Low Strain Rate Deformation Research Articles

Multi-Laminate Rate-Dependent Modelling of Static and Dynamic Concrete Behavior through Damage Formulation

Mathematical simulation of the nonlinear tri-dimensional mechanical behavior of quasi-brittle materials like concrete is one of the biggest challenges in the engineering science. It is vital to have the knowledge of the response of concrete specimens subjected to low and high strain rate deformation for the analysis of concrete structures under the static and dynamic loading cases. The behavior of this material is generally known to be strain rate sensitive. Among phenomena of different orientation, the multi plane models, like multi-laminate models using a constitutive equation in a vectorial form rather than tensorial form by means of capturing interactions, can meet this goal adequately. This paper suggests a robust rate dependent damage based model in the multi-planes framework accomplished with minimum parameters for calibration and appropriate for engineering purposes. This damage formulation has been built on the basis of two types of essential damage, axial damage and shear damage, that basically can happen on each sampling plane and based on this concept two new axial and shear damage functions are proposed. Model verification has been studied under different compressive and tensile loading rates, comparing the results of the proposed model with the experimental data and Mohr-Coulomb failure criterion envelope line.

The Formation of Strong {100} Texture by Dynamic Strain-Induced Boundary Migration in Hot Compressed Ti-5Al-5Mo-5V-1Cr-1Fe Alloy

The microstructure and texture evolution of Ti-5Al-5Mo-5V-1Cr-Fe alloy during hot compression were investigated by the electron backscatter diffraction technique. The results reveal that two main texture components containing <100> and <111> fiber textures form after the hot compression. The fraction of each component is mainly controlled by deformation and strain rate. Dynamic strain-induced boundary migration (D-SIBM) is proved to be the reason that <100>-oriented grains grow towards <111>-oriented grains. The <100>-oriented grains coarsen with the increasing <100> texture intensity. Dynamic recrystallization (DRX) occurs under a low strain rate and large deformation. The DRX grains were detected by the method of grain orientation spread. The DRX grains reserve a <100> fiber texture similar to the deformation texture; however, DRX is not the main reason causing the formation of a strong <100> texture, due to its low volume fraction.

Evidence for exceptional low temperature ductility in polycrystalline magnesium processed by severe plastic deformation

An investigation was conducted to examine the mechanical behavior and microstructure evolution during deformation of ultrafine-grained pure magnesium at low temperatures within the temperature range of 296–373 K. Discs were processed by high-pressure torsion until saturation in grain refinement. Dynamic hardness testing revealed a gradual increase in strain rate sensitivity up to m ≈ 0.2. High ductility was observed in the ultrafine-grained magnesium including an exceptional elongation of ∼360% in tension at room temperature and stable deformation in micropillar compression. Grain coarsening and an increase in frequency of grain boundaries with misorientations in the range 15°–45° occurred during deformation in tension. The experimental evidence, when combined with an analysis of the deformation behavior, suggests that grain boundary sliding plays a key role in low strain rate deformation of pure magnesium when the grain sizes are at and below ∼5 μm.

Application of Digital Image Correlation for Comparison of Deformation Response in Fusion and Friction Stir Welds

The deformation response of welded aluminum plate was evaluated at high and low strain rates. Mechanical and ballistic experiments were conducted on 2.5 cm thick samples obtained from full penetration welds for welded aluminum 5083-H131 plate. Similar experiments were also conducted for the aluminum 5083 alloy as a baseline for comparison. Experiments were designed to compare the deformation response and ballistic performance differences for fusion welds versus friction stir welds. The fusion welds were processed using gas metal arc welding. The low strain rate deformation response was evaluated with three-point bend tests at an approximate strain rate of 1 s−1. The high strain rate response of the three materials was assessed using ballistic impact experiments at a range of velocities. Digital image correlation analysis was applied to gain insight into the deformation response through quantification of the strain and deflection profiles. The deformation response differences are compared for the welds versus the baseline aluminum alloy.

