Split Hopkinson Pressure Bar Research Articles

Dynamic tensile characteristics of an artificial porous granite under various water saturation levels

Underground rocks are generally subjected to water erosion and dynamic disturbances. To exclude the effect of clay minerals on the water–rock interaction mechanism, clay-free artificial porous rock (APR) was used to investigate the coupling effect of water saturation and loading rate on tensile behavior. Dynamic Brazilian disc experiments were performed on APR with six water saturation levels (100, 80, 60, 40, 20, and 0%) using a split Hopkinson pressure bar (SHPB) combined with a high-speed camera. The results indicated that the number of cracks in the APR specimens increased with the water saturation level during the dynamic damage process, with more radial cracks at the water saturation of 80–100%. The dynamic tensile strength (DTS) and dissipated energy density exhibited different change trends with water saturation levels under different loading rates and the rate dependence was most significant in the fully saturated state (100%). At lower loading rates, DTS initially decreased rapidly and then more slowly with increasing water saturation. At higher loading rates, DTS exhibited a ‘decrease then increase’ trend. Moreover, the water-affecting factor exhibited an overall decreasing trend with the loading rate and gradually converged to 1.0. These phenomena can be attributed to the interplay between the weakening and strengthening water-effect mechanisms based on the microstructure and water distribution in the APR specimens.

Impact response of 3D printed cornstalk-inspired structures: The effect of indenter shape on penetration

This paper reports the mechanical response and damage tolerance of 3D-printed cornstalk-inspired structures subjected to impact loading. Specimens were subjected to dynamic indentation tests at multiple impact energies with flat, hemispherical, and conical indenters. The mechanical properties of the base material (ABS) were measured across varying strain rates using a Shimadzu® Universal Testing Machine and a Split Hopkinson Pressure Bar. The effect of geometrical variations of the constituents on energy-absorbing capability was also investigated. Damage characteristics were interrogated through X-ray CT scans and provided detailed failure modes associated with each indenter shapes. Further, finite element simulations provided insights into the penetration mechanisms associated with the different indenter shapes. The results demonstrated that test specimens impacted by flat indenters absorbed ∼25 % less energy than those impacted by hemispherical and conical indenters. Among the various indenters, the conical shape had the highest duration of contact force within the specimen before experiencing failure by matrix cracking and complete perforation.

Mesoscale Modeling of Microcapsule-Based Self-Healing Cementitious Composites under Dynamic Splitting Tension

Microcapsule-based self-healing cementitious composite (MSCC) offers autonomous damage repair, extending the service life of structures. However, most of the existing studies focus on static behavior and healing effectiveness but rarely explore dynamic responses. This study developed the mesoscale modeling approach to investigate MSCC behavior under dynamic split tensile loading. At the mesoscale, MSCC can be treated as a four-phase composite consisting of coarse aggregates, interfacial transition zones, cement mortar, and microcapsules. Alternatively, it can be simplified as a two-phase composite comprising a homogeneous mortar matrix and microcapsules. Four-phase and two-phase mesoscale MSCC models were developed for 2D simulations, while a two-phase 3D model was also developed for comparison. Mesoscale numerical simulations were conducted based on Split-Hopkinson Pressure Bar Brazilian disk-splitting tests, considering various strain rates. Simulation results of different mesoscale models were compared with experimental results. All of the models accurately predicted the tensile strength of MSCC, with the 2D four-phase model providing the best representation of failure modes and crack propagation. Both experimental and numerical data exhibited obvious strain rate effects, indicating that MSCC’s mechanical properties were sensitive to the loading rate. Dynamic increase factors were obtained, quantifying rate sensitivity. The obtained dynamic mechanical properties of MSCC provide insights for designing MSCC components and structures that can better withstand collisions or explosions.

