Showing papers on "Shear band published in 1981"

Influence of voids on shear band instabilities under plane strain conditions

Abstract: The effect of microscopic voids on the failure mechanism of a ductile material is investigated by considering an elastic-plastic medium containing a boubly periodic array of circular cylindrical voids. For this voided material under uniaxial or biaxial plane strain tension the state of stresses and deformations is determined numerically. Bifurcation away from the fundamental state of deformation is analysed with special interest in a repetitive pattern that represents the state of deformation inside a shear band. Both in the fundamental state and in the bifurcation analysis the interaction between voids and the details of the stress distribution around voids are fully accounted for. Comparison is made with the shear band instabilities predicted by a continuum model of a ductile porous medium. Based on the numerical results an adjustment is suggested for the approximate yield condition in this model of dilatant, pressure sensitive plastic behaviour.

Finite element analysis of deformation of strain‐softening materials

Abstract: A finite element analysis of strain-softening materials is presented in which the shear band of prescribed thickness is assumed to exist within elements where maximal stress intensity is reached. The incremental stiffness matrix of the element is derived including shear band deformation. Examples presented in the Paper demonstrate that the load-displacement curve and the displacement field are not sensitive to the mesh size used in the solution.

Flow Localization in the Plane Strain Tensile Test

Abstract: The deformations in a plane strain tensile test are analyzed numerically, both for a solid characterized by a phenomenological corner theory of plasticity and for a nonlinear elastic solid As opposed to the simplest flow theory of plasticity with a smooth yield surface, both these material models exhibit shear band instabilities at a realistic level of strain Initial imperfections are specified in the form of thickness inhomogeneities A long-wavelength imperfection grows into the well-known necking mode and subsequently, at a sufficiently high local strain level, bands of intense shear deformations develop in the necking region The location of these shear bands is strongly influenced by the location of small strain concentrations near the surface, induced by various short-wave patterns of initial thickness imperfections In accord with the non-uniform straining in the neck it is found that the intensity of the localized deformations varies along the bands, and some of the shear bands end inside the material

Deformation and unstable flow in hot forging of Ti-6Ai-2Sn-4Zr-2Mo-0.1Si

Abstract: The stable and unstable plastic flow of Ti-6Al-2Sn-4Zr-2Mo-0.1Si (Ti-6242) has been investigated at temperatures from 816 to 1010 °C (1500 to 1850 °F) and at strain rates from 0.001 to 10 s-1 in order to establish its hot forging characteristics. In hot, isothermal compression, Ti-6242 with an equiaxed a structure deforms stably and has a flow stress which decreases with straining due to adiabatic heating. With a transformed-β microstructure, unstable flow in hot compression is observed and concluded to arise from large degrees of flow softening caused by microstructural modification during deformation and, to a small extent, by adiabatic heating. Both microstructures have a sharp dependence of flow stress on temperature. Using the concepts of thermally-activated processes, it was shown analytically that this dependence is related to the large strain-rate sensitivity of the flow stress exhibited by the alloy. From lateral sidepressing results, the large dependence of flow stress on temperature was surmised to be a major factor leading to the shear bands occurring in nonisothermal forging of the alloy. Shear bands were also observed in isothermal forging. A model was developed to define the effect of material properties such as flow softening rate and strain-rate sensitivity on shear band development and was applied successfully to predict the occurrence of shear bands in isothermal forging.

Metallurgical Effects on Impact Loaded Materials

Abstract: The change of microstructure is a common process in impact-loaded materials. Around the path of projectile, for example, cracks and white streaks are visible. From the point of view of protection this deformation behavior of an armor plate must be considered an undesired process. In order to investigate these streaks a specimen was designed by which it is possible - using a Hopkinson bar - to produce white streaks in a laboratory with the advantage of good reproducibility. With different materials load versus time measurements using strain gauges were carried out. Comparing these results with those of the examination of the streaks by light and electron microscopy methods a relationship could be given between the load-time-diagram and the correspondent microstructure of the streaks. Furthermore the effects of material parameters on the intensity of deformation and the dimensions of the streaks are reported.

Material Factors in Adiabatic Shearing in Steels

Abstract: Impact studies were carried out on several steels using flat- nosed projectiles. An air gun was employed to produce projectile velocities in the range of 100–250 feet per second. The target structures were studied using microhardness measurements and optical microscopy. Deformed shear bands were found in most steels and ferrous alloys. In most of the ferritic alloys studied, these deformed bands act as “precursor bands” to transformed bands which frequently follow when the strain concentration is sufficiently great or the strain rate sufficiently high.

Strain hardening at high strain in aluminum alloys and its effect on strain localization

Abstract: The modes of strain localization in the tensile testing of a sheet sample are diffuse necking, localized necking and, in some materials, localization in an unstable shear band. In a tensile test of a rate insensitive material, the normalized strain hardening parameter,H = (1/σ)(dσ/de) has the values ofH = 1 for diffuse necking andH = 0.5 for localized necking. Curves ofH vs strain were obtained up to large values of plastic strain using the hydraulic bulge test. The materials selected were commercially important sheet alloys in the condition normally used for forming. It is shown that the materials have similarH vs strain curves in the range of uniform tensile straining, but the curves diverge widely at higher strains whereH falls below 1. This has important consequences on strain localization behavior. The limit strains of the alloys in simple tension and punch stretching show reasonable correlation with their values ofH and those alloys which are susceptible to catastrophic shear failure have low values ofH at high strains. Strain rate sensitivity adds to or subtracts from theH values obtained in this study and has an additional influence on strain localization.

