Strain Hardening Rate Sensitivity Research Articles

Abnormal strain rate strengthening and strain hardening with constitutive modeling in body-centered cubic {332} TWIP titanium alloy

Metastable body-centered cubic titanium alloys exhibiting {332}<113> twinning-induced plasticity (TWIP) have attracted extensive attention owing to their remarkable strain hardenability and unprecedented impact performance. However, the intrinsic correlation between the strain rate, {332}<113> twinning, and flow stress remains to be fully elucidated. In this study, the instantaneous strain rate sensitivity (ISRS) and strain rate sensitivity of strain hardening (SRSS) corresponding to strain rate strengthening and strain hardening, respectively, were systematically clarified in {332}<113> Ti-15Mo alloy exhibiting TWIP effect by conducting strain-rate-jump tests and constitutive modeling analyses. The positive ISRS with an increasing trend correlated well with the change in the dislocation activation volume, which was mainly controlled by local obstacles of interstitial O atoms, and was affected by geometrically necessary dislocations due to twin formation. The negative SRSS with a decreasing trend correlated well with the evolution of deformation microstructures, which was mainly due to the decrease in the twinning rate and an increase in the dislocation annihilation rate. The established constitutive model reasonably described not only the abnormal strain rate strengthening from the thermal activation of dislocation but also the abnormal strain hardening from concurrent back stress strengthening due to twin formation and dislocation strengthening.

Strain hardening rate sensitivity and strain rate sensitivity in TWIP steels

TWIP steels are materials with very high strength and exceptional strain hardening capability, parameters leading to large energy absorption before failure. However, TWIP steels also exhibit reduced (often negative) strain rate sensitivity (SRS) which limits the post-necking deformation. In this study we demonstrate for an austenitic TWIP steel with 18% Mn a strong dependence of the twinning rate on the strain rate, which results in negative strain hardening rate sensitivity (SHRS). The instantaneous component of SHRS is large and negative, while its transient is close to zero. The SRS is observed to decrease with strain, becoming negative for larger strains. Direct observations of the strain rate dependence of the twinning rate are made using electron microscopy and electron backscatter diffraction, which substantiate the proposed mechanism for the observed negative SHRS.

Strain-Rate Sensitivity of Aluminum Alloys AA1200 and AA3103

Tensile tests are carried out for the aluminum alloys AA1200 and AA3103 at various strain-rates in the range from 10−4 s−1 to 1 s−1. Tests with constant nominal strain-rate and strain-rate jump tests are conducted, and the instantaneous rate sensitivity and the rate sensitivity of strain hardening are investigated. For both materials, the instantaneous rate sensitivity is found to be rather independent of strain, while the rate sensitivity of the strain hardening is important and the saturation stress increases with increasing strain-rate. A phenomenological constitutive model is described that comprises a kinetic equation governing the instantaneous rate sensitivity of the flow stress and a structural parameter that determines the mechanical state of the material. The evolution of the structure parameter is assumed to depend on strain-rate. The model parameters are determined for the two materials using the available experimental information. It is found that the constitutive model provides a good representation of the experimental results.

Strain path change effects in the local necking of aluminum sheet and in the tension of internally pressurized tubes

It is possible to increase significantly the uniform elongation achieved in the uniaxial tension of commercial purity aluminum by accelerating the testing rate. This effect is linked to a significant rate sensitivity of strain hardening. However, very little increase in the strains associated with the final stages of localization in sheet specimens were achieved by this means. The importance of the change in strain rate and path on local necking has been investigated by introducing path changes of appropriate magnitudes in tubular tensile specimens by internal pressurization. The path change led to a decrease in strain-hardening rate which was not compensated for by an increase in strain rate. The potential consequences of this effect upon ductility in sheets are significant and limit the potential usefulness of any rate sensitivity of strain hardening in increasing formability.

