Dynamic Flow Stress Research Articles

Dynamic flow stress of pure polycrystalline aluminum: Pressure-shear plate impact experiments and extension of dislocation-based modeling to large strains

The dynamic thermo-mechanical behavior of pure aluminum has attracted renewed interest lately due to experimental observations of an anomalous increase in Hugoniot Elastic Limit (HEL) at incipient plasticity and elevated temperatures in polycrystalline pure metals. In context of current dislocation-mediated plasticity models for metals, this increase in dynamic strength is indicative of a transition in the rate controlling mechanism for dislocation glide from being thermally assisted to phonon-drag restricted due to increase in phonon viscosity at elevated temperatures. Though these studies have helped to shed light on these important mechanisms operative in FCC metals at incipient plasticity, the extent to which they contribute to flow stress, particularly at larger strains, remains unclear. In the present study, we address these questions through a combined experimental and modeling effort focused on investigating the evolution of dynamic flow stress in polycrystalline aluminum using experimental data gathered from a series of combined pressure-and-shear plate impact (PSPI) experiments designed to reveal the flow stress of pure aluminum at strain rates ~ 105/s, plastic strains of up to 40% and temperatures ranging from room to 866 K. In all cases, the flow stress of aluminum, as inferred from the measured transverse particle velocity histories at the free surface of an elastic tungsten carbide target plate, reveals saturation with increasing plastic strains at stress levels that decrease with increasing test temperatures. Numerical simulations are performed to correlate the experimentally observed temperature and strain rate dependence of flow stress at small and large plastic strains using the Austin-McDowell dislocation-mediated plasticity model parametrized to normal plate impact experiments conducted in an earlier study by Zaretsky and Kanel (2012). Extensions to the model are made to better represent dynamic behavior of pure aluminum at larger plastic strains as observed in elevated temperature split Hopkinson Pressure bar (SPHB) experiments of Samanta (1971) and Lindholm and Yeakly (1965), and the combined pressure-and-shear plate-impact experiments conducted in the present study. The main theoretical extension to the Austin-McDowell model made in this paper is the introduction of a new rate- and temperature- dependent dynamic recovery function, which can potentially allow for an effective reduction in the rate of accumulation of dislocations at large plastic strains. The numerical predictions of the revised and re-calibrated plasticity model are brought into agreement with the experimental observations, correlating sufficiently well with dynamic yield stress at incipient plasticity and the flow stress levels at larger plastic strains, plastic strain rates in the range 103 – 106 /s, and elevated temperatures up to near melt. The model, in concert with the experimental measurements, suggests that at incipient plasticity the rate governing mechanism for plastic flow is phonon-drag restricted dislocation glide, whereas, at higher magnitudes of plastic strain, it transitions to stress-assisted thermally activated glide. The transition between these two rate governing mechanisms is controlled by the evolution of dislocations throughout the deformation process.

A Comparative Study on the Ballistic Performance and Failure Mechanisms of High-Nitrogen Steel and RHA Steel Against Tungsten Heavy Alloy Penetrators

Traditionally, medium-carbon, low-alloy steels with tempered martensitic microstructures are widely used in structural armor applications. There have been continuous attempts to develop alternative structural armor materials that can provide further weight reduction of armored vehicles. In this context, high-nitrogen steel (HNS) plates with austenitic microstructures were studied against a full-scale tungsten heavy alloy penetrator (500 mm in length and 25 mm in diameter), and the results were compared with those of rolled homogeneous armor steels with tempered martensitic microstructures. The ballistic trials on HNS and RHA steel plates were carried out against a WHA penetrator at velocities of 1630 ± 20 m/s (at a 0° angle of attack and a distance of 100 m) to determine the depth of penetration. HNS exhibited higher ballistic performance (i.e., a lower normalized depth of penetration) against WHA long-rod projectiles than RHA steel. The ballistic results were analyzed with the help of the initial mechanical properties and operating failure mechanisms. The better ballistic performance of HNS against tungsten heavy alloy can be primarily attributed to its higher dynamic flow stress. Post-ballistic hardness measurements on crater cross sections indicated that a higher volume of material was involved in energy dissipation in the HNS target than in the RHA steel target. Microstructural analysis showed that adiabatic shear band-induced cracking played an important role in the failure of both steel targets.