Fracture of shear bands in atactic polystyrene

Abstract: Thick shear bands in polystyrene formed by compression could cause fracture or the formation of cracks by intersecting with themselves, by relaxing after the removal of the load, by propagating all the way to the side surfaces and by subsequent tensile deformation. The microstructural mechanisms involved in all these fracture processes are discussed.

A model of the mechanics of shear-crack propagation in tearing for amorphous metals II. Kinetics of inhomogeneous flow

Abstract: This paper presents the dynamical mechanics of inhomogeneous plastic flow in the tear test, which can account for the kinetics of slip in thin sheets of a variety of amorphous alloys. Based on the newly derived formula for shear-crack propagation, the propagation rate of a shear-crack-tip with a single shear band in steady tearing is modelled in terms of tile mobility (v) of the front of a slip band: this is the fundamental flow unit in amorphous metals at available resolutions. The thermodynamical motion of serrations in tearing is also modelled as a mechanic process and can be represented by a kinetic equation, v = A exp (-H/kT). The thermodynamical flow in amorphous alloys of various compositions is investigated in terms of the proposed mechanics. All of the amorphous alloys studied display serrated flow and obey the above kinetic equation. The activation energies (H) for onset of serrations in amorphous Pd78Cu6Si16, Ni70Fe8Si10B12 and Co78Si10B10, alloys arc 0.35, 0.40 and 0.48 eV respectivel...

Effects of Strain Rate on Deformation-Induced Martensite in 304 Stainless Steel

Abstract: Sheet specimens of 304 stainless steel were tested in uniaxial tension over a temperature range of −173 to +100°C at low strain rate (10−3/s) and high strain rate (103/s). Two shook loading experiments were conducted; one at −105°C and one at ambient temperature. The amount of strain-induced α′ martensite was measured magnetically, the temperature increase during deformation was measured with a thermocouple, and the substructure was examined by transmission electron microscopy. At room temperature and small strain levels, more α′ martensite was produced at high rate than at low rate. At strains >0.25, low-rate deformation produced significantly more α′ martensite. Tests over a range of temperatures showed that the decrease in α′ at large strains and high strain rates resulted from an increase in temperature due to adiabatic heating. Micro structurally, we found that α′ nucleates at shear band intersections. At high rate, there are more shear bands, but the growth of the α′ embryos is restricted by the temperature increase. During shock loading, α′ martensite formed at both test temperatures, thereby demonstrating that transformation is possible at such high rates, and under the predominantly compressive stress state.

Some Metallurgical Aspects of the Dynamic Expansion of Shells

Abstract: Observations have been performed on fragments recovered after the dynamic expansion and rupture of relatively thin metallic shells under the action of explosives. Two main types of rupture have been observed: a very ductile one, sometimes by chisel-point (e.g. in copper or aluminum alloys) and a brittle one associated with adiabatic shear bands (in uranium and titanium alloys). For these two types of rupture we have reviewed the different theories proposed to take into account the experimental results concerning the rupture elongation.

Diffusion of methanol in PMMA shear bands

Abstract: The rate of penetration of liquid methanol in PMMA shear bands was found to be much faster than that in the undeformed region. In the shear bands, the transport process seems to be Fickian diffusion, while in the undeformed region it is Case 11. The diffusion coefficient of methanol in the PMMA shear band was found to be 2·4 × 10 −8 cm2/s at 24·5°C.

A test to evaluate flow localization during forging

Abstract: This paper describes a new test which measures a material’s propensity for localized flow via an empirical parameter, percent Distributed Gage Volume (pct DGV). The specimens, which are deformed uniaxially in compression, are cylindrical and feature a reduced gage section. The measurement and calculation of pct DGV are presented in detail. Low values of pct DGV are associated with localized plastic flow and the formation of shear bands and gradients in hardness, microstructure, and flow in the test specimens. The test, therefore, offers a means to evaluate material and operating conditions that will minimize nonuniformity in production forgings of complex shapes. Test results on austenitic steels, a precipitation hardening stain less steel, 6061 aluminum, and Ti-6Al-4V show that deformation behavior depends on the alloy as well as on the microstructural condition of the alloy. The localized flow and shear band formation are related to dynamic recovery, dynamic recrystalli zation, and precipitation.

Metallurgical influences on shear band activity

Abstract: Metallurgical effects on shear band behavior were studied by applying a state of uniform dynamic shear strain and then suddenly removing it by explosively expanding a hollow specimen cylinder to a diameter determined by a massive confining cylinder. Because geometry-induced stress concentrations are absent and because shear band development can be stopped at various stages, the procedure emphasizes the role of metallurgical features on shear band activity.

Tensile failure of crystalline polymers

Abstract: In the creep experiment the brittle fracture of the unoriented semicrystalline polymers at very small and very high tensile load with the intermediate ductile region may be explained by the competition between crazing and shear band formation during the microcrack growth phase. The former type of microcrack growth leads to brittle fracture while the latter type yields necking which transforms the original lamellar structure into the final fibrous structure. The actual fate of the strained sample depends on the growth time of the craze, tg, and of the shear band formation time, ts. If tg ts, the material will deform plastically. The failure of the fibrous material seems to occur when the ratio between the average distance and diameter of the microcracks reaches a value about 3. The microcracks seem to form primarily at defects of the microfibrillar structure, i.e., at the ends of microfibrils where the axial connection of subsequent crys...

Formation of Adiabatic Shear Bands During Upsetting of 18-4-1 Alloy Steel at High Strain Rates

Abstract: Optical Metallographic studies on the specimens deformed at different temperatures (Room temperature to 900°C) and mean strain rates of 0.015, 0.6, 11.0 and 470 s−1 using compression testing machine, double action hydraulic press, friction screw press and high velocity gravity drop hammer respectively were carried out. The specimens were heated to the required temperatures by means of a tubular furnace included in the tool set-up itself.