High Strain Rate Conditions Research Articles

Mechanical characterization of alloys in extreme conditions of high strain rates and high temperature

The aim of this paper is the description of the mechanical characterization of alloys under extreme conditions of temperature and loading. In fact, in the frame of the Cost Action CA15102 “Solutions for Critical Raw Materials Under Extreme Conditions (CRM-EXTREME)” this aspect is crucial and many industrial applications have to consider the dynamic response of materials. Indeed, for a reduction and substitution of CRMs in alloys is necessary to design the materials and understand if the new materials behave better or if the substitution or reduction badly affect their performance. For this reason, a deep knowledge of the mechanical behaviour at high strain-rates of considered materials is required. In general, machinery manufacturing industry or transport industry as well as energy industry have important dynamic phenomena that are simultaneously affected by extended strain, high strain-rate, damage and pressure, as well as conspicuous temperature gradients. The experimental results in extreme conditions of high strain rate and high temperature of an austenitic stainless steel as well as a high-chromium tempered martensitic reduced activation steel Eurofer97 are presented.

Studying the acoustic emission response of an Indian monumental sandstone under varying temperatures and strains

Study of rocks under high temperature and strain-rate condition can serve as vital information in the restoration process of fire damaged buildings. In this study, the mechanical response of thermally treated fine-grained Dholpur sandstone was observed under increasing strain rates. Dholpur sandstone, a popular construction material, has been used to build some of the iconic monuments in India, a list which includes the Parliament, House of Indian President and the Buddhist Stupas of Sanchi. Thermal treatment spanned across ten days which included heating of the samples (200, 400, 600, 800 and 1000 °C) for five days followed by cooling, in room condition, for the exact same duration. The samples were then tested to failure at three different strain rates (2 × 10−5, 1 × 10−4 and 2 × 10−4 S−1). The elastic modulus was measured using non-contact laser extensometer. Acoustic emission (AE) technique was used to observe the microcrack development under compressive loading. The results from AE were used to demarcate the various stress thresholds and the stress regimes. The result of the experimental analysis suggests that the strength and elastic properties of rocks tend to increase till 400–600 °C followed by a fall in the mechanical characteristics. Onset of plasticity can be observed at high temperature. Strength of the rocks decrease at high strain rates due to the influence of strain rate dependency. The threshold values follow the behaviour of the mechanical properties under the influence of temperature and strain rate.

Structural transformations of carbon and boron nitride nanoscrolls at high impact collisions.

The behavior of nanostructures under high strain-rate conditions has been the object of theoretical and experimental investigations in recent years. For instance, it has been shown that carbon and boron nitride nanotubes can be unzipped into nanoribbons at high-velocity impacts. However, the response of many nanostructures to high strain-rate conditions is still unknown. In this work, we have investigated the mechanical behavior of carbon (CNS) and boron nitride nanoscrolls (BNS) colliding against solid targets at high velocities, using fully atomistic reactive (ReaxFF) molecular dynamics (MD) simulations. CNS (BNS) are graphene (boron nitride) membranes rolled up into papyrus-like structures. Their open-ended topology leads to unique properties not found in their close-ended analogs, such as nanotubes. Our results show that collision products are mainly determined by impact velocities and by two orientation angles, which define the position of the scroll (i) axis and (ii) open edge relative to the target. Our MD results showed that for appropriate velocities and orientations, large-scale deformations and nanoscroll fractures could occur. We also observed unscrolling (scrolls going back to quasi-planar membranes), scroll unzipping into nanoribbons, and significant reconstruction due to breaking and/or formation of new chemical bonds. For particular edge orientations and velocities, conversion from open to close-ended topology is also possible, due to the fusion of nanoscroll walls.

Effect of grain size inhomogeneity of ingot on dynamic softening behavior and processing map of Al-8Zn-2Mg-2Cu alloy

The effect of grain size inhomogeneity of large ingot on the hot deformation behavior of Al-8Zn-2Mg-2Cu alloy were investigated using hot compression tests over a temperature range from 300 °C to 450 °C with strain rate from 0.1 s−1 to 10 s−1. It was found that the grain size (100 μm) in surface layer of the ingot is finer than that of the central layer (>200 μm). The surface specimen exhibited lower flow stress than that of central one in the temperature range from 350 °C to 450 °C and the strain rate range from 0.1 s−1 to 1 s−1. The softening mechanism is dynamic recovery, together with a partial dynamic recrystallization (DRX) at high temperature and low strain rate condition regardless the grain size. The nucleation mechanism of DRX is the strain induced grain boundary migration. Therefore, the DRX fraction is higher in surface specimen because the higher grain boundary density provides more nucleation sites. Based on processing maps, the applicable hot deformation parameters for the large size ingot are determined to be at the temperature range of 440–450 °C and the strain rate of 0.1–0.3 s−1 when taking the grain size inhomogeneity into account.

