Positive Strain Rate Dependence Research Articles

Effect of strain rates on mechanical behavior, microstructure evolution and failure mechanism of extruded-annealed AZ91 magnesium alloy under room-temperature tension

Loading rate is usually considered as a significant factor affecting formability of AZ91 magnesium alloy structural components. Herein, uniaxial tensile tests are conducted on extruded-annealed AZ91 alloy under room temperature with strain rates ranging from 1 × 10−4 s−1 to 1 × 10−2 s−1. Mechanical properties, microstructure characteristics, and fracture morphologies tensioned at these different strain rates are investigated. Extruded-annealed AZ91 alloy exhibits a positive strain-rate dependence of strength and a negative strain-rate dependence of elongation. With strain rate increasing, strain rate sensitivity basically decreases while strain-hardening rate increases. At higher strain rate, the larger strength is mainly attributed to the more rapid accumulation of dislocation and the larger dislocation density, and the lower elongation is mostly owing to the more elevated level of inhomogeneous deformation related with dislocations accumulation. As strain rate increases, the area fraction of basal texture increases while that of prismatic and pyramid texture decreases in tensioned samples, which is associated with the dominant room-temperature deformation mode of basal-slip and twinning co-induced lattice rotation. With tension strain rate increasing, there are three kinds of fracture modes including ductile facture with more and deeper dimples at low strain rate, ductile-brittle facture with some dimples and cleavage planes at mediate strain rate and brittle facture with dominant cleavage planes at high strain rate.

Phase Composition Effects on Dynamic Behavior and Strain Rate Sensitivity in Metastable β-Ti Alloys.

In this study, high strain rate tension tests are conducted to determine and compare the dynamic mechanical behaviors and deformation mechanisms of different phase composition α-β metastable β-Ti alloys using a split Hopkinson tension bar. Two typical bimodal equiaxed αp + β and lamellar αs + β Ti-45551 alloy microstructures are formed through different hot working and thermal processing for investigating the effect of phase composition or microstructure on mechanical properties and strain rate sensitivity. It is demonstrated that dislocation nucleation and motion in the α/β phase and dislocation tangle or pile up at the α/β interface are typical deformation modes in both of the typical dual-phase Ti alloys at quasi-static loading conditions. Under dynamic loading, both the strength and ductility show a clearly positive strain rate dependence, which is directly related to dislocation activation in the α + β Ti-45551 alloy. Based on microstructure characterizations, it is shown that deformation twinning starts to become a major deformation mechanism in equiaxed αp + β microstructures under dynamic loading conditions. However, deformation twins are not favored in the lamellar αs + β Ti-45551 alloy due to its nano phase size. Finally, the mechanical behaviors and strain rate sensitivity are strongly dependent on the phase composition of metastable β-Ti alloys.

Strength differential effect and anisotropy of Mg–1.9Mn–0.3Ce (wt-%) alloy subjected to high-rate loadings

Tension and compression tests for a rare-earth-containing alloy sheet, Mg–1.9Mn–0.3Ce (wt-%), are performed under quasi-static and high-rate loadings along the rolling, transverse and normal direction, and stress – strain responses exhibit positive strain-rate dependence. Empirically based Johnson-Cook constitutive model is modified to describe the rate-dependent plastic deformation in multiple loading directions. A strong strength differential effect persists, and such effect is shown to be stronger at high strain rates. A weak in-plane anisotropy exists in the initial yielding and subsequent strain hardening behaviour as well as strength differential effect. In contrast, the out-of-plane anisotropy in compression is apparent and is approximately rate insensitive. Microstructural observations indicate that twinning plays the key role for the enhancement of strain-hardening rate under high strain-rate deformation.