Dynamic compressive behaviour of selective laser melted AlSi12 alloy: Effect of elevated temperature and heat treatment

Selective Laser Melting (SLM) is a near net shaped additive manufacturing process. Parts processed by SLM can experience dynamic loading and high temperature in service, in applications such as defence, aerospace, automotive and high-speed machining. This study investigates the dynamic behaviour of SLM-processed and heat treated eutectic AlSi12 alloy under compression. The dynamic high strain rate experiments were carried out in the range of 1 × 103 to 2 × 103 /s using the split Hopkinson pressure bar (SHPB) equipment. A significant reduction in dynamic yield strength and ultimate compressive strength was observed when the as-built AlSi12 samples were tested at elevated temperatures (200 °C and 400 °C). Moreover, the dynamic flow stress was found to decrease with the increase in temperature at which the samples were heat treated prior to testing. This reduction in flow stress was attributed to the softening of the samples due to the agglomeration and growth of Si-rich precipitates at increased temperatures. The deformation behaviour of dynamically tested AlSi12 samples were also investigated for ellipticity with respect to elevated temperatures, heat treatment and different strain rates.

Elasto-Visco behavior of Al7075-T6 using a numerical technique

The transition in temperature and associated stress induced during machining, may affect the structural integrity of the Al7075-T6 alloy. In the present study, Steinberg Guinan material parameters were used to investigate the dynamic behaviour viz chip morphology (fracture mechanism), temperature change and flow stress in the elasto-visco material Al7075-T6. An explicit algorithm, was adopted to investigate the outcomes. Results showed chip formation and the mode of fracture was closely related with depth of cut and can influence the temperature and flow stress during the machining process. Work hardening and edge dislocation slip in the secondary shear zone was observed to have a notable influence. An optimal machining condition was found between 0.6 and 0.7 mm depth of cut.

Dynamic tensile properties and microstructural evolution of extruded EW75 magnesium alloy at high strain rates

Abstract The dynamic tensile properties and microstructural evolution of an extruded EW75 magnesium alloy deformed at ambient temperature and different high strain rates (from 1000 to 3000 s-1) along extrusion direction (ED) were investigated by Split Hopkinson Tension Bar (SHTB). The corresponding deformation mechanisms, texture evolution and microstructure changes were analyzed by optical microscope (OM), electron backscatter diffraction (EBSD) and transmission electron microscope (TEM). The results show that the extruded EW75 magnesium alloy along ED exhibits a conventional positive strain rate sensitivity that the dynamic flow stresses increase with increasing strain rate. Texture measurements show that after dynamic tension, the initial weak texture of extruded EW75 magnesium alloy tansforms to a relatively strong //ED texture with increasing strain rates. The microstructural analysis demonstrates that dislocation motion are main deformatin mode to accommodate dynamic tensile deformation at high strain rates. In addition, the interactions of dislocation-dislocation and dislocation-second phase lead to the increase of flow stress and strain hardening with increasing strain rate.

Constitutive modeling of dynamic strain aging for HCP metals

Dynamic strain aging hugely affects the microstructural mechanical behavior of metallic materials when activated. It has been extensively reported in the literature that the dynamic strain aging causes significant gaps between model predictions and experimental measurements. Without considering this phenomenon, a constitutive model is unable to accurately capture the material behavior when it is active. The new constitutive model for hexagonal close-packed metals is developed in this work to predict the dynamic strain aging-induced hardening based on the probability function. The proposed model consists of three elements, i.e. dynamic strain aging-induced flow stress as well as thermal and athermal flow stresses. Physically motivated derivation of all the three elements is presented. The proposed model is applied to pure hexagonal close-packed titanium under quasi-static loading (ε˙=0.001/s and ε˙=0.1/s) to dynamic loading (ε˙=2200/s and ε˙=8000/s) across a broad range of temperatures (77 −1000 K).

Dynamic Flow Stress Behavior of Hypo-Eutectoid Ferrite-Pearlite Steels Under Rapid Heating

In the machining process, the workpiece undergoes large plastic deformation at high strain rate and is heated rapidly by plastic work and friction. Rapid temperature excursions brought about during this process may result in non-typical microstructures whose mechanical behavior differs from what has traditionally been observed and modeled. This paper presents dynamic stress-strain measurements on three hypo-eutectoid ferrite-pearlite carbon steels of increasing carbon content (AISI 1018, 1045 and 1075) under rapidly heated conditions, with total heating times less under 4 s, up to 1000 °C. The mechanical behavior of these steels is broken down into four regions: low temperature thermal softening, followed by dynamic strain aging, pearlite decomposition and, finally, ferrite-austenite thermal softening. The present rapidly heated high strain rate results are generally commensurate with literature data up through dynamic strain aging to about 700 °C, indicating limited effects of short heating times below the pearlite decomposition temperature (A1). Above A1, however, the results diverge significantly, owing to the limited time for diffusion processes that govern the transformation from ferrite-pearlite to ferrite-austenite and finally austenite. The divergence includes an inversion of the effect of carbon content on flow stress above A1 compared to previous studies with longer heating times.

