Strain Rate Sensitivity Behavior Research Articles

Dynamic Strain Aging and Negative Strain Rate Sensitivity in Coarse-grained Al0.3CoCrFeNi High Entropy Alloy Under Hot Compression

This study investigates the deformation behavior of coarse-grained (CG) Al0.3CoCrFeNi alloy under hot compression, revealing notable strain rate sensitivity (SRS) and dynamic strain aging (DSA) phenomena. DSA is observed at temperatures between 1223 K and 1373 K, exclusively at lower strain rates (0.5 s-1 – 1 s-1), while higher strain rates (≥ 5 s-1) exhibit DSA absence. Non-compact {112} slip activation on dislocation forest structures is proposed as the driving force behind DSA. Negative SRS behavior at 1223 K is attributed to Ni-Al cluster-based solid solution strengthening. Activation energy calculations highlight lattice diffusion and partial dislocation constriction as contributors to hot deformation. Consistency is observed in the activation volume (V*) calculation, indicating dislocation forest formation (127 – 519·b3, b = Bureger’s vector), diminishing to 22 – 46·b3 upon DSA initiation. The alloy displays delayed dislocation hardening/softening transition due to CG's impact on retarding dislocation accumulation. This behavior stems from a higher V* at yielding, prolonging the typical transition observed in fine-grained alloys with lower V*. The study elucidates the intricate interplay of microstructure, strain rate, and deformation mechanisms in CG Al0.3CoCrFeNi.

Strain rate-dependent failure mechanics of the intervertebral disc under tension/compression and constitutive analysis

BackgroundThe intervertebral disc exhibits not only strain rate dependence (viscoelasticity), but also significant asymmetry under tensile and compressive loads, which is of great significance for understanding the mechanism of lumbar disc injury under physiological loads. ObjectiveIn this study, the strain rate sensitive and tension-compression asymmetry of the intervertebral disc were analyzed by experiments and constitutive equation. MethodThe Sheep intervertebral disc samples were divided into three groups, in order to test the strain rate sensitive mechanical behavior, and the internal displacement as well as pressure distribution. ResultsThe tensile stiffness is one order of magnitude smaller than the compression stiffness, and the logarithm of the elastic modulus is approximately linear with the logarithm of the strain rate, showing obvious tension-compression asymmetry and rate-related characteristics. In addition, the sensitivity to the strain rate is the same under these two loading conditions. The stress–strain curves of unloading and loading usually do not coincide, and form a Mullins effect hysteresis loop. The radial displacement distribution is opposite between the anterior and posterior region, which is consistent with the stress distribution. By introducing the damage factor into ZWT constitutive equation, the rate-dependent viscoelastic and weakening behavior of the intervertebral disc can be well described.

Experimental study and crystal plasticity modeling of additive manufacturing IN718 superalloy considering negative strain rate sensitivity behavior

Revealing underlying mechanisms of IN718 superalloy's negative strain rate sensitivity behavior at elevated temperatures is significantly important for its further applications in extreme conditions. This work investigates the microstructural mechanisms of the temperature-dependent strain rate sensitivity using experimental methods and crystal plasticity characterization model. Firstly, Electron backscatter diffraction (EBSD) and scanning electron microscopy (SEM) techniques are utilized to observe microstructural characteristics of additively manufactured IN718 alloy at different temperatures. The influence of solute precipitation and dislocation slip interaction on the mechanical properties is discussed. Subsequently, the representative volume element (RVE) model is constructed by using acquired orientation, grain size, and dislocation density data. Finally, the crystal plasticity model incorporating dynamic strain aging and dislocation density evolution is developed to characterize the relationship between strain rate sensitivity and temperature. The model can effectively simulate the transition of strain rate sensitivity of additive manufacturing materials. The results indicate that the increase in dislocation pinning caused by intensified carbon element precipitation at high temperatures is the primary reason of the strain rate sensitivity change observed in IN718 metal materials. The carbon precipitated at the dislocation nucleus induces serration flow in IN718 superalloy under high strain. This study contributes to understanding and characterizing the microscopic mechanisms of deformation in metal materials subjected to thermal environments.