Strain rate dependence of SM490B at room temperature and application to Cowper-Symonds constitutive models

Steel is widely used as a constituent material for various structures such as automobiles and ships. To perform high precision analysis including high strain rate behavior, an understanding for the strain rate dependence of material strength becomes very important. The purpose of this study is to evaluate the strain rate dependence of material strength with rolled steels for welded structure, JIS SM490B (ASTM E). We investigated the deformation characteristics at room temperature by performing compression tests at a wide range of strain rates and applied the obtained experimental results to the material constitutive model using an optimization method of Nelder-Mead method. The quasi-static tests were conducted using a universal testing machine at the strain rate of 10-3, 10-2 and 10-1 s-1. The impact test was conducted using a split Hopkinson pressure bar apparatus at the strain rate of approximately 103 s-1. As the results of the compression tests, it was confirmed that SM490B has a positive strain rate dependence of material strength. The Cowper-Symonds constitutive model showed good agreement with the experimental results up to the strain of 20%. However, the error became larger between experimental results and CS approximation as the strain increases to 20% or more.

Rate dependent tensile behavior of polyurethane under varying strain rates

Elastomeric polymers, such as polyurethane (PU), are being used in novel applications to enhance the load-carrying capacity, ductility, and the survivability of structures under dynamic loading. The mechanical response of elastomeric materials is highly rate and pressure dependent, and exhibits stress-strain non-linearity. The objective of the current study is to identify the effect of the strain rate on the uniaxial tensile behavior of elastomeric PU. To this end, four types of PUs differing in their plasticizer content were used. The uniaxial tensile characteristics under loading and unloading conditions and the cyclic softening behavior were examined under varying strain rate regimes (ranging from 0.001 s−1 to 0.33 s−1). The experimental results showed that the stress-strain behavior of all PUs is non-linear and rate dependent. Young’s module, yield stress, tangent module, ultimate tensile stress, failure stress, failure strain, resilience module, toughness module and residual strain of PU6 at 0.33 s−1 are 0.37–4.13 times compared to the values at 0.001 s−1. It also exhibits hysteresis and cyclic softening. Increasing strain rates resulted in a dramatic transition in behavior from rubbery to leathery for all PUs. This behavior was described as positive strain-rate dependence. The behavior of PUs was defined as hyper-viscoelastic material.

Characterization of Aged Li-Ion Battery Electrodes for Direct Recycling Process Design

Developing a recycling protocol for lithium-ion batteries (LIBs) at their end of life is crucial to both supply chain stability and waste mitigation. Novel nondestructive recycling methods are under investigation, but lack the process engineering specifications required for full-scale operation. Specifically, the ability of end-of-life LIB components (i.e., recycling feedstock) to withstand the mechanical stresses inherent in industrial reprocessing techniques has not been established. To this end, we have mechanically characterized the electrodes of both fresh cells and “cycled-aged” (C-A) cells (cells that have been cycled and subsequently calendar-aged, and thus reflect a representative recycling input). We report the components’ performance under uniaxial tensile stress and compressive loads, and determine the effects of tensile strain rate and electrode coating on mechanical response. Further, we explore how these measured mechanical properties affect design parameters for a roll-to-roll (R2R) direct recycling process. Cycle-aging is found to significantly reduce the tensile strength of coated electrodes across all strain rates, and electrodes are found to have significantly lower elastic moduli than their respective current collectors. Positive strain rate dependence is observed for bare current collectors but not for coated electrodes, implying that the coating of active material permanently alters the current collector matrix. Compressive failure is not observed for any of the samples; however, C-A cathodes reach elastic deformation at a lower strain than do fresh cathodes. In the context of direct recycling, these findings suggest that roll tension may need to be reduced by 13-65% for C-A components relative to fresh components to avoid irreversible damage. Additionally, C-A cathodes are expected to permanently deform at a lower calendering force than fresh cathodes. Finally, we have investigated each component’s contribution to overall cell capacity loss, and have probed the degradation mechanisms of cathodes to inform recycling method development. Electrochemical analysis suggests that phase shift (layered to spinel phase) and buildup of electrolyte residues at both the primary particle and in the inter-particle pore space may significantly contribute to cathode degradation. Washing C-A cathodes with aprotic solvents is found to improve half-cell cycling performance, likely due to the removal of electrolyte residues, but does not reverse structural degradation. These electrochemical results suggest the importance of a washing step prior to chemical relithiation in direct recycling methods. Thus the present work, which combines mechanical and electrochemical analyses of cycle-aged LIB components, informs the process design of industrial-scale nondestructive LIB recycling.