Research on Dynamic Mechanical Behavior of Reinforcing Steel Under Ultrahigh Strain Rate

The strain rate effect significantly influences the dynamic mechanical behavior of reinforcing steel under impact loads. To investigate the effect of strain rate on the dynamic mechanical behavior of reinforcing steel, this study conducts dynamic compression tests on four types of reinforcing steel using a Split Hopkinson pressure bar. The results show that there is a significant strain rate effect on the reinforcing steel when the strain rate reaches 103–104 s−1. The experimental results are analyzed using the Johnson–Cook model (J–C model), and the original model is modified based on the error between the model predictions and the experimental results. The results indicate that the modified J–C model, which includes an enhanced strain rate effect term, better predicts the dynamic mechanical behavior of reinforcing steel at ultrahigh strain rates.

Characteristics of dynamic mechanics and energy loss in reef limestone concrete during dry-wet carbonation periods

Coral reef limestone is a unique type of rock and soil body characterized by high porosity. Its dynamic mechanical properties under impact loads differ significantly from those of conventional land-sourced aggregate concrete.This study utilizes coral reef limestone as both coarse and fine aggregates to prepare C40 strength concrete. The research investigates the effects of dry-wet carbonation cycles on its dynamic mechanical behavior and energy evolution characteristics using a Split Hopkinson Pressure Bar (SHPB) mechanical testing system.The findings reveal that increasing the number of dry-wet carbonation cycles leads to a significant weakening of the internal structural bonding in coral reef limestone concrete. Notably, the degree of phenolphthalein color change diminishes, while uniaxial compressive strength and tensile strength demonstrate an overall downward trend. The reduction in tensile strength is less pronounced than the decrease in compressive strength. Additionally, the relative dynamic elastic modulus gradually decreases, and a size effect is noted, with a rapid acceleration in mass loss. As the number of dry-wet carbonation cycles increases, dynamic compressive strength declines, and failure modes shift from surface cracking to crush-type failure.The dynamic increase factor (DIF) of the coral reef limestone concrete indicates a high sensitivity to strain rate, with a significant rise in DIF value as the strain rate increases. Various energies generated under impact load exhibit clear strain rate effects. Furthermore, the effects of dry-wet carbonation cycling enhance energy dissipation, especially at 30 cycles, where energy dissipation increases sharply, while a hindering effect on transmitted energy is observed.

Quantification of energy consumed in simulated percussive drilling process using dynamic indentation experiment

The high cost of drilling deep wells is the main barrier to the widespread exploitation of deep geothermal energy. Percussive drilling is one of the significant drilling technologies used in energy exploration projects. However, there is no good quantitative understanding of how much energy in percussive drilling is consumed in pulverization, heating, the kinetic energy of particles, acoustic emission, etc. In this study, energy efficiency is quantitatively investigated to understand the percussive drilling process better. The dynamic percussive drilling was evaluated using a modified split Hopkinson pressure bar (SHPB) system and non-contact measurements. The amount of energy dissipated in different processes and the overall energy efficiency was estimated for Kuru granite, Balmoral granite, and Kivijärvi gabbro. The energy spent on the kinetic energy Ek of fragments was evaluated using a high-speed camera, whereas the energy consumed on heat or the thermal energy Et was obtained by high-speed infrared imaging. The cracking energy Ec was measured by using the surface energy of rock and the total newly created surface areas. The results indicate that the fragment size distribution of these three rocks generally varies with the penetration speed, and the fragmentation level of these rocks increases with the penetration speed. The input energy and the energy consumption grow with the increase of the penetration speed. The proportions of Et, Ek, and Ec in the total energy consumption for these three rocks increase with the penetration speed. The energy efficiency obtained from the dynamic indentation experiments for the three rocks generally increases with the penetration speed and almost approaches a limit value when the penetration speed is high. A model is improved to describe the relationship between energy efficiency and penetration speed quantitatively. Therefore, the penetration process should be optimized to balance the high drilling efficiency and the low energy consumption.

