High Strain Rate Regime Research Articles

High-velocity deformation of Al0.3CoCrFeNi high-entropy alloy: Remarkable resistance to shear failure

The mechanical behavior of a single phase (fcc) Al0.3CoCrFeNi high-entropy alloy (HEA) was studied in the low and high strain-rate regimes. The combination of multiple strengthening mechanisms such as solid solution hardening, forest dislocation hardening, as well as mechanical twinning leads to a high work hardening rate, which is significantly larger than that for Al and is retained in the dynamic regime. The resistance to shear localization was studied by dynamically-loading hat-shaped specimens to induce forced shear localization. However, no adiabatic shear band could be observed. It is therefore proposed that the excellent strain hardening ability gives rise to remarkable resistance to shear localization, which makes this material an excellent candidate for penetration protection applications such as armors.

Effects of strain rate on the microstructure evolution and mechanical response of magnesium alloy AZ31

The effects of strain rate on the mechanical behaviour and microstructure evolution of extruded Mg AZ31 alloy are investigated here for strain rates ranging from 0.0001s−1 to 4000s−1. High strain rate compression testing was performed using a split Hopkinson pressure bar apparatus. The microstructure and texture evolution were characterised by using electron backscatter diffraction (EBSD) and neutron diffraction. The results show that the yield strength increases by roughly 55% from the quasi-static to the high strain rate regime while the plateau stress increases by roughly 35%. After compression along the extrusion direction, the deformed textures of both quasi-static and high strain rate loading samples are similar, showing that a large portion of grains have been reoriented through the activation of tensile twinning. However, a detail EBSD analysis suggests that there is a higher fraction of secondary compressive twins in high strain rate deformed specimens at 16% strain. A visco-plastic self-consistent (VPSC) model, which incorporates the composite grain scheme to account for twinning, is employed to model the mechanical response and texture evolution, and to elucidate the collaborative nature of the slip-twinning deformation. Using only a single set of parameters, the VPSC model successfully reproduces the stress-strain curves at all tested strain rates, and more importantly, captures the texture and twin volume fraction evolutions. The simulation results suggest that under quasi-static and high strain rate loadings, the dominant slip systems are different in both untwinned grains and tensile twins. The results also suggest that twins are formed at a lower strain at high strain rates when compared to quasi-static loading.

Strain rate sensitivity and fracture mechanism of AlSi10Mg parts produced by Selective Laser Melting

Implementation of Additive Manufacturing (AM) parts in the growing applications within the automotive and aerospace industries encourages further investigations of the material behavior under various strain rates, spanning from quasi-static to the high strain rate regimes. Although mechanical properties of AM-Selective Laser Melting (SLM) AlSi10Mg parts under a static regime have been investigated, the strain rate sensitivity of these materials, to the best of our knowledge, has not been discussed in the literature. In this work, the properties of AM-SLM AlSi10Mg material were systematically investigated under a wide range of strain rates, spanning from 2.77×10−6 to 2.77×10−1 S−1. The AM-SLM AlSi10Mg alloy, as opposed to Al alloys fabricated by conventional methods, was found to be strain rate sensitive with significant changes to the flow stress and strain hardening exponents with an increase in strain rate. The fracture mechanisms of these specimens, built in different orientations, are discussed.

Impact erosion prediction using the finite volume particle method with improved constitutive models

Erosion damage in hydraulic turbines is a common problem caused by the high- velocity impact of small particles entrained in the fluid. In this investigation, the Finite Volume Particle Method is used to simulate the three-dimensional impact of rigid spherical particles on a metallic surface. Three different constitutive models are compared: the linear strainhardening (L-H), Cowper-Symonds (C-S) and Johnson-Cook (J-C) models. They are assessed in terms of the predicted erosion rate and its dependence on impact angle and velocity, as compared to experimental data. It has been shown that a model accounting for strain rate is necessary, since the response of the material is significantly tougher at the very high strain rate regime caused by impacts. High sensitivity to the friction coefficient, which models the cutting wear mechanism, has been noticed. The J-C damage model also shows a high sensitivity to the parameter related to triaxiality, whose calibration appears to be scale-dependent, not exclusively material-determined. After calibration, the J-C model is capable of capturing the material's erosion response to both impact velocity and angle, whereas both C-S and L-H fail.

