Low Strain Rate Range Research Articles

Creep deformation behaviour of AZ31 magnesium alloy over wide range of temperature and stress

The creep deformation behaviour of coarse grained AZ31 magnesium alloy was examined in the temperature range from 423 to 673 K (0·46–0·73Tm) under various constant stresses covering low strain rate range from 4×10−9 to 2×10−2 s−1. Most shape of the creep curve was typical of class II behaviour. However, only at low stress and low temperature, the shape of the creep curve was typical of class I behaviour. At very low stress at 673 K, the stress exponent for the secondary creep rate was ∼2. At low stress level, the stress exponent was ∼3 and the present results were in good agreement with the prediction of Takeuchi and Argon model. At high stress level, the stress exponent was ∼5 and the present results were in good agreement with the prediction of Weertman model. The transition of deformation mechanism from solute drag creep to dislocation climb creep could be explained in terms of solute atmosphere breakaway concept.

A study of the hot and cold deformation of twin-roll cast magnesium alloy AZ31

Recent advances in twin-roll casting (TRC) technology of magnesium have demonstrated the feasibility of producing magnesium sheets in the range of widths needed for automotive applications. However, challenges in the areas of manufacturing, material processing and modelling need to be resolved in order to fully utilize magnesium alloys. Despite the limited formability of magnesium alloys at room temperature due to their hexagonal close-packed crystalline structure, studies have shown that the formability of magnesium alloys can be significantly improved by processing the material at elevated temperatures and by modifying their microstructure to increase ductility. Such improvements can potentially be achieved by processes such as superplastic forming along with manufacturing techniques such as TRC. In this work, we investigate the superplastic behaviour of twin-roll cast AZ31 through mechanical testing, microstructure characterization and computational modelling. Validated by the experimental results, a novel continuum dislocation dynamics-based constitutive model is developed and coupled with viscoplastic self-consistent model to simulate the deformation behaviour. The model integrates the main microstructural features such as dislocation densities, grain shape and grain orientations within a self-consistent viscoplasticity theory with internal variables. Simulations of the deformation process at room temperature show large activity of the basal and prismatic systems at the early stages of deformation and increasing activity of pyramidal systems due to twinning at the later stages. The predicted texture at room temperature is consistent with the experimental results. Using appropriate model parameters at high temperatures, the stress–strain relationship can be described accurately over the range of low strain rates.

The strain-rate effect of engineering materials and its unified model

The test data of metals, brittle materials and polymers in high, medium and low strain-rate range were summarized. It was found that the dynamic strength or yield stress of these materials was not sensitive to strain-rate in the low and high strain-rate range (corresponding to weak sensitivity area and saturated zone). But in the medium strain-rate range (strong sensitivity area), the dynamic strength or yield stress of these materials was extremely sensitive to strain-rate. The strain-rate effect function in classic Johnson-Cook constitutive model could not describe these phenomena in the range of low to high strain-rate. Therefore, the strain-rate effect function in classic Johnson-Cook constitutive model has been modified to a strain-rate effect expression which is suitable for different materials. This new model can better describe the phenomena of the weak sensitivity area, strong sensitivity area and saturated zone. With this model, the fitted curves are in good agreement with the test data, and the parameters are different for different materials. Finally, the fitting parameters are given.

Strain Rate Sensitivity of Pre‐Strained AISI 301LN2B Metastable Austenitic Stainless Steel

AbstractThe dynamic behavior of AISI 301LN2B (EN 1.4318) metastable austenitic steel grade has been investigated at 296 K by means of servohydraulic tensile and split Hopkinson bar testing in the strain rate range 0.005–1000 s−1. As delivered, as well as 10% uniaxial, biaxial, and plane strain pre‐strained conditions, without subsequent heat treatment have been tested. A negative strain rate sensitivity is observed in the low strain rate range between 10−4 and 1–10 s−1. Pre‐straining reduces the magnitude of the adiabatic tensile strength softening, especially in the plane strain condition with higher triaxility. The thermal activation related dynamic flow stress increase is not dependent on pre‐straining. The γ → α′ induced additional flow stress increase, however, is highly strain rate and pre‐straining sensitive. The amount of pre‐straining determines the overall ductility at fracture, and therefore the adiabatic temperature increase. The pre‐straining stress state influences the amount of α′‐martensite formed before dynamic testing, and consequently the maximum intensity of the TRIP induced flow stress increase by subsequent dynamic testing.

