Showing papers on "Strain rate published in 2020"

Ultrahigh high-strain-rate superplasticity in a nanostructured high-entropy alloy

Abstract: Superplasticity describes a material’s ability to sustain large plastic deformation in the form of a tensile elongation to over 400% of its original length, but is generally observed only at a low strain rate (~10−4 s−1), which results in long processing times that are economically undesirable for mass production. Superplasticity at high strain rates in excess of 10−2 s−1, required for viable industry-scale application, has usually only been achieved in low-strength aluminium and magnesium alloys. Here, we present a superplastic elongation to 2000% of the original length at a high strain rate of 5 × 10−2 s−1 in an Al9(CoCrFeMnNi)91 (at%) high-entropy alloy nanostructured using high-pressure torsion. The high-pressure torsion induced grain refinement in the multi-phase alloy combined with limited grain growth during hot plastic deformation enables high strain rate superplasticity through grain boundary sliding accommodated by dislocation activity. Superplasticity at high strain rates is challenging to achieve in high strength materials. Here, the authors show superplastic elongation in excess of 2000% in a high entropy alloy nanostructured by high-pressure torsion.

Strain rate dependency of dislocation plasticity

Abstract: Dislocation slip is a general deformation mode and governs 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 single crystals using 192 simulations spanning over 10 orders of magnitude in strain rate and 9 orders of magnitude in dislocation density. Based on these large set of simulations and theoretical analysis, a new analytical relationship between material strength, dislocation density, strain rate and dislocation mobility is proposed, which is in excellent agreement with the current simulations as well as with experimental data. The results show that the material strength is a non-monotonic function of dislocation density and displays two universal regimes (first decreasing, then increasing) as the dislocation density increases. The first regime is a result of strain rate hardening, while the second regime is dominated by the classical Taylor forest hardening. Accordingly, the strength displays universally, as a function of strain rate, a rate-independent regime at low strain rates (governed by forest hardening) followed by a rate hardening regime at high strain rates (governed by strain rate hardening). All the results can be captured by a single scaling function. Finally, the fluctuations of dislocation flow are analyzed in terms of the strain rate dependent distribution of dislocation segment velocities. It is found that the fluctuations are governed by another universal scaling function and diverge in the rate independent limit, indicating a critical behavior. The current analysis provides a comprehensive understanding on how collective dislocation motions are governed by the competition between the internal elastic interactions of dislocations, and the stress required to drive dislocation fluxes at a given externally imposed strain rate.

Mechanical and Volumetric Fracturing Behaviour of Three-Dimensional Printing Rock-like Samples Under Dynamic Loading

Abstract: Heterogeneous rock contains numerous pre-existing three-dimensional (3D) cracks, which control its mechanical and fracturing properties. Considerable effort has been devoted to studying the volumetric fracturing behaviour of rock under static loading conditions. Although rock masses are often subject to dynamic impacts such as earthquakes and blasting, the mechanical and volumetric fracturing behaviour of rock under dynamic loading is still poorly understood. In this paper, dynamic laboratory tests were performed on 3D-printed artificial rock samples with 3D embedded flaws created during three-dimensional printing (3DP), with the aim of studying the volumetric fracturing and mechanical properties of these samples under impact with high strain rate. The results show that the dynamic compressive strength and the tangent modulus decrease with an increasing number of flaws, but have very limited effects on the ratio of the fracture initiation stress of the first crack to the peak stress of the sample, the maximum axial strain of the sample and the volumetric fracturing behaviour of the sample. The tensile failure of a sample is caused by the continuous extension of wing cracks from the outer flaw tips. The mechanical and volumetric fracturing behaviour of samples with 3D embedded flaws are strain rate dependent. The tangential modulus and the ratio of the fracture initiation stress of the crack to the peak stress increase significantly when the loading type changes from static compression to dynamic compression. Under dynamic compression, wing cracks can continuously extend to the sample ends, whereas under static compression, wing cracks can intermittently extend only a limited distance. Moreover, the fracturing behaviour of 3D flaw differs from that of 2D flaws under dynamic loading. Under high strain rate loading, wing cracks generated at 3D flaw tips lead to splitting failure of the sample, while shear cracks formed at 2D flaw tips result predominant shear failure of the sample. The findings in this paper could facilitate a better understanding of rock failure subjected to dynamic loading conditions.