Strain hardening rate and strain rate sensitivity behavior of bcc/fcc-dual-phase tungsten heavy alloy

90W-7Ni-3Fe alloy is a heterogeneous biphasic alloy, and it is essential to investigate the impact of the biphasic microstructure on key mechanical properties at varying strain rates. To address this issue, the quasi-static and dynamic compressive mechanical properties of dual-phase 90W-7Ni-3Fe alloy, single-phase pure W, and single-phase NiFe alloy were comparatively analyzed in conjunction with characterization analysis. The results indicate that the flow stress, strain hardening rate (SHR), and strain rate sensitivity (SRS) of the 90W-7Ni-3Fe alloy at various strain rates are mainly due to the involvement of W particles during the initial strain, although the overall plastic deformation of the alloy is attributed to the continuous coordinated deformation of the W particles with γ-(Ni, Fe) phases in various regions. In addition, the compressed W phase shows a typical 〈111〉 and 〈100〉 BCC compression texture and the compressed γ-(Ni, Fe) phase shows a typical 〈110〉 FCC compression texture, both with high Schmidt factors. These characteristics are essential for slip transfer and strain transfer from the γ-(Ni, Fe) phase to the W phase. Notably, the increased number of slip systems at high strain rates makes them more susceptible to both types of transfer, resulting in higher flow stress, SHR, and SRS during initial deformation.

MECHANICAL ANISOTROPY OF COLD-ROLLED ST-37 STEEL PLATE UNDER HIGH STRAIN RATES LOADINGS

Cold rolling process in metal could increase its strength and hardness, but also induces mechanical anisotropy. This is caused by unresolved plastic strain and microstructural changes caused by plastic deformation exerted by pair of rolls at a relatively low temperature. This research aims to provide understandings on anisotropic and strain rate sensitive behaviour of St-37 mild steel. The microstructure of rolled and unrolled St-37 plate were observed in 3 directions relative to the rolling direction. The cylindrical specimens were then prepared from rolled plate with 33.3% reduction in the orientation of 0o, 45o, and 90o relative to the rolling direction. Specimens were also prepared from unrolled plate as comparison in the same directions. These specimens were then tested in compression loading, first in quasi-static condition, and then by using Split-Hopkinson Pressure Bar (SHPB) at the strain rate of 1200 s-1. The experimental results in the form of stress-strain curves are used to obtain the parameters of Simplified Johnson-Cook viscoplastic model. The anisotropy of yield strength in rolled specimens could be seen on both quasi-static and high strain rate condition, where the highest strength found on the specimens perpendicular to the rolling direction. In addition, the effect of cold rolling to the strain rate sensitivity of the material were also captured successfully in this study, where specimens from rolled plate show less strain rate sensitivity compared to the unrolled specimens.

Self-Consistent Crystal Plasticity Modeling of Slip-Twin Interactions in Mg Alloys

Parsing the effect of slip-twin interactions on the strain rate and thermal sensitivities of Magnesium (Mg) alloys has been a challenging endeavor for scientists preoccupied with the mechanical behavior of hexagonal close-packed alloys, especially those with great latent economic potential such as Mg. One of the main barriers is the travail entailed in fitting the various stress−strain behaviors at different temperatures, strain rates, loading directions applied to different starting textures. Taking on this task for two different Mg alloys presenting different textures and as such various levels of slip-twin interactions were modeled using visco-plastic self-consistent (VPSC) code. A recently developed routine that captures dislocation transmutation by twinning interfaces on strain hardening within the twin lamellae was employed. While the strong texture was exemplified by traditional rolled AZ31 Mg alloys, the weak texture was represented by ZEK100 Mg alloy sheets. The transmutation model incorporated within a dislocation density based hardening model showed enhanced flexibility in predicting the complex strain rate and thermal sensitive behavior of Mg textures’ response to various mechanical loading schemes.

