Stain Rate Research Articles

Calculations of uniaxial tensile strength of Al–Cu–Ni based metallic glasses using molecular dynamics simulations

Molecular dynamics (MD) simulations have been performed to investigate the mechanical properties (MPs) of Al–Cu–Ni based metallic glasses (MGs). The embedded atom method interaction potential has been employed for present MD technique. Due to brittle nature of Cu50Zr50 we studied the effect of Al (2 at%) on the MPs. Moreover, effect of Co and Ni (2 at%) on MPs of two component MGs (NiAl and FeCu respectively) is studied that are generated by melt quenching processes. The structure of samples is changed at melting and glass transition temperatures and the radial distribution function provides the appropriate outcomes for crystalline and glassy states. The mechanical analysis of three MGs has been investigated by employing uniaxial tensile deformation for a wide range of varying stain rates. It has been shown that the MPs of MGs are significantly expanded by the accumulation of Al, Co and Ni concentrations. It is revealed from the MD results that yield strength, Young modulus and ultimate tensile strength increase with an increase of stain rates at constant temperature. The computer software LAAMPS is used in order to investigate these MPs of MGs. In addition, computer software (OVITO) has been used for the visualization of crystalline and glassy states. The numerical outcomes obtained from MD method are found to be in satisfactory agreement with that obtained through the earlier known numerical MD results.

Effect of compressive strain rate on the deformation behaviour of austenitic stainless steel foam produced by space holder technique

316L austenitic stainless steel foam (ASSF) with 55–81% porosities was made by P/M route using urea as space holder. The cold compacted at 500 MPa pressure and sintered at 1200 °C for 60 min. Microstructure, cell size, cell walls thickness and micro-hardness were evaluated. Twins boundaries are visible in cell wall regions. XRD, FESEM, and EDX analysis proved that no residue of space holder particles remained in the sintered foam samples. The compressive deformation behaviour of sintered foam samples are studied under three different strain rate (0.001, 0.01, and 0.1 s−1). The yield stress (σy), average plateaus stress (σpl), elastic modulus (Ef) and energy absorption (Eab) increases with increase in relative density (ρrd), and also increased with increasing strain rate. Densification strain decreased with increasing strain rate and follows linear relationship with ρrd. The yield stress, elastic modulus, and densification strain were compared with Gibson-Ashby's model and are found to be in closed agreement. The stain rate parameter (Σ) marginally decreases with increase in ρrd and follows linear relationship with ρrd.

ANALYSIS OF STRENGTH OF BRITTLE MATERIALS UNDER DIFFERENT STRAIN RATES USING LDEM SIMULATIONS

This paper presents a study on the variation of strain rate influence with the scale effect in quasi-brittle materials. Works in the literature report an increase of dynamic resistance with strain rate. To study this relation, a numerical model that combines the Finite Element Method and the Lattice Discrete Element Method is employed. This mixed model was implemented on the commercial software Abaqus/Explicit. The samples are prepared with FEM at the extremity and LDEM for the central part, which has a reduction in the cross-section, so the rupture occurs in the middle. In the simulations, four model specimens with dimensional variations were subjected to direct tensile testing with different strain rates. The results showed that with the increase of applied strain rate, the resistances also increase and the body dimension influences this behavior. The variation of the tensile strength is found without modifications on the elemental constitutive relationship or material parameters. At last, a method that could lead to an independent relationship between the dynamic intensification and the stain rate of scale effect is also proposed.

Deformation behavior and softening mechanism of TiB reinforced near-α titanium matrix composite during hot compression

