Positive Strain Rate Sensitivity Research Articles

Microstructure evolution and dynamic recrystallization mechanism of tantalum-tungsten alloys under hot compression

Ta-W alloys exhibit outstanding comprehensive properties at high temperatures, allowing them to have great application prospects in the high-temperature field. High-temperature compression tests (1573~1873 K) were conducted to evaluate the high-temperature mechanical properties of Ta-12W alloy, and the strain-compensated Arrhenius-Type model and hot processing map were developed to predict the optimal processing window. The electron backscatter diffraction, Transmission electron microscope, and DEFORM-3D software were performed to investigate the microstructure evolution and distributions of strain. The results demonstrated that the flow stress of Ta-12W alloy presented apparent negative temperature sensitivity and positive strain rate sensitivity. Based on the thermal processing diagrams and EBSD analysis, the results show that the low deformation temperature and high strain rate introduce stress concentration and deformation instability in the Ta-12W alloy. The optimized processing temperature and strain rate are at 1800 ~1873 K/0.001 s-1~0.05 s-1, and the microstructure at 1873 K/0.001 s-1 is more uniform and exhibits a low residual stress level. As the temperature increases, dynamic recovery and dynamic recrystallization cause significant softening effects, and continuous dynamic recrystallization dominates the high-temperature deformation behavior. Moreover, multiple continuous dynamic recrystallizations occur simultaneously, including sub-grain nucleation inside large grains, sub-grain nucleation at grain boundaries, and grain fragmentation accompanied by grain rotation. This study revealed the mechanisms of microstructural evolution during hot deformation of Ta-12W alloy, providing important guidance for optimizing the thermomechanical processing parameters.

Microstructure evolution and mechanical response of hetero-induced SiCp addition in Al-6061 under high strain rate compressive loading

Al/SiC composites having a high-volume fraction of SiC are eye-catching materials in the automobile and aerospace industries. However, their behavior under stringent environments i.e., under high strain rate compression is much different than quasi-static compression. For such conditions, the morphology, interfacial bonding, and the fraction of the SiC play a decisive role in achieving the high performance of the Al/SiC composites. Here, in this study, we reported that a transitional effect of de-bonding and intergranular crack in SiC has occurred in Al–SiC composites having SiC particle size >0.7 μm. Below the 0.7 μm, the diffusion of the Al dislocations is characterized on the boundary of SiC along with the strong interfacial bonding. To prove it, we have fabricated an Al6061/30 vol% SiC composite through powder metallurgy route and subsequently subjected to compression in the range of strain rate 0.001–2100 s−1. In support of our observations, the microstructure was characterized by optical microscopy, scanning electron microscopy, X-ray diffraction, electron backscattered diffraction, and transmission electron microscopy analysis. The result also revealed a positive strain rate sensitivity up to a strain rate of 1800 s−1, while for 2100 s−1, the flow stress decreased, which is the cause of adiabatic temperature rise, dynamic recrystallization in Al, and massive cracks and deboning in coarse SiC particles. We also characterized that the SiC having particle size 1–4 μm showed stacking fault and dislocation induced in the SiC due to strong interfacial bonding. A quantitate analysis of dislocation was also established by using the Williamson-Hall technique which further confirmed the dislocation activity and dynamic recrystallization in 2100 s−1, characterized by EBSD and TEM. Thus, this study enables our understating that using the fine SiC particles is promising for achieving the high performance of the Al/SiC composites for use in stringent environments.

The Designs and Testing of Biodegradable Energy-Absorbing Inserts for Enhanced Crashworthiness in Sports Helmets.