Hot Deformation Behaviors and Microstructure Evolution of SiCp/Al Composites

In order to explore the compressive properties of aluminium matrix composite reinforced with middle content SiC particles, hot compression behavior of 30%SiCp/2024A1 composite was investigated using Gleeble-1500 system at a temperatures range from 350 to 500°C and strain rates from 0.01 to 10 s−1. The associated structural changes were studied by OM, SEM and TEM observations. The results show that the true stress–true strain curves exhibited a peak stress at a small strain (<0.1), after which the flow stresses decreased monotonically until high strains, showing a dynamic flow softening. The stress level decreased with increasing deformation temperature and decreasing strain rate, indicating that the composite is a positive strain rate sensitive material. And therefore there will be a enough time for dynamic recrystallization to complete nucleation and growth at low strain rate and high deformation temperatures.

Fracture Behavior of AZ31 Magnesium Alloy During Low-Stress High-Temperature Deformation

Investigation of creep behavior of AZ31 magnesium alloy at three different temperatures (230, 270, and 350 °C) and stresses of 1–13 MPa reveals that grain boundary sliding (GBS) is the dominant creep mechanism at elevated temperatures and low stresses. GBS and Mg17Al12 precipitates in Mg–Al alloys result in stress concentration sites for cavity formation during high-temperature low-strain rate deformation leading to premature failures. Analysis of fractured surfaces of samples deformed at 350 °C reveals that brittle-type fracture (inter-granular and trans-granular) is the dominant mechanism at low stresses (σ = 1–5 MPa) while at higher stresses (σ = 7–13 MPa) dimple ruptures are predominant. Grain growth, dynamic recovery, and a decrease in dislocation density are characteristics of low-stress deformation of AZ31 alloys in GBS region whereas increase in dislocation density and dynamic recrystallization is noted during deformation under higher stresses where dislocation creep was noted to be predominant.

Microstructural Evolution in Fe-22Mn-0.4C Twinning-Induced Plasticity Steel During High Strain Rate Deformation

Microstructural evolution of an Fe-22 pct Mn-0.4 pct C twinning-induced plasticity (TWIP) steel during high strain rate deformation has been investigated. When subjected to low strain rate deformation, the steel shows the typical TWIP phenomenon. On the other hand, when subjected to high strain rate deformation, there is a formation of nanostructured austenite, due to the occurrence of deformation twinning forming nanoscale twin/matrix lamellae followed by dynamic recovery induced by adiabatic heating during high strain rate deformation.

Analysis of Strain Rate Sensitivity of Ultrafine-Grained AA1050 by Stress Relaxation Test

Commercially pure aluminum sheets, AA 1050, are processed by accumulative roll bonding (ARB) up to eight cycles to achieve ultrafine-grained (UFG) aluminum as primary material for mechanical testing. Optical microscopy and electron backscattering diffraction analysis are used for microstructural analysis of the processed sheets. Strain rate sensitivity (m-value) of the specimens is measured over a wide range of strain rates by stress relaxation test under plane strain compression. It is shown that the flow stress activation volume is reduced by decrease of the grain size. This reduction which follows a linear relation for UFG specimens, is thought to enhance the required effective (or thermal) component of flow stress. This results in increase of the m-value with the number of ARB cycles. Strain rate sensitivity is also obtained as a monotonic function of strain rate. The results show that this parameter increases monotonically by decrease of the strain rate, in particular for specimens processed by more ARB cycles. This increase is mainly linked to enhanced grain boundary sliding as a competing mechanism of deformation acting besides the common dislocation glide at low strain rate deformation of UFGed aluminum. Recovery of the internal (athermal) component of flow stress during the relaxation of these specimens seems also to cause further increase of the m-value by decrease of the strain rate.

A texture and microstructure analysis of high speed rolling of AZ31 using split Hopkinson pressure bar results

This study used very high strain rate uniaxial compression testing to analyze the microstructure and texture evolution during high speed rolling of as-cast AZ31B alloy. A split Hopkinson pressure bar equipped with induction radiation furnace was used to attain a strain rate of 1200 s -1 in the temperature range of 25-350 C and the result was compared with low strain rate (0.01 s -1 ) behavior. As well, high speed rolling at 500 m min -1 was employed to successfully roll AZ31 alloy in one pass with 71 % reduction at 200 C. During rolling, the mill was suddenly stopped and the sheet was withdrawn from rolling gap and the microstructure and texture evolution was observed. Grain boundary misorientation analysis shows that coincident site lattice boundaries related to contraction twins and secondary twins are more numerous in the samples deformed at high strain rate. With increasing strain for both rolling and compression at 200 C, the splitting of basal poles was observed, indicating the activation of more contraction twins and secondary twins compared to low strain rate deformation. Also, the recrystallized volume fraction increased significantly with strain rate, probably due to increasing the twin-induced recrystallization frac- tion. On annealing of the samples compressed at 200 C, secondary twins and their vicinity were observed to be the preferential sites for nucleation and it seems that rapid recrystallization on secondary twins contributes to the basal texture weakening. Therefore, an increasing number of such twins increase the texture weakening.