In order to investigate the plastic workability of TiB reinforced near-α high-temperature titanium matrix composite, 3vol.% TiB/Ti-6Al-4Sn-10Zr-1Mo-1Nb-1W-0.3Si composite were hot compressed over a temperature range of 850–1100°C and a strain rate range of 0.01–0.5s−1. Microstructures of the composite after hot compression were observed by scanning electron microscope (SEM), electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM). The constitutive equations of this composite were calculated and the processing map was established. The results show that the activation energy of (α+β) phase and β phase region were 758550J · mol−1 and 220510J · mol−1, respectively. In the stable regions, equiaxed α phase accounting for a high proportion was formed in the matrix at low temperature and strain rate, and became more obvious with increasing of temperature and stain rate. Whereas, in the unstable region the microstructure is mainly characterized by the debonding of TiB closely associated with strain hardening in the matrix, and incomplete recrystallization of β phase in the β phase region at high strain rate. More specially, softening mechanisms in stable during deformation were different under different parameters. The softening mechanism is dominated by the dynamic recrystallization when the process is about 900°C/0.1–0.01s−1, but gradually becoming by periodic competition of dynamic recrystallization and dynamic recovery when the processes is 950–1100°C/0.1s−1. This difference is mainly attributed to the decreasing of activation energy of at elevated temperature. Finally, the hot deformation processing parameters are optimized as temperature range of 900–950°C and strain rate of 0.01–0.1s−1.

Effects of stress states and strain rates on mechanical behavior and texture evolution of the CoCrFeNi high-entropy alloy: Experiment and simulation

The effects of stress states and strain rates on mechanical behaviors and texture characteristics of the CoCrFeNi high-entropy alloy (HEA) were investigated via shear-tension specimens and Split-Hopkinson Tensile Bar (SHTB). The results revealed that the stress states and strain rates have an obvious influence on the mechanical properties and texture evolution. The volume fractions of textures S {123}<634> and Brass {110}<112> both increase with the decline of shear stress component under different stain rates, but their volume fractions are larger at high strain rates. The strength and elongation of the CoCrFeNi HEA are simultaneously increased at high strain rates and complex stress states. In addition, a crystal plasticity finite element model (CPFEM) with a damage factor was established to simulate the stress distribution and damage evolution at quasi-static condition. The results showed that the CPFEM can exactly predict the texture and damage characteristic of the CoCrFeNi HEA under different stress states at quasi-static deformation.

Determination of Constitutive Equation and Thermo–Mechanical Processing Map for Pure Iridium

Deformation behavior of pure iridium has been studied during thermal compression testing with the help of Gleeble-1500D in the temperature range of 1200 °C~1500 °C and strain rate range of 10−1 s−1~10−2 s−1. Resistance to deformation, microstructural evolution and hot workability of pure iridium have been used to analyze in detail. Frictional coefficient has been used to modify the experimental stress–strain curve of thermal compression test, and it has been found effective in reducing the influence of friction during thermo–mechanical testing. The hyperbolic sine constitutive equation of pure iridium has been established to give a material processing model for numerical simulation. A very high value of activation energy for iridium, 573 KJ/mol, clearly indicates that it is very hard to deform this material. The deformation mechanism of pure iridium is dependent upon temperature as well as strain rate. At low temperature and strain rate (temperature range of 1200 °C~1300 °C and strain rate range of 10−1 s−1~10−2 s−1), dynamic recovery is active while dynamic recrystallization becomes operative as temperature and stain rate are increased. On further increase in temperature and decrease in strain rate (temperature range of 1400 °C~1500 °C and strain rates of 10−2 s−1~10−3 s−1), abnormal grain growth takes place. On the basis of a constitutive model and processing map, suitable forming process parameters (temperature range of 1400 °C~1500 °C and strain rate range of 0.1 s−1~0.05 s−1) for pure iridium have been worked out.

Study of accelerated shear creep behavior and fracture process of micro-scale ball grid array (BGA) structure Cu/Sn–3.0Ag–0.5Cu/Cu joints under coupled electro-thermo-mechanical loads

Shear creep deformation and fracture behavior of micro-scale ball grid array (BGA) structure Cu/Sn–3.0Ag–0.5Cu/Cu joints under electro-thermo-mechanical coupled loads with high-density current stressing (6.0 × 103 A/cm2) at different temperatures (40, 60, 80, 100 and 120 °C) and various shear stress levels (12.5, 15, 17.5, 20 and 22.5 MPa) were studied intensively. The results show that electric current stressing can significantly accelerate the creep deformation and fracture process of BGA solder joints in terms of increasing steady-state creep stain rate and shortening creep lifetime of the joints. Garofalo hyperbolic-sine function has shown to be a suitable creep constitutive equation, which can be well used to formulate the creep deformation behavior of micro-scale Cu/Sn–3.0Ag–0.5Cu/Cu joints under electro-thermo-mechanical coupled loads, and the equation has the increased stress exponent of 8.6 and creep activation energy of 90.8 kJ/mol, respectively. Combined theoretical analysis and experimental characterization suggest that electric current stressing leads to the change of the dominant steady-state creep deformation mechanism from dislocation climb to lattice diffusion. The interruption creep tests demonstrate that crack initiates at the interface between the solder and intermetallic compounds (IMC) near the position of electrons entering or exiting the solder matrix when the steady-state creep stage approaches the limit, and then the crack first propagates along the solder/IMC interface and gradually turns to the solder matrix under combined shear and tensile stresses. The joint fractographies exhibit a mixed brittle-ductile fracture mode with one part in the interface and the other part in the solder matrix.

