High Rate Deformation Research Articles

Dynamical modeling of bi-layer Aluminium adhesive tape for laser shock applications

The presented work covers the response of Aluminium tape (Al tape) under high strain rate of deformation (order of 106s−1) using laser shock. High power laser (J) with a short pulse duration (ns) is used to create laser shock within the water confinement regime on two Al tape configurations in order to apply low and high pressure (order of MPa and GPa). Al tape has been modeled using Johnson-Cook (J-C) material model for the Al layer, and Steinberg–Cochran–Guinan (SCG) material model (elastic with pressure dependence) for the adhesive layer, both material models are coupled with Grüneisen equation of state. The Al tape model has been validated by comparing the simulated Back Face Velocity (BFV) of the target with the measured one by the Velocity Interferometer System for Any Reflector (VISAR). In addition, the validated material model is used to conduct the sensitivity studies about the transmitted pressure depending on the acoustical impedance of the target and adhesive thickness. Moreover, location and magnitude of maximum tensile stress within the target are calculated in function of the adhesive thickness of the Al tape. Finally, it has been proved that using one laser beam configuration, maximum tensile zone could appears close to the front face by increasing the adhesive thickness.

Thermal softening behavior up to fracture initiation during high-rate deformation

This work proposes an innovative method to analyze the softening behavior due to adiabatic heat up to the fracture onset during high-rate deformation of AISI4340 steel at room temperature. To derive the rate- and temperature-dependent constitutive equation considering the post-necking behavior, uniaxial (UT) and notched tension (NT) specimens were tested at three strain rates of 10-3/s, 100/s and 103/s. The full fields of strain and temperature were measured until fracture initiation with high-speed digital and thermal imaging cameras, respectively. In particular, this research focused on observing the abrupt localized temperature increase in the post-necking phase, which was achieved through the use of a newly configured split Hopkinson tension bar (SHTB) system. It enabled an experimental investigation of the effect of adiabatic heat on localized behavior up to fracture at high strain rates (103/s). As the strain rate increased, the strain-localization occurred more quickly, and a higher temperature was experimentally observed at the location where fracture initiated. The experimentally measured thermal field was newly utilized for inverse optimization, to determine the thermal softening effect of the constitutive model considering adiabatic heat up to the onset of fracture including post-necking. Finally, a hybrid experimental-numerical approach was used to investigate the physical quantities of local strain, strain rate, temperature inside the specimen where fracture actually began.

Direct comparison between experiments and dislocation dynamics simulations of high rate deformation of single crystal copper

A long standing challenge in computational materials science is to establish a quantitative connection between the macroscopic properties of plastic deformation with the microscopic mechanisms of dislocations in crystalline materials. Although the discrete dislocation dynamics (DDD) simulation method has been developed for several decades with the goal of addressing this challenge, a one-to-one comparison between the DDD predictions on single crystal stress–strain curves and experimental measurements under identical conditions has not been possible to date. Such a comparison is an essential step towards establishing a dislocation-physics based theory of plasticity and a multiscale framework of the plastic behaviors of crystalline materials. Here we provide direct comparisons between the stress–strain curves of Cu single crystals under high strain rate loading in the [001] and [011] directions obtained from miniaturized desktop Kolsky bar experiments and those from DDD simulations under identical loading conditions. With an appropriate set of parameters, DDD simulations can produce stress–strain curves that are in reasonable agreement with the experimental results. However, the dislocation mobility values needed to achieve this agreement are an order of magnitude lower than expected based on previous measurements and atomistic simulations. We hypothesize that this discrepancy could be caused by drag forces from jogs and point defects produced during the plastic deformation. Cross-slip of screw dislocations is also found to be necessary to capture the experimental stress–strain behavior, especially for the [011] loading direction. This work provides an example of how direct comparisons between DDD simulations and experimental measurements can provide new insight into the fundamental mechanisms of plastic deformation.