Modeling of the mutual effect of dynamic precipitation and dislocation density in age hardenable aluminum alloys

A model has been proposed to capture the complex strain rate effect on dynamic precipitation of GP zones in an age-hardenable aluminum alloy. The contributions of vacancies and dislocations to dynamically formed GP zones have been specified in the model. It has been demonstrated that the proposed model is capable for predicting the contribution of each dynamic precipitation mechanisms, accurately, which are acting during deformation. Furthermore, the vacancy and dislocation evolutions during deformation have been considered in this modeling. The effect of strain rate by considering different mechanisms of dynamic precipitation of GP zones has been studied and confirmed by experimental data reported in the work of other researchers. Both experimental results and model outputs demonstrate that at high strain rate regime, strain rate dependency of dynamic precipitation is increased and in comparison with vacancy-assisted nucleation, dislocation-assisted nucleation of GP zones is encouraged. In this study, the mutual effects of dislocation density and dynamic precipitation have been considered. Outputs of model demonstrate that there is a weak negative strain rate sensitivity in the age-hardenable supersaturated aluminum alloy.

Numerical simulation of systems of shear bands in ductile metal with inclusions

We develop a method for numerical simulations of high strain-rate loading of mesoscale samples of ductile metal with inclusions. Because of its small-scale inhomogeneity, the composite material is prone to localized shear deformation (adiabatic shear bands). This method employs the Generalized Method of Cells of Paley and Aboudi [Mech. Materials, vol. 14, pp. 127–139, 1992] to ensure that the micro mechanical behavior of the metal and inclusions is reflected properly in the behavior of the composite at the mesoscale. To find the effective plastic strain rate when shear bands are present, we extend and apply the analytic and numerical analysis of shear bands of Glimm, Plohr, and Sharp [Mech. Materials, vol. 24, pp. 31–41, 1996]. Our tests of the method focus on the stress/strain response in uniaxial-strain flow, both compressive and tensile, of depleted uranium metal containing silicon carbide inclusions. We use the Preston-Tonks-Wallace viscoplasticity model [J. Appl. Phys., vol. 93, pp. 211–220, 2003], which applies to the high strain-rate regime of an isotropic viscoplastic solid. In results, we verify the elevated temperature and thermal softening at shear bands in our simulations of pure DU and DU/SiC composites. We also note that in composites, due the asymmetry caused by the inclusions, shear band form at different times in different subcells. In particular, in the subcells near inclusions, shear band form much earlier than they do in pure DU.

Flow softening of 253 MA austenitic stainless steel during hot compression at higher strain rates

The flow softening of 253 MA austenitic stainless steel was investigated by means of hot compression tests performed over the temperature range of 900–1150°C. Constant true strain rates of 0.01s−1, 0.1s−1, 1s−1, 10s−1 and 20s−1 were used. The effect of deformation temperature and strain rate on the flow softening and the flow softening mechanisms in the high strain-rate regime (1–20s−1) was examined. The flow curves at higher strain rates, i.e. 10s−1 and 20s−1, were characterized by a single peak followed by continuous stress softening at higher strain levels. The degree of flow softening increased with increasing strain rate and increasing temperature. The flow softening was attributed to thermal softening due to deformation heating, microstructural softening by dynamic recrystallization (DRX) and flow instability. Deformation heating produced by plastic work resulted in significant temperature rise in the steel. The thermal softening due to deformation heating was more pronounced at higher strain rates and lower temperatures. DRX was found to take place at higher deformation temperatures and higher strain rates; this was confirmed by the double-differentiation method proposed by Poliak and Jonas. Flow instability in the form of localized deformation bands occurred at the temperatures of 900°C and 950°C. The processing map developed as per Murty–Rao's instability criterion was found to yield an overestimate of the unstable flow region. Based on the flow behavior and microstructure analysis, it is concluded that this steel is suitable to be hot worked at higher strain rates and higher temperatures in order to reduce the flow stress and to refine grain size.

Rate-limited plastic deformation in nanocrystalline Ni

Numerical simulations at nanometer scales have identified several mechanisms of plastic deformation. However, high strain rate regimes are required to resolve nanometer length scales. Extrapolating these numerical predictions at high strain rates to experimental conditions remains an unresolved challenge. Phase-field dislocation dynamics (PFDD) simulations are conducted to study the strain rate sensitivity of plastic deformation in nanocrystalline metals. The PFDD simulations involve the collective behavior of partial and extended full dislocations at strain rates ranging from 1 × 106 s−1 to 1 × 109 s−1 in Ni samples with an average grain size of 15 nm. Significant differences are found in the activation and glide of dislocations over this range of strain rates. At high strain rates, there are a large number of partial dislocations that begin to glide at grain boundaries across the entire sample. On the other hand, glide events are limited to a few grain boundaries at lower strain rates. Even though, the number of events is larger at high strain rates, mainly leading partial dislocations are active, and therefore, the amount of plastic deformation is smaller. This leads to an effective delay in plastic strain at high strain rates that explain the stress upturn observed at high strain rates when plastic deformation is carried out by partial dislocations. When extended full dislocations are present at lower strain rates, the yield stress is reduced by around 40%.

