Showing papers on "Strain rate published in 2009"

In situ observation of dislocation nucleation and escape in a submicrometre aluminium single crystal.

Abstract: Nanocrystalline materials show significantly different mechanical properties than their bulk counterparts. An in situ microscopy study of Al nanocrystals is now able to directly observe the role of dislocations in tensile deformation and uncover a sensitivity to the strain rate. ‘Smaller is stronger’ does not hold true only for nanocrystalline materials1 but also for single crystals2,3,4,5. It is argued that this effect is caused by geometrical constraints on the nucleation and motion of dislocations in submicrometre-sized crystals6,7. Here, we report the first in situ transmission electron microscopy tensile tests of a submicrometre aluminium single crystal that are capable of providing direct insight into source-controlled dislocation plasticity in a submicrometre crystal. Single-ended sources emit dislocations that escape the crystal before being able to multiply. As dislocation nucleation and loss rates are counterbalanced at about 0.2 events per second, the dislocation density remains statistically constant throughout the deformation at strain rates of about 10−4 s−1. However, a sudden increase in strain rate to 10−3 s−1 causes a noticeable surge in dislocation density as the nucleation rate outweighs the loss rate. This observation indicates that the deformation of submicrometre crystals is strain-rate sensitive.

Analysis of the tensile behavior of a TWIP steel based on the texture and microstructure evolutions

Abstract: The texture and microstructure evolutions of a fine-grained TWIP steel subjected to tensile tests at room temperature were investigated in relation to the mechanical behavior. This steel combines both high ductility and strength owing to the TWIP effect. Also the steel exhibits a high strain hardening rate that evolves according to five stages, which are related to the microstructure and texture evolutions and characteristics. The formation of nano-twins in the initial stage of deformation leads to an increase in strain hardening rate. The development of the pronounced fiber in the tensile direction sustains mechanical twinning and maintains the strain hardening rate on a high level. The resulting microstructure exhibits several types of twin configurations and sub-boundaries with high misorientations due to intense activities of dislocation glide. The twin volume fraction was estimated to be 9% at the final stage of tensile deformation. The new orientations generated by mechanical twinning do not change considerably the final texture.

Constitutive equations to predict high temperature flow stress in a Ti-modified austenitic stainless steel

Abstract: The experimental stress–strain data from isothermal hot compression tests, in a wide range of temperatures (1123–1523 K) and strain rates (10−3–102 s−1), were employed to develop constitutive equations in a Ti-modified austenitic stainless steel. The effects of temperature and strain rate on deformation behaviors were represented by Zener-Holloman parameter in an exponent type equation. The influence of strain was incorporated in the constitutive analysis by considering the effect of strain on material constants. The constitutive equation (considering the compensation of strain) could precisely predict the flow stress only at 0.1 and 1 s−1 strain rates. A modified constitutive equation (incorporating both the strain and strain rate compensation), on the other hand, could predict the flow stress throughout the entire temperatures and strain rates range except at 1123 K in 10 and 100 s−1. The breakdown of the constitutive equation at these processing conditions is possibly due to adiabatic temperature rise during high strain rate deformation.

Time-dependent brittle creep in Darley Dale sandstone

Abstract: [1] The characterization of time-dependent brittle rock deformation is fundamental to understanding the long-term evolution and dynamics of the Earth's crust. The chemical influence of pore water promotes time-dependent deformation through stress corrosion cracking that allows rocks to deform at stresses far below their short-term failure strength. Here, we report results from a study of time-dependent brittle creep in water-saturated samples of Darley Dale sandstone (initial porosity, 13%) under triaxial stress conditions. Results from conventional creep experiments show that axial strain rate is heavily dependent on the applied differential stress. A reduction of only 10% in differential stress results in a decrease in strain rate of more than two orders of magnitude. However, natural sample variability means that multiple experiments must be performed to yield consistent results. Hence we also demonstrate that the use of stress-stepping creep experiments can successfully overcome this issue. We have used the stress-stepping technique to investigate the influence of confining pressure at effective confining pressures of 10, 30, and 50 MPa (while maintaining a constant 20 MPa pore fluid pressure). Our results demonstrate that the stress corrosion process appears to be significantly inhibited at higher effective pressures, with the creep strain rate reduced by multiple orders of magnitude. The influence of doubling the pore fluid pressure, however, while maintaining a constant effective confining pressure, is shown to influence the rate of stress corrosion within the range expected from sample variability. We discuss these results in the context of microstructural analysis, acoustic emission hypocenter locations, and fits to proposed macroscopic creep laws.