Effect of Strain Rate on the Tensile Strength of High-Modulus Type Poly-p-Phenylene Benzobisoxazole (PBO) Fiber

In this paper, effect of strain rate on the tensile strength of high-modulus type poly-p-phenylene benzobisoxazole (PBO) fiber was investigated in monofilament tests. Tensile tests were carried out at a gauge length of 12.5 mm. The strain rate ranged from 6.7×10-4 to 3.2×10-1 s-1. It was found that the tensile strength removed size effect in diameter direction of PBO fiber was represented by a two parameter Weibull distribution under all strain rate conditions. The mean tensile strength at high strain rate range from 4.0×10-2 to 3.2×10-1 s-1 was about 8 % higher than that at low strain rate range from 6.7×10-4 to 1.3×10-2 s-1. In addition, it was found by SEM observation that there were distinct differences between fracture surface image of specimen at high strain rate range and at low strain rate range, i.e., the crack length in fiber direction at high strain rate range was much shorter than that at low strain rate range. However the crack grew along boundaries of microfibrils at the low strain rate range, the crack grew not only along the boundaries but also in the microfibrils at the high strain rate range. Therefore, the increase in tensile strength at the high strain rate range was caused by the difference in the path of crack propagation.

Energy‐Absorbing Properties of Paper Honeycombs under Low and Intermediate Strain Rates

The effect of the strain rate on the mechanical behaviour and energy absorption of paper honeycombs is investigated by experiments on an environmental condition designed to simulate the actual logistic environment in most of South China. The strain rate varies from 3.3 × 10−4 to 1.1 × 102 s−1. The experimental results show that the load‐carrying capacity and the energy absorption performance of paper honeycombs are insensitive to the loading speed in low strain rate range (10−4–10−2 s−1). However, the initial peak stress, the plateau stress and the densification strain of paper honeycombs under intermediate strain rate impact (10−2–102 s−1) increase with impact velocity and are obviously higher than that under static compression, thus demonstrating a certain degree of strain rate sensitivity. The restoring force due to gas compression in a confined space of hexagonal cavity contributes to the increase in dynamic plateau stress. A model is developed for the prediction of dynamic plateau stress by considering this restoring force. A good agreement between observation and prediction is obtained, indicating that the model constructed here can be used to evaluate the dynamic plateau stress for paper honeycombs. Copyright © 2011 John Wiley & Sons, Ltd.

Subtransus deformation mechanisms of TC11 titanium alloy with lamellar structure

Isothermal compression tests are applied to study the deformation mechanisms of TC11 titanium alloy with lamellar structure under the deformation temperature range of 890–995 °C and strain rate range of 0.01–10 s −1. According to the flow stress data obtained by compression tests, the deformation activations are calculated based on kinetics analysis of high temperature deformation, which are then used for deformation mechanism analysis combined with microstructure investigation. The results show that deformation mechanisms vary with deformation conditions: at low strain rate range, the deformation mechanism is mainly dislocation slip; at low temperature and high strain rate range, twinning is the main mechanism; at high temperature and high strain rate range, the deformation is mainly controlled by diffusion of β phase.