Recrystallization of 304SS during and after High Strain Rate Deformation

During the hot working of austenitic stainless steels the shape of the flow curve is strongly influenced by the strain rate. Low strain rate deformation results in flow curves typical of dynamic recrystallization (DRX) but as the strain rate increases the shape changes to a ‘flat-top’ curve. This has traditionally been thought to indicate no DRX is taking place and that dynamic recovery (DRV) is the only operating softening mechanism. Examining the work-hardening behaviour and corresponding deformation microstructures showed this is not the case for austenitic stainless steel, as clear evidence of dynamic recrystallization process can be seen. The post-deformation recrystallization kinetics can be modelled using a standard Avrami equation with an Avrami exponent, n, of 1.15. With an increasing value of the Zener-Hollomon parameter it was found that the kinetics of recrystallization become less strain rate sensitive until at the highest values (highest strain rates/lowest temperatures) the recrystallization kinetics become strain rate insensitive.

Low strain rate deformation behavior of a Cr–Mn austenitic steel at −80 °C

The deformation behavior of a Cr–Mn austenitic steel during interrupted low strain rate uniaxial tensile testing at −80°C has been studied using X-ray diffraction (XRD), electron backscatter diffraction and transmission electron microscopy. Continuous γ→ε→α′ martensite transformation was observed until failure. High dislocation densities were estimated in the austenite phase (∼1015m−2), and for the α′-martensite they were even an order of magnitude higher. Dislocation character analysis indicated that increasing deformation gradually changed the dislocation character in the austenite phase to edge type, whereas the dislocations in α′-martensite were predominantly screw type. XRD analyses also revealed significant densities of stacking faults and twins in austenite, which were also seen by transmission electron microscopy. At low strains, the deformation mode in austenite was found to be dislocation glide, with an increasing contribution from twinning, as evidenced by an increasing incidence of ∑3 boundaries at high strains. The deformation mode in α′-martensite was dominated by dislocation slip.

Transformation of a High Strength Steel during Deformation at α/γTemperature

The relatively higher alloy content of AHSS leads to complex transformation behavior after finishing rolling and the transformation is unable to complete just after coiling, which causes some extent of deformation due to the transformation dilation. In the present study, low strain rate deformation was performed at γ/α temperature zone after the sample was deformed to simulate the deformation and transformation interaction after coiling by using Gleeble 3800 thermo-mechanical simulator. All the flow stress curves display a sharp decrease after certain strain when deformation temperatures below 750°C. The interaction between deformation and transformation is used to explain the phenomena.

Distributed deformation across the Rio Grande Rift, Great Plains, and Colorado Plateau

We use continuous measurements of GPS sites from across the Rio Grande Rift, Great Plains, and Colorado Plateau to estimate present-day surface velocities and strain rates. Velocity gradients from five east-west profiles suggest an average of ∼1.2 nanostrains/yr east-west extensional strain rate across these three physiographic provinces. The extensional deformation is not concentrated in a narrow zone centered on the Rio Grande Rift but rather is distributed broadly from the western edge of the Colorado Plateau well into the western Great Plains. This unexpected pattern of broadly distributed deformation at the surface has important implications for our understanding of how low strain-rate deformation within continental interiors is accommodated.