Mechanical properties of basalt fiber reinforced geopolymeric concrete under impact loading

Abstract: Impact mechanical properties of basalt fiber reinforced geopolymeric concrete (BFRGC), including dynamic compressive strength, deformation and energy absorption capacity, were studied using a 100-mm-diameter split Hopkinson pressure bar (SHPB) system. For the valid SHPB tests on BFRGC specimens, the improved pulse shaping techniques were proposed to obtain dynamic stress equilibrium and nearly constant strain rate loading over most of the test durations. Impact properties of BFRGC exhibit strong strain rate dependency, and increase approximately linearly with the strain rate. The addition of basalt fiber can significantly improve deformation and energy absorption capacities of geopolymeric concrete (GC), while there is no notable improvement in dynamic compressive strength. In addition, the optimum volume fraction of basalt fiber was presented for BFRGC.

Effects of strain rate, mixing ratio, and stress-strain definition on the mechanical behavior of the polydimethylsiloxane (PDMS) material as related to its biological applications.

Abstract: Tensile tests on Polydimethylsiloxane (PDMS) materials were conducted to illustrate the effects of mixing ratio, definition of the stress-strain curve, and the strain rate on the elastic modulus and stress-strain curve. PDMS specimens were prepared according to the ASTM standards for elastic materials. Our results indicate that the physiological elastic modulus depends strongly on the definition of the stress-strain curve, mixing ratio, and the strain rate. For various mixing ratios and strain rates, true stress-strain definition results in higher stress and elastic modulus compared with engineering stress-strain and true stress-engineering strain definitions. The elastic modulus increases as the mixing ratio increases up-to 9:1 ratio after which the elastic modulus begins to decrease even as the mixing ratio continues to increase. The results presented in this study will be helpful to assist the design of in vitro experiments to mimic blood flow in arteries and to understand the complex interaction between blood flow and the walls of arteries using PDMS elastomer.

Strain hardening behavior of a Fe―18Mn―0.6C―1.5Al TWIP steel

Abstract: The strain hardening behavior of a Fe–18Mn–0.6C–1.5Al TWIP steel was investigated through the modified Crussard–Jaoul (C–J) analysis and microstructural observations. The strain hardening rate obtained by modified C–J analysis was high up to the critical strain of 37% and then greatly decreased with further strain. The electron backscatter diffraction (EBSD) observation showed that the deformation twinning rate is greatly decreased beyond about 34% strain, indicating that the reduced strain hardening rate at the large strain region is attributed to the deceleration of deformation twinning rate. The volume fraction of twinned region was increased with tensile strain due to the increase in the number of deformation twins not to the lateral growth of each deformation twin.

Artificial neural network modeling to evaluate and predict the deformation behavior of stainless steel type AISI 304L during hot torsion

Abstract: The deformation behavior of type 304L stainless steel during hot torsion is investigated using artificial neural network (ANN). Torsion tests in the temperature range of 600-1200^oC and in the (maximum surface) strain rate range of 0.1-100s^-^1 were carried out. These experiments provided the required data for training the neural network and for subsequent testing. The input parameters of the model are strain, log strain rate and temperature while torsional flow stress is the output. A three layer feed-forward network was trained with standard back propagation (BP) and Resilient propagation (Rprop) algorithm. The paper makes a robust comparison of the performances of the above two algorithms. The network trained with Rprop algorithm is found to perform better and also needs less number of iterations for convergence. The developed ANN model employing this algorithm could efficiently track the work hardening, dynamic softening and flow localization regions of the deforming material. Sensitivity analysis showed that temperature and strain rate are the most significant parameters while strain affects the flow stress only moderately. The ANN model, described in this paper, is an efficient quantitative tool to evaluate and predict the deformation behavior of type 304L stainless steel during hot torsion.