This article addresses manufacturing structures made via injection molding from biodegradable materials. The mentioned structures can be successfully used as energy-absorbing liners of all kinds of sports helmets, replacing the previously used expanded polystyrene. This paper is focused on injection technological tests and tensile tests (in quasi-static and dynamic conditions) of several composites based on a PLA matrix with the addition of other biodegradable softening agents, such as PBAT and TPS (the blends were prepared via melt blending using a screw extruder with mass compositions of 50:50, 30:70, and 15:85). Tensile tests showed a positive strain rate sensitivity of the mixtures and a dependence of the increase in the ratio of the dynamic to static yield stress on the increase in the share of the plastic component in the mixture. Technological tests showed that increasing the amount of the plasticizing additive by 35% (from 50% to 85%) results in a decrease in the minimal thickness of the thin-walled element that can be successfully injection molded by about 32% in the case of PLA/PBAT blends (from 0.22 mm to 0.15 mm) and by about 26% in the case of PLA/TPS blends (from 0.23 mm to 0.17 mm). Next, the thin-walled elements (dimensions of 55 × 55 × 20 mm) were manufactured and evaluated using a spring-loaded drop hammer. The 60 J impact energy was tested in accordance with the EN 1078 standard. The dynamic crushing test included checking the influence of the materials' temperature (-20, 0, 20, and 40 °C) and the impact velocity. It was proven that the maximum deflection increases with increasing material temperature and an increase in the share of the plastic component in the mixture. The PLA15PBAT85 blend was selected as the most effective material in terms of its use as an energy-absorbing liner for sport helmets. Johnson-Cook and Cowper-Symonds material plasticizing models were constructed. Their use during dynamic FE simulation provided results that were in good agreement with those of the conducted experiment.

Laser powder bed fusion of Fex(CoCrMnNi)100-x medium-entropy ferrous alloys: Processability, microstructure and dynamic deformation mechanism

In this work, Fex(CoCrMnNi)100-x (x = 40, 60 and 80 at.%) medium-entropy ferrous alloys (MEFAs) were fabricated through laser powder bed fusion (LPBF) of mixed powders composed of FeCoCrNiMn and Fe powders, and the dynamic compressive properties and deformation mechanisms were studied with combination of experiment and molecular dynamics simulation. The results demonstrate that all MEFAs exhibit good processability.The Fe40 and Fe60 samples were predominantly composed of FCC phase with coarse grains, while the Fe80 mainly consisted of BCC phase with a smaller grain size. When subjected to quasi-static and dynamic compression at 3000/s, the yield strengths of Fe40, Fe60 and Fe80 increases by 131, 220 and 256 MPa, respectively, illustrating the positive strain rate sensitivity and influence of Fe on the mechanical properties of MEFAs. Further, both the single-crystal and the polycrystalline models suggested that the incompatibility between the BCC and FCC phases is responsible for the enhancement of strength. Therefore, it is more likely to produce deformation twins, dislocations and thus stress concentration around the BCC phase, which is consistent with the TEM observations.

Hot deformation behavior and hot working map of Mn–Cr–Ni–Co steel for ball mill liner forging process

To explore the thermoformability of Mn–Cr–Ni–Co steel for ball mill liner, isothermal compression tests were performed on Mn–Cr–Ni–Co steel using a Gleeble-3800 thermal compressive testing machine. Flow stress curves at 900 °C–1200 °C and 0.01s−1∼10s−1 were obtained. Results show Mn–Cr–Ni–Co steel has obvious positive strain rate sensitivity, with peak stress decreasing with increasing temperature. A constitutive equation for Mn–Cr–Ni–Co steel was established using the Arrhenius model based on strain compensation, and fitted values were compared to experimental values. A hot working map for Mn–Cr–Ni–Co steel was established and analyzed. Hot working process parameters are: deformation temperature of 980∼1200 °C, strain rate of 0.04–1.78s−1, providing a basis for selecting the hot forging process of the ball mill liner. Forging and forming experiments of the ball mill liner were carried out using the optimal parameters of Mn–Cr–Ni–Co steel from the hot working map. The ball mill liner has good forming quality with a relatively uniform martensitic structure inside. Tensile and impact tests were performed on the ball mill liner. Compared to the mechanical properties of traditional cast Mn13 ball mill liners, results show the tensile strength increased by 159.75%, elongation by 18.8%, and impact capacity by about 16.7%.