High Strain Rate Behavior of Graphene Reinforced Polyurethane Composites

A compressive split Hopkinson pressure bar (SHPB) was used to investigate the dynamic mechanical behavior of graphene (GR) reinforced polyurethane (PU) composites (GR/PU) at high strain rates ranging from approximately 1500 s−1 to 5000 s−1. Four types of GR/PU composites with different GR contents: 0.25% GR, 0.5% GR, 0.75% GR, and 1% GR were prepared by the solution mixing method and divided into two groups of unheated and postheated specimens. Experimental results show that the GR/PU composite is a strong strain rate dependent material, especially in the high strain rate regime of 3000 s−1–5000 s−1. The dynamic mechanical properties of GR/PU composite in terms of plateau stress, peak stress, and peak load carrying capacity are better than that of pristine PU at most of the applied strain rates. Among the four different GR concentrations used, the 0.5 wt.%-GR specimen shows the highest peak stress, and the 1 wt.% GR specimen has the highest plateau stress; while no significant change in peak strain with changing GR weight fraction was observed. Compared to unheated specimens, the plateau stress, peak stress, and peak strain of postheated specimens are significantly higher.

Dynamic crushing and energy absorption of sandwich structures with combined geometry shell cores

Dynamic crushing and energy absorption characteristics of sandwich structures with combined geometry shell cores were investigated experimentally and numerically. The effect of strain rate on the crushing behavior was presented by the crushing tests at quasi-static, intermediate and high strain rate regimes. It was shown that absorbed energy increased with increasing impact velocity. The effect of confinement on crushing behavior was shown by conducting confined experiments at quasi-static and dynamic rates. Higher buckling loads at lower deformation were observed in confined quasi-static crushing due to additional lateral support and friction provided by confinement wall. By using fictitious numerical models with strain rate insensitive material models, the effect of inertia and strain rate on crushing were shown. It was observed that, increase in impact velocity caused increase in inertial effects and strain rate effects were nearly independent from the impact velocity. The effects of multilayering were also investigated numerically.

Treatment of contact separation in Eulerian high-speed multimaterial dynamic simulations

SUMMARY A frictionless contact separation treatment in a sharp-interface Eulerian framework is presented to handle the general situation of high-speed impact and separation of materials. The algorithm has been developed for an established Eulerian-based Cartesian grid multimaterial flow code in which the interfaces are tracked in a sharp manner using a standard narrow-band level set approach. Boundary conditions have been applied using a modified ghost fluid method for elasto-plastic materials. The sharp-interface treatment maintains the distinct interacting interfaces without smearing the contact zone while also removing the difficulties associated with Lagrangian moving mesh entities in contact-separation situations. The algorithm has been tested and verified against experimental and numerical results for three different problems in the high strain rate regime, which involve contact, separation and sliding of materials. Copyright © 2014 John Wiley & Sons, Ltd.

Steady state flow of the FeCoNiCrMn high entropy alloy at elevated temperatures

Steady state flow behavior of the FeCoNiCrMn high-entropy alloy at temperatures ranging from 1023 to 1123 K was systematically characterized. It was found that the stress exponent (i.e., the reciprocal of strain-rate sensitivity) was dependent on the applied strain rate, and specifically the stress exponent is high (∼5) in the high strain rate regime, but decreases with decreasing strain rate. Microstructural examinations of the samples before and after deformation were performed to understand the interplay of the microstructures with the corresponding properties. Based on the observations, it was proposed that, at high strain rates, the deformation of the current high-entropy alloy was controlled by dislocation climb and the rate limiting process was the diffusion of Ni. At low strain rates, however, the deformation appeared to be controlled by the viscous glide of dislocations. Moreover, at the slowest strain rate (i.e., the longest thermal exposure time), new phases evolved, which caused elemental redistribution and weakening of the material.