Study of effect of provision of cut-outs on axial collapse behaviour of circular aluminium tubes

Geometrical modification of thin-walled tubular structure is one of the innovative ways in the development for vehicle safety design. This research is developed in accordance with the need of investigation on behaviour of tubular structures with geometrical modifications like cut-outs under dynamic loading technique. Diamond and elliptical cut-out shapes are provided in multiple number arranged circumferentially in a single row on a tube surface. Tubes with cut-outs are loaded axially at three different velocities, corresponding to three different strain rates. Energy absorption characteristics like energy efficiency, specific energy absorption, peak collapse force, mean collapse force and efficiency of peak force reduction at the initiation of tube deformation are calculated and compared using finite element (FE) based ABAQUS/Explicit® software. Split-Hopkinson Pressure Bar (SHPB) is modelled in the FE software to understand the material behaviour under high strain dynamic loading. Johnson-Cook material model parameters are incorporated in the study to understand the strain rate sensitive behaviour of aluminium alloy material. Results revealed that providing diamond and elliptical cut-outs on tube is much efficient than providing circular cut-outs on tube, in terms of percentage peak load reduction as well as energy efficiency of structure. It was observed that number of cut-outs influences the polygonal deformation pattern of tube with cut-outs. Overall study significantly helps to achieve the aim of improving vehicle crashworthiness characteristics by implementing cut-outs in a crash frame.

The strain rate sensitive and anisotropic behavior of rare-earth magnesium alloy ZEK100 sheet

To overcome the limitation in formability at room temperature, manufacturers have developed magnesium alloys with remarkable properties by adding rare-earth elements. The rare-earth magnesium alloys behave differently from the conventional alloys, especially with respect to their coupled anisotropic and strain rate sensitive behavior. In the current work, such behavior of the rare-earth Mg alloy ZEK100 sheet at room temperature is investigated with the aid of the elastic viscoplastic self-consistent polycrystal plasticity model. Different strain rate sensitivities (SRSs) for various deformation modes are employed by the model to simulate the strain rate sensitive behaviors under different loading directions and loading rates. Good agreement between the experiments and simulations reveals the importance and necessity of using different SRSs for each deformation mode in hexagonal close-packed metals. Furthermore, the relative activities of each deformation mode and the texture evolution during different loadings are discussed. The anisotropic and strain rate sensitive behavior is ascribed to the various operating deformation modes with different SRSs during loading along different directions.

High strain rate deformation behavior of 2195 Al-Li alloy: Constitutive behavior and grain fragmentation

The high-speed forming process is a promising method for the preparation of high-performance Al-Li alloy components. However, the high-strain rate deformation behavior of the material is complex and difficult to accurately predict, which significantly restricts its development. In this paper, using the split Hopkinson pressure bar apparatus, the constitutive behavior and microstructure evolution of as-annealed and solution treated 2195 Al-Li alloy during high strain rate deformation was studied. As the strain rate increases, the flow stress first decreases and then increases for both two-state materials, showing a local strain rate softening behavior. Based on the Johnson-Cook model, a modified constitutive model is proposed, in which a correction term for the C value related to strain rate and strain is introduced. The modified Johnson-Cook model can accurately capture the abnormal strain rate sensitivity behavior observed in the experiment. In addition, it was found that the heat treatment state has a significant effect on the microstructural evolution behavior of 2195 Al-Li alloy. The as-annealed material shows the deformation behavior based on the evolution of dislocation cells. In addition to dislocation cells, there is also deformation behavior based on the evolution of submicron lath deformation bands for solution treated material. Dislocation cross slip is suppressed in the solution treated material, and its slip mode tends to be the plane slip. Through the analysis of dislocation density, mean slip distance of dislocation slip, and the interaction between solid atoms and dislocation, the mechanism of the abnormal deformation behavior of the solution treated material is revealed.