Onset and nature of flow-induced vibrations in cerebral aneurysms via fluid–structure interaction simulations

Clinical, experimental, and recent computational studies have demonstrated the presence of wall vibrations in cerebral aneurysms, thought to be induced by blood flow instability. These vibrations could induce irregular, high-rate deformation of the aneurysm wall, and potentially disrupt regular cell behavior and promote deleterious wall remodeling. In order to elucidate, for the first time, the onset and nature of such flow-induced vibrations, in this study we imposed a linearly increasing flow rate on high-fidelity fluid–structure interaction models of three anatomically realistic aneurysm geometries. Prominent narrow-band vibrations in the range of 100–500 Hz were found in two out of the three aneurysm geometries tested, while the case that did not exhibit flow instability did not vibrate. Aneurysm vibrations consisted mostly of fundamental modes of the entire aneurysm sac, with the vibrations exhibiting more frequency content at higher frequencies than the flow instabilities driving those vibrations. The largest vibrations occurred in the case which exhibited strongly banded fluid frequency content, and the vibration amplitude was highest when the strongest fluid frequency band was an integer multiple of one of the natural frequencies of the aneurysm sac. Lower levels of vibration occurred in the case which exhibited turbulent-like flow with no distinct frequency bands. The current study provides a plausible mechanistic explanation for the high-frequency sounds observed in cerebral aneurysms, and suggests that narrow-band (vortex-shedding type) flow might stimulate the wall more, or at least at lower flow rates, than broad-band, turbulent-like flow.

A Comparative Study of the Skeletal Development of Epinephelus fuscoguttatus (♀) and E. tukula (♂) Hybrid Progeny and E. fuscoguttatus

As a new breed of grouper hybrid breeding, EFT has the advantages of delicious flesh, fast growth, and high deformity rate. We explored the metamorphosis and skeletal development of Epinephelus fuscoguttatus and hybrids (EFT) of E. fuscoguttatus (♀) and E. tukula (♂). EFT grew slightly slower than E. fuscoguttatus during the larval period but grew faster during the juvenile period. The sequences of skeletal development were similar in EFT and E. fuscoguttatus. Newly hatched larvae did not have skeletons until 5 days after hatching (DAH) when head cartilage started to develop and complete at 43 days later. The vertebral column started to develop at 11 DAH and was completed at 38 DAH. Pectoral fin development began at 5 DAH and ossification was completed at 48 DAH in EFT and at 43 DAH in E. fuscoguttatus. Anal fin development started at 11 DAH and ossification was completed earlier in EFT than in E. fuscoguttatus. The caudal fins of the two groupers developed at 8 DAH and ossified earlier in EFT than in E. fuscoguttatus. In contrast, E. fuscoguttatus had one additional epural than EFT. These results provide important basic biological data for the growth and skeletal development of pure and hybrid grouper.

β Grain Size Inhomogeneity of Large Scale Ti-5Al-5V-5Mo-3Cr Alloy Bulk after Multi-Cycle and Multi-Axial Forging in α + β Field

In order to fabricate homogeneous large-scale Ti-5Al-5V-5Mo-3Cr (Ti-5553) alloy bulk with fine and equiaxial β grain, we performed a series of multi-axial α + β field forging with 62 forging cycles on the large-scale Ti-5553 billet by using 12.5 MN high-speed hydraulic press. The β-annealed microstructure was the starting microstructure of the billet. After the 6th forging cycle, β grain deformed dramatically, and the grain-boundary network developed within the irregular β grain. As the forging cycle increased to 44, the volume fraction of the fine and equiaxial β grain that is less than 20 μm, which is caused by dynamic recrystallization, increased gradually. However, the incomplete dynamic recrystallization region within the original β grain could not be eliminated. As the forging cycle further increased, the volume fraction of the fine and equiaxial β grain did not increase. In contrast, the abnormal grain growth of the β phase occurred during 50th~62nd forging cycle. Here, we attribute the formation of the incomplete dynamic recrystallization region and the abnormal grain growth of the β phase to the high deformation rate of the α + β forging. The refining behavior of β grain and the abnormal coursing β grain, which is found during the multi-cycle multi-axial forging of large-scale Ti-5553 alloy billet, are seldom reported in the isothermal compression of small-scale Ti-5553 alloy specimen. The findings of the paper are instructive for improving the sub-transus forging strategy that is used to fabricate the large-scale homogeneity Ti-5553 alloy billet with fine and equiaxial β grain.