The dynamic and quasi-static mechanical response of three aluminum armor alloys: 5059, 5083 and 7039

Abstract The mechanical response and microstructural evolution of aluminum alloys 5083, 5059 and 7039 was examined in compression and shear in both the quasi-static (0.001 s −1 ) and dynamic (≈2000 s −1 ) strain rate regimes. Electron Back Scattered Diffraction was utilized for detailed post-mortem analysis of the specimens following loading. The mechanical responses in shear were found to be strain-rate sensitive. At the slowest strain rates, all of the alloys had relatively large volumes of highly deformed material with 5083 and 5059 having the largest shear affected volumes. The dynamic strain rate test samples all formed highly compact shear localized volumes across the sheared zone with 7039 consistently displaying the narrowest shear regions. The morphology of these shear bands, along with the limited hardening during deformation, indicate a mechanism change at the higher strain rates. Higher resolution orientation image mapping has shown that between the three alloys there are varying degrees of crystallographic order within the shear bands. Transmission electron microscopy revealed various stages of dynamic recrystallization were present suggesting that while low strain rate deformation is controlled by dislocation multiplication and glide, high strain and strain-rate deformation is influenced in part due to mechanical recrystallization.

Influence of grain size on high temperature fracture in a Mg AZ31 alloy

Tensile experiments at 673 K and grain sizes from ∼8 to 17 μm revealed large ductility at a low strain rate and a reduced ductility at a high strain rate, corresponding to a change from a high to a low value for the strain rate sensitivity. High strain rate deformation led to fracture by flow localization, whereas low strain rate deformation involved fracture by cavity nucleation and growth. Analysis revealed that grain boundary migration can assist significantly in reducing the stress concentrations caused by grain boundary sliding, thereby retarding cavity nucleation. Calculations demonstrate that the interlinkage of voids parallel and perpendicular to the tensile axis occurs significantly, so that it is not always possible to use the cavity shapes to distinguish between diffusion and plasticity controlled growth. Cavitation damage evolves slowly in materials with a coarser grain size because of reduced nucleation related to a reduction in the strain rate sensitivity and associated grain boundary sliding.

Deformation twinning in Ti-6Al-4 V during low strain rate deformation to moderate strains at room temperature

This study examines the possibility of twinning to occur in Ti-6Al-4 V during moderate (4 - 14%) uniaxial compression at room temperature using a slow strain rate (10 −6s −1). To do this, the starting material was manufactured to have strong basal texture components in the transverse and normal directions but not in the rolling direction. After only 6% compressive deformation parallel to the rolling direction it was possible to indentify a significant texture change introducing a strong basal texture component in the rolling direction. TEM analysis provided evidence of twins within grains, but at too low a volume fraction to explain the texture changes. Detailed EBSD mapping was carried out and showed an increasing number of entire grains to have their c-axis aligned with the loading direction in the deformed material, suggestive of entire grain twinning. The root cause for sudden complete grain twinning can only be speculated about at the moment, but it is suggested that the stress required for twin nucleation is significantly higher than that required for their propagation.

A space–time-ensemble parallel nudged elastic band algorithm for molecular kinetics simulation

A scalable parallel algorithm has been designed to study long-time dynamics of many-atom systems based on the nudged elastic band method, which performs mutually constrained molecular dynamics simulations for a sequence of atomic configurations (or states) to obtain a minimum energy path between initial and final local minimum-energy states. A directionally heated nudged elastic band method is introduced to search for thermally activated events without the knowledge of final states, which is then applied to an ensemble of bands in a path ensemble method for long-time simulation in the framework of the transition state theory. The resulting molecular kinetics (MK) simulation method is parallelized with a space–time-ensemble parallel nudged elastic band (STEP-NEB) algorithm, which employs spatial decomposition within each state, while temporal parallelism across the states within each band and band-ensemble parallelism are implemented using a hierarchy of communicator constructs in the Message Passing Interface library. The STEP-NEB algorithm exhibits good scalability with respect to spatial, temporal and ensemble decompositions on massively parallel computers. The MK simulation method is used to study low strain-rate deformation of amorphous silica.