Strain rate sensitivity and toughness behaviour of ductile iron under specific loading

An efficient assessment of the mechanical properties of ductile iron (DI) under dynamic loading is usually missing. To address this gap, a plasticity model for DI grade EN-GJS-500-14 was suggested. Fracture mechanics (FM) and Charpy impact (CI) tests, high-speed and impact tensile tests as well as tensile tests at variable temperature and stress states were carried out on DIs with silicon contents from 2.10 to 3.64 wt-%. Toughness-related results indicate a difference in the transition temperatures of 100–140 K between CI and FM tests. The reason for this is assumed to be a pronounced strain rate sensitivity, which could be confirmed in the study. The plasticity model was able to predict toughness properties in CI tests.

Dynamic increase factors of concrete-like materials at very high strain rates

Strain rate effects on the compressive and tensile strengths of a concrete-like material play an important part in the construction of its dynamic constitutive model. So far no data for the same material covering a wide range of strain rates were reported in the literature, not to mention the data for compression at strain rates greater than 103 s−1. This paper presents a combined theoretical and experimental study on the dynamic increase factors of concrete-like materials at very high strain rates. First, a method is proposed to determine dynamic increase factor for a concrete-like material in compression due to strain rate effect only (that is to say to eliminate the inertial (containment) effect) at very high strain rates (>103 s−1) by in-depth analysis of its Hugoniot elastic limit (HEL). Then, various material tests (i.e. quasi-static uniaxial compression and tensile tests, SHPB and SHTB tests, plate impact test and dynamic compression-shear test) were performed on a certain type of cement mortar in a wider range of strain rates (i.e. 10-5 s−1 ∼ 106 s−1). Finally, the test data are compared with the previously proposed semi-empirical equations for dynamic increase factors. Furthermore, the plate impact test results for ultra high performance concrete (UHPC) available in the literature are also analyzed using the proposed method. Dynamic increase factor for tension at strain rate of 103 s−1 is obtained indirectly by the relationship between dynamic uniaxial tensile strength and dynamic shear strength through the shear wave tracking (SWT) technique which is based on dynamic compression-shear test. It is found that dynamic increase factor for concrete-like materials in compression due to strain rate effect only can be regarded as a constant at very high strain rates. It is also found that the previously proposed semi-empirical equations can accurately describe the strain rate sensitive behavior of concrete-like materials in both tension and compression.

Crystal plasticity modeling of strain-induced martensitic transformations to predict strain rate and temperature sensitive behavior of 304 L steels: Applications to tension, compression, torsion, and impact

This paper advances crystallographically-based Olson-Cohen (direct γ → α’) and deformation mechanism (indirect γ→ε→α’) phase transformation models for predicting strain-induced austenite to martensite transformation. The advanced transformation models enable predictions of not only strain-path sensitive, but also of strain-rate and temperature sensitive deformation of polycrystalline stainless steels (SSs). The deformation of constituent grains in SSs is modeled as a combination of anisotropic elasticity, crystallographic slip, and phase transformation, while the hardening is based on the evolution of dislocation density and explicit shifts in phase fractions. Such grain-scale deformation is implemented within the meso‑scale elasto-plastic self-consistent (EPSC) homogenization model, which is coupled with the implicit finite element (FE) method to provide a constitutive response at each FE integration point for solving boundary value problems at the macro-scale. Parameters pertaining to the hardening and transformation models within FE-EPSC are calibrated and validated on a suite of data including flow curves and phase fractions for monotonic compression, tension, and torsion as a function of strain-rate and temperature for wrought and additively manufactured (AM) SS304L. To illustrate the potential and accuracy of the integrated multi-level FE-EPSC simulation framework, geometry, mechanical response, phase fractions, and texture evolution are simulated during gas-gun impact deformation of a cylinder and quasi-static tension of a notched specimen made of AM SS304L. Details of the simulation framework, comparison between experimental and simulation results, and insights from the results are presented and discussed.

