The yield and fracture behaviour of high-purity iron single crystals at high rates of strain

1967 ◽  
Vol 299 (1459) ◽  
pp. 464-490 ◽  
Keyword(s):  
Strain Rate ◽  
Single Crystals ◽  
Yield Stress ◽  
Tensile Axis ◽  
Shear Stresses ◽  
A Value ◽  
Twinning System ◽  
Purity Iron ◽  
Mere Presence ◽  
The Impact

Single crystals of iron were deformed in tension at an impact strain rate of about 1000 per second at temperatures between 20 and -196 °C. The results were compared with those of tests at a conventional rate of strain. The effect of raising the strain rate was to increase the yield stress, to increase the temperature range over which the specimens deformed by twinning and to increase the range of orientation showing brittle failure at -196°C. There was no significant effect of strain rate, however, upon the fracture stress level. A few specimens cleaved at a temperature higher than -196°C in tests at the impact strain rate, but there w as no indication of a ductile-brittle orientation transition at these temperatures. The yield stress, in the absence of twinning, was found to be independent of crystal orientation. The twinning stress at the high strain rate was found to be independent of temperature but was a little higher than at the conventional rate of strain. While twinning always took place on those systems supporting the higher resolved shear stresses under tensile loading, it did so at a value of shear stress on the most favoured twinning system which was not independent of orientation but increased as the tensile axis of the crystal approached [001]. In many tests on crystals which did not fracture twin intersections of the type that would be expected to initiate cleavage were found. It is concluded that the mere presence of such critical intersections is not enough to cause fracture. The records from tests at the impact strain rate, however, provide direct evidence that when cleavage fracture was obtained it was always preceded by twinning.

The mechanical properties of pure iron tested in compression over the temperature range 2 to 293 °K

1967 ◽  
Vol 261 (1121) ◽  
pp. 253-287 ◽  
Keyword(s):  
Mechanical Properties ◽  
Flow Stress ◽  
Strain Rate ◽  
Single Crystals ◽  
Yield Stress ◽  
Low Temperatures ◽  
Pure Iron ◽  
Constant Strain Rate ◽  
Frictional Stress ◽  
Strain And Strain Rate

The mechanical properties of pure iron single crystals and of polycrystalline specimens of a zone-refined iron have been measured in compression over the temperature and strain rate ranges 2.2 to 293 °K and 7 x 10 -7 to 7 x 10 -3 s -1 respectively. Various yield stress parameters were determined as functions of both temperature and strain rate, and the reversible changes in flow stress produced by isothermal changes of strain rate or by changes of temperature at constant strain rate were also measured as functions of temperature, strain and strain rate. Both the temperature variation of the flow stress and the strain rate sensitivity of the flow stress were generally identical for the single crystals ( ca. 0.005/M carbon) and the polycrystalline specimens ( ca. 9/M carbon). At low temperatures, the temperature dependence of the yield stress was smaller than that of the flow stress at high strains, probably because of the effects of mechanical twinning, but once again the behaviour of single and polycrystalline specimens was very similar. Below 10 °K, both the flow stress and the extrapolated yield stress were independent of temperature. The results show that macroscopic yielding and flow at low temperatures are both governed by the same deformation mechanism, which is not very impurity sensitive, even in the very low carbon range covered by the experiments. The flow stress near 0 °K is ca. 5.8 x 10 -3 u where [i is the shear modulus. On the basis of a model for thermally activated flow, the activation volume at low temperatures (high stresses) is found to be ca. 5 b 3 . The exponent in the empirical power law for the dislocation velocity against stress relation is ca. 3 near room temperature, but becomes quite large at low temperatures. The results indicate that macroscopic deformation at low temperatures is governed by some kind of lattice frictional stress (Peierls-Nabarro force) acting on dislocations.

Strain Rate Dependence of the Flow Stress and Work-Hardening of Single Crystals of NI3(Al,Hf)B

1994 ◽  
Vol 364 ◽  
Author(s):  
S. S. Ezz ◽  
Y. Q. Sun ◽  
P. B. Hirsch
Keyword(s):  
Flow Stress ◽  
Strain Rate ◽  
Temperature Dependence ◽  
Single Crystals ◽  
Yield Stress ◽  
Strain Rate Sensitivity ◽  
Work Hardening ◽  
Room Temperature ◽  
Rate Sensitivity ◽  
Controlling Mechanism

AbstractThe strain rate sensitivity ß of the flow stress τ is associated with workhardening and β=(δτ/δln ε) is proportional to the workhardening increment τh = τ - τy, where τy is the strain rate independent yield stress. The temperature dependence of β/τh reflects changes in the rate controlling mechanism. At intermediate and high temperatures, the hardening correlates with the density of [101] dislocations on (010). The nature of the local obstacles at room temperature is not established.

