcation load and on the slope of the band. These results are compared with those obtained on materials whose incrementally linear behaviour is defined by linearization of the preceding nonlinear relations along the direction given by the fundamental loading path at the time of bifurcation.

Modeling of Instability at the Tip of a Shear Band

Abstract: The present work focusses on the tip of the shear band, and assumes that the critical phenomena dictating the propagation or arrest of a shear band occur at the tip. This approach is analogous to fracture mechanics in which the crack tip is the region where the relevant processes are taking place, while the crack surfaces are merely the product. The driving energy for the extension of the tip comes from an increase of the imposed displacement, which generates shear stresses and strains. In the analysis presented in this paper the plastic deformation ahead of a shear band is calculated as a function of imposed displacement. A number of assumptions are required to render the problem tractable. The principal assumptions are given and justified below.