Showing papers on "Strain rate published in 2021"

Strain rate dependency of dislocation plasticity.

Abstract: Dislocation glide is a general deformation mode, governing the strength of metals. Via discrete dislocation dynamics and molecular dynamics simulations, we investigate the strain rate and dislocation density dependence of the strength of bulk copper and aluminum single crystals. An analytical relationship between material strength, dislocation density, strain rate and dislocation mobility is proposed, which agrees well with current simulations and published experiments. Results show that material strength displays a decreasing regime (strain rate hardening) and then increasing regime (classical forest hardening) as the dislocation density increases. Accordingly, the strength displays universally, as the strain rate increases, a strain rate-independent regime followed by a strain rate hardening regime. All results are captured by a single scaling function, which relates the scaled strength to a coupling parameter between dislocation density and strain rate. Such coupling parameter also controls the localization of plasticity, fluctuations of dislocation flow and distribution of dislocation velocity. The relationship between the strain rate and micro-scale deformation in metals is still poorly understood. Here the authors use discrete dislocation dynamics and molecular dynamics to establish a universal relationship between material strength, dislocation density, strain rate and dislocation mobility in fcc metals.

Hot deformation behavior and mechanism of a new metastable β titanium alloy Ti–6Cr–5Mo–5V–4Al in single phase region

Abstract: A new metastable β titanium alloy Ti–6Cr–5Mo–5V–4Al (Ti-6554) has shown good application potential in large parts. In order to study the hot deformation behavior and mechanism of the Ti-6554 alloy in the β single phase region, hot compression tests were carried out at temperatures of 800–950 °C and strain rates of 0.001–10 s−1 on the Gleeble-3500 thermal simulation machine. Strain-compensated Arrhenius constitutive model was used to predict the flow behavior of the alloy, and the correlation coefficient between the experimental and predicted values reached 0.982. Based on the hot processing map, it was found that the instability region was mainly concentrated in the high strain rate area, and the peak efficiency of power dissipation (η) region occurs in the temperature range of 913–928 °C and the strain rate of 0.001–0.0025 s-1. There was an obvious DRX phenomenon in the stability region, while the instability region was dominated by deformation band (DB) and flow localization (FL). The continuous dynamic recrystallization (CDRX) by progressive rotation of subgrains and discontinuous dynamic recrystallization (DDRX) by grain boundaries bulging could be observed. DDRX mainly occurred in high temperature and low strain rate regions, while the CDRX process occurred in the high strain rate region. As the η decreased, the deformation mechanism changed from DDRX to CDRX and further to dynamic recovery (DRV), DB, FL.

Creep feed grinding induced gradient microstructures in the superficial layer of turbine blade root of single crystal nickel-based superalloy

Abstract: The service performance of the turbine blade root of an aero-engine depends on the microstructures in its superficial layer. This work investigated the surface deformation structures of turbine blade root of single crystal nickel-based superalloy produced under different creep feed grinding conditions. Gradient microstructures in the superficial layer were clarified and composed of a severely deformed layer (DFL) with nano-sized grains (48–67 nm) at the topmost surface, a DFL with submicron-sized grains (66–158 nm) and micron-sized laminated structures at the subsurface, and a dislocation accumulated layer extending to the bulk material. The formation of such gradient microstructures was found to be related to the graded variations in the plastic strain and strain rate induced in the creep feed grinding process, which were as high as 6.67 and 8.17×107 s-1, respectively. In the current study, the evolution of surface gradient microstructures was essentially a transition process from a coarse single crystal to nano-sized grains and, simultaneously, from one orientation of a single crystal to random orientations of polycrystals, during which the dislocation slips dominated the creep feed grinding induced microstructure deformation of single crystal nickel-based superalloy.