A physically based constitutive model for dynamic strain aging in Inconel 718 alloy at a wide range of temperatures and strain rates

Dynamic strain aging has a huge effect on the microstructural mechanical behavior of Inconel 718 high-performance alloy when activated. In a number of experimental researches, significant additional hardening due to the dynamic strain aging phenomenon was reported. A constitutive model without considering dynamic strain aging is insufficient to accurately predict the material behavior. In this paper, a new constitutive model for Inconel 718 high-performance alloy is proposed to capture the additional hardening, which is caused by dynamic strain aging, by means of the Weibull distribution probability density function. The derivation of the proposed constitutive relation for the dynamic strain aging-induced flow stress, the athermal flow stress and the thermal flow stress is physically motivated. The developed model is applied to Inconel 718 high-performance alloy to demonstrate its ability to capture the dynamic strain aging behavior, which was observed in the literature across a wide range of temperatures (300–1200 K) and strain rates from quasi-static loading (0.001/s) to dynamic loading (1100/s).

Grain size effect on strain-rate dependence of mechanical properties of polycrystalline copper

ABSTRACTThis work illustrates the grain size effect on strain-rate dependence of mechanical properties of polycrystalline copper through experimental characterisations and numerical calculations. The as-received and annealed samples with different grain sizes are prepared. Mechanical properties at high strain rates are experimentally attained by using a split Hopkinson pressure bar device. With the increase of grain size, dynamic flow stress decreases, but strain-rate dependence of flow stress increases. Johnson–Cook constitutive model is applied to carry out a numerical analysis about the grain size effect on strain-rate dependence of mechanical data and the quantificational illustrations are given.

Dynamic tensile deformation and microstructural evolution of AlxCrMnFeCoNi high-entropy alloys

The influence of microstructure and strain rate on tensile behavior of AlxCrMnFeCoNi (x = 0, 0.4 and 0.6 in molar ratio) high-entropy alloys was investigated. The alloys with lower Al content exhibited microstructures of simple fcc solid solution whereas the Al0.6CrMnFeCoNi alloy consisted of fcc + bcc mixed solutions after thermomechanical processing. Al0.6CrMnFeCoNi showed higher tensile strength at quasi-static condition due to the combinative contributions from solution strengthening, grain-boundary strengthening and precipitation hardening. Under dynamic loading conditions, both the yield strength and work hardening increased remarkably with increasing strain rate for the two Al-containing alloys. However, the Al0.4CrMnFeCoNi alloy showed a larger strain-rate sensitivity than Al0.6CrMnFeCoNi alloy owing to the existence of abundant bcc phase and interphase boundaries. Johnson-Cook constitutive model can be utilized to describe the effects of strain rate and strain on the dynamic flow stress. Compared to quasi-static condition, the higher density dislocations and interaction were the main characteristics of microstructures for dynamic deformation, leading to the increased flow stress. High volume fraction of bcc phase and strain rate led to a transition from ductile to quasi-cleavage fracture.

Microstructure-topology relationship effects on the quasi-static and dynamic behavior of additively manufactured lattice structures

This study demonstrates a relationship between manufacturing variables including design topology and post-processing heat treatment on the porosity distribution, quasi-static, and dynamic behavior of additively manufactured lattice structures (AMLS). Lattice structures were manufactured out of Inconel 718 using selective laser melting technique with four different topologies. The effect of heat treatment on the porosity size and distribution was examined using X-ray computed tomography for as-built (AB), stress relieved (SR), and hot isostatic pressed (HIP) plus solution aged (SA) heat-treatment conditions. It was noticed that reduction of porosity in the as-built samples, as a result of SR, was greater compared the porosity reduction due to the subsequent HIP plus SA. Quasi-static and dynamic loading was conducted and it was found that the deformation trends of each topology were independent of the strain rate. It was also found that the stress relieving heat treatment process enhances the quasi-static and dynamic flow stress after yielding. However, further heat-treating, including HIP and SA, for the same topology were not as effective as the initial SR process. Furthermore, the validity of digital image correlation in measuring average global strain and the validity of using a Kolsky bar for measuring dynamic mechanical behavior of AMLS are discussed.