Shear Band Formation with Split Hopkinson Bar Experiments

The essence of dynamic failure is closely linked to dramatic shear deformations which often lead to the formation of adiabatic shear bands (ASB). Under high loading velocities and the subsequent rapid temperature increase, the localization of shear strain is crucial in view of safety issues of systems in mechanical and aircraft engineering, especially with respect to fast rotating components and diverse crash scenarios. In this research, we perform high speed impact tests at the split Hopkinson pressure bar (SHPB) setup and use particular hat-shaped specimen geometries that resemble the stresses and failure conditions at the component level.In the first step, we specify a notched specimen geometry using finite element (FE) simulations to ensure pure shear. Further, quasi-static compressive tests and a series of impact tests at high strain rates of 103−104s−1 are conducted on specimens manufactured from a fine-grain structural steel with the properties of S355. Optical microscopy and electron backscatter diffraction (EBSD) of the sheared zones unveil significant localization to maximal shear strains of about 0.9 accompanied by grain refinement by factors 5 to 14. The displacements across the surface of the specimens are captured with subset-based local digital image correlation (DIC) during the impact time, and serve as an objective to validate a viscoplastic constitutive relationship. More precisely, the deformation distribution is accurately reproduced by the widely recognized Johnson-Cook (JC) model, which features an enhanced description of damage evolution. Thus, combining experimental and characterization techniques, continuum mechanics and reasonable optimization strategies for the identification of model parameters provides an efficient approach for comprehensive insights into the strain localization behaviour and its impact on the mechanical performance of S355 under extreme strain rates and deformations.

Modeling and Characterization of Li-Ion 18650 Nickel–Cobalt–Alumina Battery Jellyroll Subjected to Static and Dynamic Compression Loading

This study presents a comprehensive experimental investigation of the mechanical response of the jellyroll and complete Li-ion 18650 Nickel–Cobalt–Alumina (NCA) battery under axial compression, highlighting the effects of strain rate and state-of-charge (SOC). The jellyroll was subjected to both static (1 mm/min) and dynamic (10–30 m/s) axial compression using a Split-Hopkinson Pressure Bar (SHPB). A key innovation of this work is the investigation of the role of electrolytes under both static and dynamic conditions, revealing their significant impact on stress and strain behavior due to hydrostatic pressure. Additionally, the complete NCA battery was tested under various SOC levels (0–75%) using flat plate compression. The results demonstrate the jellyroll’s sensitivity to strain rate, with increased stress responses at higher loading speeds. Furthermore, the inclusion of electrolytes markedly amplified the stress and strain response. The Fu-Chang model was successfully employed to numerically replicate the observed static and dynamic behaviors. Critically, the full battery tests revealed a negative correlation between voltage cutoff and SOC, with the risk of fire and explosion increasing at higher SOC levels. This research provides novel insights into the safety and mechanical resilience of Li-ion batteries under compression.

Dynamic failures at the metal-glass interface under impact loading

To investigate dynamic fracture behavior in the metal, three metal spheres (e.g., steel sphere, high purity tungsten sphere, and high purity lead sphere) are accelerated by the gas gun devices to impact glass spheres under the critical speed range (i.e., from 70 m/s to 210 m/s). The velocity interferometer system for any reflector (VISAR) devices are employed to measure the particle velocities at the back surface of glass sphere, and high-speed photographs are utilized to capture the failure process at the metal-glass interface. Due to the asynchronous evolutions of stress fields and strain fields in the violent failure process, the results illustrate quite different failure mechanisms from those by the Split Hopkinson Pressure Bar (SHPB) impacting. Fragmentations of the glass sphere are caused mainly by the radial cracks and the lateral cracks around the metal-glass interface and the edges of the sphere with increasing impact velocity. Dynamic failures in the three metal impactors exhibit different modes, e.g., tensile fracture in the steel impactor, shear fracture in the tungsten impactor, and compressed yielding in the lead impactor. The transferring of strain energy releasing is introduced to describe the failure behavior at the metal-glass interface, and a relaxation-diffusion equation of strain energy releasing is then established based on the experimental results and the numeric results by the discrete element method (DEM). The evolutions of failures at the metal-glass interface are discussed. Further investigation is conducted to describe the dynamic fractures in tungsten impactors and steel impactors based on the dimensional analyses, and the quantitative expressions of these strain rate dependent fracture strains and crack width in the metal impactors are obtained. The results are helpful for the profound understanding of the dynamic fracture in the metal structures and the dynamic fragmentations in the brittle material when subjected to impact loading.

