Experimental analysis of the multiaxial failure stress locus of commercially pure titanium at low and high rates of strain

The mechanical response of commercially pure copper under multiaxial loading at low and high strain rates

Yield and fracture of polyethylene have been studied in torsion tests under superposed hydrostatic pressures. Two ductile-to-brittle transitions have been observed. At high strain rates and pressures, a conventional ductile-to-brittle transition was found with increasing strain rate and pressure. A second ductile-to-brittle transition was observed at low strain rates with decreasing strain rate. The yield stress showed a region of low, relatively constant, rate dependence at low strain rates, high temperatures and low pressures and a second region of higher strain-rate dependence at high strain rates and pressures. In contrast, the fracture stress was found throughout to have a relatively constant strain-rate dependence of intermediate value between those obtained for the yield stress. These features confirmed that failure can be considered as competition between yield and fracture processes. The fracture stress became lower than the yield stress at both high and low strain rates where brittle fracture was observed, with fully ductile behaviour resulting in intermediate conditions where the fracture stress exceeded the yield stress. The pressure, strain rate and temperature dependence of the yield stress was well described by two Eyring processes acting in parallel, both processes being pressure dependent.

Constitutive modeling and microstructure characterization of 2196 Al-Li alloy in various hot deformation conditions

In order to study the mechanical properties of NEPE propellant at low and high strain rates, the quasi-static and impact eperiments of NEPE propellant were carried out by the electronic universal testing machine and split Hopkinson bar, and the stress-strain curves of NEPE propellant under different strain rates (1.667×10−4−4 500 s−1) were obtained by processing the experiment data. By analyzing the stress-strain curve of low and high strain rates experiment, it can be found that NEPE propellant has obvious nonlinear elasticity and strain rate sensitivity. With the increase of strain rate, the strength, yield stress and elastic modulus of the material increase significantly. Compared with low strain rate, the strain rate sensitivity of the material at high strain rate is higher. Under the high speed impact, a large amount of heat is generated inside the material and cannot be released in time, which makes the internal temperature of the material rise, leading to softening effect of the material and reduction of mechanical properties. In this paper, a nonlinear visco-hyperelastic constitutive model is established to describe the mechanical properties of NEPE propellant at low and high strain rates, in which the Rivlin strain energy function is used to describe the static hyperelastic behaviour, and an integral constitutive model is used to characterize the dynamic response of the material. Considering that the relaxation time has strain rate correlation, a rate-dependent relaxation function is adopted in this paper to replace the traditional Prony series. The hyperelastic parameters were obtained by fitting the hyperelastic part of the constitutive model with extremely slow compression experiment data, and then the other parameters were obtained by fitting the constitutive model with quasi-static and dynamic experiment data. It was proved that the model could well describe the mechanical properties of NEPE propellant at low and high strain rates by the good coincidence degree between the prediction curve and the experiment curve under different strain rates.

Layer dependence in strain distribution and chondrocyte damage in porcine articular cartilage exposed to excessive compressive stress loading

This paper presents an experimental study of the strain-rate and temperature dependence of the yield stress for three low carbon steels (percent c = 0.2, 0.3, and 0.4). Tension test results using notched specimens are reported for temperatures of −20°C, 0°C, and 25°C and strain rates from 10−4 s−1 to 200 s−1. Two regions with different kinetics have been observed. The yield stress shows a low sensitivity to strain-rate at higher temperatures and lower strain rates (region I), but the rate sensitivity is high at lower temperatures and higher strain rates (region II). An approximately linear dependence of yield stress on log strain rate is displayed in both regions. The carbon content of the steel strongly affects strengthening in region I, but weakening appears as the strain rate increases in region II. Deformation in the two regions is analyzed according to standard formalisms. When the yield stress is separated into athermal and thermally activated components, the carbon concentration dependence is shown to isolate to the athermal stress component, which becomes rate controlling at high temperatures and low strain rates.