Effect of build orientation on the quasi-static and dynamic response of SLM AlSi10Mg

Abstract: Selective Laser Melting (SLM) allows the fabrication of complex geometries with high resolution and robust mechanical properties. However, the manner of manufacture – melting of metallic powder with a laser power source – affects microstructure and results in mechanical anisotropy. While some studies have sought to characterise the microstructure and performance of SLM AlSi10Mg, the dynamic response, particularly with regard to anisotropic effects, remains relatively undefined. To overcome this deficit AlSi10Mg specimens were fabricated using SLM with three different build orientations, and quasi-static and dynamic split-Hopkinson tensile bar tests were performed to characterise the tensile properties of the material at strain rates ranging from 3.33 x 10-2 to 2.4 x 103 s-1. The microstructure of as-manufactured specimens and fracture surfaces of failed specimens were analysed. Quasi-static and dynamic results showed little difference between build orientations with regard to strength, but components loaded perpendicular to the build direction were found to be more ductile than other build orientations. Significant scatter was observed in dynamic results, suggesting no strain rate sensitivity of the material in the tested strain rate range. Build orientation was found to affect fracture surface morphology of dynamically tested specimens due to fracture paths following melt pool boundaries. These results assist in the characterisation of the anisotropic effects of build orientation on quasi-static and dynamic behaviours of SLM AlSi10Mg towards the further commercial adoption of the manufacturing technique and material.

On the Strain Rate Sensitivity of Fused Filament Fabrication (FFF) Processed PLA, ABS, PETG, PA6, and PP Thermoplastic Polymers.

Abstract: In this study, the strain rate sensitivity of five different thermoplastic polymers processed via Fused Filament Fabrication (FFF) Additive Manufacturing (AM) is reported. Namely, Polylactic Acid (PLA), Acrylonitrile-Butadiene-Styrene (ABS), Polyethylene Terephthalate Glycol (PETG), Polyamide 6 (PA6), and Polypropylene (PP) were thoroughly investigated under static tensile loading conditions at different strain rates. Strain rates have been selected representing the most common applications of polymeric materials manufactured by Three-Dimensional (3D) Printing. Each polymer was exposed to five different strain rates in order to elucidate the dependency and sensitivity of the tensile properties, i.e., stiffness, strength, and toughness on the applied strain rate. Scanning Electron Microscopy (SEM) was employed to investigate the 3D printed samples' fractured surfaces, as a means to derive important information regarding the fracture process, the type of fracture (brittle or ductile), as well as correlate the fractured surface characteristics with the mechanical response under certain strain rate conditions. An Expectation-Maximization (EM) analysis was carried out. Finally, a comparison is presented calculating the strain rate sensitivity index "m" and toughness of all materials at the different applied strain rates.

Strain rate-dependent mechanical metamaterials.

Abstract: Mechanical metamaterials are usually designed to exhibit novel properties and functionalities that are rare or even unprecedented. What is common among most previous designs is the quasi-static nature of their mechanical behavior. Here, we introduce a previously unidentified class of strain rate-dependent mechanical metamaterials. The principal idea is to laterally attach two beams with very different levels of strain rate-dependencies to make them act as a single bi-beam. We use an analytical model and multiple computational models to explore the instability modes of such a bi-beam construct, demonstrating how different combinations of hyperelastic and viscoelastic properties of both beams, as well as purposefully introduced geometric imperfections, could be used to create robust and highly predictable strain rate-dependent behaviors of bi-beams. We then use the bi-beams to design and experimentally realize lattice structures with unique strain rate-dependent properties including switching between auxetic and conventional behaviors and negative viscoelasticity.