Characterization of AZ91 magnesium alloy processed by cyclic contraction/expansion extrusion using the experimental and micromechanical cellular automaton finite element approach

The feasibility to fabricate ultra-fine-grained AZ91 Mg alloy is investigated with a newly presented severe plastic deformation method entitled cyclic contraction/expansion extrusion. In this method, an initial cylindrical AZ91 Mg billet is placed into a die and the moving punch causes the large deformations by extruding the material in two different regions entitled contraction and extrusion. The evolution of AZ91 microstructure and mechanical properties during cyclic contraction/expansion extrusion was studied through different experimental observations. The microstructure observations showed the ultra-fine-grained structure of AZ91 at the end of the third pass where the average grain size of 600 nm obtained from the initial value of 130 µm. The tensile tests showed that the ultimate tensile strength, yield strength, hardness, and elongation of AZ91 cyclic contraction/expansion extrusion-processed samples are increased significantly. Discontinuous dynamic recrystallization has a main role in the grain refinement of Mg alloys during hot deformations. The evolution of grains in microlevel is analyzed by the cellular automaton finite element method in the DEFORM software environment. The macroscopic flow parameters including effective plastic strain, stain rate, and temperature were calculated in finite element. By tracing these data in defined domain of cellular automaton, the discontinuous dynamic recrystallization of material is analyzed through a devised cellular automaton finite element post-processing step. The imposed plastic strain and variation of dislocation density are the two main driven forces in discontinuous dynamic recrystallization of AZ91 samples during cyclic contraction/expansion extrusion processing. The experimentally observed grains and the cellular automaton finite element predicted microstructure were reasonably in good agreement.

Nonlocal modeling and analysis of spatiotemporal patterns in non-isothermal phase transformation of NiTi strips

The spatiotemporal evolution of macro-domain patterns in a NiTi thin strip during stress-induced non-isothermal phase transformation (PT) are investigated through 2D FEM simulations using nonconvex nonlocal continuum model and 1D theoretical analysis. The FEM simulations shows that, with the increase of applied stretching rate (nominal strain rate), the way of PT in a thin strip (30 mm × 2.5 mm × 0.5 mm) changes from the sequential nucleation and growth mode in the low rates range (≤0.03/s) to the emergence of periodic patterns in the medium rates range (0.03/s~3.0/s) and eventually to stable homogeneous deformation mode in the high rates range (≥3.0/s). The domain spacing is governed by a power-law scaling to the strain rate, where the exponent changes from −1/2 (low rates range) to −1/6 (medium rates range). The 1D theoretical analysis further demonstrates that the rate-dependent patterns are essentially governed by a nondimensional internal material parameter A-0 and a nondimensional external driving parameter k-. A-0 represents the ratio of adiabatic hardening to isothermal softening in PT of the material, while k- controls the time scale competition of heat release and heat conduction, through which the thermodynamic condition of PT changes from near isothermal (k-→∞) to near adiabatic (k-→0) with the increase of the strain rate. It is shown that, as long as the material has the properties of isothermal softening and adiabatic hardening (i.e., A-0>1), the conventional nucleation-growth paradigm of PT will eventually break down for small k- (or large stain rate). This is due to the fact that the increase of the energy barrier caused by the thermal and the interfacial effects can convexify the energy landscape of the system near the adiabatic condition and suppress the nucleation events at the macroscopic level.