Mechanical and microstructural responses in molybdenum-rhenium alloys under hot compressions

To study temperature and rate-dependent behaviors of molybdenum-rhenium (Mo–Re) alloys, compressive tests at variable temperatures and strain rates were conducted. After the same machining and thermal annealing processes, fully recrystallization happens in Mo and Mo–Re alloys with low Re content (5 wt %), while recovery occurs in Mo–Re alloys with high Re content (14 wt% and 42 wt %). The strength increases as Re content rises at all temperatures (300 °C, 600 °C and 1200 °C), meaning sufficient strengthening effect of Re. The rate-dependent behaviors, however, show different trends between Mo and Mo–Re alloys. Positive strain rate sensitivity is detected in Mo and low Re Mo–Re alloys in all temperatures, which is consistent to other research. While, near zero strain rate sensitivity is found in high Re Mo–Re alloys at 600 °C. Such an unusual phenomenon may be contributed to high stored dislocation density during mechanical processes. The stored dislocations are activated at a proper temperature to accommodate plastic flow, which weakens the influence of strain rate on strength.

Quasi-static and dynamic response of AA-2219-T87 aluminium alloy

Quasi-static and high-strain rate uniaxial compression testing on AA-2219-T87 aluminium alloy were carried out under varying strain rates (ranging from 1×10−3 to 7×103 s−1) and temperatures. Quasi-static testing showcased no strain-rate sensitivity (SRS) at room temperature, while positive SRS was observed at elevated temperatures (180, 230, and 300°C), showcasing higher flow stress with an increase in strain rate. This temperature-dependent SRS is attributed to the typical activation of thermally activated processes, greater dynamic recovery, and reduced effectiveness of traditional strengthening mechanisms at higher temperatures. High-strain rate testing, performed using a Split Hopkinson pressure bar (SHPB), revealed no SRS across all tested temperatures (25, 180, and 230°C). At high-strain rates (>103), once the flow stress reaches a common flow stress, it undergoes either a steady state deformation or minor softening. This behaviour is explained by the interplay between strain hardening and thermal softening phenomena. Specifically, the formation of intense shear flow zones (including dynamic recrystallization) during impact loading at strain rates of 4×103 and 7×103 s−1 promoted thermal softening over strain hardening, resulting in flow stress levels similar to those observed at a strain rate of 1×103 s−1. The intense shear flow volume fraction increased with an increase in strain rate, which is linked to no SRS and higher strain at 4×103 s−1 and 7×103 s−1. The SHPB testing data was utilized to develop the constitutive Johnson-Cook model, enabling the determination of the plastic behaviour of the AA-2219-T87 alloy at high-strain rates and temperatures. These findings enhance our understanding of the alloy's mechanical response and provide valuable insights for optimizing its performance in high-strain-rate applications.

Modeling and simulation of dynamic compression of Bulk Metallic Glasses at room and elevated temperatures using split Hopkinson pressure bar setup

This paper presents a modeling and simulation study of dynamic compression behavior of Bulk Metallic Glasses (BMGs) at room and elevated temperatures using the split Hopkinson pressure bar (SHPB) setup. The primary objective of this study is to develop and validate a constitutive model and simulation methodology capable of predicting the high strain rate response of BMGs at different temperatures. We propose a constitutive model for BMGs that accounts for the effects of high strain rates and elevated temperatures. We numerically implemented this model in ABAQUS/Explicit Finite Element Analysis software by writing a Vectorized User Material (VUMAT) subroutine. The methodology for modeling and simulation of dynamic compression of BMG specimens using the SHPB setup is developed. The present simulations are able to correctly predict the rate-independent response of Zr41.2Ti13.8Cu12.5Ni10Be22.5 (Vitreloy-1) BMG under dynamic compression at room and elevated temperatures. Furthermore, the present simulations are also able to correctly predict the negative strain rate sensitivity (SRS) of Zr52.5Cu17.9Ni14.6Al10Ti5 (Vitreloy-105) BMG at room temperature. Finally, the present simulations correctly predict that the failure stress of Zr64.13Cu15.75Ni10.12Al10 BMG decreases with increasing temperature and exhibits a minimal positive SRS. The present study is the first successful attempt to model the mechanical response of various BMGs under dynamic compression and at room and elevated temperatures. In particular, the experimentally observed negative SRS in some BMGs has been successfully simulated. The present work has important implications for the design of next generation spacecraft shields that are based on BMGs for mitigating the effects of hypervelocity impacts from debris in space.