Plastic behavior of aluminum in high strain rate regime

The aim of this experiment was to study the plastic response of Al to dynamic loading at high strain rates. Dynamic loading was applied by direct laser ablation of the sample with pulse width of 3 ns long. The free surface velocity histories of shock loaded samples, 60-310 μm thick at room temperature, and 150 μm thick with initial temperature from 293 to 893 K, have been recorded using a line velocity interferometer for any reflections (VISAR) system. The line VISAR could measure free surface velocity profile with high temporal resolution (∼100 ps). The measured amplitudes of the elastic precursor waves have been approximated by power functions of the propagation distance with the power index of 0.581, and these data have been converted into relationships between the shear stress at Hugoniot elastic limit and the initial plastic strain rate. The peak longitudinal elastic stress and the strain rate meet a power law dependency with the power index of 0.44. Samples were recovered for post-shot metallographic analysis. The metallographic analysis leads to the conclusion that the spall strength of preheated aluminum is determined more by the rate of void nucleation rather than its growth.

An analysis of microstructural and thermal softening effects in dynamic necking

The competition between material and thermal induced destabilizing effects in dynamic shear loading has been previously addressed in detail using a fully numerical approach in Osovski et al. (2013). This paper presents an analytical solution to the related problem of dynamic tensile instability in a material that undergoes both twinning and dynamic recrystallization. A special prescription of the initial and loading conditions precludes wave propagation in the specimen which retains nevertheless its inertia. This allows for a clear separation of material versus structural effects on the investigated localization. The outcome of this analysis confirms the dominant role of microstructural softening in the lower strain-rate regime (of the order of 103s-1), irrespective of the extent of prescribed thermal softening. By contrast, the high strain-rate regime is found to be dominated by inertia as a stabilizing factor, irrespective of the material’s thermo-physical conditions, a result that goes along the predictions of Rodríguez-Martínez et al. (2013a) regarding dynamically expanding rings.

Formability of dual phase steels in electrohydraulic forming

Electrohydraulic forming (EHF) is based upon the electro-hydraulic effect: a complex phenomenon related to the discharge of high voltage electrical current through a liquid. In EHF, electrical energy is stored in a bank of capacitors and is converted into the kinetic energy needed to form sheet metal by rapidly discharging that energy across a pair of electrodes submerged in a fluid. The objective of this paper is to report the results of formability testing of dual phase steels in EHF conditions and to provide an explanation of the observed formability improvement based on analysis of the experimental results and through the use of a modeling technique developed as part of this work for simulating the EHF process. Comparison of the maximum strains resulting from EHF into a conical die and a v-shape die to the maximum strains resulting from quasistatic LDH testing (limiting dome height) indicated that substantially higher strains can be accomplished in the EHF process. In order to obtain the maximum benefits of increased formability with EHF technology, it is necessary to get to the high strain rate regime and the accompanying high hydrostatic stresses in the sheet, which occurs mostly in the contact area between the blank and the die. If the blank decelerates and gets to quasistatic forming conditions before filling the die cavity completely, very limited improvement in formability can be anticipated. A numerical model of the EHF process has been developed, incorporating four distinct models that are integrated into one: (1) an electrical model of the discharge channel, (2) a model of the plasma channel, (3) a model for the liquid as a pressure-transmitting medium, and (4) a deformable sheet metal blank in contact with a rigid die. The relative improvement in plane strain formability in EHF conditions was between 63% and 190%, depending on the grade of dual phase steel. Numerical modeling showed that the peak strain rates occurring in EHF are approximately 20,000 units per second.

Large scale Optimal Transportation Meshfree (OTM) Simulations of Hypervelocity Impact

Large scale three-dimensional numerical simulations of hypervelocity impact of Aluminum alloy 6061-T6 plates by Nylon 6/6 cylindrical projectile have been performed using the Optimal Transportation Meshfree (OTM) method of Li et al.[7] along with the seizing contact and variational material point failure algorithm [17,18]. The dynamic response of the Al6061-T6 plate including phase transition in the high strain rate, high pressure and high temperature regime expected in our numerical analysis is described by the use of a variational thermomechanical coupling constitutive model with SESAME equation of state, rate-dependent J2 plasticity with power law hardening and thermal softening and temperature dependent Newtonian viscosity. A polytropic type of equation of state fit to in-house ReaxFF calculations is employed to model the Nylon 6/6 projectile under extreme conditions. The evaluation of the performance of the numerical model takes the form of a conventional validation analysis. In support of the analysis, we have conducted experiments over a range of plate thicknesses of [0.5, 3.0] mm, a range of impact velocities of [5.0, 7.0]km/s and a range of obliquities of [0,70]o at Caltech's Small Particle Hypervelocity Range (SPHIR) Facility. Large scale three-dimensional OTM simulations of hypervelocity impact are performed on departmental class systems using a dynamic load balancing MPI/PThreads parallel implementation of the OTM method. We find excellent full field agreement between measured and computed perforation areas, debris cloud and temperature field.