Stretchable strain sensors with dentate groove structure for enhanced sensing recoverability

Abstract: Stretchable strain sensors based on conductive polymer composites commonly utilize elastic polymers as the matrix. However, elastic polymers always show strong mechanical hysteresis effect leading to shoulder peak phenomenon and thereby poor recoverability of strain sensors. Herein, we design a stretchable rough filament strain sensor with dentate groove structure to eliminate the shoulder peak phenomenon and improve recoverability. The filament strain sensor is fabricated by the extrusion of poly(styrene-b-ethylene-b-butylene-b-styrene) (SEBS) filament constructing dentate groove structure and the subsequent ultrasonic treatment decorating carbon nanotubes (CNTs) on the surface of the SEBS filament. It is interesting to find that the strain sensing range of rough SEBS/CNTs filaments with dentate groove structure is wider than that of smooth filaments. More importantly, the rough filament strain sensors exhibit significantly enhanced recoverability without shoulder peak during the releasing process while the rough dentate groove structure has minor effects on the mechanical properties of SEBS filaments. The great improvement is ascribed to the uniform distribution of deformation because of the dentate groove structure, which induces reduction of the mechanical hysteresis effect and thereby decreases residual strain. Moreover, the rough filament strain sensors have a favorable integration of good stability, fast response time of 300 ms (0.5% strain is applied with a high strain rate of 500 mm/min) and excellent durability (1976 cycles at the strain of 50%). The rough filament strain sensors can accurately and stably monitor both large and subtle human motions (such as body motion, expression and phonation), showing broad application prospects in wearable devices.

Contribution of fiber orientation to enhancing dynamic properties of UHPC under impact loading

Abstract: This paper investigates the contribution of fiber orientation to enhancing dynamic properties of ultra-high performance concrete (UHPC) under impact loading. Dynamic properties of 27 cylindrical core samples taken from prisms that have predominantly perpendicular, random, and parallel fiber orientations relative to the loading direction were tested. The Spilt Hopkinson Pressure Bar test method was used with strain rates of 160–290 s−1. Fiber orientation of samples after impact was evaluated using computed tomography scan. Test results indicated that the dynamic compressive strength, peak strain, and energy absorption capacity increased by up to 65%, 105%, and 295%, respectively, with the increase in strain rate. For a given strain rate, samples with predominant fiber orientations perpendicular and parallel to the loading direction showed the highest and lowest dynamic properties, respectively. Such spread in dynamic compressive strength, peak strain, and energy absorption capacity due to fiber orientation was up to 40%, 90%, and 135%, respectively. Fiber orientation coefficients along the direction of tensile stress were around 0.7 and less than 0.3 for samples with predominantly perpendicular and parallel fiber orientations, respectively. Good relationships were established to estimate the dynamic properties of UHPC given the quasi-static compressive strength, strain rate, and fiber orientation. A constitutive model to estimate the stress-strain curves of UHPC subjected to impact loading was developed. The model considers the influence of fiber orientation and strain rate. The model was successfully validated using the experimental data and was found to provide accurate prediction of the ascending portions of the stress-strain curves of UHPC under impact loading. The maximum spread between predicted and experimental dynamic compressive strength was limited to 10%.

Meso-scale modelling of static and dynamic tensile fracture of concrete accounting for real-shape aggregates

Abstract: This paper presents a computational framework for modelling the fracture process in concrete under static and dynamic tensile loading considering its mesostructural characteristics. 3D mesostructure of concrete composed of real-shape coarse aggregates, mortar, interfacial transition zone between them and voids was developed using an in-house code based on Voronoi tessellation and splining techniques. Cohesive zone model was then employed to simulate the tensile fracture behaviour of concrete in terms of stress- and energy dissipation-displacement responses and crack mechanisms against the shape (spherical and irregular) and volume fraction (30%, 35% and 40%) of aggregate and strain rate (0, 1, 10 and 50 s−1). Results indicate that the irregular shape of aggregate has an important role in the micro-crack nucleation and ultimate fracture pattern but exhibits an insignificant effect on the tensile strength of concrete, which is mainly dependent on the strain rate and the random location and size distribution of aggregate.

High-temperature tensile characteristics and constitutive models of ultrahigh strength steel

Abstract: In this investigation, the isothermal tensile experiments over wide ranges of deformation parameters (strain rate and tensile temperature) are conducted for studying the high-temperature tensile behaviors of an ultrahigh strength steel. The influences of deformation parameter on high-temperature tensile behaviors, fracture characteristics and deformation mechanisms are analyzed. Moreover, Arrhenius-type phenomenological (AP) model developed by the regression method or the Nelder-Mead (NM) simplex method, and the artificial-neural-network (ANN) model developed by combining genetic algorithm (GA) and back propagation learning algorithm (BP) are proposed, respectively. The results show that the high-temperature tensile behavior of the studied steel exhibits the typical work hardening and dynamic recovery characteristics. The necking capability increases with the strain rate decreasing and tensile temperature increasing. However, the large deep dimples dramatically deteriorate the loading capability during the localized necking, leading to the poor elongation to fracture at low strain rate. Both for the modeling and verifying data, the AP model developed by the NM simplex method shows the relatively high relative coefficient (higher than 0.9963), low average absolute relative error (lower than 1.6692%) and narrow error band (controlled in ±6.8MPa), compared with the AP model developed by the regression method and the GA-BP ANN model.