Thickness effects on microstructure, mechanical and soft magnetic properties of sputtered Fe[sbnd]Zr thin film metallic glass

In this work, we report the thickness effects on microstructure, mechanical and soft magnetic properties of Fe75Zr25 thin film metallic glasses (TFMGs) prepared by DC magnetron sputtering. The Fe75Zr25 TFMGs exhibit a nano-granular structure and the particle size is growing with the increase of thickness. The hardness (H) and the elastic modulus (E) evaluated by nanoindentation, both show a decreasing trend with the increase of film thickness. The smooth P-h curves reflect a homogenous deformation mode, which is attributed to the activation of shear transformation zones (STZs) in the nanoscale granular thin films that could reasonably restrain the formation of shear bands. Furthermore, a positive strain rate dependence was observed for all films with highest strain rate sensitivity SRS (m) value of 0.037 for thicker film, more likely due to the free volumes accumulation during deformation. In addition, Fe75Zr25 TFMGs show anomalous strong ferromagnetism at low and high temperatures (10 K and 300 K). It is found that in a critical thickness of 270 nm, the Fe75Zr25 TFMGs manifest the desired combination of soft magnetism and mechanics. The underlying mechanism of variation in mechanical and soft magnetic properties could be rationalized in light of the interplay of nanoscale granular size and structural evolution with increase of film thickness. This study offers a paradigm to design Fe-based TFMGs with simultaneously superior mechanical performance and excellent soft magnetic properties.

Anomalous effects of strain rate on the room-temperature ductility of a cast Mg-Gd-Y-Zr alloy

The loading rate significantly affects the mechanical properties of Mg alloys. However, the effects of strain rate on newly developed Mg–Gd–Y alloys at room temperature (RT) have rarely been investigated. Here, uniaxial tensile tests were conducted on a cast Mg-10Gd-3Y-0.5Zr (wt. %) alloy (GW103) in both as-aged and as-solutionized states with different grains sizes at RT and various strain rates. The results showed an anomalous positive strain-rate dependence of elongation in all the GW103 alloys investigated from 1 × 10−5 to 1 × 10−1 s−1. Careful microstructural characterization by slip trace analysis revealed that the exclusive deformation mechanism of pyramidal <c+a> slip, non-Schmid behavior of basal slip, and slip transfer at 1 × 10−1 s−1; these induced the high RT ductility. The occurrence of pyramidal slip activity at high strain rates was suggested to be associated with rare earth solutes, fine precipitates, and local high stresses at grain boundaries due to dislocation pile-up. It was proposed that the increasing strain rate was approximately equivalent to decreases in grain size, enabling the generation of local stresses at grain boundaries, thus causing this anomalous phenomenon.

Strain Rate Dependence of Material Strength in AA5xxx Series Aluminum Alloys and Evaluation of Their Constitutive Equation

The effect of strain rate on the mechanical properties of AA5xxx series aluminum alloys containing solute Mg atoms (AA5005, AA5021, AA5082 and AA5182) and pure aluminum (A1070) was investigated within a wide strain rate range of 1.0 × 10−4 to 1.0 × 103 s−1 at room temperature. The A1070 exhibited a positive strain rate dependence of material strength at the investigated strain rates. However, the AA5xxx series aluminum alloys primarily exhibited the negative strain rate dependence of material strength and serration caused by the Portevin-Le Chatelier effect on the Mg content and strain rate. As a result of using the material constitutive equation for the negative strain rate dependence, it was found that the flow stress may change in the dynamic strain rate range. However, it was found that the strain rate dependence of material strength differed in the AA5082 and the AA5182 alloys. It would be caused by less solute Mg of the Al phase in the AA5182 alloy than in the AA5082 alloy, because more Mg2Si compounds precipitated on Mn bearing particles as precipitation sites in the AA5182 alloy.