Phenomenological model for the effect of strain rate on recrystallization and grain growth kinetics in the 617B alloy

The recrystallization behavior of alloy 617B is investigated by isothermal hot compression tests in a temperature range of 1120–1210 °C with a strain rate scope of 0.01–20 s−1. The effect of deformation parameters on DRX behavior is studied by optical microscopy and electron backscatter diffraction (EBSD) technique. The major dynamic recrystallization (DRX) mechanism of alloy 617B is confirmed to be discontinuous dynamic recrystallization (DDRX), accompanied by some characteristics of continuous dynamic recrystallization (CDRX). The DRX process is apparently influenced by strain rate and is accelerated in high strain rate condition, which is different from traditional awareness that high strain rate can inhibit DRX. As a result, a segmented model of strain rate is proposed to describe the DRX kinetics of alloy 617B. Furthermore, a grain growth model is also developed for the simulation of actual manufacture process. DEFORM-2D software is adopted for hot extrusion simulation and FORTRAN language is used for secondary development to invoke the developed model effectively. The simulated microstructure agrees well with the actual manufactured pipe of alloy 617B, demonstrating that the models and simulation method proposed in present study can be used for microstructure prediction and optimization for alloy 617B during hot deformation.

Hot deformation characteristics and microstructure evolution of Hastelloy C-276

High temperature deformation characteristics of Hastelloy C-276 was investigated using compression tests at elevated temperature ranging from 900°C to 1200°C and strain rates covering quasi-static to quasi-dynamic regions (0.001s−1-10s−1) to a final true strain 0.69 (50% Engineering strain). The flow curves at all strain rates and high temperatures (1100°C, 1200°C) exhibit a peak stress, confirming the dynamic recrystallization (DRX) phenomena. To predict and establish the safe hot workability window, processing maps were evaluated based on the dynamic material model and plotted for strain of 0.65. Based on the detailed microstructure analysis and the identification of the active softening mechanisms, the optimal hot workability window was found to be 1200°C and 0.001s−1. Bulk texture analysis revealed presence of fiber textures <011> and <001> parallel to deformation axis (ND), which were prevalent at high temperature and low strain rate conditions. Using Arrhenius model, the apparent hot working activation energy (QHW) was determined as 474kJ/mol. Zener-Holloman parameter was calculated to gauge the deformation resistance and found to have a linear relationship with peak flow stress at given deformation conditions.

Unusual plasticity and strength of metals at ultra-short load durations

This paper briefly reviews recent experimental results on the temperature-rate dependences of flow and fracture stresses in metals under high strain rate conditions for pulsed shock-wave loads with durations from tens of picoseconds up to microseconds. In the experiments, ultimate (‘ideal’) values of the shear and tensile strengths have been approached and anomalous growth of the yield stress with temperature at high strain rates has been confirmed for some metals. New evidence is obtained for the intense dislocation multiplication immediately originating in the elastic precursor of a compression shock wave. It is found that under these conditions inclusions and other strengthening factors may have a softening effect. Novel and unexpected features are observed in the evolution of elastoplastic compression shock waves.

Tensile behavior and microstructural evolution for AZ31 magnesium alloys sheet at high strain rate

AbstractMechanical behavior and microstructural evolution of AZ31 sheet at high strain rates and elevated temperatures were investigated using a tensile Split-Hopkinson bar. Flow stress and elongation increased with increasing strain rate under high strain rate conditions, but temperature had little effect. Microstructural evolution was analyzed by optical and scanning electron microscopy. Twinning was observed at high strain rates, even at 200°C and 250°C, but not under quasi-static tension. Dynamic recrystallization was observed under quasi-static conditions with increasing temperature, but not at high strain rates. Dimples were observed over the entire temperature range at high strain rates. Increasing ductility was limited compared with that under quasi-static conditions. Dynamic recrystallization occurred at strain rates of 2 000 s−1 and 3 000 s−1 at 250 °C, but the recrystallized grains were very small.