Study on the Fractal Characteristics of the Pomegranate Biotite Schist under Impact Loading

Abstract: In order to study the fractal characteristics of the pomegranate biotite schist under the effect of blasting loads, a one-dimensional SHPB impact test was carried out to test the dynamic compressive strength, damage morphology, fracture energy dissipation density, and other parameters of the rocks under different strain rates; besides, sieve tests were conducted to count the mass fractal characteristics of the crushed masses under different strain rates to calculate the fractal dimension of the crushed rock . Finally, the relationships between fractal dimension and dynamic compressive strength, crushing characteristics, and energy dissipation characteristics were analysed. The results show that under different impact loads, the strain rate effect of the rock is significant and the dynamic compressive strength increases with the increasing strain rate, and they show a multiplicative power relationship. The higher the strain rate of the rock, the deeper the fragmentation and the higher the fractal dimension, and the fractal dimension and rock crushing energy density are multiplied by a power relationship. By performing the comparative analysis of the pomegranate biotite schist, a reasonable strain rate range of 78.75 s-1~82.51 s-1 and a reasonable crushing energy consumption density range of 0.78 J·cm-3~0.92 J·cm-3 were determined. This research provides a great reference for the analysis of dynamic crushing mechanism, crushing block size distribution, and crushing energy consumption of the roadway surrounding rock.

Dynamic Cracking Behaviors and Energy Evolution of Multi-flawed Rocks Under Static Pre-compression

Abstract: Understanding the dynamic cracking behaviors and energy evolution of flawed rocks is highly relevant to underground rock engineering. In this study, multi-flawed rock specimens are tested under coupled static–dynamic compression using a modified SHPB system combined with high-speed photography and DIC monitoring. We systematically investigated the influences of pre-stress ratio, flaw inclination angle and strain rate on the dynamic progressive cracking mechanism and energy evolution of multi-flawed rocks. Experimental results show that the dynamic/total strength generally increases with increasing strain rate, featuring evident rate-dependence. With increasing flaw inclination angle from 15° to 60°, the dynamic/total strength initially decreases and subsequently increases with the minimum achieved around 45°. With the pre-stress ratio increasing from 0.2 to 0.8, the dynamic strength persistently decreases while the total strength initially increases and subsequently decreases with the maximum achieved at 0.6. Furthermore, based on the displacement trend lines method, a novel crack classification method is developed to analyze the progressive cracking mechanism of multi-flawed rocks using high-speed photography and DIC technique. Generally, mixed cracking dominates the failure of multi-flawed rocks under coupled static–dynamic compression. With increasing flaw inclination angle form 15°–60°, the predominant cracking mechanism changes from mixed tensile-shear cracking to mixed compression-shear cracking. The increasing pre-stress ratio promotes shear cracking under lower flaw inclination angles while facilitates tensile cracking under higher flaw inclination angles. In addition, the energy evolution for coupled static–dynamic SHPB tests is re-evaluated and a new energy calculation formula is proposed. The results show that the increasing strain rate reduces the energy utilization while promotes the energy dissipation density. Both the energy utilization and energy dissipation density increase with increasing pre-stress ratios.

Deformation mechanisms in ultrafine-grained metals with an emphasis on the Hall–Petch relationship and strain rate sensitivity

Abstract: Ultrafine-grained materials display almost no strain hardening, an enhanced strain rate sensitivity and grain boundary offsets during plastic deformation. It is expected that dislocation climb is active in order to enable prompt recovery. The present analysis proposes a deformation mechanism that includes these effects and follows from the mechanism for high temperature grain boundary sliding. This mechanism predicts the relationship between strain rate, flow stress, grain size, temperature and basic material properties such as the Burgers vector modulus, the shear modulus and the grain boundary diffusion coefficient. The model may be used to estimate the final grain size achieved by severe plastic deformation and the strain rate sensitivity. An analysis shows that the predicted behavior agrees with the data from multiple experimental investigations and provides a good estimate of the Hall–Petch slope for different materials which includes breakdown and inverse Hall–Petch behavior under some conditions. The incorporation of a threshold stress provides an opportunity to predict the relationship between flow stress and grain size for a broad range of grain sizes, strain rates and temperatures. An excellent agreement is observed between the predictions of the model and experimental data for Al, Cu, Fe (α), Fe(γ), Mg, Ni, Ti and Zn.