Microstructure and Strain Rate-Dependent Tensile Deformation Behavior of Fiber Laser-Welded Butt Joints of Dual-Phase Steels

The microstructure and tensile deformation behavior of the fiber laser-welded similar and dissimilar dual-phase (DP) steel joints over a wide range of strain rates from 10−3 to 103 s−1 were investigated for the further applications on the lightweight design of vehicles. The high strain rate dynamic tensile deformation process and full-field strain distribution of the base metals and welded joints were examined using the digital image correlation method and high-speed photography. The strain rate effects on the stress–strain responses, tensile properties, deformation, and fracture behavior of the investigated materials were analyzed. The yield stress (YS) and ultimate tensile strength (UTS) of the dissimilar DP780/DP980 welded joints were lying in-between those of the DP780 and DP980 base metals, and all materials exhibited positive strain rate dependence on the YS and UTS. Owing to the microstructure heterogeneity, the welded joints showed relatively lower ductility in terms of total elongation (TE) than those of the corresponding base metals. The strain localization started before the maximum load was reached, and the strain localization occurred earlier during the whole deformation process with increasing strain rate. As for the dissimilar welded joint, the strain localization tended to occur in the vicinity of the lowest hardness value across the welded joint, which was in the subcritical HAZ at the DP780 side. As the strain rate increased, the typical ductile failure characteristic of the investigated materials did not change.

Crystal plasticity modeling and characterization of the deformation twinning and strain hardening in Hadfield steels

The deformation and strain hardening due to dislocation slip and twinning in Hadfield steels is investigated with a crystal plasticity model. A phenomenological interaction and hardening formulation is incorporated to the numerical model based on the microscopic characterization and deformation behavior. Single and polycrystal simulations on a Hadfield steel are conducted to prove the soundness of the model in describing the deformation and hardening behavior of the steel. The competition between dislocation slip and twin dominated deformation plays an essential role in the asymmetry between tension-compression as well as in the strain hardening behavior of the steel. The simulations performed on another Hadfield alloy verifies the model's capability to represent the strain rate sensitivity of the material, when a positive strain rate dependence exists. The use of realistic 3D polycrystalline aggregates imitating the microstructure of the Hadfield steels provides new insight into the inter-grain and intra-grain behavior of austenitic microstructures exhibiting twinning, particularly revealing the nature of local stress and twin concentrations. The strain rate sensitivity of the alloyed Hadfield steel is observed also at the grain scale, intensifying twinning at higher strain rates depending on the loading direction. The local gradients were found to arise from the neighbouring grains and from the intra-grain reorientation, explaining the experimental observations of only partially twinned grains. The simulation results and the 3D aggregate approach used in this paper provide information for the validation of crystal plasticity model as well as about the local deformation behavior of Hadfield steels.

Dynamic behaviour of Al-Mg aluminum alloy at a wide range of strain rates

The effect of strain rate on mechanical properties of Al-2.3wt.%Mg alloy (AA5021) and commercial pure aluminum (purity 99.7wt.%: A1070) was investigated at room temperature. The tensile tests were conducted at strain rates from 1.0×10−4 to 1.0×103 s−1. The universal testing machine was used for strain rate 1.0×10-4 to 1.0×10−1 s−1. For the strain rate 1.0×100 s-1, the servohydraulic testing machine, which was developed by our laboratory, was used. The impact strain rate 1.0×103 s−1 was obtained using the split Hopkinson pressure bar method. The pure aluminum showed positive strain rate dependence of material strength at the investigated strain rates. In contrast, the Al-2.3wt.%Mg alloy showed the negative strain rate dependence at strain rates from 1.0×10−4 to 1.0×100 s−1. However, Al-2.3wt.%Mg alloy showed the positive strain rate dependence at strain rates from 1.0×100 to 1.0×103 s−1. It was surmised that the effect of dislocation locking by the solute Mg atoms became negligible at strain rate of approximately 1.0×100 s−1. It was confirmed that material properties for the Al-Mg alloy at the strain rate of 1.0×100 s−1 were important, since the strain rate dependence changed negative to positive around this strain rate.