Modeling of flow stress of 42CrMo steel under hot compression

Abstract: The compressive deformation behavior of 42CrMo steel was investigated at temperatures from 850 °C to 1150 °C and strain rates from 0.01 s −1 to 50 s −1 on a Gleeble-1500 thermo-simulation machine. The results show that the true stress–true strain curves exhibit peak stresses at small strains, then the flow stresses decrease monotonically until high strains, showing a dynamic flow softening. The stress level decreases with increasing deformation temperature and decreasing strain rate, which can be represented by a Zener–Hollomon parameter in an exponent-type equation. A revised model describing the relationships of the flow stress, strain rate and temperature of the 42CrMo steel at elevated temperatures is proposed by compensation of strain. The stress–strain relations of 42CrMo steel predicted by the proposed models agree well with experimental results.

Deformation behavior of TRIP and DP steels in tension at different temperatures over a wide range of strain rates

Abstract: The dependence of the mechanical behavior of DP and TRIP steels on temperature and strain rate is still not completely understood. Therefore, the mechanical properties of a DP 600 and a TRIP 700 steel were characterized by tensile tests over a temperature range of −100 ≤ T ≤ 235 °C at strain rates ranging from 10−3 to 1250 s−1. The results show that the strain hardening behavior of the TRIP steel depends strongly on the initial test temperature, to which the DP steel seems to be relatively insensitive in the studied temperature interval. On the other hand, the tensile strength of the TRIP steel appears to be much less sensitive to strain rate than that of the DP steel. This is explained by the effects of deformation induced heating on the martensite transformation based hardening of the TRIP steel, partially offsetting the direct effects of strain rate.

Effect of strain-induced martensite on the formation of nanocrystalline 316L stainless steel after cold rolling and annealing

Abstract: This work aimed to study the effects of cold rolling temperature and pre-strain on the volume fraction of strain-induced martensite in order to obtain nanocrystalline structures of 316L stainless steel. Hot rolling and cold rolling followed by annealing treatments were conducted under different conditions. The microstructures and the volume fraction of phases were characterized by scanning electron microscopy and feritscope tests, respectively. The hardness and tensile properties of the specimens were also measured. The results showed that decreasing the rolling temperature while increasing pre-strain leads to increased the volume fraction of martensite accompanied by decreased saturating strain and, further, that this behavior affects the degree of grain refinement. The smallest grain size of about 30–40 nm was obtained via 30% pre-strain at 523 K and subsequent conventional cold rolling at 258 K with a strain and a strain rate of 95% and 0.5 s −1 , respectively, followed by annealing at 1023 K for 300 s. Uniaxial tensile tests at room temperature showed that this specimen exhibits very high tensile strength of about 1385 MPa.

A thermo-viscoplastic constitutive model to predict elevated-temperature flow behaviour in a titanium-modified austenitic stainless steel

Abstract: The experimental stress–strain data from isothermal hot compression tests over a wide range of temperatures (1073–1473 K), strains (0.1–0.5) and strain rates (0.001–1 s−1) were employed to formulate a suitable constitutive model to predict the elevated-temperature deformation behaviour in a Ti-modified austenitic stainless steel (alloy D9). It was observed that the Johnson–Cook (JC) model in its original form is inadequate to provide good description of flow behaviour of alloy D9 in the above hot working domain. This has been attributed to the inadequacy of the JC model to incorporate the coupled effects of strain and temperature and of strain rate and temperature. A modified constitutive model based on the Zerilli–Armstrong model has been proposed for considering the effects of thermal softening, strain rate hardening and isotropic hardening as well as the coupled effects of temperature and strain and of strain rate and temperature on flow stress. The proposed modified constitutive model could predict the elevated-temperature flow behaviour of alloy D9 over the specified hot working domain of alloy D9 with good correlation and generalization.