The impact of flash sintering on densification and plasticity of strontium titanate: High heating rates, dislocation nucleation and plastic flow

Flash sintering involves very rapid densification of ceramic powder compacts during a thermal runaway induced by an applied voltage and current. The mechanisms of fast densification are still not well-understood. The present study investigates the impact of high heating rates during flash sintering on densification, dislocation density and plasticity of SrTiO3. After flash sintering, a high dislocation density of almost 1014 m−2 was observed by TEM. Uniaxial compression at 1150 °C revealed very high deformation rates. It is argued that for SrTiO3, dislocations are generated and migrate during flash sintering. This becomes possible by the very high heating rates, which conserve high driving forces for sintering up to high temperatures. High driving forces of several 10 MPa are preserved up to high temperatures. Thus, the sintering stress can be above the flow stress of SrTiO3 (5 MPa), and the nucleation of dislocations occurs, paving the path for plastic flow.

Simulation of hot deformation of cast super duplex corrosion-resistant steel

The deformation behavior of cast super duplex steel has been studied in the temperature range 1100–1250 °C and strain rates 0.1–10 s–1. Hot deformation was carried out by uniaxial compression of cylindrical samples on a Gleeble 3800 thermomechanical simulator. It is shown that the flow stresses decrease with an increase in temperature and a decrease in the strain rate in accordance with a change in the Zener‒Hollomon parameter of the temperature-speed mode of deformation. The shape of the flow curves indicates that hot deformation is accompanied by intense dy-namic softening. As a result, the flow stresses experience a drop or remain at a constant level after reaching peak values. Ferrite in all studied modes of hot deformation acquires a dynamically recrystallized structure. At the lowest deformation temperature (1100 °C) and relatively high deformation rates (1–10 s–1), the austenite softening mechanism is dynamic recovery. A decrease in the strain rate, as well as an increase in the deformation temperature, causes a partial dynamic recrystallization of austenite. Under the same deformation conditions, the level of plastic flow stresses in the steel under study is significantly higher than in standard duplex stainless steels. From the analysis of the peak stresses of the flow, the value of the effective activation energy Q = 501.31 kJ/mol of hot deformation, which is necessary for calculating the Zener‒Hollomon parameter, was determined. An expression for the peak flow stress is obtained in the form of a hyperbolic function of the Zener‒Hollomon parameter. The resulting expression describes the array of experimental data with high accuracy and can be used to estimate the required energy-power parameters of forging and rolling equipment. A comparative assessment of the hot ductility of super duplex steel is carried out by finding the amount of deformation corresponding to the appearance of the first macroscopic cracks on the sample surface. It is shown that at a strain rate of 10 s–1, which is typical for hot stamping processes, the safest temperature range of deformation of the steel under study is the temperature range of 1150–1250 °C, in which austenite undergoes partial dynamic recrystallization, and the risk of crack formation decreases.

An intra‐articular injectable phospholipids‐based gel for the treatment of rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic inflammatory and destructive arthropathy with a high deformity rate. Despite numerous studies and clinical trials, no curative treatment is available for large weight-bearing joints. Intra-articular (IA) injections could deliver high concentrations of drug to the afflicted joint and improve the drug efficacy while reducing systemic toxicity. However, free drugs are rapidly cleared from synovial fluid and do not significantly halt the progression of joint disease. Herein, a phospholipids-based controlled-release gel was prepared for sustained IA delivery of celastrol (CEL) and the therapeutic efficiency was evaluated in a rheumatoid arthritis rabbit model. The CEL-loaded gel (CEL-gel) contained up to 70% phospholipids yet was easy to inject. After injecting into the joint cavity, CEL-gel achieved sol to gel phase transition without special stimuli and gelling agent. In vitro release and in vivo pharmacokinetic studies evidenced the stable and sustained release action of CEL-gel. A single IA injection of CEL-gel could maintain therapeutic efficiency for about 25 d and showed much better anti-arthritic efficacy compared to repeated injections of free drug solution (CEL-sol). Furthermore, the IA injection of CEL-gel greatly reduced the systemic toxicity of CEL. With good biocompatibility and biodegradability, CEL-gel might be a promising IA drug delivery system.