Study on high temperature mechanical behavior and microstructure evolution of Ni3Al-based superalloy JG4246A

Ni3Al-based polycrystalline intermetallic compound is a kind of engineering superalloy, which have been used in the adjustment flaps of aeroengine tail nozzle. In order to explore its mechanical properties and microstructure evolution in service, the high temperature flow characteristics and microstructural changes of alloys were investigated by uniaxial isothermal compression tests on Gleeble-1500. The true stress-true strain curves were obtained and the microstructures were analyzed by OM, SEM, and EBSD. Based on the experimental data, three different constitutive models, belonging to phenomenological model, physical-based model, and radial basis function (RBF) neural network model, separately, were established to predict the deformation stress. The results show that the true stress is very sensitive to temperature and stain rate. Flow instability occurred during the deformation process, which caused some decline in the stress. Microstructure evolution under different deformation conditions indicated that the γ' to γn' phase transformation was also related to the degree of deformation and the four stage evolutionary process was identified. The SEM and EBSD results showed that the DRX appeared at the later stage of the deformation. The prediction capabilities of the three models were assessed by correlation coefficient (R), the absolute value of relative error (ARE), and the absolute average error (AARE). The result demonstrated that the RBF neural network had the best ability to predict the flow stress and the coefficient.

Dynamic compressive properties of dry and saturated coral rocks at high strain rates

Coral rock is characterized by a high porosity of 48.51 ± 5.43% and a low compressive strength of 9.91 MPa due to its biological origin. To experimentally investigate the uniaxial compressive behaviors of both air-dried and fully saturated coral rocks, the quasi-static and dynamic compression tests were performed employing an electro-hydraulic servo-controlled device and a split Hopkinson pressure bar (SHPB) apparatus. The combined effects of strain rate and water saturation on the mechanical properties of coral rock were assessed in terms of the uniaxial compressive strength, Young's modulus, brittleness, energy dissipation as well as the failure pattern. The dynamic uniaxial compressive strength and Young's modulus of saturated coral rocks is consistently lower than those under dry condition, while the losses in them reduces with an increase in stain rate. The coral rock behaves in a more brittle manner in dry state than in saturated state, which is validated by the digital image correlation (DIC) in conjunction with high-speed camera images. Likewise, the change in brittleness or specific energy absorption (SEA) with strain rate under dry and saturated conditions exhibits a similar trend to the dynamic compressive strength. Furthermore, the dynamic failure of dry coral rock appears to be more dependent on the inherent flaws and defects than the saturated one, as the failure pattern doesn't follow a fixed shearing or splitting fracture mode. Overall, the difference of dynamic behaviors between dry and saturated coral rocks are mainly controlled by the coupling weakening and strengthening effects induced by free water. The strengthening effect of water saturation is manifested by making the strength and deformability of coral rock more sensitive to the loading strain rate.

Influence of twinning-induced recrystallization on texture evolution in a high strain rate compressed Mg-Zn alloy

Recrystallization texture evolution in a high stain rate compressed Mg-1Zn alloy was investigated using electron backscattered diffraction (EBSD). Besides of the 101¯2 tension twins (T), 101¯1 compression twins (C) and 101¯1-101¯2 double twins (CT), the 53°101¯0 tension-tension double twins (TT) and deviated 75°112¯0 tension twin boundaries (T′) were observed in the as-deformed sample. When annealed, twinning-induced recrystallization occurred in C and CT twins at the very beginning, which was proved to strongly weaken the basal texture due to a strong orientation scattering by the recrystallized grains. In contrast, the recrystallization of TT twins weakened the basal texture insignificantly. In addition, the lenticular T twins tended to de-twin rather than recrystallize, which remained after 24 h annealing.

Metadynamic recrystallization behavior of β-solidified TiAl alloy during post-annealing after hot deformation

The metadynamic recrystallization (MDRX) of β-solidified TiAl alloy and its effects on microstructural homogeneity were investigated in the present work. Based on the comprehensive microstructure characterizations in terms of scanning electron microscope in back-scattered electron (BSE) mode and electron back scattered diffraction (EBSD) technique, it is found that the occurrence of MDRX during the post-annealing process after hot compression leads to the decomposition of lamellar colonies, grain equalization and reduction of geometrically necessary dislocation. The MDRX in present alloy during interpass annealing process has a hardening effect on the subsequent flow behavior, while it is restricted due to the phase transformation γ to α when annealing at 1200 °C. Moreover, the occurrence of MDRX can reduce the microstructure heterogeneity among various areas induced by the different strains and stain rates, thus, to result in the microstructural homogenization. Through investigating the MDRX behavior, the present work supports important information for optimizing forged microstructure of TiAl alloys.