Analysis and constitutive modelling of high strain rate deformation behaviour of wire–arc additive-manufactured ATI 718Plus superalloy

A fundamental prerequisite for obtaining realistic finite element simulation of machining processes, which has become a key machinability assessment for metals and alloys, is the establishment of a reliable material model. To obtain the constitutive model for wire–arc additive-manufactured ATI 718Plus, Hopkinson pressure bar is used to characterise the flow stress of the alloy over a wide range of temperatures and strain rates. Experiment results show that the deformation behaviours of as-deposited ATI 718Plus superalloy are influenced by the applied strain rate, test temperature and strain. Post-deformation microstructures show localised deformation within the deposit, which is attributable to the heterogeneous distribution of the strengthening precipitates in as-deposited ATI 718Plus. Furthermore, cracks are observed to be preferentially initiated at the brittle eutectic solidification constituents within the localised band. Constitutive models, based on the strain-compensated Arrhenius-type model and the modified Johnson–Cook model, are developed for the deposit based on experimental data. Standard statistical parameters, correlation coefficient (R), root-mean-square error (RMSE) and average absolute relative error (AARE) are used to assess the reliability of the models. The results show that the modified Johnson–Cook model has better reliability in predicting the dynamic flow stress of wire–arc-deposited ATI 718Plus superalloy.

Dynamic flow stress of shock loaded low carbon steel

This study identifies the role of twin volume fraction in the dynamic stress-strain relationship of shock loaded low carbon steel. This relationship is modeled using thermal activation concepts based on dislocations interactions with thermal and athermal stress barriers. Model parameters are determined by first subjecting a set of as-received and 3% pre-strained steel specimens to shock loads of 6 GPa, and 11 GPa using a gas gun. These specimens, as well as ones with no prior shock loading, were subjected to dynamic compression tests at temperatures in the range 293–923 K and at strain rates between 1E+ 3 s−1 and 5E+ 3 s−1 using a split Hopkinson pressure bar. The resulting stress-strain curves were partitioned into thermal and athermal stress components in which the role of twin boundaries is discussed in terms of their relation with the interaction of dislocation motion and grain subdivision. Results of this work show that the thermal stress component of both the as-received and pre-strained materials is insensitive to the impact history. The athermal stress component for as-received specimens which have been previously shock loaded is shown to be proportional to the shock loading pressure. For specimens, however, which have been pre-strained before shock loading, the athermal stress did not vary for the two shock load levels. This difference in athermal stress between as received and pre-strained materials is explained in terms of the twin volume fraction generated as a function of the impact load. Model results are presented and discussed in relation to the proposed stress-strain formulations.

Hot Deformation Behavior and Dynamic Recrystallization Characteristics of 12Cr Ultra-Super-Critical Rotor Steel

In this paper, the constitutive model and dynamic recrystallization characteristics of 12Cr ultra-super-critical rotor steel were investigated quantitatively during hot deformation. A series of axisymmetric hot compression tests at temperatures from 900 to 1200 °C under strain rates of 0.001–1 s−1 were conducted on a Gleeble−1500D thermal simulator. Based on the experimental true stress–strain curves varying with temperature and strain rates, a complete constitutive model was established and all material parameters in the model could be expressed as a function of strain using a fifth order polynomial fit. The proposed model was verified so as to have the capability of accurately predicting the flow behaviour with an average absolute relative error of < 2.82%. Meanwhile, after hot deformation the microstructure was observed via electron backscatter diffraction technology. Then, the dependence of the characteristic parameters on the Zener–Hollomon parameter were confirmed. Furthermore, the kinetics equation of dynamic recrystallization was obtained, which included the flow stress calculated based on the evolution equation of the dislocation density during the work hardening-dynamic recovery stage. The result indicated that the predicted values for dynamic recrystallization volume fraction and flow stress were in line with the experimental values.