Dispersion of zein into pea protein with alkaline agents imparts cohesive and viscoelastic properties for plant-based food analogues

The use of plant-based proteins to replace animal proteins is gaining interest from consumers and food processors, with plant proteins now having become a major ingredient commodity. Even though the commercial plant proteins used today (e.g., soybean and pea) are good texturizing proteins as evidenced by their widespread use in plant-based analogues, they do not show good cohesive and viscoelastic properties. In this study, we describe some natural and food-friendly ways to promote cohesiveness and viscoelasticity using the corn protein, zein, in plant-based protein alternatives. Small deformation tests showed that arginine and/or calcium hydroxide addition to zein-pea protein blends significantly increased elasticity and cohesiveness. Arginine did not perform as well as calcium hydroxide to provide strength to pea protein, though its effect on improving elasticity was good. A lubricated squeezing flow test showed that all blends had strain rate thickening/hardening behavior and blends were highly resistant even at higher deformation rates. Textural attributes of the blends were able to be modified with combination treatments to achieve different textural product specifications. Secondary structure composition analysis by FT-IR emphasized the importance of β-sheet content in enhancing the material viscoelastic properties. Arginine and calcium hydroxide-incorporated blends presented the highest β-sheet content. Imaging on blends using scanning electron and confocal microscopy showed that zein formed viscoelastic fibrils holding the structure together through a dispersed network. In conclusion, zein with an added alkaline agent provides cohesiveness and viscoelasticity to plant-based formulations for potential replacement of currently used chemically-modified hydrocolloids or wheat gluten in formulations for meat substitutes, such as analogues of chicken tenders, burger patties, and hotdogs. • Alkaline treatments were studied to promote cohesiveness and viscoelasticity of zein. • Arginine and calcium hydroxide increased cohesiveness and elasticity of zein blends. • Zein formed viscoelastic fibrils holding the structure together through a dispersed network. • Zein confers cohesiveness and viscoelasticity to plant-based formulations.

Studying the high-rate deformation of soft materials via laser-induced membrane expansion.

Cavitation is a phenomenon that occurs when the internal pressure of a material exceeds the resistance to deformation provided by the surrounding medium. Several measurements, such as the blister test, bubble inflation, and cavitation rheology, take advantage of this phenomenon to measure the local mechanical properties of soft materials at relatively low deformation rates. Here, we introduce a new measurement called laser-induced membrane expansion (LIME) that measures the shear modulus of a thin membrane at high strain rates (≈106 s-1 to 108 s-1) by using laser ablation to rapidly expand a thin (tens of microns) elastomeric membrane. To demonstrate the capabilities of this measurement, we use LIME to study the mechanical properties of poly(dimethylsiloxane) (PDMS) membranes at several thicknesses (from 10 μm to 60 μm) and crosslink densities. We find that the shear modulus of the PDMS measured by LIME was weakly dependent on the crosslink density, but was strongly strain rate dependent with values ranging from 106 Pa to 108 Pa. This measurement platform presents a new approach to studying the mechanical properties of soft but thin materials over a range of deformation rates.