Dynamic mechanical behavior of ultra-high strength steel 30CrMnSiNi2 A at high strain rates and elevated temperatures

During high speed machining in the field of manufacture, chip formation is a severe plastic deformation process including large strain, high strain rate and high temperature. And the strain rate in high speed cutting process can be achieved to 105 s−1. 30CrMnSiNi2 A steel is a kind of important high-strength low-alloy structural steel with wide application range. Obtaining the dynamic mechanical properties of 30CrMnSiNi2 A under the conditions of high strain rate and high temperature is necessary to construct the constitutive relation model for high speed machining. The dynamic compressive mechanical properties of 30CrMnSiNi2 A steel were studied using split Hopkinson pressure bar (SHPB) tests at 30 – 700 °C and 3000 – 10000 s−1. The stress-strain curves of 30CrMnSiNi2 A steel at different temperatures and strain rates were investigated, and the strain hardening effect and temperature effect were discussed. Experimental results show that 30CrMnSiNi2 A has obvious temperature sensitivity at 300 °C. Moreover, the flow stress decreased significantly with the increase of temperature. The strain hardening effect of the material at high strain rate is not significant with the increase of strain. The strain rate hardening effect is obvious with increasing the temperature. According to the experimental results, the established Johnson-Cook (J-C) constitutive model of 30CrMnSiNi2 A steel could be used at high strain rate and high temperature.

Tailoring the rate-sensitivity of low density polyurea foams through cell wall aperture size

The plateau stress and energy absorption of low density (≤300 kg/m3) polyurea (PU) foams and expanded polystyrene (EPS) were measured at deformation rates ranging from 0.004 s−1 to 5000 s−1. Low (≤10−1 s−1) strain rate testing was performed using an Instron load frame, intermediate (101–102 s−1) strain rates using a drop-weight impact tower, and high (≥103 s−1) strain rate conditions using a modified split-Hopkinson pressure bar. The plateau stress and energy absorption of low density PU foams exhibit a strong rate dependence across all deformation rates. This result has been previously unreported for low density polymer foams under low and intermediate strain rates. The strain rate sensitivity of PU foams was found to be strongly dependent on cell size for low strain rates and cell wall aperture size for intermediate and high strain rates. EPS type foam, however, remained nearly insensitive to strain rate. At low and intermediate strain rates, the plastic crushing in the EPS and the high plateau stress yield a much higher energy absorption capability than the viscoelastic dissipation in the PU foams. However, PU foams were found to display similar energy absorption properties as EPS based foams under high strain rates. Thus, controlling the strain rate sensitivity of PU foams through aperture diameter can lead to an increase in energy absorption properties at high strain rates, while simultaneously maintaining the peak stress below certain injury thresholds. Additionally, unlike EPS, which undergo plastic crushing after first impact, flexible polyurea foams will recover fully after each impact and thus will have multiple hit capabilities. This will allow these materials to have a wide range of applications, in advance body armors and protective headgears to use in low-cost protection systems for a wide range of military platforms, civilian, and space applications.

Thermal-Mechanical Coupling in Plasma-Sprayed Hydroxyapatite Coating on Ti-6Al-4V Substrate

Abstract After considering the interactions of the temperature and deformation during plasma spraying, the classical Fourier transient heat conduction equation was modified, and the thermal elastic-plastic nonlinear constitutive equation, suitable for high temperature and high strain rate conditions, was employed. Thus, a thermal-mechanical coupling model was developed according to the special case of plasma-sprayed hydroxyapatite coating on Ti-6Al-4V substrate. Based on the above coupling model, the temperature field and residual stress field were coupling solved by the finite element method. The effect of substrate preheating temperature on residual stress components was also simulated. Moreover, the material removal method was used to evaluate the residual stress of coating near the interface to get a quantitative comparison. The results show that, the calculated result is consistent with the experimental result; obvious stress concentration appears near the interface edge; it is helpful to reduce the residual stress by increasing properly the substrate preheating temperature.

Research on constitutive models and hot workability of as-homogenized Al-Zn-Mg-Cu alloy during isothermal compression

Hot compression tests of as-homogenized Al-Zn-Mg-Cu alloy were performed at the deformation temperature range of 350-450 °C and the strain rate range of 0.001-1 s-1. The Arrhenius-type constitutive equation and the Avrami-type model were established to predict the flow behaviors of the alloy respectively. The processing map at the true strain of 0.92 was developed to evaluate the workability of the alloy and the related microstructures were investigated. The results show that the Avrami-type model has a higher accuracy to predict flow stress than the Arrhenius-type constitutive equation. The stable deformation occurs under high temperature or low strain rate mainly owing to the dynamic recrystallization. Flow instability is prone to occur under the condition of low temperature and high strain rate due to the initiation and the propagation of micro-cracks. According to the processing map and corresponding microstructure characteristics, the optimum processing parameters are in the temperature range of 380-405 °C and the strain rate range of 0.006-0.035 s-1.