Microstructure- and Strain Rate-Dependent Tensile Behavior of Fiber Laser-Welded DP980 Steel Joint

DP980 steels were butt-welded by fiber laser welding. The microstructures, microhardness distribution, and tensile behavior of the joint were investigated. The results showed that the fusion zone (FZ) consisted of fully martensite with higher hardness compared to the base metal (BM). A softened zone (20 HV0.2 drop) was produced in heat-affected zone due to martensite tempering during the laser welding. The ultimate tensile strength (UTS) and yield strength (YS) of the laser-welded joint were not degraded compared to BM with the existence of softened zone. The UTS and YS of the welded joint increased with the increase of tensile strain rate. The work hardening exponents of the BM and welded joint showed weak positive strain rate dependence. The deformation of softened zone was restrained by the hardened FZ during loading, resulting in a higher work hardening rate of softened zone than that of BM. The failure of welded joint occurred in the BM instead of softened zone. The fracture surfaces of the joint exhibited typical ductile fracture over strain rate from 0.0001 to 0.1 s−1.

Strain rate dependent deformation and failure behavior of laser welded DP780 steel joint under dynamic tensile loading

Laser welded DP steel joints are used widely in the automotive industry for weight reduction. Understanding the deformation and fracture behavior of the base metal (BM) and its welded joint (WJ), especially at high strain rates, is critical for the design of vehicle structures. This paper is concerned with the effects of strain rate on the tensile properties, deformation and fracture behavior of the laser welded DP780 steel joint. Quasi-static and dynamic tensile tests were performed on the WJ and BM of the DP780 steel using an electromechanical universal testing machine and a high-speed tensile testing machine over a wide range of strain rate (0.0001–1142s−1). The microstructure change and microhardness distribution of the DP780 steel after laser welding were examined. Digital image correlation (DIC) and high-speed photography were employed for the strain measurement of the DP780 WJ during dynamic tensile tests. The DP780 WJ is a heterogeneous structure with hardening in fusion zone (FZ) and inner heat-affected zone (HAZ), and softening in outer HAZ. The DP780 BM and WJ exhibit positive strain rate dependence on the YS and UTS, which is smaller at lower strain rates and becomes larger with increasing strain rate, while ductility in terms of total elongation (TE) tends to increase under dynamic loading. Laser welding leads to an overall reduction in the ductility of the DP780 steel. However, the WJ exhibits a similar changing trend of the ductility to that of the BM with respect to the strain rate over the whole strain rate range. As for the DP780 WJ, the distance of tensile failure location from the weld centerline decreases with increasing strain rate. The typical ductile failure characteristics of the DP780 BM and WJ do not change with increasing strain rate. DIC measurements reveal that the strain localization starts even before the maximum load is attained in the DP780 WJ and gradual transition from uniform strains to severely localized strains occurs at high strain rates. The diffuse necking of the DP780 WJ occurs earlier during the tensile deformation process at higher strain rates under dynamic loadings.

Dynamic Strain Ageing in a Twin-Induced Plasticity Steel

This work was to clarify the characteristics of serrated flow in an austenitic FeMnC twin-induced plasticity (TWIP) steel at room temperature (RT) using both strain- and crosshead displacement-controlled tensile tests. Three types of serrations were observed in strain-controlled but not in displacement-controlled tests, indicating that strain-controlled tensile tests provide more deformation details. The occurrence of the different types of serrations depends on both strain rate and strain level. Type C serrations were observed in TWIP steels at RT for the first time. The critical strain for the onset of serrations exhibits a positive strain rate dependence at higher strain rates, whereas an “inverse” critical strain behavior was observed in the lower strain rate region.