Microstructure evolution of a Ti–Al–Sn–Zr based alloy during the hot compression deformation

The Ti–Al–Sn–Zr based alloy was fabricated and its deformation behavior was investigated with increasing of temperature and strain rate. Microstructure and phase of the as-fabricated, heat treated and deformed alloy was characterized by OM, XRD and TEM. The results exhibited that the addition of Mo, Nb and Ta promoted the precipitate of β phase at relative low heat treatment temperature. And the β phase has relative good phase interface with the αphase. During the compression test, the compressive strength of the alloy dropped with the strain rate decreasing and the testing temperature rising. When the compression deformation was preformed above α phase transformation temperature, the high strain rate would promote the phase transformation. When the alloy was compressed with strain rate of 1 s–1, it almost had the similar compressive strengths at 960 °C and 940 °C. Moreover, the same phenomenon appeared when the compression was carried out at 960 °C and 980 °C with stain rate of 0.01 s–1. Such a phenomenon should be ascribed to the existence of high amount of β phase which could contribute to the strength.

Superplastic Deformation and Dynamic Recrystallization of a Novel Disc Superalloy GH4151.

The superplastic deformation of a hot-extruded GH4151 billet was investigated by means of tensile tests with the strain rates of 10−4 s−1, 5 × 10−4 s−1 and 10−3 s−1 and at temperatures at 1060 °C, 1080 °C and 1100 °C. The superplastic deformation of the GH4151 alloy was reported here for the first time. The results reveal that the uniform fine-grained GH4151 alloy exhibited an excellent superplasticity and high strain rate sensitivity (exceeded 0.5) under all experimental conditions. It was found that the increase of strain rate resulted in an increased average activation energy for superplastic deformation. A maximum elongation of 760.4% was determined at a temperature of 1080 °C and strain rate of 10−3 s−1. The average activation energy under different conditions suggested that the superplastic deformation with 1 × 10−4 s−1 in this experiment is mainly deemed as the grain boundary sliding controlled by grain boundary diffusion. However, with a higher stain rate of 5 × 10−4 s−1 and 1 × 10−3 s−1, the superplastic deformation is considered to be grain boundary sliding controlled by lattice diffusion. Based on the systematically microstructural examination using optical microscope (OM), SEM, electron backscatter diffraction (EBSD) and TEM techniques, the failure and dynamic recrystallization (DRX) nucleation mechanisms were proposed. The dominant nucleation mechanism of dynamic recrystallization (DRX) is the bulging of original grain boundaries, which is the typical feature of discontinuous dynamic recrystallization (DDRX), and continuous dynamic recrystallization (CDRX) is merely an assistant mechanism of DRX. The main contributions of DRX on superplasticity elongation were derived from its grain refinement process.

Influence of dynamic recrystallization on microstructure and mechanical properties of welding zone in Al–Mg–Si aluminum profile during porthole die extrusion

Abstract The effect of dynamic recrystallization (DRX) on the microstructure and mechanical properties of 6063 aluminum alloy profile during porthole die extrusion was studied through experiment and simulation. The grain morphology was observed by means of electron backscatter diffraction (EBSD) technology. The results show that, at low ram speeds, increasing the ram speed caused an increase in DRX fraction due to the increase of temperature and strain rate. In contrast, at high ram speeds, further increasing ram speed had much less effect on the temperature, and the DRX faction decreased due to high stain rates. The microhardness and fraction of low angle boundaries in the welding zones were lower than those in the matrix zones. The grain size in the welding zone was smaller than that in the matrix zone due to lower DRX fraction. The decrease of grain size and increase of extrudate temperature were beneficial to the improvement of microhardness.