Mechanical properties and rate-sensitive deformation of AA6063 aluminum alloys at 298 K, 78 K, and 4 K

The mechanical properties and rate-sensitive deformation of T4 and T6 AA6063 aluminum alloys have been analyzed at 298 K, 78 K, and 4 K. The strength, ductility, and work hardening capacity of two alloys increase with decreasing deformation temperature. Plastic deformation of AA6063-T4 is accompanied by adiabatic flow stress instabilities at 4 K and the Portevin-Le Chatelier (PLC) effect at 298 K. The alloy shows a better synergy of flow stress and work hardening than the T6 condition, determined by the underlying microstructure. The limited ability of dislocation storage in T6 alloy arises from shearing β″ particles and the enhanced dislocation annihilation occurring in pure Al matrix. The failure of the alloys is promoted by void nucleation at intermetallic particles before the Considére criterion is fulfilled. AA6063-T6 exhibits positive strain rate sensitivity (SRS) at three temperatures. Dislocations-β″ particle interactions determine its SRS and are a rate-controlling process at 298 K. AA6063-T4 shows negative strain rate sensitivity (nSRS) at 298 K controlled by dislocation-solute interactions. Data for SRS and thermodynamic deformation parameters characterizing different mechanisms in T4 and T6 alloys between 4 K and 298 K are discussed. The results indicate that plastic deformation produces vacancies by jogs' dragging at 298 K, but this process does not occur at 78 K and 4 K.

Plasticity of solid solution Al-Mn and Al-Mn-Si alloys

The influence of Mn and Si on the mechanical properties of solid solution Al-Mn and Al-Mn-Si alloys have been studied at 4 K, 78 K, and 298 K. The solute strengthening potency of the yield strength increases with decreasing temperature. The addition of Si stimulates the precipitation of Mn-containing dispersoids and decreases the solubility of Mn in the Al matrix, reducing the strengthening effect. The alloys exhibit Stage III of declining work hardening rate with the flow stress during the plastic flow at three temperatures. The mean slip length Λ in the work hardening model of Nabarro, Basinski, and Holt (NBH) evaluated from σΘ curves correlates with the scale of the microstructure. The strain rate sensitivity (SRS) measurements conducted at 78 K show positive SRS of Al and the alloys. At 298 K, one observes a change in SRS due to the change of the mechanism from solute-dislocation to dislocation-dislocation dominated interactions. Mn and Si decrease SRS at 298 K. Al-Mn-Si alloys exhibit negative SRS and jerk flow at a sufficiently low strain rate at 298 K. Analysis of the apparent activation volume, distance, and work has been conducted at 78 K and 298 K to understand the rate-controlling mechanisms. The results indicate that dislocation intersections are rate-limiting processes, but the atomistic mechanism of dislocation cutting varies vastly depending on the temperature and alloy composition. It is suggested that at 78 K, forest obstacles are passed by the formation of jogs and sequential activation of individual partials without making constriction to the perfect defect. At 298 K, the cutting involves constriction, the creation of jogs, and the creation of point defects at each intersection site. The calculated activation work and distance provide signatures of these processes.

Strain rate and anisotropic effects on incipient plastic deformation of Zn–Cu–Ti alloy sheets

Zinc alloys shows strong strain rate dependent and anisotropic mechanical behaviours, the mechanistic origin of which is however not fully understood. In this work, a combination of the experiments and crystal plasticity finite element simulations were used to gain an insight into the relation between deformation mechanisms, tensile deformation behaviour and texture evolution, especially for the incipient plastic deformation stage. To this end, specimens were cut from Zn–Cu–Ti alloy sheets at various orientations with respect to the rolling direction, and uniaxial tensile tests were conducted at different strain rates ranging from 10−3 to 5 × 10−2 s−1. The microstructure evolution with the deformation was examined by the electron backscatter diffraction technique. Crystal plasticity model was calibrated considering the effect of strain rate on the flow stress, and finite element simulations were carried out based on a representative volume element of the polycrystal zinc alloy. It is found out that the flow stress increases significantly, as the loading direction approaches the transverse direction, which results in a lower relative activity of basal slip and higher relative activity of non-basal ones. This, in turn, reduces the rotation of the c-axis of the crystallites towards ND during tensile tests. On the other hand, a positive strain rate sensitivity is found in the zinc alloy. Meanwhile, increasing strain rate leads to an obvious decrease in the activity of the pyramidal π-II slip systems, accompanied by more activated twinning systems. The lower activity of the pyramidal π-II slip at a higher strain rate probably hinders the development of CDRX and grain fragmentation at large strain levels.