On the Effect of Strain Rate and Temperature on the Yield Strength Anomaly in L21-structured Fe2AlMn

2008 ◽  
Vol 1128 ◽  
Author(s):  
Markus W. Wittmann ◽  
Janelle M. Chang ◽  
Yifeng Liao ◽  
Ian Baker
Keyword(s):  
Yield Strength ◽  
Strain Rate ◽  
Single Crystals ◽  
Yield Stress ◽  
Strain Rate Sensitivity ◽  
Rate Sensitivity ◽  
Temperature Range ◽  
Effects Of Temperature ◽  
Increasing Temperature

AbstractThe effects of strain rate and temperature on the yield strength of near-stoichiometric Fe2AlMn single crystals were investigated. In the temperature range 600-800K the yield stress increased with increasing temperature, a response commonly referred to as a yield strength anomaly. No strain rate sensitivity was observed below 750K, but at higher temperatures the yield stress increased with increasing strain rate. Possible mechanisms to explaining the effects of temperature and strain rate are discussed.

EFFECTS OF STRAIN RATE DEPENDENCY OF MATERIAL PROPERTIES IN LOW VELOCITY IMPACT

2008 ◽  
Vol 22 (09n11) ◽  
pp. 1165-1170 ◽  
Author(s):  
HIROFUMI MINAMOTO ◽  
ROBERT SEIFRIED ◽  
PETER EBERHARD ◽  
SHOZO KAWAMURA
Keyword(s):  
Finite Element ◽  
Strain Rate ◽  
Yield Stress ◽  
Coefficient Of Restitution ◽  
Finite Element Simulations ◽  
Material Behavior ◽  
Rate Dependency ◽  
Rate Dependent ◽  
Strain Rate Dependency ◽  
The Impact

Impact processes are often analyzed using the coefficient of restitution which represents the kinetic energy loss during impact. In this paper the effect of strain rate dependency of the yield stress on the coefficient of restitution is investigated experimentally and numerically for the impact of a steel sphere against a steel rod. Finite Element simulations using strain-rate dependent material behavior are carried out. In addition, Finite Element simulations with elastic-plastic material behavior, which ignore the strain rate dependency, are carried out as well as elastic material behavior. Comparisons between the experiments and the simulations using strain-rate dependent material behavior show good agreement, and also prove the strong dependency of the coefficient of restitution on the strain rate dependency of the yield stress for steel. The results from both, the experiments and the simulations show also the strong influence of the wave propagation in the rod on the coefficient of restitution.

Computer Simulation of Tensile Deformation of Titanium Small Single Crystals with Stacking Faults

1993 ◽  
Vol 319 ◽  
Author(s):  
M. Aoshima ◽  
T. Kusube ◽  
J. Ida ◽  
Masao Doyama
Keyword(s):  
Molecular Dynamics ◽  
Computer Simulation ◽  
Single Crystals ◽  
Yield Stress ◽  
Tensile Deformation ◽  
Tensile Axis ◽  
Stacking Faults ◽  
Stacking Fault ◽  
Molecular Dynamics Method ◽  
Dynamics Method

AbstractSmall single crystals of titanium with and without stacking faults have been pulled by use of the molecular dynamics method. The tensile axis was [0001] and the stacking fault was introduced on (0001) pianes. The yield stress was higher in the crystal with stacking faults. The deformation was complicated in the crystal with stacking fault. Dislocations were created near the tip of a crack and moved on (1122).

Viscoplastic Effects Occurring in Impacts of Aluminum and Steel Bodies and Their Influence on the Coefficient of Restitution

2010 ◽  
Vol 77 (4) ◽  
Author(s):  
Robert Seifried ◽  
Hirofumi Minamoto ◽  
Peter Eberhard
Keyword(s):  
Kinetic Energy ◽  
Strain Rate ◽  
Yield Stress ◽  
Split Hopkinson Pressure Bar ◽  
Material Model ◽  
Coefficient Of Restitution ◽  
Material Behavior ◽  
Steel Sphere ◽  
Elastic Plastic ◽  
The Impact

Generally speaking, impacts are events of very short duration and a common problem in machine dynamics. During impact, kinetic energy is lost due to plastic deformation near the contact area and excitation of waves. Macromechanically, these kinetic energy losses are often summarized and expressed by a coefficient of restitution, which is then used for impact treatment in the analysis of the overall motion of machines. Traditionally, the coefficient of restitution has to be roughly estimated or measured by experiments. However, more recently finite element (FE) simulations have been used for its evaluation. Thereby, the micromechanical plastic effects and wave propagation effects must be understood in detail and included in the simulations. The plastic flow, and thus the yield stress of a material, might be independent or dependent of the strain-rate. The first material type is called elastic-plastic and the second type is called elastic-viscoplastic. In this paper, the influence of viscoplasticity of aluminum and steel on the impact process and the consequences for the coefficient of restitution is analyzed. Therefore, longitudinal impacts of an elastic, hardened steel sphere on aluminum AL6060 rods and steel S235 rods are investigated numerically and experimentally. The dynamic material behavior of the specimens is evaluated by split Hopkinson pressure bar tests and a Perzyna-like material model is identified. Then, FE impact simulations and impact experiments with laser-doppler-vibrometers are performed. From these investigations it is shown that strain-rate effects of the yield stress are extremely small for impacts on aluminum but are significant in impacts on steel. In addition, it is demonstrated that it is possible to evaluate for both impact systems the coefficient of restitution numerically, whereas for the aluminum body a simple elastic-plastic material model is sufficient. However, for the steel body an elastic-viscoplastic material model must be included.