Boronic ester-based vitrimeric methylvinyl silicone elastomer with “solid-liquid” feature and rate-dependent mechanical performance

Vitrimer chemistry can integrate reprocessability, recyclability, self-healing and good creep-resistance into polymers through covalent adaptive network structure. In particular, reactive processing shows great potential to convert commercial polymers into vitrimers in industrial scale. In this work, vitrimeric silicone elastomer was successfully prepared in a one-step peroxide-induced thiol-ene click reaction between thiol-terminated boronic ester and vinyl groups of commercial methylvinyl silicone rubber (MVQ) via reactive processing. It was found that MVQ vitrimer elastomer shows good reprocessability, self-healing property, excellent elasticity and a similar gel content to that of traditional peroxide-crosslinked MVQ rubbers. Based on classical Arrhenius theory, the calculated activation energy (Ea) of B–O dynamic bond in MVQ vitrimer is 28.5 kJ/mol along with a topology-freezing transition temperature (Tv) of 6.4 °C. Intriguingly, the fabricated MVQ vitrimer shows a strongly rate-dependent mechanical response, i.e., larger mechanical strength at higher deformation rate, which is mainly due to fast bond-exchange rate of the B–O bonds even at room temperature. MVQ vitrimer shows a “solid-liquid” feature, that is, exhibiting liquid-like behavior at longer obeservation time or when stretched at low strain rate while acting as solid elastomer at high strain rate. Notably, the tensile toughness (@500% strain) can obtain nearly 1.6-fold increase to 47.59 MJ/m3 from 18.58 MJ/m3 when increasing tensile speed, indicating an excellent energy-dissipative behavior especially at high deformation rate. Therefore, our work can provide guidance for molecular design and reactive processing of high-performance vitrimeric silicone elastomers toward industrial applications.

Rheological changes in melts and magmas induced by crystallization and strain rate

This review highlights the rheological and phase proportions variation induced by cooling events from superliquidus temperature (melt) to subliquidus temperatures. It provides a comprehensive view of the rheological response of magmatic systems undergoing dynamic cooling and shear deformation. The two main parameters which are of importance to model the rheological properties of such crystallizing systems and which are simultaneously poorly investigated so far are crystallization and strain rates. The response to relatively high deformation rates results in shear thinning behavior in partly crystallized systems under variable shear rate and it should be considered in magmatic processes. Due to the sluggish crystallization of SiO 2 -rich melts, data are mainly available for mafic systems, which does not allow a general reappraisal. An attempt to model available literature data for less evolved systems in dynamic scenarios and a comparison with MELTS algorithm approach (thermodynamic equilibrium conditions) is provided. Since there are difficulties in comparing experimental data gained using different methodologies, we focus mainly on data obtained with the concentric cylinder technique. This highlights the fact that a general experimental protocol is needed in order to compare and model viscosity data to predict the dynamic rheological evolution for volcanic rocks.

Flow of polymer blends—Developing a deformation-induced morphology followed by analytical scanning electron microscopy

We applied a new experimental method for studying the evolution of the morphology in the flow of polymer blends. This so-called analytic scanning electron microscopy is based on spectroscopy of Auger electron transitions, which is widely used for catching foreign atoms in metals, alloys, and ceramic articles but has remained unknown in polymer science. The possibility of applying this approach to polymer blends became possible when choosing two polymers that are composed of different atoms. We have used polysulfone with sulfur in the main chain and a thermotropic liquid crystalline copolymer (LCP) of polyethylene terephthalate and p-hydroxybenzoic acid, which has significantly more oxygen atoms in its composition than polysulfone. In addition, LCP has lower viscosity. The blends with different ratios of components were extruded through a capillary at different shear rates. The components are randomly distributed, and no significant changes in the spatial distribution or the radial migration over the volume were observed during the low shear deformations. At high shear rates, phase separation takes place, emulsification of dispersed droplets of a low-viscous phase occurs, and these droplets were drawn along the streamlines, forming a self-arranged bundle of the strings. The increase in the radial transfer, leading to the phase separation, is most likely determined by the deformations and not by the shear rate. If the concentration of the low-viscous phase is high enough, it squeezed out of the extrudate body, forming a coverlayer on the convergent stream at the capillary inlet. The apparent viscosity of the blend strongly depends on the concentration of the low-viscous component. This can be explained by the combination of the predominant flow inside low-viscous strings and the existence of the low viscous “lubricant” on the periphery of the stream. The structure of the blend is destroyed at sufficiently high deformation rates apparently due to the emergence of the elastic turbulence.