High speed tensile test of steel sheets for the stress-strain curve at the intermediate strain rate

Abstract: This paper presents stress-strain curves of steel sheets for an auto-body obtained at intermediate strain rates with a servo-hydraulic type high speed tensile testing machine. The apparatus has the maximum stroke velocity of 7.8 m/sec to obtain the tensile material properties at a strain rate of up to 500/sec. A special jig fixture is specially designed for accurate acquisition of tensile loads with reduction of the load-ringing phenomenon induced by unstable stress wave propagation at high strain rates. Tensile testing of steel sheets for an auto-body was carried out to obtain stress-strain curves of mild steel and advanced high strength steels at strain rates ranged from 1/sec to 200/sec. The test results provide interesting information regarding the stress-strain curves at intermediate strain rates ranged from 1/sec to 200/sec and demonstrate that strain rate hardening is strongly coupled with strain hardening.

Effect of strain ratio and strain rate on low cycle fatigue behavior of AZ31 wrought magnesium alloy

Abstract: Magnesium alloys are increasingly used in automotive and aerospace industries for weight reduction and fuel economy improvement. Low cycle fatigue (LCF) behavior of these alloys is an important consideration for the structural applications. The objective of the present investigation was to identify influences of strain ratio and strain rate on cyclic deformation characteristics and fatigue life of an AZ31 extruded alloy. As the strain ratio decreased, stronger cyclic hardening rate, more asymmetric hysteresis loop, smaller stress amplitude, lower mean stress, and higher initial plastic strain amplitude were observed due to increasing compressive stresses. This was considered to be associated with the twinning during cyclic deformation in the compressive phase, and detwinning in the tensile phase. The residual twins acting as barriers to dislocation slip and pile-up were considered to be the main cause for the occurrence of cyclic hardening. Fatigue life increased with decreasing strain ratio and increasing strain rate. Fatigue crack initiation occurred at the specimen surface due to the presence of larger grains near the surface, and fatigue crack propagation was characterized by a mixture of striations and dimple-like ductile fracture features.

Influence of temperature on brittle creep in sandstones

Abstract: The characterization of time-dependent brittle creep, promoted by chemically active pore fluids, is fundamental to our understanding of the long-term evolution and dynamics of the Earth's crust. Here we report results from a study of the influence of temperature on both short-term strength and time-dependent brittle creep in three sandstones under triaxial stress conditions. We show that an increase in temperature from 20 degrees to 75 degrees C significantly enhances stress corrosion cracking in all three sandstones, leading to (1) a systematic reduction in strength during constant strain rate experiments and (2) an increase by several orders of magnitude in brittle creep strain rates during stress-stepping creep experiments. We also show that a conventional creep experiment performed at 75 degrees C exhibits a qualitatively similar three-stage brittle creep curve as that observed at ambient temperature. Extrapolation of our results suggests that temperature is likely to be the dominant influence on the evolution of creep strain rate with depth in the shallow crust. Citation: Heap, M. J., P. Baud, and P. G. Meredith (2009), Influence of temperature on brittle creep in sandstones, Geophys. Res. Lett., 36, L19305, doi: 10.1029/2009GL039373.

Impact characterization of basalt fiber reinforced geopolymeric concrete using a 100-mm-diameter split Hopkinson pressure bar

Abstract: Industrial wastes, slag and fly ash, were used to produce geopolymeric concrete (GC), and which was reinforced with short basalt fiber. Impact mechanical properties of basalt fiber reinforced geopolymeric concrete (BFRGC) of three different matrix strengths were investigated using a 100-mm-diameter split Hopkinson pressure bar (SHPB), and strain rate effects on dynamic compressive strength, critical strain and specific energy absorption were studied. For the valid SHPB tests on BFRGC specimens, the improved pulse shaping techniques were proposed to obtain dynamic stress equilibrium and nearly constant strain rate loading over most of test durations. The results show that impact properties of BFRGC exhibit strong strain rate dependency, and increase approximately linearly with strain rate. The transition point from low strain rate sensitivity to high sensitivity decreases with the increase of matrix strength. The addition of basalt fiber can significantly improve deformation and energy absorption properties of GC, while there is no notable enhancement in dynamic compressive strength. Increase of matrix strength results in decrease of deformation capacity and increase of energy absorption capacity for BFRGC.