Evaluation of Tensile properties of FRP Composite Laminates under Varying Strain Rates and Temperatures

The present investigation deals with the characterization of tensile behavior of various Fiber Reinforced Polymer composites under Thermo-Mechanical loading. Five different types of Uni-Directional (UD) composites of Carbon, Glass, Carbon-Glass hybrid and Metal Laminates of Carbon & Glass were tested for tensile behavior. Tensile tests were performed at strain rates of 10-3, 10-2, & 10-1 s-1 at Room Temperature,250 0C and 450 0C. Stress-strain relations reveal the strain rate and temperature sensitive behavior of composites. Glass, Glass-Carbon, Glass-metal epoxy composites showed higher peak tensile stress under room temperature with varying strain rates as compared to neat carbon epoxy composites. Also, high strain rate tensile properties such as peak stress and peak strain of Glass-Carbon-Epoxy specimens were 26%, and 60% higher than that of the neat carbon epoxy composite. The failure mechanisms of both the composites were analyzed through scanning electron microscopy. The composites mainly failed due to matrix crack within elastic range under room temperature and failed with significant plastic deformation of matrix and fibers under test temperatures 250 0C and 450 0C. Finally, this study reveals that the continuous phase of metal layer embedded between Uni-Directional Glass and Carbon fiber, based composite system can be tailored to act as an energy-absorbing material system under both elastic and plastic stress strain regimes.

Positive Strain Rate Sensitivity and Deformation Behavior of a Fe–29Mn–3Al–3Si TWIP Steel

Positive Strain Rate Sensitivity and Deformation Behavior of a Fe–29Mn–3Al–3Si TWIP Steel

Room temperature strain rate sensitivity of as-built 3D printed AlSi10Mg by laser powder bed fusion

Tensile and compression loading behavior of as-built AlSi10Mg alloy printed by laser powder bed fusion were studied at quasi-static to medium strain-rate regimes. A non-linear strain rate sensitivity behaviour was observed at all strain rates, a response linked to the extra-fine, non-equilibrium as-built microstructure. For both loading conditions, except for a negative strain rate sensitivity in 10−2–10−1 1/s range, a positive trend was observed. At the equivalent strain of 0.04, a strain rate sensitivity parameter of 0.010 and 0.019 (in 10−4–10−2 1/s range) was measured for tensile and compression loading, respectively, indicating a strong dependency especially in compression. For the medium regime (1–5 1/s), the strain rate sensitivity parameter in compression was further increased to 0.046.

Characterisation and modelling of micro- and macroscale creep and strain rate sensitivity in Zircaloy-4

Intrinsic crystal slip system properties which control stress, stress relaxation and strain rate sensitivity (SRS) are investigated using combined experimental bend testing, digital image correlation strain measurement and electron backscatter detection with crystal plasticity modelling. Intrinsic properties (activation energy and volume) have been determined from room temperature experiments on Zircaloy-4 which has been shown to creep at 20 °C with non-negligible SRS of ∼0.03. Polycrystal plasticity modelling has been shown to capture DIC-measured intra- and transgranular creep strain and SRS evolution, and intragranular stresses have been calculated and shown to quantitatively reproduce those estimated from differential DIC strain measurements. The modelling also reproduces polycrystal sample-measured stress, stress relaxation and SRS behaviour remarkably well demonstrating consistency from slip system properties to polycrystal strain rate sensitivity. Polycrystal plasticity finally predicts diffraction-measured effects of texture and slip system activation on lattice strain rate sensitivities.

Twinning-Induced Abnormal Strain Rate Sensitivity and Indentation Creep Behavior in Nanocrystalline Mg Alloy.

Nanocrystalline materials exhibit many unique physical and chemical properties with respect to their coarse-grained counterparts due to the high volume fraction of grain boundaries. Research interests on nanocrystalline materials around the world have been lasting over the past decades. In this study, we explored the room temperature strain rate sensitivity and creep behavior of the nanocrystalline Mg–Gd–Y–Zr alloy by using a nanoindentation technique. Results showed that the hardness and creep displacements of the nanocrystalline Mg–Gd–Y–Zr alloy decreased with increasing loading strain rate. That is, the nanocrystalline Mg–Gd–Y–Zr alloy showed negative strain rate sensitivity and its creep behavior also exhibited negative rate dependence. It was revealed that the enhanced twinning activities at higher loading strain rates resulted in reduced hardness and creep displacements. The dominant creep mechanism of the nanocrystalline Mg–Gd–Y–Zr alloy is discussed based on a work-of-indentation theory in this paper.