Application of Four Different Models for Predicting the High-Temperature Flow Behavior of 1420 Al–Li Alloy

In this paper, the high-temperature rheological behavior of 1420 aluminum–lithium alloy under experimental conditions (temperatures of 350–475 °C and strain rates of 0.01–10 s−1) was systematically investigated using a Gleeble-3500 thermal simulation tester (temperature 350~475 °C, strain rate of 0.01~10 s−1). Based on the flow stress curves of this alloy, four different types of high-temperature constitutive models of the alloy were constructed: the Arrhenius (AR) model, the Modified Johnson–Cook (MJC) model, the Modified Zerilli–Armstrong (MZA) model, and the VOCE model. The prediction accuracy of the four constitutive models was compared, and the response of the accuracy of the four constitutive models to the deformation parameters (temperature, deformation rate, and strain) was analyzed. The results showed that the VOCE, AR, and MZA models had high overall prediction accuracy with average absolute relative error (AARE) of 1.8933%, 3.9912%, and 7.8422%, respectively. The VOCE model could achieve the prediction of large strain deformation resistance under small strain with small batch experimental conditions for the corresponding conditions. The AR model had optimal prediction accuracy for the high-rate deformation process. The MJC model had the optimal prediction accuracy for the low-temperature low-rate deformation process. The MZA model had better prediction accuracy for the low-rate high-temperature deformation process. The 1420 aluminum–lithium alloy process parameters selection area constitutive model matching diagram was constructed.

Deformation Behavior and Connection Mechanism of EMP Connections in Aluminum Pipe Joints

The joint is a key component of the aviation piping system, with severe performance requirements and better requirements for connection technology. With a focus on the manufacturing demand of AA6061 aerospace pipe joints, as well as the characteristics of EMP forming technology, this paper investigates the deformation behavior of the EMP forming on AA6061 aerospace pipe joints, the influence of process parameters on the deformation behavior, and the deformation mechanism of the tube wall. The results show that under the conditions of this paper, with an increase in the initial tube-sleeve gap and discharge voltage, the degree of local deformation of the AA6061 tube wall and the trench embedding rate increase. Keeping the width and depth of the grooves as 1.14 mm and 0.23 mm, the embedding rate of the grooves is less than 85% under the clearance conditions of 0.11 mm and 0.5 mm, while the lowest voltage for the embedding rate of the grooves to reach more than 85% under the clearance conditions of 1 mm, 1.5 mm and 2 mm is 7 kV, 6 kV, and 5 kV, respectively. The metallographic organization of the deformation area shows that the tube is deformed by the intense shear at the edge of the groove of the tube sleeve, thereby showing streamlined organization characteristics and deformation characteristics. The electromagnetic pulse forming process of the AA6061 tube is mainly divided into two stages: free bulging and local deformation; the inertia of high-rate deformation causes the groove filling to exhibit volume deformation characteristics in the local deformation stage.

High frame rate deformation analysis of knee cartilage by spiral dualMRI and relaxation mapping.

Daily activities including walking impose high-frequency cyclic forces on cartilage and repetitive compressive deformation. Analyzing cartilage deformation during walking would provide spatial maps of displacement and strain and enable viscoelastic characterization, which may serve as imaging biomarkers for early cartilage degeneration when the damage is still reversible. However, the time-dependent biomechanics of cartilage is not well described, and how defects in the joint impact the viscoelastic response is unclear. We used spiral acquisition with displacement-encoding MRI to quantify displacement and strain maps at a high frame rate (25 frames/s) in tibiofemoral joints. We also employed relaxometry methods (T1 , T1ρ , T2 , T2 *) on the cartilage. Normal and shear strains were concentrated on the bovine tibiofemoral contact area during loading, and the defected joint exhibited larger compressive strains. We also determined a positive correlation between the change of T1ρ in cartilage after cyclic loading and increased compressive strain on the defected joint. Viscoelastic behavior was quantified by the time-dependent displacement, where the damaged joint showed increased creep behavior compared to the intact joint. This technique was also successfully demonstrated on an in vivo human knee showing the gradual change of displacement during varus load. Our results indicate that spiral scanning with displacement encoding can quantitatively differentiate the damaged from intact joint using the strain and creep response. The viscoelastic response identified with this methodology could serve as biomarkers to detect defects in joints in vivo and facilitate the early diagnosis of joint diseases such as osteoarthritis.