768 Reproducibility of gene expression measurements in microarray studies relies on filtering of expressed genes: implications for the understanding of childhood papillary thyroid cancer transcriptome

In this paper, a new experimental technique is reported to study behavior of sheet metals in the range of low and high strain rates. The main purpose of this work was to analyse visco-plastic behavior of sheet metals for an industrial application. It may be mentioned that a variety of industrial processes, such as drawing or even crashworthiness, involve a wide range of strain rates. The test method which is based on previous work, (Klepaczko, J.R., 1994. An experimental technique for shear testing at high and very high strain rates. The case of mild steel. Int. J. Impact Engng. 15, 25–39), is a combination of two experimental techniques, that enables one to obtain the range of strain rates from 10−4 to 103 s−1, (Rusinek, A., Klepaczko, J.R., 1998. Etude experimentale du double cisaillement des tôles, modélisation de l'écoulement viscoplastique. Rapport interne, LPMM). The double-shear specimen is loaded by a fast hydraulic machine, and in the second case a specimen of the same geometry can be loaded by the direct impact of a projectile. The combination of direct impact with the double shear specimen, which is based on the Hopkinson tube technique of force measurement, permits to reduce the rise time of loading in comparison to the bar-tube configuration (Campbell, J.D., Ferguson, W.G., 1970. The temperature and strain rate dependence of shear strength of mild steel. Phil. Mag. 21, 63–82). The combination of two schemes of loading assures a complete determination of the shear stress τ(t), the shear strain Γ(t) and shear strain rate Γ̇(t) as a function of time at wide range of strain rates. The test method has been applied to determine the visco-plastic behavior of cold rolled, Al-calmed sheet steel used in the automotive industry. The experimental results obtained with those two experimental techniques are in agreement with other reported data. An original visco-plastic constitutive relation has been proposed. It permits to model the viscoplastic behavior of the sheet metal tested in the whole range of strain rates considered. A general procedure for the identification of the material constants is proposed (Rusinek, A., Klepaczko, J.R., 1998. Etude experimentale du double cisaillement des tôles, modélisation de l'ecoulement viscoplatique. Rapport interne, LPMM.). It offers the advantage of subdividing the total number of constants into groups, this procedure simplifies the problem of identification. The constitutive relation proposed takes into account the strain hardening, the strain rate sensitivity and temperature effects. A comparison with experimental results is presented in the form of stress–strain curves at different strain rates, up to 103 s−1.

Optimum parameters and rate-controlling mechanisms for hot working of extruded Mg–3Sn–1Ca alloy

The hot working behavior of extruded Mg–3Sn–1Ca alloy has been characterized by compression testing in the temperature range of 300–550 °C and strain rate range of 0.0003–10 s −1 with a view to evaluate the optimum processing parameters as well as the rate-controlling mechanisms. Processing maps, developed on the basis of the temperature and strain rate dependence of flow stress, exhibited two domains in which dynamic recrystallization occurs. Both these are in the temperature range 325–500 °C, with one in the lower strain rate range (0.0003–0.003 s −1) and the other in the higher strain rate range (1–10 s −1), the optimum temperature being 400 °C. Kinetic analysis in the above two domains yielded apparent activation energy values of 196 and 168 kJ/mole, respectively, which are higher than that for self-diffusion in pure magnesium suggesting that the large volume fraction of CaMgSn intermetallic particles in the matrix causes significant back stress. In the change-over strain rate range (0.003–0.3 s −1), unusual grain size changes have occurred which may render microstructural control difficult.

Hot working parameters and mechanisms in as-cast Mg–3Sn–1Ca alloy

The hot working characteristics of as-cast Mg–3Sn–1Ca alloy have been studied using processing-map technique and the kinetic rate equation. The map exhibited two domains; one in the lower strain rate range (0.0003–0.01 s − 1 ) and the other in the higher strain rate range (0.1–10 s − 1 )—both falling in the same temperature range of 350–550 s − 1 . Hot extrusion at 500 °C and a speed corresponding to an average strain rate of 3 s − 1 exhibited dynamically recrystallized microstructure. The estimated apparent activation energy values are higher than those for self-diffusion in pure magnesium suggesting that the large volume fraction of MgSnCa intermetallic particles causes significant back stress.