Dynamic compressive behavior of functionally graded triply periodic minimal surface meta-structures

Triply periodic minimal surface (TPMS) lattice structures have gained considerable attention because of their light weight, high strength, and excellent energy absorption capabilities. However, the effect of the amplitude that can control the topological morphology of a TPMS on the dynamic properties of the TPMS structure is not yet fully understood, as previous studies have focused on the relative density and size as well as their quasi-static mechanical properties. In this study, three types of uniform sheet-based TPMS structures with different amplitudes and three types of functionally graded sheet-based TPMS structures were proposed. Experiments and numerical simulations were conducted under quasi-static and dynamic loading conditions. Six types of TPMS lattice structures made of 316L stainless steel were manufactured via powder bed fusion. Quasi-static compression tests were performed at a strain rate of 0.001 s⁻¹. The experimental results indicate that increasing the amplitude can increase the elastic modulus, plateau stress, and energy absorption capacity of a structure. Moreover, the functional gradient amplitude structure has a higher energy absorption capacity, and the structures with line and log gradient strategies improved by 17.38% and 35.43%, respectively, compared to the uniform structure with an amplitude of 1. Additionally, an idealized rigid-linear plastic hardening (R-LPH) model was proposed to predict the mechanical response of the structures. The finite element method (FEM) was used to construct dynamic compression numerical models, and their validity was verified through split Hopkinson pressure bar (SHPB) tests at a strain rate of 695 s⁻¹. The mechanical response, deformation modes, and stress enhancement effects of the structures under dynamic compression were systematically studied. The results show that the mechanical performance and energy absorption capacity of the structures under dynamic impact loading increase with increasing strain rate. The critical velocity for the transition from the quasi-static mode to the impact mode increases with amplitude. For strain rates below 6000 s⁻¹, the strain rate effect is the main factor influencing the dynamic stress enhancement. As the strain rate continues to increase, the dynamic stress enhancement results from the combined effects of inertia and strain rate, with inertia effects gradually becoming the dominant factor. This study shows that functional gradient TPMS meta-structures have excellent mechanical and energy absorbing properties under quasi-static compression and dynamic compression, with potential applications in passive safety protection.

Investigation of the dynamic mechanical response of corroded ultra-high performance fiber reinforced concrete (UHPFRC) with initial defects

This study addresses the dynamic mechanical response of the corroded ultra-high-performance fiber-reinforced concrete (UHPFRC) with initial defects, considering the possibility of corrosion deterioration induced by various pre-existing cracks during the long-term service life. For this purpose, an integrated accelerated corrosion method and Split Hopkinson Pressure Bar (SHPB)/high-speed camera etc. techniques are employed. Results show that increasing pre-impacting damage promotes the crack density and maximum width by 32.4%–62.3 % and 1.11–1.8 times, respectively. In terms of mechanical properties, coupling damages of initial defects and corrosion have adverse effects on the dynamic mechanical response. Typical fib Model Code 2010 applies to predict the DIF evolution of the corroded UHPFRC with initial defects. Numerous shear cracks are created at an angle along the weak interface as the corroded specimens with initial defects are again subjected to axial loading, revealing the associated failure mechanism. These results shed light on the dynamic response of corroded UHPFRC containing various initial defects and the failure mechanism gives some reference to service status evaluation.

Dynamic behavior of fractured gabbro treated by high temperatures

Rock masses in deep engineering are often subjected to high geothermal effect, and blasting is a key technique for deep rock excavation. It is crucial to study the dynamic characteristics of fractured rock masses under high geothermal conditions and blasting excavation. This paper focuses on fractured gabbro, examining the effects of high temperatures on its surface chromaticity and mass. Dynamic splitting tests were conducted using a Split Hopkinson Pressure Bar (SHPB) apparatus, and the microstructure was analyzed with scanning electron microscopy (SEM). The results indicate that: (1) High temperatures significantly affect the surface chromaticity and mass of gabbro samples. (2) Elevated temperatures lead to the progression from a microstructure with no noticeable microcracks to the development of extensive microcracks and mineral particle fragmentation. (3) The impacts of high temperatures on the splitting strength, peak strain, and dynamic elastic modulus of gabbro samples reach a critical point around 500 °C. At this temperature, the strengthening effect of high temperature is evident, whereas at 800 °C, these properties significantly decrease due to extensive mineral crystal fragmentation and microcrack expansion, weakening the sample's strength, stiffness, and deformation resistance. (4) From 25 °C to 600 °C, the fissure angle significantly influences the splitting strength, peak strain, and dynamic elastic modulus of gabbro. Although the fissure angle still impacts these properties at 800 °C, the overall effect of high temperature is more pronounced. (5) The failure of fractured gabbro samples is primarily characterized by tensile and shear failures, with the crack initiation angle closely related to the fissure angle.