Rate-dependences of deformation mechanisms in NiTi shape memory alloys

In the NiTi shape memory alloys (SMAs), the macro-mechanical deformations and the microstructural evolutions at different strain-rates (0.001-1200 s-1) are investigated. It is found that the detwinning stress of martensitic twin increases with strain-rate increasing, which indicates that the detwinning stress has the positive strain-rate dependence. A large number of detwinning regions are found in the NiTi specimen which is deformed at the strain-rate of 10 s-1 under tension. However, with the strain-rate further increasing up to 100 s-1 and 1200 s-1, no detwinning region is observed and many twins still exist. It is shown that the detwinning rates of martensitic twin in NiTi SMAs are in a range of 10-100 s-1. Simultaneously, thermally-induced austenite is detected in the NiTi specimens deformed at high strain-rates (≥qslant10 s-1). It is ascribed to the fact that there is a change from the isothermal process to the adiabatic process when the tensile strain-rate goes up to a critical value. Additionally, a small shoulder peak is detected in differential scanning calorimeter peak of 1200 s-1 strain-rate specimen, indicating that the two-stage phase transformation occurs.

Fracture Behavior Accompanying Electromagnetic Waves of Granite in Dynamic Three Point Bending

A series of three-point bending tests for granite consisting of quartz, plagioclase, alkali feldspar and biotite was carried out at quasi-static and dynamic rates to examine the relation between its mechanical properties and electromagnetic phenomena during fracture. The output of ferrite-core antenna, which was located close to the specimen in a shielding box made of Permalloy plates, was measured through a band-pass filter. The dynamic bending strength was larger than the static one, i.e. the positive strain-rate dependence was observed in the strength of granite. It was also found that the intensity of electromagnetic waves measured in dynamic tests was much greater than that observed in static tests. This means that the electromagnetic phenomenon strongly depends on the loading rate, too.

Mechanical behavior of glass-fiber reinforced thermoplastic materials under high strain rates

Glass-fiber reinforced polypropylene (PP) and polybutene-1 (PB-1) materials were investigated in a high-speed tensile test. The glass-fiber content of the materials and the strain rate were varied in the range between 0 and 40 wt.-% and 0.007 and 174 s −1, respectively. The aim of the investigations was to show in which way the glass-fiber content, and especially the strain rate, influence the material behavior, in this case the stress–strain behavior, the tensile strength and the fracture appearance. The strain-rate dependent material behavior is described with the phenomenological G'Sell–Jonas model. It was found that a transition from isothermal to adiabatic behavior occurs and, therefore, two different regression parameter sets are necessary to describe the material behavior accurately. An increase of tensile strength could be found for the PP and PB-1 materials examined with increasing strain rate. This behavior is described in the literature as positive strain-rate dependence.

The positive strain-rate dependence of ductility in a 50Mo-50Re alloy

This paper is a review of the recent studies of deformation and fracture behaviors, especially the anomalous strain-rate effect on plasticity of a 50Mo-50Re alloy. The ductility of this alloy was found to increase with increase in strain rate within the range of 10−6 s−1 to 1 s−1 at room temperature in air. The fracture surfaces in the alloy changed from brittle to ductile in nature with increasing strain rate. A damage-toughening phenomenon was also observed in this alloy.

Effect of strain rate on compressive behavior of Ti-based bulk metallic glass at room temperature

Compressive deformation behavior of the Ti 40Zr 25Ni 8Cu 9Be 18 bulk metallic glass was investigated over a wide strain rate ranging from 10 −4 to 10 3 s −1 at room temperature. Fracture stress was found to increase and fracture strain decrease with increasing applied strain rate, which were due to the decrease of the density of shear bands. Serrated flow was observed at lower strain rate. The appearance of a large number of shear bands was probably associated with flow serration observed during compression. At high strain rates, the rate of shear band nucleation was not sufficient to accommodate the applied strain rate and thus caused an early fracture of the test sample. The positive strain rate dependence of compressive strength might be associated with the different microstructure on the atomic scale in the Ti-based amorphous matrix.