On Coble creep in Mg–9Al–1Zn alloy with ultrafine-grained microstructure

High-temperature deformation behavior of AZ91 alloy with ultrafine grains was studied at 473–573 K. Analyses based on the deformation mechanism maps for Mg and comparison of the experimental data with the values predicted by equations for several deformation mechanisms indicated that Coble creep was the rate-controlling deformation mechanism in low strain rate range. The critical grain size where Coble creep can be detected in Mg at a given strain rate and temperature was calculated.

Hot workability characteristics of cast and homogenized Mg–3Sn–1Ca alloy

Abstract Hot working behavior of Mg–3Sn–1Ca alloy has been evaluated with a view to explore its potential as a wrought alloy. The temperature and strain rate dependence of steady state flow stress has been analyzed. The processing maps exhibited two domains both in the temperature range 350–550 °C with one in the lower strain rate range (0.0003–0.01 s−1) and the other at higher strain rates (1–10 s−1). The apparent activation energies estimated in these two domains respectively are 217 kJ/mole and 154 kJ/mole. The activation area values obtained, after making a correction for the back-stress generated by the hard intermetallic CaMgSn particles, are in the range 13–100 b2 and decreased with increasing effective stress. The corrected results suggest that the lower strain rate domain is controlled by lattice self-diffusion and grain boundary diffusion occurs at higher strain rates. Extrusion at 500 °C and at a speed that corresponds to an average strain rate of 3 s−1 yielded a sound product with a dynamically recrystallized microstructure.

High Strain Rate Deformation Behavior of Mg–Al–Zn Alloys at Elevated Temperatures

High temperature deformation behavior of AZ31 and AZ91 magnesium alloys was examined by compression tests over a wide strain rate range from 10–3 to 103 s–1 with emphasis on the behavior at high strain rates. The dominant deformation mechanism in the low strain rate range below 10–1 s–1 was suggested to be climb-controlled dislocation creep. On the other hand, experimental results indicated that the deformation at a high strain rate of ~103 s–1 proceeds by conventional plastic flow of dislocation glide and twinning even at elevated temperatures. The solid-solution strengthening was operative for high temperature deformation at ~103 s–1.

Unconfined compression of white matter

The porous properties of brain tissue are important for understanding normal and abnormal cerebrospinal fluid flow in the brain. In this study, a poroviscoelastic model was fitted to the stress relaxation response of white matter in unconfined compression performed under a range of low strain rates. A set of experiments was also performed on the tissue samples using a no-slip boundary condition. Results from these experiments demonstrated that the rheological response of the white matter is primarily governed by the intrinsic viscoelastic properties of the solid phase. The permeability of white matter was found to be of the order of 10 −12 m 4/Ns.

Deformation mechanisms responsible for the high ductility in a Mg AZ31 alloy analyzed by electron backscattered diffraction

The microstructural evolution during tensile deformation of an AZ31 alloy with grain size ranging from 17 to 40 µm, at intermediate temperatures, has been studied using electron backscattered diffraction (EBSD) and optical microscopy (OM) as the main characterization tools. Two deformation regimes could be distinguished. In the high-strain-rate regime, the stress exponent was found to be about 6, and the activation energy is close to that for Mg self-diffusion. These values are indicative of climb-controlled creep. In the lower strain rate range, elongations higher than 300 pct were measured. In this range, significant dynamic grain growth takes place during the test, and thus, the predominant deformation mechanisms have been investigated by means of strain-rate-change tests. It was found that the stress exponent varied during the test between 1.7 and 2.5, while the activation energy remains close to that for grain-boundary diffusion. The EBSD analysis revealed, additionally, the appearance of low to moderately misoriented boundaries that tend to lay perpendicular to the tensile axis. The enhanced ductility of this AZ31 alloy in this regime is attributed to the operation of a sequence of deformation mechanisms. Initially, grain-boundary sliding governs deformation; once dynamic grain growth occurs, dislocation slip becomes gradually more important. Dislocation interaction gives rise to the appearance of new interfaces by continuous dynamic recrystallization (CDRX).