Assessment of bending waves in Torsion Hopkinson Bar experiments using Photon Doppler Velocimetry

A Photon Doppler Velocimetry system that measures the propagation of elastic shear waves in a Torsion Hopkinson Bar (THB) system is presented. The method uses multiple fiber optic probes located symmetrically on opposing sides of the apparatus bars, and provides data with high spatial (a laser irradiated spot size of 35μm) and temporal resolution that is ultimately limited by the data acquisition system and used electronic components. A series of validation experiments simulating the movement of the bar subjected to bending and misalignments demonstrated that this approach is effective in detecting and accounting for the bending waves. The THB experiment under non-ideal conditions, where a combination of shear and bending waves propagates in the system, conclusively confirmed that the disturbance in the acquired signals can be properly addressed with the proposed arrangement of the PDV probes. It was reflected in similar measurements of the component of tangential velocity to the strain gauges. This approach shown to be complementary to the conventional strain gauge technique, but can provide better precision and be more robust under loading and/or temperature conditions that may affect the reliability of strain gauge measurements.

Longitudinal wave propagation in FG rods under impact force

A well-known fact is that one-dimensional wave analysis is the theoretical basis of the famous Hopkinson bar dynamic testing technique. The current one-dimensional wave theory is mostly confined to the slender rods of isotropic materials. It is not easy to obtain an analytical solution to the wave equation of an anisotropic rod. In this work, rods with both elastic modulus and density graded in the length direction are presented and analyzed. The one-dimensional variable coefficient wave equation corresponding to the functionally graded rod is constructed and converted into a second-order variable coefficient partial differential equation using the Laplace approach. Then, the details of solving the partial differential equation of the second-order variable coefficients are given. It is worth noting that here we construct a variable coefficient equation that satisfies the form of Euler's equation. It is still difficult to obtain analytical solutions for other equations that do not satisfy this form. Subsequently, studies of the wave propagation characteristics of rods with different graded configurations are carried out. The theoretical results show that the wave propagation behavior and post-impact vibration of the rod are significantly influenced by the graded configuration. It is possible to adjust not only the impact response at the end but also the impact response in the middle of the rod by optimizing the design of the rod's graded configuration.

Dynamic tensile intralaminar fracture and continuum damage evolution of 2D woven composite laminates at high loading rate

The intralaminar tensile failure of 2D woven composites under dynamic tensile load was investigated in this paper. Compact tension samples were tested at high loading rate with an electromagnetic Hopkinson bar system. The strain field was obtained with high-speed imaging and digital image correlation, and the J-integral method was employed to obtain the fracture toughness and corresponding R-curve. The continuum damage evolution of intralaminar failure was then analyzed by tracking the opening near the initial crack tip. It is found that the dynamic intralaminar fracture toughness is decreased by 51% compared to the quasi-static condition, the continuum damage evolution and its dependence on loading rate have been reported as well. The failure mechanisms were studied with thermal imaging and scanned electron microscopy, shorter fibre pull-out length and thinner failure process zone may be responsible for the reduced toughness at high loading rate.