Coupling effect of strain rate and specimen size on the compressive properties of coral aggregate concrete: A 3D mesoscopic study

Abstract: The dynamic compressive behavior of concrete is closely associated with the specimen size effect and the coupling effect of strain rate and specimen size on the compressive properties is something there worthy studying for the development and application of coral aggregate concrete (CAC). A novel 3D mesoscale modelling approach taking the random characteristics of aggregates is developed for the numerical study in the present work. The specimen diameter of numerical cylinder with the slenderness of 1/2 varies from 50 mm to 200 mm, and the strain rate is within the range of 10−3–200s−1. Employing the 3D mesoscale model, the dynamic compressive responses of CAC, i.e., stress-strain relation, failure pattern and process, energy evolution process, etc. have been simulated and analyzed systematically. Both the effect of strain rate and specimen size have been qualitatively and quantitatively discussed. Through the comparison of numerical and existing test data, it is verified that the compressive behaviors of CAC under different conditions could be characterized and predicted using the 3D mesoscale model. In addition, all above-listed properties are related to the strain rate and specimen size at different degrees. The strain rate effect law on the CAC's strength and absorbed energy can be well expressed using a quadratic function and an exponential function, respectively. And the relationship between strength and specimen size can be mathematically formulated by a power function. Furthermore, a coupling effect law of strain rate and specimen size on the strength of CAC has been proposed and validated based on numerical and test data, which is meaningful for the properties prediction of CAC.

Energy Transfer from Large to Small Scales in Turbulence by Multiscale Nonlinear Strain and Vorticity Interactions

Abstract: An intrinsic feature of turbulent flows is an enhanced rate of mixing and kinetic energy dissipation due to the rapid generation of small-scale motions from large-scale excitation. The transfer of kinetic energy from large to small scales is commonly attributed to the stretching of vorticity by the strain rate, but strain self-amplification also plays a role. Previous treatments of this connection are phenomenological or inexact, or cannot distinguish the contribution of vorticity stretching from that of strain self-amplification. In this Letter, an exact relationship is derived which quantitatively establishes how intuitive multiscale mechanisms such as vorticity stretching and strain self-amplification together actuate the interscale transfer of energy in turbulence. Numerical evidence verifies this result and uses it to demonstrate that the contribution of strain self-amplification to energy transfer is higher than that of vorticity stretching, but not overwhelmingly so.

Constitutive analysis, twinning, recrystallization, and crack in fine-grained ZK61 Mg alloy during high strain rate compression over a wide range of temperatures

Abstract: This study aimed to explain the uniaxial compression of T5 heat-treated extruded ZK61 magnesium alloy at the temperatures of 298 − 673 K and the strain rates of 0.001 − 10 s−1. Flow stress data were analyzed to establish a constitutive equation by means of numerical simulation and activation energy was estimated to be 141 ± 1 kJ/mol. Microstructure analysis revealed a low fraction of primary and secondary twinning at temperature 623 K in the early stages of deformation and dense twinning at low temperatures ≤ 473 K. Pole figure analysis suggested virgin fiber texture where c − axis of HCP is tilted at angle ~25° with respect to extrusion direction. During early stages of deformation at a strain 0.1 and at high-temperature c-axis of the crystal reorients and changes to ~40° along transverse direction, while at strain 0.45 highly aligned typical extruded texture was developed, which proposed that {0002} basal planes are perfectly parallel to the extrusion direction. Moreover, the results indicated that at high temperature > 548 K the dominant discontinuous dynamic recrystallization was changed into continuous dynamic recrystallization with the increase in strain; this phenomenon is attributed to initial processing history; fine grain size and activation energy 141 ± 1 kJ/mol. The twin dynamic recrystallization was dominant at temperatures ≤548 K. Thus, hot deformation in the strain rate range of 0.001 − 1 s−1 at temperature 548 K or in the strain rate range of 0.001 − 10 s−1 and in the temperature range of 573 − 623 K is optimum for the ZK61 Mg alloy (T5). Fracture behavior revealed that principal and secondary cracks were nucleated and propagated owing to the low fraction of dynamic recrystallization, twin − twin, twin − dislocation interaction and voids at triple junctions of grain boundaries.

Tensile Behavior of High-Density Polyethylene Including the Effects of Processing Technique, Thickness, Temperature, and Strain Rate.