The plateau stress and energy absorption of low density (≤300 kg/m3) polyurea (PU) foams and expanded polystyrene (EPS) were measured at deformation rates ranging from 0.004 s−1 to 5000 s−1. Low (≤10−1 s−1) strain rate testing was performed using an Instron load frame, intermediate (101–102 s−1) strain rates using a drop-weight impact tower, and high (≥103 s−1) strain rate conditions using a modified split-Hopkinson pressure bar. The plateau stress and energy absorption of low density PU foams exhibit a strong rate dependence across all deformation rates. This result has been previously unreported for low density polymer foams under low and intermediate strain rates. The strain rate sensitivity of PU foams was found to be strongly dependent on cell size for low strain rates and cell wall aperture size for intermediate and high strain rates. EPS type foam, however, remained nearly insensitive to strain rate. At low and intermediate strain rates, the plastic crushing in the EPS and the high plateau stress yield a much higher energy absorption capability than the viscoelastic dissipation in the PU foams. However, PU foams were found to display similar energy absorption properties as EPS based foams under high strain rates. Thus, controlling the strain rate sensitivity of PU foams through aperture diameter can lead to an increase in energy absorption properties at high strain rates, while simultaneously maintaining the peak stress below certain injury thresholds. Additionally, unlike EPS, which undergo plastic crushing after first impact, flexible polyurea foams will recover fully after each impact and thus will have multiple hit capabilities. This will allow these materials to have a wide range of applications, in advance body armors and protective headgears to use in low-cost protection systems for a wide range of military platforms, civilian, and space applications.

Opposite grain size dependence of strain rate sensitivity of copper at low vs high strain rates

In this study, pure titanium equivalent to Grade 1 was subjected to tensile tests at strain rates ranging from 10−6 to 100 s−1 to investigate the relationship between its mechanical properties and its twinning and slip. Deformation properties and microstructures of samples having average grain sizes of 210 μm (Ti-210), 30 μm (Ti-30), and 5 μm (Ti-5) were evaluated. With increasing strain rates, the 0.2% proof stress and ultimate tensile strength increased for all samples; the fracture strain increased for Ti-210, decreased for Ti-5, and changed negligibly for Ti-30. Comparing high (100 s−1) and low (10−6 s−1) strain rates, twinning occurred more frequently in Ti-30 and Ti-210 at high strain rates, but the frequency did not change in Ti-5. The frequency of 1st order pyramidal slip tended to be higher in Ti-30 and Ti-5 at low strain rates. The higher ductility exhibited by Ti-210 at high strain rates was attributed to the high frequency of twinning. In contrast, the higher ductility of Ti-5 at low strain rates was attributed to the activity of the 1st order pyramidal slip.

Experiments and numerical simulations were conducted to investigate the effects of the burner diameter on the flame structure and extinction limit of counterflow non-premixed methane flames in normal gravity and microgravity. Experiments were performed for counterflow flames with a large inner diameter ( d) of 50 mm in normal gravity to compare the extinction limits with those obtained by previous studies where a small burner ( d < 25 mm) was used. Two-dimensional (2D) simulations were performed to clarify the flame structure and extinction limits of counterflow non-premixed flame with a three-step global reaction mechanism. One-dimensional (1D) simulations were also performed with the same three-step global reaction mechanism to provide reference data for the 2D simulation and experiment. For microgravity, the effect of the burner diameter on the flame location at the centerline was negligible at both high ( ag = 50 s−1) and low ( ag = 10 s−1) strain rates. However, a small burner flame ( d = 15 mm) in microgravity showed large differences in the maximum flame temperature and the flame size in radial direction compared to a large burner flame ( d = 50 mm) at low strain rate. In addition, for normal gravity, a small burner flame ( d = 23.4 mm) showed differences in the flame thickness, flame location, local strain rate, and maximum heat release rate compared to a large burner flame ( d = 50 mm) at low strain rate. Counterflow non-premixed flames with low and high strain rates that were established in a large burner were approximated by 1D simulation for normal gravity and microgravity. However, a counterflow non-premixed flame with a low strain rate in a small burner could not be approximated by 1D simulation for normal gravity due to buoyancy effects. The 2D simulations of the extinction limits correlated well with experiments for small and large burner flames. For microgravity, the extinction limit of a small burner flame ( d = 15 mm) was much lower than that of a large burner flame when ag ≤ 20 s−1. For normal gravity, the extinction limit of a small burner flame ( d = 23.4 mm) was also much lower than that of the large burner flame when ag ≤ 35 s−1. The effects of the burner diameter on the flame structure and extinction limit of counterflow non-premixed methane flames were more important in normal gravity than in microgravity.