Pressure-shear impact investigation of strain rate history effects in oxygen-free high-conductivity copper

A combined experimental and computational investigation of the behavior of oxygen-free high-conductivity copper under very high shear rates is presented. Pressure-shear plate impact is used for conducting constant strain rate tests and strain rate change texts in which the specimen is strained at shear rates up to 10 6s −1 for 1μs and then strained at substatially lower shear rates for another microsecond. The specimen is sandwiched between two hard elastic plates to impose conditions of simple shear at very high strain rates and constant hydrostatic pressure. Marked increases in flow stresses are observed at strain rates of 10 5s −1 and higher. Flow stresses decrease gradually after a sharp drop in strain rate in all strain rate change tests. Homogeneous equiaxed dislocation cells are found as the predominant substructure in the deformed specimens. Theoretical analyses of the nonlinear wave propagation within the specimen are carried out using a general internal variable formulation in which the hardening rate depends on the rate of deformation. The governing system of hyperbolic partial differential equations is solved using a finite-difference scheme; computational results are compared with the experimental results. Both small- and finite-deformation formulations are considered. Only the internal variable model which incorporates a strong rate sensitivity of strain hardening is successful in describing the observed response to the change in strain rate. The enhanced rate sensitivity at high strain rates is concluded to be related primarily to the rate sensitivity of strain hardening, not the rate sensitivity of the flow stress at constant dislocation structure. The generation and evolution of dislocation cells appears to be the dominant micromechanical process during the high- rate deformation of pure metals.

High Strain Rate Behavior of Metals

Experimental results on the high strain rate response of polycrystalline metals are reviewed, with emphasis on the behavior of pure metals. A strong increase in flow stress with increasing strain rate is reported for strain rates of approximately 105s−1 and higher. This increase is observed in pressure-shear plate impact experiments at nominally constant strain rates from 105s−1 to 106s−1. To improve understanding of the increased rate sensitivity at high strain rates, pressure-shear, strain-rate-change experiments have been conducted on OFHC copper specimens. These experiments have been analyzed using a conventional viscoplasticity formulation and an internal variable formulation in which the hardening rate depends on the rate of deformation. Only the latter formulation is successful in describing the observed response to the change in strain rate. This observation is discussed in terms of its implications for interpreting other dynamic plasticity experiments and for improved understanding of the underlying dislocation mechanisms. The enhanced rate sensitivity at high strain rates is concluded to be related primarily to the rate sensitivity of strain hardening, not the rate sensitivity of the flow stress at constant structure.

On rate sensitivity of f.c.c. metals, instantaneous rate sensitivity and rate sensitivity of strain hardening

A n up-to-date approach to the phenomenon of rate sensitivity observed in f.c.c. metals is discussed. It is shown that the rate sensitivity of strain hardening, which so far has been neglected, plays an equal or even dominant role in an estimation of the total rate sensitivity. It is suggested that the presence of the rate sensitivity of strain hardening is developed by the athermal generation of structural defects (dislocations), while at the same time collision and partial annihilation of dislocations occurs with the assistance of thermal activation. These micromechanisms of plastic deformation are capable of developing so-called strain rate and temperature history effects. Some experimental evidence of strain rate history effects, for both polycrystals and monocrystals, are provided in this paper; they are discussed within the framework of instantaneous rate sensitivity versus rate sensitivity of strain hardening. Both rate sensitivities are reviewed within the framework of thermal activation strain rate analysis. Experimental data are provided for aluminium, copper and lead. They conclusively demonstrate the importance of rate sensitivity of strain hardening which is developed by dynamic recovery (annihilation of defects during plastic deformation). Some fundamentals of how to construct constitutive relations have been discussed on the basis that the total flow stress τ is the sum of the effective stress τ∗ and the internal stress τ μ. General relations for structural evolution have been analysed, which are able to describe strain rate and temperature history effects. On the basis of earlier results a general relationship for structural evolution has been proposed using the concept of the effective dislocation multiplication coefficient M eff. It is shown that an evolutionary relationship should be of differential type with generation and annihilation terms. Finally, some recommendations are provided as to constitutive modeling and future studies of both instantaneous rate sensitivity and rate sensitivity of strain hardening.