Dynamic Recrystallization Behavior and Texture Evolution of NiAl Intermetallic During Hot Deformation

B2-ordered intermetallic NiAl was fabricated by hot pressing sintering and studied in terms of dynamic recrystallization behavior and texture evolution during isothermal compression over a temperature range of 1100-1300 °C and strain rate of 10−2 s−1 as well as strain rate of 10−3 to 10−1 s−1 and temperature of 1250 °C. It is found that DDRX mechanism operates predominantly at a high temperature and medium strain rate, while CDRX process is enhanced in the deformation condition of medium temperature and high strain rate yet in a localized way. In particular, progressive subgrain rotation takes place at 1250 °C/10−1 s−1. Conversely, dynamic recrystallization process is insufficient in the case of 1250 °C/10−3 s−1 and 1100 °C/10−2 s−1. Original hot pressing sintered billets are microstructurally homogeneous. Orientation distribution function sections reveal that the strong {110}〈111〉 and {112}〈111〉 texture components appear after deformation. Furthermore, the textures are strengthened with increasing temperature and strain rate.

Effect of thermomechanical processing on microstructural evolution and deformation behaviour of polycrystalline magnesium under quasi-static and high strain rate conditions

The effect of thermomechanical processing on microstructure evolution and room temperature flow behaviour of polycrystalline magnesium in compression at strain rates of ~10−2 and ~103 s−1 was investigated. Different initial microstructures were produced by optimising rolling and annealing cycles. Prior to annealing for 1 h at 350 °C, Mg samples were processed by two different treatments such as (i) hot rolling at 350 °C and (ii) hot rolling at 350 °C plus cold rolling at room temperature. Introduction of cold working step led to an increased fraction of hard oriented grains with a marginal grain size difference in post-annealed samples. A profound effect of thermomechanical processing on strain hardening rate as well as rate-sensitive flow behaviour of Mg was observed. The influence of prior processing history and strain rate on flow behaviour of Mg was clearly reflected in terms of texture strengthening/weakening phenomena and formation of microstructural deformation bands.

High strain rate response of nanofiber interlayered structural composites

Nanofibrous interlayer toughening strategy for laminated composite materials typically demonstrated at quasi-static loading is here evaluated under high strain rate deformation. Carbon fiber reinforced composite laminates of (0/90)25s stacking sequence are interlayered by polystyrene-co-glycidyl methacrylate (P(St-co-GMA)) nanofibers which are chemically tuned for interfacial compatibility when embedded in epoxy matrix. The cubical composite specimens are cut and subjected to high strain-rate deformation via Split Hopkinson pressure bar testing. Specimens are hit at their through-the-thickness (stacking) and side-to-side (in-plane) directions. The change in the dissipation of energy due to altered interlaminar microstructure is monitored and reported. Enhancement in the capacity of the energy dissipation due to the nanofibrous interlayers is as high as 80% in-plane and 40% through thickness directions, depending on the strain rate. The results overall suggest that interlayer toughening strategy used in this work prevents the formation of critical matrix cracks that can cause the formation of instantaneous mode II delamination. Incorporation of the nanofibers without causing notable weight penalty effectively toughen the matrix dominant interlaminar zones under high strain rate conditions as well.

Coupled ASB-and-microvoiding-assisted Dynamic Ductile Failure

Adiabatic shear banding (ASB) is a dynamic localization phenomenon resulting from thermomechanical instability under high strain rate and low stress triaxiality loading conditions. High strength, steels and lightweight alloys of titanium and aluminum are highly susceptible to this phenomenon which leads to premature material failure. At an advanced stage of the localization process, the adiabatic shear bands have been shown to contain micro-voids which coalesce to form cracks and ultimately lead to the fracture of the structure. A physics-motivated, unified constitutive model accounting for the coupled effects of ASB and micro-voiding has been developed in the context of large deformation, high strain rate and high temperature rise. The enlarged model is herein implemented as user material into the engineering finite element (FE) computation code LS-DYNA in the context of standard FE kinematic formulation and its performances are assessed.