Fracture behavior of a rapidly solidified thin-strip continuous cast AA5182 Al-Mg alloy with the Portevin-Le Chatelier effect under varying strain rates

This study investigates the strain-rate triggered PLC effect and fracture behavior of a rapidly solidified thin strip (TS) cast AA5182 alloy in comparison with those of direct chill (DC) counterparts. Higher cooling rates obtained via TS casting suppress the formation of phases in Al-Mg alloys and significantly enhance the matrix solute supersaturation beyond the equilibrium level, thereby impacting the interaction dynamics of mobile dislocations and solute atoms. This in turn affects the evolution of typical stress fluctuations in Al-Mg alloys, referred to as the Portevin-Le Chatelier (PLC) effect, occurring in specific strain rate-temperature domains, i.e., with type and characteristics dependent upon the microstructure. Tensile testing demonstrated a strain-rate dependent PLC effect, also aligning with the ductility trend in both negative and positive strain-rate sensitivity domains. The characteristics of PLC bands, as well as the magnitude of stress fluctuations, were shown to evolve with strain rate, facilitating distinct fracture mechanisms. In particular, the TS sample exhibited a higher strain rate sensitivity of the fracture mechanisms and PLC band characteristics and, thus, a narrower range of deformation rates for optimum ductility (attributed to increased solute in the matrix). Controlling the deformation rate to favor predominantly Type-B serrations and ensuring their stability with minimal fluctuations was shown to postpone necking and prevent material failure. The significance of rate sensitivity in alloys with reduced microsegregation is emphasized, suggesting the need for further exploration of mechanical properties.

An insight into mechanical response and twinning behavior of bimodal textured AZ31 magnesium alloy under quasi-static and high strain rate compression

In this work, the dynamic, quasi-static response and twinning behavior of bimodal textured Mg–3Al–1Zn alloy is comparatively investigated at room temperature. The relationship between yield stress and bimodal texture is quantified and characterized as an increase of the yield stress with the increasing of the percentage of <0002>⊥ED (extrusion direction) component; the ultimate strength under high strain rate (103 s−1) was quite higher than that under quasi-static condition (10−3 s−1); but, the yield stress was just the reverse. The competition between twin nucleation and growth is significantly affected by a coupled effect of strain rate and texture. Twin nucleation shows positive strain rate sensitivity and is largely enhanced by dynamic deformation. However, the twin growth tendency is bimodal texture and strain rate dependent: it is favorable under quasi-static condition for samples with more soft orientations, while for samples with more hard orientations, dynamic deformation promotes twin growth. It is also found that a higher strain rate can stimulate the activation of more twin variant types in bimodal textured magnesium alloys.

An integrated computational and experimental study of the failure mode in Mg/Al laminated composite under three-point bending

The deformation behavior of hot-roll-bonded Mg/Al laminated metal composites (LMCs) is investigated, focusing on the crack formation around the interface. The material demonstrates a positive strain rate sensitivity mainly arising from the Mg matrix. A diffusion layer was formed at the Mg/Al interface with a serrated morphology during the two-pass rolling process, which consists of the Al3Mg2 and Mg17Al12 phases. The mechanical responses of the LMCs are evaluated using uniaxial tension, interfacial debonding tests (in normal and shear modes), and microstructure characterization techniques, which clarify the interfacial heterogeneity. The fracture constants of the interface were calibrated and incorporated in the finite element models to predict the failure modes of heterogeneous Mg/Al interface under three-point bending. With the increased thickness ratio of the Mg layer, the damage accumulation rate decreases at the interface and increases in the Mg matrix. As a result, the failure mode transferred from interface failure to matrix fracture. The findings of the present work provide new insight into the failure mechanism in laminated metal composites under bending conditions and can provide theoretical guidance for the plastic forming of such materials.