High temperature compressive properties over a wide range of strain rates in an AZ31 magnesium alloy

High temperature compressive properties in AZ31 magnesium alloy were examined over a wide strain rate range from 10−3 to 103 s−1. It was suggested that the dominant deformation mechanism in the low strain rate range below 10−1 s−1 was dislocation creep controlled by pipe diffusion at low temperatures, and by lattice diffusion at high temperatures. On the other hand, analysis of the flow behavior and microstructural observations indicated that the deformation at high strain rates of ∼103 s−1 proceeds by conventional plastic flow of dislocation glide and twinning even at elevated temperatures.

Tensile Properties of MoSi<sub>2</sub> at Elevated Temperatures

The tensile properties of two MoSi2 alloys with different grain sizes (1 micrometer and 10 micrometer) were evaluated in vacuum at temperatures ranging from 1400 to 1600K and initial strain rates ranging from 1×10-5/s to 1×10-3/s. For the alloy with 10micron grain size an m vale of 0.35 and an activation energy value of 350 kJ/mol were observed in the lower strain rate range while an m value of 0.12 and an activation energy value of 760 kJ/mol were observed in the higher strain rate range. For the alloy with 1micron grain size, a uniform m value of 0.55 and an activation energy value of 160 kJ/mol were observed. Moreover these two alloys showed remarkable ductility (maximum 33%) in the test temperatures. The deformation mechanism and the remarkable ductility are discussed in the light of the microstructural observations through SEM and TEM.

Temperature and grain size dependence of superplasticity in a Zn−0.3wt.%Al alloy

The superplastic deformation behavior of quasi-single phase Zn-0.3 wt. %Al was investigated. A series of load relaxation and tensile tests was conducted at various temperatures ranging from RT (20 °C) to 200 °C. The recently proposed internal variable theory of structural superplasticity was applied. The flow curves obtained from load relaxation tests were shown to consist of contributions from interface sliding (IS) and accommodating plastic deformation. In the case of quasi-single phase Zn-0.3 wt.% Al alloy with an average agrain size of 1 μm, the IS behavior could be described as a viscous flow process characterized by a power index of Mg=0.5. A large elongation of about 1400% was obtained at room temperature and the strain rate sensitivity parameter was about 0.4. Although relatively large-grained (10 μm) single phase alloy showed a high value of strain rate sensitivity comparable to that of fine-grained alloy at very low strain rate range, IS was not expected from the analysis based on the internal variable theory of structural superplasticity at room temperature. As the temperature increased above 100 °C, however, the contribution from IS was observed at a very low strain rate range. A high elongation of ∼400% was obtained in a specimen of 10-μm-grain-size at 200 °C under a strain rate of 2×10−4/sec.

522 高温低サイクル疲労破壊における SUS316 鋼の粒界すべり挙動の評価

In the case of low-cycle fatigue failure at high temperature, it is known that the reduction in fatigue life and Interguranular damage become prominent as the strain rate becomes low. In addition the reduction in fatigue life comes about in low strain rate range. In this study, grain boundary sliding displacement of SUS316 was estimated under low-cycle fatigue at 600,700 degrees C. One of the causes of the reduction in fatigue life due to lower strain rate is dependent on increasing the grain boundary sliding displacement per cycle. And Grain boundary sliding displacement was accumulated by compression-side deformation of grain boundary sliding deformation.

Thermo-mechanical behavior associated with pseudoelastic transformationof shape memory alloys

We have performed a simulation analysis of the pseudoelasticbehavior of a NiTi shape memory alloy wire placed in air, subjected to tensilecyclic loads over a range of low strain rates. Thermomechanical interactions,such as energy dissipation, latent heat and heat transfer, are accounted forin a one-dimensional thermomechanical model based on the phase interactionenergy function which was introduced in our previous analyses. The interactionenergy function was numerically determined so as to make an analyticalconstitutive relationship fit available experimental stress-strain curves.Numerical results show that the loading strain rate is influential on changesin the stress-strain characteristics and the temperature of the wire. Goodagreement between experiment and analysis was obtained, and the effectivenessof the thermomechanical model proposed here has been confirmed.