Abstract: The primary goal of this study was to investigate the monotonic tensile behavior of high-density polyethylene (HDPE) in its virgin, regrind, and laminated forms. HDPE is the most commonly used polymer in many industries. A variety of tensile tests were performed using plate-type specimens made of rectangular plaques. Several factors can affect the tensile behavior such as thickness, processing technique, temperature, and strain rate. Testing temperatures were chosen at −40, 23 (room temperature, RT), 53, and 82 °C to investigate temperature effect. Tensile properties, including elastic modulus, yield strength, and ultimate tensile strength, were obtained for all conditions. Tensile properties significantly reduced by increasing temperature while elastic modulus and ultimate tensile strength linearly increased at higher strain rates. A significant effect of thickness on tensile properties was observed for injection molding specimens at 23 °C, but no thickness effect was observed for compression molded specimens at either 23 or 82 °C. The aforementioned effects and discussion of their influence on tensile properties are presented in this paper. Polynomial relations for tensile properties, including elastic modulus, yield strength, and ultimate tensile strength, were developed as functions of temperature and strain rate. Such relations can be used to estimate tensile properties of HDPE as a function of temperature and/or strain rate for application in designing parts with this material.

A dislocation density-based model and processing maps of Ti-55511 alloy with bimodal microstructures during hot compression in α+β region

Abstract: Hot compression features of Ti-55511 alloy are investigated by high-temperature compression tests in α+β region. It is found that the flow stress and softening mechanisms are obviously influenced by deformation conditions. The true stress decreases with the reduced strain rate or the raised temperature. The spheroidization of α phase and dynamic recrystallization (DRX) of β phase easily occur at low temperatures such as 973, 1003 and 1033 K, while the dynamic recovery (DRV) of β phase mainly occurs at high temperatures such as 1063 K because of the transformation from α phase to β phase at relatively high temperatures A dislocation density-based constitutive model, which is associated with DRV, work hardening mechanisms and the spheroidization of α phases, is established and validated to describe flow behavior. The correlation coefficient (R) and average absolute relative error (AARE) of the established model are 0.9924 and 6.8%, respectively. 3D power dissipation efficiency maps and processing maps are established to determine the appropriate processing window, i.e., too low temperatures (lower than 973 K) or too high strain rates (higher than 1 s−1) easily induce flow instability. Therefore, the medium temperature (1003–1063 K) and the low strain rate (0.001–0.1 s−1) are applicable for thermal compression of the studied titanium alloy.

Hot deformation characteristics and dynamic recrystallization mechanisms of a Co–Ni-based superalloy

Abstract: The hot deformation behavior of a Co–Ni-based superalloy was systematically investigated using thermal compression tests. Stress–strain curves showed a typical dynamic softening after peak stress, especially at high temperatures and low strain rates. An Arrhenius-type constitutive equation was developed to reveal the relationship between the flow stress, strain rate, and temperature, while a processing map was constructed based on the calculations from the stress-strain curves combined with microstructural observations to determine the optimum thermal deformation conditions. The extent of recrystallization was found to increase with increasing temperature, a decreasing strain rate, or an increasing strain. A complete dynamic recrystallization (DRX) condition was reached at 1050 °C/0.01 s−1/0.7. In addition, pre-existing annealing twins were replaced by discontinuous dynamic recrystallization (DDRX) grains along the twin boundaries and the twin-DRX (TDRX) grains in the twin interior. In the case of an un-twinned matrix, a combined DDRX and continuous DRX (CDRX) process occurred at high strain rates, in contrasted with a single DDRX process taking place at low strain rates.