High strain rate constitutive and fracture characterization of AA7075-T6 sheet under various stress states

The stress state and strain rate dependent fracture behavior of high strength aluminum sheet, AA7075-T6, is characterized experimentally and numerically. Experiments were performed using shear, hole tension, notch tension, and groove tension specimen geometries at strain rates in the regime 10−2 – 500 s−1. The temperature change on the specimen surface was measured utilizing high speed thermal imaging, while strains were acquired using DIC-based optical measurements. Mild positive rate sensitivity was observed in the flow response for equivalent strains up to approximately 20%, while the flow stress decreased at higher (500 s−1) strain rates as the strain levels increased. An increase in temperature was observed within the gauge region of the specimens during elevated strain rate (500 s−1) tests leading to thermal softening. Positive strain rate sensitivity was observed in the measured axial force response for the hole tension and notch tension specimens. The fracture limits of the AA7075-T6 in the shear, hole tension, and notch specimens decreased between strain rates of 0.01 and 10 s−1, then increased at strain rates between 10 and 500 s−1. A strain rate and temperature dependent Hockett–Sherby (HS-SRT) constitutive model was proposed to capture the dependency of the flow stress on the equivalent plastic strain, strain rate, and temperature. The generalized Drucker-Prager (GDP) fracture model was extended to capture the effect of strain rate on fracture initiation. Numerical simulations of the coupon experiments were then performed to extract the local stress and strain states to calibrate the GDP fracture model across the range of strains considered.

Dynamic Tensile Behavior of Laser-Directed Energy Deposition and Additive Friction Stir-Deposited AerMet 100

Quasi-static and high-rate tensile experiments were used to examine the strain rate sensitivity of laser-directed energy deposition (L-DED)- and additive friction stir deposition (AFSD)-formed AerMet 100 ultrahigh-strength steel-additive manufactured builds. Electron backscattered diffraction (EBSD) revealed similar as-deposited grain sizes between the two AM processes at approximately 24 μm and 17 μm for the L-DED and AFSD samples, respectively. The strain hardening rate, θ, revealed little change in the overall hardening observed in the L-DED and AFSD materials, with a consistent hardening in the quasi-static samples and three identifiable regions in that of the high-rate tested materials. The L-DED deposited materials displayed average ultimate tensile strength values of 1835 and 2902 MPa for the 0.001 s−1 and 2500 s−1 strain rates, respectively and the AFSD deposited materials displayed ultimate tensile strength values of 1928 and 3080 MPa for the 0.001 s−1 and 2500 s−1 strain rates, respectively. Overall, the strength for both processes displayed a positive strain rate sensitivity, with increases in strength of ~1000 MPa for both processes. Fractography revealed significant solidification voids in the laser DED material and poor layer adhesion in the AFSD material.

Investigation of dynamic tensile mechanical responses and deformation mechanism at high strain rates in a TWIP steel

In view of the potential demand for the materials with excellent dynamic mechanical behavior in protective construction, the dynamic tensile mechanical properities and deformation mechanism of twinning-induced plasticity (TWIP) steel at various high strain rates were investigated in this paper by combining test research and numerical simulation. In order to explore the impact of strain rates on mechanical behaviors within the tested range, a split Hopkinson tensile bar (SHTB) was utilized to measure the mechanical responses of TWIP steel ranging from 1000 s−1 to 6500 s−1. The Johnson–Cook (J-C) dynamic constitutive model was established based on the mechanical test results, and the mechanical properties were studied by using ANSYS/LS-DYNA. EBSD and TEM were employed to investigate the microstructure evolution of TWIP steel under dynamic loading. The results demonstrate that both strength and plasticity increase with increasing strain rate, demonstrating positive strain rate sensitivity (PSRS) which is primarily due to the activation of multiple twin systems at higher strain rates. As the strain rate increases, the mean free path of dislocation slip reduces, causing a higher strain hardening rate. The critical shear stress required for twinning is almost unchanged with an increase in strain rate, while the shear stress required for dislocation increases consistently. As the strain rate exceeds 3000 s−1, the critical shear stress required for dislocation becomes greater than that for twinning. With increasing strain rate, the proportion of secondary twins in TWIP steel rises and the deformation twins are bent when the strain rate is 6500 s−1 owing to the effect of dislocations and stacking faults. The production of nanoscale twins at higher strain rates is a beneficial phenomenon, as it enhances both strength and plasticity. The deformation twins become thinner with an increase in strain rate, leading to a rise in the stress needed for the motion of dislocations to pass the twins, which contributes to the improvement of strength in TWIP steel. The slip systems that deforms at a disadvange have new orientations because of defromation twins. The established J-C model is appropriate well for analyzing the mechanical properties of TWIP steel in the tested range of strain rates accoring to the fingdings of test and numerical simulation. Establishing the connection of microstructure and macroscopic behavior of TWIP steel could provide a guidance for the its further study and the application in the fields of impact resistance and anti-explosion.