A Micromechanical Model for Simulation of Rock Failure Under High Strain Rate Loading

Abstract: Quasi-brittle materials such as rock are rate sensitive materials and their behaviour under dynamic loading is not identical with that under static loading. In this study, numerical Brazilian tensile tests are conducted using a Split Hopkinson Pressure Bar system in an attempt to reproduce the dynamic increase factors (DIF) of the experimental tests. The rock is modelled by a bonded particle system made of spherical particles which interact at the contact points. The numerical results indicate that while the bonded particle system with a simple contact bond model can closely mimic the static behaviour of the sandstone specimens, it lacks what is needed for a rate dependent material. Therefore, a micromechanical model in which the contact bond strength is allowed to vary in proportion to the relative velocity of the involved particles is introduced. It is shown that the modified model can reproduce the physical tests data reported in the literature. In particular, with the application of strength enhancement coefficients in the range of 0–16 × 105, DIF values of 1.1–13 are obtained in the indirect tensile Brazilian tests, and the induced strain rate in the specimen is in 10–1000 s−1 range. Our preliminary study indicates that the model, consistent with the fact reported for the quasi-brittle materials, shows different rate-dependent sensitivity and dynamic strength enhancement in tension and compression. The micromechanical parameters in the proposed model can be adjusted to reproduce the physical rock strength, and that the shape of the reflected and transmitted numerical waves can be modified to approach those in the physical tests.

High strain rate compressive strength behavior of cemented paste backfill using split Hopkinson pressure bar

Abstract: The stability of cemented paste backfill (CPB) is threatened by dynamic disturbance, but the conventional low strain rate laboratory pressure test has difficulty achieving this research purpose. Therefore, a split Hopkinson pressure bar (SHPB) was utilized to investigate the high strain rate compressive behavior of CPB with dynamic loads of 0.4, 0.8, and 1.2 MPa. And the failure modes were determined by macro and micro analysis. CPB with different cement-to-tailings ratios, solid mass concentrations, and curing ages was prepared to conduct the SHPB test. The results showed that increasing the cement content, tailings content, and curing age can improve the dynamic compressive strength and elastic modulus. Under an impact load, a higher strain rate can lead to larger increasing times of the dynamic compressive strength when compared with static loading. And the dynamic compressive strength of CPB has an exponential correlation with the strain rate. The macroscopic failure modes indicated that CPB is more seriously damaged under dynamic loading. The local damage was enhanced, and fine cracks were formed in the interior of the CPB. This is because the CPB cannot dissipate the energy of the high strain rate stress wave in a short loading period.

Dynamic precipitation and recrystallization mechanism during hot compression of Mg-Gd-Y-Zr alloy

Abstract: Hot compression tests of Mg-Gd-Y-Zr alloy at different deformation temperatures and strains were carried out with Gleeble-1500 simulator at strain rate of 0.002 s−1. The microstructure evolution of 400 °C/0.002 s−1 sample under different strain was analyzed emphatically by transmission electron microscopy and electron backscatter diffraction technology. Dynamic precipitation characteristics and nucleation-expansion mechanism of dynamic recrystallization (DRX) were discussed in detail. The results indicated that the dynamic precipitation takes place prior to DRX, and the morphology, size and distribution of precipitates change with increasing strain. At the later stage of deformation, the large-size hard phase β-Mg5(Gd,Y) can induce the particle-stimulated nucleation (PSN) mechanism under large strain and act as an effective nucleation site for DRX. In addition, we constructed a schematic diagram of the DRX nucleation-expansion mechanism of Mg-Gd-Y-Zr alloy under high temperature deformation. The first layer of DRX grains is formed by discontinuous dynamic recrystallization (DDRX) mechanism characterized by grain boundary bulge out, and the precipitates distributed at the original grain boundary promote DDRX nucleation; The expansion of necklace structure depends on continuous dynamic recrystallization (CDRX) mechanism; Subgrain division can further refine DRX grains.