Using FEM simulations of cutting for evaluating the performance of different johnson cook parameter sets acquired with inverse methods

Material model parameters are the primary source of error in the finite element analysis (FEM) of cutting processes. Expensive and time consuming material testing is required in order to describe the material's behavior in high temperature and high strain rate conditions during cutting. An alternative approach has been suggested in research papers; inverse analysis using cutting experiments together with FE analysis or analytical models. The latest approach is to combine an analytical model together with a material model capable of describing flow stress in terms of strain, strain rate and temperature, and using cutting experiments to acquire input parameters for inverse analysis, from which the material model parameters can be solved. In this paper, performance evaluation is done for five different sets of Johnson Cook parameters for AISI 1045, acquired with materials testing, inverse analysis with FEM, and the proposed combined inverse analysis with an analytical model and cutting experiments. The performance is evaluated by running simulations with a wide range of cutting parameters and comparing the simulated results of cutting forces and temperature to known experimental results found in literature. It was found that the proposed inverse method produces better performing model parameters than those found in literature.

Study on Hot Workability of Al-5.87Zn-2.07Mg-2.28Cu Alloy Using Processing Map

Hot compression tests of as-homogenized Al-5.87Zn-2.07Mg-2.28Cu alloy were performed at the deformation temperature range of 350–450°C and a strain rate range of 0.001–1 s−1. Based on the dynamic materials model and Prasad’s instability criterion, the processing map at the true strain of 0.69 was established to evaluate the workability of the alloy, and the related microstructures were investigated. The results show that the flow stress declines with increasing temperature and decreasing strain rate. The high-power dissipation efficiency of stable domains can be mainly attributed to the occurrence of dynamic recrystallization. Flow instability is prone to occur under the condition of low temperature and high strain rate due to the initiation and the propagation of micro-cracks. According to the processing map and corresponding microstructure characteristics, the optimum processing parameters are in the temperature range of 390–430°C and the strain rate range of 0.005–0.032 s−1.

Mathematical Modeling of High-Temperature Constitutive Equations and Hot Processing Maps for As-Cast SA508-3 Steel

The hot deformation behavior and hot workability characteristics of as-cast SA508-3 steel were studied by modeling the constitutive equations and developing hot processing maps. The isothermal compression experiments were carried out at temperatures of 950°C, 1050°C, 1150°C, and 1250°C and strain rates of 0.001 s−1, 0.01 s−1, 0.1 s−1, and 1 s−1 respectively. The two-stage flow stress models were established through the classical theories on work hardening and softening, and the solution of activation energy for hot deformation was 355.0 kJ mol−1 K−1. Based on the dynamic material model, the power dissipation and instability maps were developed separately at strains of 0.2, 0.4, 0.6 and 0.8. The power dissipation rate increases with both the increase of temperature and the decrease of strain rate, and the instable region mainly appears on the conditions of low temperature and high strain rate. The optimal hot working parameters for as-cast SA508-3 steel are 1050–1200°C/0.001–0.1 s−1, with about 25–40% peak efficiency of power dissipation.

A Study on the Effect of Strain Rate on the Dynamic Recrystallization Mechanism of Alloy 617B

The effect of strain rate on dynamic recrystallization (DRX) behavior and mechanism of alloy 617B was investigated by isothermal compression test in a temperature range of 1393 K to 1483 K (1120 °C to 1210 °C) with a wide strain rate scope of 0.01 to 20 s−1. The microstructure evolution was investigated comprehensively by optical microscopy, electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI), and transmission electron microscopy (TEM) to provide detailed insight into the effect of strain rate on DRX mechanism. The study shows that DRX is accelerated at both low strain rate and high strain rate conditions with an apparent sluggish kinetics at the intermediate strain rate of 1 s−1. In the low strain rate condition (i.e., <1 s−1), DRX is mainly controlled by the growth of DRX nuclei due to the sufficient time. When the strain rate is higher than 1 s−1, besides the commonly accepted reason of adiabatic heat generated by high strain rate, enhanced DRX nucleation mechanism is crucial for the promotion of DRX at high strain rate. High strain rate could lead to enhanced pile-up of dislocation and higher stored energy, which can facilitate the process of DRX. In addition, distortion or subdivision of twins and “grain fragment” are detected when the strain rate is higher than 1 s−1, which provide additional DRX nucleation mechanism. As a result, the combined effect leads to the higher DRX nucleation rate to promote DRX at high strain rate. The effect of strain rate on DRX is the completion result between sufficiency of time on the one hand and adiabatic heat and enhanced nucleation mechanism on the other.