Scalar predictors of premixed gas ignition by a suddenly-starting hot jet

Hot-jet ignition is usually designed to reduce emissions in lean-burn combustion engines, and has potential in enabling novel pressure-gain combustion. Inspired by our experimental observations related to wave-rotor combustion chamber ignition, this work employs a numerical method to examine the sudden injection of a hot jet into a quiescent mixture of CH4–H2-air and the subsequent ignition. The goal is to provide the range of thermo-physical scalars that are supportive of successful ignition. The evolution of scalar fields is evaluated using large-eddy simulation (LES). The temporal evolution of mixture fraction, the squared gradient of mixture fraction (as indicative of scalar dissipation rate), strain rate, and intermediate species are investigated in order to find the appropriate physical conditions which support ignition. Independent distribution of stain rate and squared gradient of mixture fraction, especially in the leading head vortex, shows the necessity of correlated scalar analysis of the ignition process. Experimental and numerical methods are then employed to provide the qualitative and quantitative understanding of ignition process for fuels with two distinct hydrogen contents. Results show the meaningful difference in spatial distribution of local ignition as hydrogen content of the fuel increases.

Dynamic Stress-strain Compressive Response of Soft Tissue using Polymeric split- Hopkinson Pressure Bar

The characterization of soft tissues subjected to higher compressive strain rates is of increasing importance as the material properties of soft tissues were commonly used in impact applications such as automotive safety and crashworthiness for biofidible numerical modeling of the human body. Due to less availability of stain rate dependent data in the open literature and various challenges in the characterisation of soft tissues under impacts, need were felt for characterisation of soft tissues at higher strain rates. The purpose of this study is to investigate the dynamic mechanical behavior of soft tissues at varying compressive strain-rates. The viscoelastic split Hopkinson pressure bar (SHPB) is designed and developed to predict the high rate compressive behavior of soft tissues. The primary benefit in using viscoelastic bars is the reduced bar impedance allowing for high-quality measurements of the transmitted and reflected stress wave signals. The dynamic behavior of goat muscles has been measured at higher strain rates ~500-1200 s-1 using a polymeric SHPB apparatus. The dynamic stress-strain response of muscle tissue exhibits non-linear behavior under compressive loadings and is strain-rate dependent.

Investigation into cooling-rate dependent residual stresses in ZrB2–SiC composites using improved Raman spectroscopy method

Thermal residual stresses in spark-plasma sintered ZrB2–20 vol% SiC composites were investigated for different cooling rates employed after sintering. A new method using barycenter shift of Raman emission band was developed to determine the surface residual stress in the SiC phase, with improved accuracy compared to measurements using the traditional peak shift method. Effect of cooling rate on thermal residual stress was quantitatively identified and analyzed. The results showed that the Raman spectroscopy method based on the barycenter shift can reduce the measurement scatter caused by small grain sizes and thus has better accuracy for ZrB2–SiC composites with nano-sized SiC particles. It was found that a lower cooling rate reduced the average residual stress, which was attributed to the smaller thermal gradient, lower stain rate and more diffusional creep with reduction in cooling rate. In addition, decreasing grain size could also reduce the residual stress. This study provides an improved measurement method and a better understanding of the development and distribution of thermal residual stresses in ZrB2–SiC composites.

Numerical study on the low-velocity impact response of ultra-high-performance fiber reinforced concrete beams

The material and structural behaviors of Ultra High-Performance Fiber-Reinforced Concrete (UHP-FRC) under a high strain rate loading are different than a quasi-static loading condition. Though, UHP-FRC has very high compressive strength compared to conventional concrete, the failure strain of UHP-FRC is not enough to withstand large plastic deformations under high stain rate loading such as impact and blast loading. Due to the high cost of large-scale experimental research of structural elements under impact loading, modeling of UHP-FRC girders using computer aided program has become a need. Hence, this study aims to develop a finite element model of UHP-FRC to simulate low-velocity impact phenomenon. The responses obtained from the numerical study are in good agreement with the experimental results under impact loads. Five different types of UHP-FRC beams were simulated under impact loading to observe the global and local material response. The key parameters investigated were the reinforcement ratio (ρ), impact load under various drop heights (h), and the failure phenomena. It was observed that higher reinforcement ratio showed better deflection recovery under the proposed impact. Also, for a specific reinforcement ratio, the maximum deflection increases approximately 15% when drop height decreases from 100 mm to 25 mm. Moreover, the applicability of concrete damage plasticity model for impact loading is investigated. The results also provide recommendations for predicting the location of the local damage in UHPFRC beams under impact loading.