scholarly journals Deformation of Ice Single Crystals Close to the Melting Point

1978 ◽  
Vol 21 (85) ◽  
pp. 445-455 ◽  
Author(s):  
Stephen J. Jones ◽  
Jean-Guy Brunet
Keyword(s):  
Activation Energy ◽  
Strain Rate ◽  
Single Crystals ◽  
Constant Strain Rate ◽  
Mean Value ◽  
Compression Tests ◽  
A Value ◽  
Polycrystalline Ice ◽  
Strain Rate Compression ◽  
Activation Energy Of Deformation

Abstract Constant strain-rate compression tests on ice single crystals at temperatures between –20°C and -0.2°C are described. The power-law dependence of yield stress on strain-rate gives a value of n which varies from 1.95±0.04 at –0.2°C to 2.07±0.08 at –20°C. The activation energy of deformation varies with strain-rate, but a mean value of 70±2 kJ mol–1 is obtained, with no indication of any increase close to maleting point,as has been found polycrystalline ice. An apparent work-hardening effect, at strains greater than 15%, is explained as being due to bending of the crystal changing the orientation of the basal planes.

The tensile properties of single crystals of high-purity iron at temperatures from 100 to ─253°C

1956 ◽  
Vol 234 (1197) ◽  
pp. 221-246 ◽  
Keyword(s):  
Single Crystals ◽  
High Purity ◽  
Stress Axis ◽  
Shear Stresses ◽  
Fracture Characteristics ◽  
Deformation And Fracture ◽  
Unit Triangle ◽  
Purity Iron ◽  
High Purity Iron ◽  
Reduction In Area

An account is given of the deformation and fracture characteristics of single crystals of high-purity iron of various orientations when tested in tension at temperatures from 100 to ─253°C. At temperatures down to ─124°C, the crystals were fully ductile, giving chisel-edge fractures and 100% reduction in area. At ─196°C, depending on the orientation of the stress axis, the behaviour covered the whole range from fully ductile with 100% reduction in area to completely brittle with cleavage fractures and no apparent deformation. Between these limits, mixtures of slip, twinning and cleavage were obtained. At ─253°C, the crystals gave cleavage fractures, and, over most of the orientation range studied, this occurred without prior deformation. It is shown that the resolved shear stresses required to produce slip or twinning at ─196°C vary with the orientation; in particular, the values are higher for orientations within 20 to 25° of the [001] than elsewhere in the unit triangle, and these values are not reached in this region before failure occurs by cleavage. A mechanism is put forward to explain this in terms of ‘locking’ of dislocations. The cleavage strength resolved normal to the cleavage plane is not constant with change in temperature, as is commonly supposed, but is substantially higher at ─253°C than at ─196°C.

Stress relief during solid-state transformations in minerals

1992 ◽  
Vol 436 (1897) ◽  
pp. 203-216 ◽  
Keyword(s):  
Solid State ◽  
Strain Rate ◽  
Yield Stress ◽  
Transition Layer ◽  
Deviatoric Stress ◽  
Small Scale ◽  
Strain History ◽  
Shear Stresses ◽  
The Matrix ◽  
Kinetics Of

This work estimates the power dissipated by solid-state phase changes in the transition layer of the mantle. Earlier studies have shown these transitions occur by nucleation and growth, and that dislocations are induced in the matrix around the grain during growth. The small-scale flow allowed by the dislocations dissipates power as a sample transforms. This dissipation is easily found in high-pressure experiments by studying the growth of an isolated grain in a rigid, perfectly plastic solid, and using the observed dislocation densities to infer the yield stress. Extrapolation of this result to the mantle raises a difficulty. If the deviatoric stress depends on the strain rate, the high shear stresses inferred around the grains in the experiments may not occur at the low dilatation rates typical of the transition layer. This difficulty is treated here by studying growth in two limits. Either the growth rate is determined by: (i) the kinetics of interface propagation, or (ii) the kinetics of matrix deformation. In case (i) it is argued that the deviatoric stress in the matrix can be taken as independent of strain rate, and the problem of extrapolation is trivial. It is shown that some form of layered convection is likely in case (i), because the yield stress inferred from experiments is such that if all the volume flow in the mantle were to cross the transition layer, the extra power loss would exceed the total viscous dissipation outside the transition layer. In case (ii), the difficulty of extrapolation is significant, and a test is given to identify this case in practice. Such ‘ slip-controlled ’ growth is shown to have a characteristic signature: if the pressure is fixed, the volume of an isolated spherical grain grows exponentially with time if the matrix is isotropic, and the deviatoric stress is a functional of the strain history. The small-scale flow can then be used as a rheometer.

Sign in / Sign up

Export Citation Format

Share Document