A novel assessment of dynamic mechanical properties in different articular cartilage layers

Human articular cartilage, a nonlinear, viscoelastic solid-liquid biphasic tissue, has different mechanical properties and undertakes different functions in different layer. The compression deformation and fatigue damage mechanism of each layer of cartilage is also different when in injury. Compression properties of cartilage can predict cartilage integrity and the likelihood of osteoarthritis. Therefore, the confined and unconfined compression of cartilage is taken to systematically study the dynamic mechanical properties of each layer and reveal the relationship between the dynamic mechanical properties of each cartilage layer and the function of the corresponding cartilage layer. Under confined and unconfined compression conditions, the larger the cyclic loading rate, the greater the deformation rate of each layer. In addition, under the same cyclic loading rate, the superficial layer had the highest deformation rate, followed by the middle layer and the deep layer, which indicates that the deep layer mainly assumes the compression load. Furthermore, under the same loading displacement, the loading stress of cartilage and the deformation rate of each layer in confined compression were greater than those in unconfined compression. Simultaneously, with the increase in the number of loading cycles, the deformation rate in different layers increased first and then stabilized.

Comparative numerical study of rate-dependent continuum-based plasticity models for high-velocity impacts of copper particles against a substrate

The problem of high-rate elastic-plastic deformation of micro-sized copper particles impacting against a copper substrate was investigated by applying the continuum-based formulation and finite element thermomechanical analysis. Comparative study of selected plasticity models was performed. The aim of the paper was to study strain rate-dependant plasticity for a wide range of strain rates. The strain-rate-dependant Johnson-Cook and Cowper-Symonds models were studied by comparing displacements, velocities, strains, strain rates, stresses, contact forces, and temperatures and their contribution to material yield stress. The study shows the importance of the high-strain rate yielding model and its adequacy for experimental data. Both models complement each other and may be regarded as soft and hard bounds of the solution. A new, combined, two-function model, containing two independent functions for each of the two ranges, is suggested. The proposed model describes a low strain rate sensitivity range using the Johnson-Cook expression, while allows fitting of the model for experimental results in a high strain rate sensitivity range, using a modified Cowper-Symonds expression. This combination is capable of describing both low and high strain rate regimes, giving the minimum deviation from experimental results.

Impact-induced deformation and ignition related to localized viscous shear flow heating for high-ductility composite energetic materials

Understanding the impact-induced deformation and ignition properties of high-ductility composite energetic material (CEM), including high-energy propellants and casted high explosives, is fundamental to the safety design and usage of rockets and warheads and has been a broad concern in the fields of aerospace and defence. In this study, an impact ignition experiment is designed using a Split Hopkinson Pressure Bar to quantitate the continuum deformation and reaction responses of a high-ductility CEM sample undergone periodic impact loading pulses. The relation between macroscopic shear deformation rate and localized viscous shear flow heating, melting, and reaction is further investigated utilizing a macro-mesoscopic thermomechanical model. The results show that CEM sample is severely crushed to 2% of the original thickness and laterally extruded with a high shear rate (∼105 s−1). High shear deformation rate contributes to the localized viscous shear flow heating, thereby inducing temperature rise spike (∼200 K) and following ignition. Ignition of CEM sample occurs in the third compression pulse because the thinner sample exhibits a high velocity gradient, which causes a high viscous shear flow heating rate.