High Strain Rate Behavior of Ultrafine Grained AA2519 Processed via Multi Axial Cryogenic Forging

The present work deals with studies on the dynamic behavior of ultrafine grained AA2519 alloy synthesized via cryogenic forging (CF) and room temperature forging (RTF) techniques. A split-Hopkinson pressure bar was used to perform high strain rate tests on the processed samples and the microstructures of the samples were characterized before and after impact tests. Electron backscatter diffraction (EBSD) maps demonstrated a significant grain size refinement from ~740 nm to ~250 nm as a result of cryogenic plastic deformation showing higher dislocation densities and stored strains in the CF sample when compared to the RTF sample. This microstructure modification caused the increase of dynamic flow stress in this alloy. In addition, the aluminum matrix of the CF alloy is more densely populated with fragmented particles than the RTF alloy due to the heavier plastic deformation applied to the cryogenically forged alloy. The results obtained from the stress–strain curve for the RTF sample showed intense thermomechanical instabilities in the RTF sample which led to a severe thermal softening and the subsequent sharp drop in the flow stress. However, no significant decrease was observed in the stress–strain curve of the CF alloys with ultrafine grains which means that thermal softening would probably not be the most effective failure mechanism. Furthermore, higher level of sensitivity of CF alloys to strain rates was observed which is ascribed to transition of rate-controlling plastic deformation mechanisms. In the post-mortem microstructure investigation, deformed and transformed adiabatic shear bands (ASBs) were identified on the RTF alloy when the strain rate is over 4000 s−1 at which it had experienced a significant thermal softening. On the other hand, circular path and aligned split arcs are the various shapes of the deformed ASB seen at no earlier than 4500 s−1 in the CF alloys. This is associated with the crack failure caused by grain boundary sliding.

Strain Rate Dependence of Dynamic Flow Stress of 1070 Aluminum at High Strain Rates

Strain Rate Dependence of Dynamic Flow Stress of 1070 Aluminum at High Strain Rates

Mechanical behavior of a DD5 single crystal superalloy with different misorientations under quasi-static and dynamic compression

Abstract

Effect of Hot-rolling Temperature on Microstructure and Dynamic Mechanical Properties of Ti-6Al-4V Alloy

The equiaxed Ti-6Al-4V alloys were rolled with an overall reduction ratio of 0.78 at 840, 870, 900 and 930 °C. The microstructure, texture and anisotropy of dynamic mechanical properties of the rolled alloy were investigated. Results show that the recrystallization and phase transformation do not occur until the rolling is performed at a temperature higher than 900 °C, and then the microstructure of the rolled alloy turns into bimodal, while the amount of lamellar αs+β structure and recrystallized α grains shows an increasing tendency with the increased rolling temperature. Moreover, it is interesting to find that although the texture varies with the rolling temperature, the <0001> direction of α phase keeps parallel to the normal direction (ND) during the hot-rolling process at various temperatures. Based on the analyses of dynamic mechanical properties, Ti-6Al-4V alloy exhibits anisotropy of dynamic mechanical properties after hot-rolling at different temperatures. However, resulting from the rolling-temperature-depended texture and distribution of dislocation, the anisotropic tendency of dynamic mechanical properties varies with the rolling temperature. Moreover, with the increased rolling temperature, the dynamic flow stress loaded along ND decreases, while the adiabatic shearing failure strain slightly increases. The dynamic flow stress loaded along RD remains constant, while the adiabatic shearing failure strain exhibits an obviously decreasing tendency with the increased rolling temperature. The dynamic mechanical properties loaded along Transverse Direction (TD) remain constant when the rolling temperature is in the range of 840 °C to 900 °C, while the dynamic flow stress loaded along TD increases evidently when the rolling temperature reaches 930 °C, but the adiabatic shearing failure strain decreases greatly.

Identification of Dynamic Flow Stress Curves Using the Virtual Fields Methods: Theoretical Feasibility Analysis

An inverse approach based on the virtual fields method (VFM) is presented to identify the material hardening parameters under dynamic deformation. This dynamic-VFM (D-VFM) method does not require load information for the parameter identification. Instead, it utilizes acceleration fields in a specimen’s gage region. To investigate the feasibility of the proposed inverse approach for dynamic deformation, the virtual experiments using dynamic finite element simulations were conducted. The simulation could provide all the necessary data for the identification such as displacement, strain, and acceleration fields. The accuracy of the identification results was evaluated by changing several parameters such as specimen geometry, velocity, and traction boundary conditions. The analysis clearly shows that the D-VFM which utilizes acceleration fields can be a good alternative to the conventional identification procedure that uses load information. Also, it was found that proper deformation conditions are required for generating sufficient acceleration fields during dynamic deformation to enhance the identification accuracy with the D-VFM.

Relation between Activation Distance and Dynamic Flow Stress of Aluminum at Very High Strain Rates

Relation between Activation Distance and Dynamic Flow Stress of Aluminum at Very High Strain Rates