Interaction between the Nazca and South American plates and formation of the Altiplano–Puna plateau: Review of a flexural analysis along the Andean margin (15°–34°S)

The results of a two-dimensional flexural analysis applied to the Andean margin, which is based on the correlation between topography and Bouguer anomaly, are here reviewed in order to characterize rigidity variations across and along the forearc–arc transition of the Central Andes and to understand the role of the forearc in the formation of the Altiplano Plateau. The forearc has maximum rigidities between 15° and 23°S. Forearc rigidity decreases gradually southward and sharply toward the plateau. The main orogen (elevations higher than 3000 m) is very weak along the entire Central Andes. A semi-quantitative interpretation of these trends, based on the relationship between flexural rigidity and the thermo-mechanically- and compositionally-controlled strength of the lithosphere, allows the following conclusions to be made: (1) across-strike rigidity variations are dominated by the thermal structure derived from the subduction process; (2) the forearc constitutes a strong, cold and rigid geotectonic element; (3) southward weakening of the forearc is directly related to the decreasing thermal age of the subducted slab; (4) very low rigidities along the main orogen are caused by the existence of a thick, quartz-rich crust with a low strain rate-to-heat flow ratio; (5) the strength of the plateau lithosphere is localized in an upper-crustal layer whose base at ∼15 km could be correlated with a P-to-S seismic wave converter (TRAC1 of Yuan et al., 2000 [Yuan, X., Sobolev, S., Kind, R., Oncken, O. et al. 2000. Subduction and collision processes in the Central Andes constrained by converted seismic phases. Nature, V 408, 21/28 Diciembre, p. 958–961]); (6) the forearc–plateau rigidity boundary corresponds to a zone of changing thermal conditions, eastward-increasing crustal thickness and felsic component in the crust, and low strain-rate deformation, which correlates with a west-verging structural system at the surface. These conclusions suggest that the rigid forearc acts as a pseudo-indenter against the weak plateau and allows the accumulation of ductile crustal material that moves westward from the eastern foreland. This pseudo-indenter is geometrically represented by a crustal-scale triangular zone rooted at TRAC1. This model allows the integration of existing contradictory ideas on the dynamics of forearc–plateau interaction that are related to the relative importance of upper-crustal compressive structures and lower crustal accumulation below the forearc.

Effect of high strain rate deformation on microstructure of strip steels tested under dynamic tensile conditions

The correlation between low and high strain rate deformation characteristics and the microstructure of novel strip steel products employed by the automotive industry has been assessed. Research has been carried out to establish whether microstructural changes during high speed deformation could explain the relative attributes of mild steel, carbon manganese steel and novel dual phase steel grades of increasing yield and tensile strength, in relation to their crash behaviour. The results obtained indicate that the dual phase steel grades having similar microstructures, although with varying strength levels, displayed the same response to high strain rate loading conditions. They both exhibited a reduction in the level of hardness at high strain rate compared with low strain rate deformation. The strain rate softening effect observed with the dual phase steel was accompanied by an increase in plastic deformation of the ferrite areas around the martensitic constituent regions.

Effects of Dynamic Recrystallization on .GAMMA. Grain Refinement and Improvement of Micro Segregation of As Cast Austenite in 9% Ni Steel

Effects of dynamic recrystallization on γ grain refinement and improvement in micro segregation of elements such as Ni and Mn of as cast austenite in 9% Ni steel were investigated by two kinds of experimental methods. The first one was a hot compression test using the specimens prepared from the strand cast slab and the hot rolled plate of 9% Ni steel, and the other was a hot tensile straining test in the austenitic region after levitation melting and solidification. In the hot compression test, variations in onset strain and flow stress of steady state flow of dynamic recrystallization with hot deformation conditions were investigated. The onset strain was found to decrease below 0.25 at the temperatures above 1523 K and the strain rate below 1×10-2s-1. The activation energy value obtained from steady state flow stress was 421 kJ/mol. Dynamically recrystallized γ grain size in as cast austenite of this steel was controlled simply by Z value with no dependence on the initial γ grain size. This beneficial feature of dynamic recrystallization was confirmed by the experiment of tensile straining in austenite formed after levitation melting and solidification, where extremely coarse initial γ grain size of around 1.9 mm was markedly refined down to 140 μm by straining such a small strain as 0.40. Micro segregation ratio of Ni and Mn in the strand cast 9% Ni steel examined by EPMA analysis was 1.20 and 1.36, respectively. These values were found to decrease continuously with reduction in strain rate in hot deformation of austenite. That is, dynamic recrystallization in austenite taken place at lower strain rate deformation is much more effective for homogenization of segregated elements compared with high strain rate deformation.