Strain rate sensitivity behaviour of Fe–21Cr-1.5Ni–5Mn alloy and its constitutive modelling

In the present investigation, hot deformation behaviour of Fe–21Cr-1.5Ni–5Mn alloy was studied by performing isothermal unidirectional hot compression tests. The tests were carried out at strain rates of 0.01, 0.1, 1, 5 and 10 s−1 and at temperatures of 675, 750, 825, 900, 975 and 1050 °C. The samples were deformed up to 50% height reduction (ε = 0.7). The flow softening behaviour was modelled using Arrhenius hyperbolic sine law approach. The predicted flow stresses and the experimental flow stresses were found to be comparable indicating that the developed model can accurately portray flow behaviour of Fe–21Cr-1.5Ni–5Mn alloy. As-received and deformed specimens showed elongated grain structure. The correlation coefficient (R2) of the experimental and predicted results for Arrhenius hyperbolic sine law model was ~0.97, while the average absolute relative error (AARE) was found to be 7.31%. The hot deformation activation energy for peak stress was found to be 508.3 kJ mol−1. The maximum value of ‘m’ (0.227) was obtained in the temperature range 975 °C–1050 °C and strain rates between 0.1 s−1 to 1 s−1. ‘m’ value between 0.1 and 0.2 is considered to be optimum for hot working. Deformation at 975 °C also gave the best match between experimental and predicted flow stress with m values between 0.1 and 0.2 and hence was considered optimum.

Deformation and tribological behavior of ductile refractory high-entropy alloys

Refractory high-entropy alloys have attracted widespread attention as potential structural materials for high temperature applications. However, there is limited understanding of their high temperature deformation behavior and tribological response. Here, the temperature dependence of strain rate sensitivity and wear behavior was evaluated for two refractory high-entropy alloys, namely HfNbTiZr and HfNbTaTiZr. HfNbTaTiZr showed higher hardness, strength, and lower strain rate sensitivity compared to HfNbTiZr. The extrinsic strain rate sensitivity of both alloys obtained from micro-pillar compression was higher compared to the intrinsic values from nanoindentation due to the larger surface to volume ratio in micro-pillars leading to annihilation of dislocations and their easier movement. The decrease in strain rate sensitivity with increase in temperature was attributed to activation of both edge and screw dislocations as well as phonon drag controlled screw dislocation motion at high temperatures. Wear rate for both alloys increased with increase in sliding cycle frequency due to frictional heating. Wear rate also increased with increase in temperature from 298 K to 423 K and then decreased with further increase in temperature to 573 K. The underlying mechanism changed from abrasive wear at room temperature to extensive ploughing with fine wear debris at intermediate temperatures and oxidative wear at the higher temperatures.

The Strain Rate Sensitivity and Creep Behavior for the Tripler Plane of Potassium Dihydrogen Phosphate Crystal by Nanoindentation.

As an excellent multifunctional single crystal, potassium dihydrogen phosphate (KDP) is a well-known, difficult-to-process material for its soft-brittle and deliquescent nature. The surface mechanical properties are critical to the machining process; however, the characteristics of deformation behavior for KDP crystals have not been well studied. In this work, the strain rate effect on hardness was investigated on the mechanically polished tripler plane of a KDP crystal relying on nanoindentation technology. By increasing the strain rate from 0.001 to 0.1 s−1, hardness increased from 1.67 to 2.07 GPa. Hence, the strain rate sensitivity was determined as 0.053, and the activation volume of dislocation nucleation was 169 Å3. Based on the constant load-holding method, creep deformation was studied at various holding depths at room temperature. Under the spherical tip, creep deformation could be greatly enhanced with increasing holding depth, which was mainly due to the enlarged holding strain. Under the self-similar Berkovich indenter, creep strain could be reduced at a deeper location. Such an indentation size effect on creep deformation was firstly reported for KDP crystals. The strain rate sensitivity of the steady-state creep flow was estimated, and the creep mechanism was qualitatively discussed.