Development of high performance lightweight aluminum alloy/SiC hollow sphere syntactic foams and compressive characterization at quasi-static and high strain rates

Aluminum alloy A356 filled with silicon carbide hollow spheres (SiCHS) is investigated for quasi-static (10−3s−1) and high strain rate (up to 1520s−1) compressive properties. Such closed cell composite foams, called syntactic foams, are of interest in weight sensitive structural applications. The present work is focused on understanding the compressive failure mechanism and relating them with the material microstructure. The compressive and plateau strengths of syntactic foams with SiCHS are found to be 163 and 110MPa, respectively. The measured properties are considerably higher than the existing fly ash cenosphere filled aluminum matrix syntactic foams. Compressive failure mechanisms are studied for A356/SiCHS syntactic foams and direct evidence of hollow sphere crushing at the end of the elastic regions is obtained. The predictions of compressive strength obtained from an existing model are validated with the experimental results. Extensive analysis of data on open and closed cell foams containing gas porosity and syntactic foams is presented. A clear advantage in terms of low density and high yield strength is observed in A356/SiCHS syntactic foams compared to other foams. Yield strength of aluminum foams may be different at high strain rate compression compared to quasi-static values but most of the foams do not show strong evidence of strain rate sensitivity within the high strain rate regime.

Rate-controlling mechanisms of hot deformation in a medium carbon vanadium microalloy steel

Isothermal compression tests were carried out on a medium carbon vanadium microalloy steel (roughly Fe-0.33C-1.5Mn-0.1V, wt%) by using a Gleeble-1500 simulator. Based on constitutive analysis including an Arrhenius term, activation energy for hot working was calculated and used to evaluate the rate-controlling mechanism of hot deformation. At low strain rates (0.1–1s−1), the activation energy for hot working (287.4kJ/mol) is very close to the austenite lattice self-diffusion activation energy, indicating that the rate-controlling mechanism is dislocation climb. While at high strain rates (10–30s−1), the activation energy becomes very high (500.6kJ/mol), and activation volume is better used under such conditions. Then, activation volume analysis based on both Schöck model and Kocks–Argon–Ashby model demonstrates that the rate-controlling mechanism at high strain rates is cross slip. That is, the rate-controlling mechanisms of hot deformation for the medium carbon vanadium microalloy steel at high and low strain rates are intrinsically different. Inspired by the findings above, processing map analysis based on dynamic materials model was further preceded and different peak domains of power dissipation efficiency in high and low strain rate regimes were found.

On the strain rate limits in dynamic testing of concrete-like heterogeneous materials

The bulk dynamic behaviour of concrete in generally known to be strain rate dependent. However, explanation of the underlying mechanisms has been a subject of debate in the research community. Among other factors, the extent of validity of existing experimental data for concrete-like materials in the high strain rate regime is believed to have further complicated the situation. This paper is concerned about the strain rate limits for dynamic testing of concrete specimens in association with the size requirement for representation of the composite material. The strain rate limits as may be established by elastic theories are discussed firstly. Representative simulation results from numerical experiments using a mesoscale model are then described, and non-uniform distributions of stress and damage under excessively high strain rates are highlighted, and this leads to discussion on the correlation between the externally measured data and the real responses within the sample material. Both dynamic compression and dynamic splitting tension are considered.

Dynamic Behaviour of Reinforcing Steel Bars in Tension

In this paper the preliminary results of the tensile behavior of reinforced steel in a large range of strain rates are presented. Tensile testing at several strain rates, using different experimental set-ups, was carried out. For the quasi-static tests a universal electromechanical testing machine with the maximum load-bearing capacity of 50 kN was used, while for the intermediate and high-strain rate regimes a hydro-pneumatic apparatus and a JRC-Split Hopkinson Tensile Bar respectively were used. The target strain rates were set at the following five levels: 10-3, 30, 250, 500, and 1000 1/s. The specimens used in this research were round samples having 3mm in diameter and 5mm of gauge length obtained from reinforcing bars. Finally, the material parameters for Cowper-Symonds and Johnson-Cook models were determined.