Compressive response and microstructural evolution of in-situ TiB2 particle-reinforced 7075 aluminum matrix composite

In this paper, the applicability of mechanical tests for biomass pellet characterisation was investigated. Pellet durability, quasi-static (low strain rate), and dynamic (high strain rate) mechanical tests were applied to mixed wood, eucalyptus, sunflower, miscanthus, and steam exploded and microwaved pellets, and compared to their Hardgrove Grindability Index (HGI), and milling energies for knife and ring-roller mills. The dynamic mechanical response of biomass pellets was obtained using a novel application of the Split Hopkinson pressure bar. Similar mechanical properties were obtained for all pellets, apart from steam-exploded pellets, which were significantly higher. The quasi-static rigidity (Young’s modulus) was highest in the axial orientation and lowest in flexure. The dynamic mechanical strength and rigidity were highest in the diametral orientation. Pellet strength was found to be greater at high strain rates. The diametral Young’s Modulus was virtually identical at low and high strain rates for eucalyptus, mixed wood, sunflower, and microwave pellets, while the axial Young’s Modulus was lower at high strain rates. Correlations were derived between the milling energy in knife and ring roller mills for pellet durability, and quasi-static and dynamic pellet strength. Pellet durability and diametral quasi-static strain was correlated with HGI. In summary, pellet durability and mechanical tests at low and high strain rates can provide an indication of how a pellet will break down in a mill.

Deformation and fracture behavior of 316L sintered stainless steel under various strain rate and relative sintered density conditions

The high temperature compressive tests of squeeze casting ZK60 magnesium alloy in the testing temperature range of 523-723K and strain rate range of 0.001-10s-1 were performed on Gleeble-1500D thermal simulator testing machine. Optical microscopy was performed to elaborate on the dynamic recrystallization (DRX) grain growth. TEM observation indicated that the mechanical twinning, dislocation slip, and dynamic recrystallization are the materials typical deformation features. Variations of flow behavior with deformation temperature as well as strain rate were analyzed. Analysis of the flowing deformation behavior and microstructure observations indicated that the flow localization was observed at lower testing temperature and higher strain rates. Dynamic recrystallization occurred at higher testing temperature and moderate strain rates, which improved the ductility of the material. The results indicated that at the testing temperatures lower than 573K and strain rates higher than 1s-1, the material exhibited flow instability manifesting as bands of flow localizations. These temperatures and strain rates should be avoided in processing the material. Dynamic recrystallization occurs in the temperature range 573-723K and the strain rate range 0.001-0.1s-1. The number of dynamic recrystallization grains is less at lower temperature and higher strain rate than higher temperature and lower strain rate. The dynamic recrystallization is inadequate at 573-623K while the dynamic recrystallization grain growth has been observed in the temperature range of 673-723K. Therefore it may be considered that the optimum processing parameters for hot working of squeeze casting ZK60 magnesium alloy are 648K and 0.001-0.01s-1, at which fine dynamic recrystallization microstructure can be obtained.

Microstructural evolution at high strain rates in solution-hardened interstitial free steels