Evaluation of modelling parameters for AA2014-T6 in simulating dynamic fracture event

The material behavior of the aluminum alloy AA2014-T6 under different strain rates and temperatures must be examined in order to simulate their responses under extreme loading conditions like dynamic fracture and impact. In the present study, several uniaxial tensile experiments, including quasi-static, dynamic, and triaxiality, were carried out at different temperatures to evaluate the material constants for the Johnson cook constitutive and failure models of AA2014-T6. It was observed that material flow stress exhibited positive strain rate sensitivity at high strain rates and negative sensitivity when temperature increased from 25 to 250 °C. From the triaxiality experiments, it was also observed that the plastic flow increased while the ductility of the material decreased with increasing stress triaxiality. The dynamic fracture behavior of a pre-cracked 3-point bend specimen of AA2014-T6 was studied at different loading rates. Applied load and load point displacement (LPD) were measured using strain gauges. In addition, crack mouth opening displacement (CMOD) and crack initiation time were calculated using the 3D digital image correlation technique. It was found that the loading rates highly influence the dynamic fracture initiation and propagation toughness of the material. A 3D finite element simulation of dynamic 3-point bend experiments was performed in ABAQUS using the J-C damage evaluation criteria, and incident stress obtained from the dynamic experiments was used as an input load to minimize the error. In accordance, with the actual experiments conducted, the material geometry, boundary conditions, and contact circumstances were considered. The authors found good agreement between both the experimental and numerical results.

Dynamic constitutive behavior of LPBFed metal alloys

Laser powder bed fused (LPBFed) metal alloys, such as titanium, aluminium, and steel exhibit high specific strength and hold great potential for utilization in the automotive and aerospace sectors. Since automotive and aerospace structures require the materials to withstand dynamic impacts, accurately predicting the flow behavior of LPBF-based alloys under high strain rates becomes critical. In this paper, the dynamic behavior of LPBFed CPTi, AlSi10Mg, and 316L stainless steel under strain rates of approximately 500–3000 s−1 was investigated using a Split Hopkinson Pressure Bar (SHPB). The study compared the stress-strain response and strain rate sensitivity of LPBFed alloys to those of conventionally produced counterparts. Furthermore, Johnson-Cook constitutive model was adopted to predict the flow behavior. The results showed that all alloys exhibit positive strain rate sensitivity under the tested strain rate range, with LPBFed CPTi demonstrating higher flow stress than conventionally built counterparts due to the fine grain size resulting from the additive manufacturing method. Additionally, differences in the performance of AlSi10Mg observed in this study, as compared to the literature, are attributed to differences in building direction and post-processing. Finally, it was demonstrated that the Johnson-Cook model accurately predicts flow behavior for the selected alloys with relatively low error.

Hot Deformation Behavior and Processing Maps of Pure Copper during Isothermal Compression.

In this study, pure copper's hot deformation behavior was studied through isothermal compression tests at deformation temperatures of 350~750 °C with strain rates of 0.01~5 s-1 on a Gleeble-3500 isothermal simulator. Metallographic observation and microhardness measurement were carried out of the hot compressed specimens. By analyzing the true stress-strain curves of pure copper under various deformation conditions during the hot deformation process, the constitutive equation was established based on the strain-compensated Arrhenius model. On the basis of the dynamic material model proposed by Prasad, the hot-processing maps were acquired under different strains. Meanwhile, the effect of deformation temperature and strain rate on the microstructure characteristics was studied by observing the hot-compressed microstructure. The results demonstrate that the flow stress of pure copper has positive strain rate sensitivity and negative temperature correlation. The average hardness value of pure copper has no obvious change trend with the strain rate. The flow stress can be predicted with excellent accuracy via the Arrhenius model based on strain compensation. The suitable deforming process parameters for pure copper were determined to be at a deformation temperature range of 700~750 °C and strain rate range of 0.1~1 s-1.