Visco-hyperelastic constitutive modeling of strain rate sensitive soft materials

Abstract: A novel viscous dissipation potential is proposed for the visco-hyperelastic constitutive modeling of short-time memory responses of soft materials, which can capture both linear and nonlinear large deformation behaviors over a wide range of strain rates. The proposed potential is compatible with objectivity and continuum thermodynamics principles, consists of physically motivated model parameters, and adds the capability of modeling strain rate sensitivity in the small strain regime, which is currently not possible with available external state variable driven viscous dissipation potentials. By combining the proposed viscous dissipation potential with the Mooney-Rivlin strain energy density function, a visco-hyperelastic relation is formulated and fit to the rate-dependent tensile stress-strain data of human patellar tendon, which was previously modeled using an existing viscous dissipation potential. It is demonstrated that the proposed model offers improvements in fitting accuracy and prevents possible thermodynamic instabilities in quasi-static hyperelastic models from corrupting the dynamic response. In addition, the uncomplicated mathematical form of the model and the accompanying multi-step multi-start optimization procedure helps prevent numerical instabilities. Multi-deformation mode fitting of human brain gray matter under all three primary deformation modes (compression, tension and shear) is also considered using a visco-hyperelastic model based on the proposed potential and the semi-empirical Gent-Gent strain energy density function. It is shown that visco-hyperelastic models based on the proposed viscous dissipation potential capture all the essential features of the stress-strain data with unique optimal model parameters, giving reasonable accuracy in both single and multiple deformation mode cases. Further, it is demonstrated that the proposed model is stable and robust with respect to both the choice of the hyperelastic strain energy density and the availability of data from multiple deformation modes.

Effect of temperature on the hot deformation behavior of AZ80 magnesium alloy

Abstract: The present research studies the hot forgeability of a cast AZ80 alloy by means of uniaxial compression tests using a Gleeble® 3500 thermal-mechanical simulator. Compression tests were conducted for temperatures ranging from 300 °C to 450 °C, and constant true strain rates ranging from 0.001 s−1 to 1 s−1, while the characterization of deformed samples for microstructure and texture was performed only for the samples deformed within 300 °C–400 °C, at a strain rate of 0.001 s−1. The samples were deformed to true strains of 0.05, 0.15, 0.4 and 1.0, to study the microstructure and texture evolution with deformation strain, while the characterization was performed using optical microscopy, SEM-EDX, and EBSD. Analysis of the flow stress data suggested a transition in the rate controlling deformation mechanism from dislocation cross-slip at 300 °C to dislocation climb at 400 °C, with a corresponding transition in the dominant DRX mechanism from continuous dynamic recrystallization (CDRX) to discontinuous dynamic recrystallization (DDRX). The thermodynamic stability of the Mg17Al12 phase (which is the dominant secondary phase in the AZ80 alloy) varied over the test temperature range, and significantly impacted the microstructure and texture evolution at different test temperatures. At 300 °C, multiple morphologies of the Mg17Al12 precipitates were present in the microstructure during deformation, and affected the DRX behavior of the material differently. At 400 °C, the precipitates rapidly dissolved in the α-Mg matrix, while the DRX took place by the grain boundary bulging mechanism, with the newly formed DRXed grains preserving the deformation texture. The study shows that the effect of temperature on the hot deformation behavior of the AZ80 alloy is especially pronounced due to a changing amount of the Mg17Al12 phase in the microstructure.

Experimental and three-dimensional mesoscopic investigation of coral aggregate concrete under dynamic splitting-tensile loading

Abstract: An investigation that combines both experimental tests and mesoscopic modelling is conducted to characterize the dynamic splitting-tensile behavior of coral aggregate concrete (CAC). Static and dynamic splitting-tensile strength and failure patterns of CAC with different uniaxial compressive strength (30–70 MPa) are tested by means of MTS machine and Split-Hopkinson pressure bar device, respectively. A three-dimensional (3D) randomly mesoscopic model for the simulation of the splitting-tensile strength and failure of CAC under different strain rates (1–200 s−1) is developed and validated by contrasting tested and numerical results. The experimental and numerical results indicate that the splitting-tensile strength and failure pattern are significantly affected by concrete strength and strain rate. The dynamic splitting failure mechanism that the damage outside the specimen is more serious than the inside, and the fracture in the center of the specimen is more severe than the edge, has been explained from the localized failure patterns of concrete and aggregates. Furthermore, it can be learned from the tensile dynamic increase factor of CAC is sensitive to strain rate significantly, which has a profound significance in the further investigation of reef CAC structures.