Nonlinear mechanical behaviour and visco-hyperelastic constitutive description of isotropic-genesis, polydomain liquid crystal elastomers at high strain rates

The mechanical behaviour of isotropic-genesis, polydomain liquid crystal elastomers (I-PLCEs) at various strain rates is systematically investigated via experiments, theoretical analysis, and numerical modelling. Experiments encompassing SEM (scanning electron microscope), DSC (differential scanning calorimetry), TGA (thermogravimetric analyser), quasi-static and dynamic (SHPB – split Hopkinson pressure bar) mechanical tests, as well as drop-weight impact tests, are undertaken to identify the nonlinear, large-strain, rate-dependent relationship between compressive stress and deformation of the I-PLCEs studied. Subsequently, a three-dimensional compressible visco-hyperelastic constitutive model for the material is established based on the summation of Cauchy stress components. The as-used model yields good agreement with experimental data, particularly an excellent description of the mechanical responses at high strain rates of 103∼104 s−1. The fully-calibrated constitutive model is implemented in the commercial finite element code ABAQUS via a virtual user-defined material (VUMAT) subroutine. The inhomogeneous deformation processes of the I-PLCEs, corresponding to impact by a hemispherically-tipped drop weight, which induces complex stress states, are also well described. Finally, when evaluated by two dimensionless physical parameters, the I-PLCEs demonstrate a more pronounced strain rate sensitivity in terms of dynamic strength and impact toughness compared to other commonly used materials, highlighting their superior performance in dynamic loading scenarios. The present study is helpful for the design and development of impact-resistant LCE-based materials and structures.

Experimental Research on Dynamic Mechanical Properties of High-Density Foamed Concrete.

Foamed concrete is increasingly utilized in protection engineering because it offers a high energy absorption ratio and a relatively low construction cost. To investigate the dynamic properties of foamed concrete, a series of dynamic compression tests are carried out on high-density foamed concrete with densities of 800 kg/m3, 1000 kg/m3, and 1100 kg/m3 under a strain rate range of 59.05 s-1~302.17 s-1 by using a Φ-100 mm split Hopkinson pressure bar (SHPB) device. The effects of strain rate on the stress-strain relationship, dynamic compressive strength, and dynamic increase factor of foamed concrete are discussed in detail. The results show that the dynamic mechanical characteristics of foamed concrete with different densities exhibit a significant strain rate enhancement effect. Additionally, the energy absorption characteristics of foamed concrete are investigated, demonstrating that it can effectively prevent the transmission of incident energy and that its energy absorption efficiency declines as the strain rate increases. A high-speed camera was also employed to capture the failure process of foamed concrete. The results exhibit that fracture production and development induce the failure of foamed concrete, the failure process of foamed concrete advances as the strain rate increases, and the failure mode becomes increasingly severe.

Microstructural evolution and dynamic mechanical behavior of PM 2195 Al-Li alloy with resistance to shear instability in high strain rate deformation

The risk of adiabatic shear instability is common in traditional alloys covering high-strength steels, copper alloys and aluminum alloys during high strain rate deformation. However, the research on shear instability of 2195 Al-Li alloy prepared by powder metallurgy (PM) at high strain rates is still blank. The purpose is to explore its microstructure evolution and dynamic compression behavior. In this work, the 2195 Al-Li alloy prepared by PM amazingly demonstrates excellent resistance to shear instability at high strain rates during Split-Hopkinson pressure bar (SHPB) experiment. The microstructural evolution of grains and T1 (Al2CuLi) precipitate phase, and dynamic compression behavior of PM 2195 Al-Li alloy at various strain rates from 3000 s−1 to 9000 s−1 are investigated in detail employing correlative electron-backscatter diffraction (EBSD) and transmission electron microscopy (TEM). During high strain rate deformation, the alloy undergoes grain fragmentation and rotation, increasing dislocation density around the grains. When the strain rate is from 3000 s−1 to 7500 s−1, the Kernel Average Misorientation (KAM) value increases, indicating intensified localized strain. At a strain rate of 9000 s−1, a small number of grains are surrounded by nanocrystals (around 200 nm) that undergo stress release, while the average size of micrometer-sized grains is 0.81 μm. As the strain rate increases, the T1 phases in the alloy extends, vibrates, and fractures before dissolving in the matrix, while high-rate deformation promotes dislocation entanglement and defects, leading to the decomposition or ordering of precipitated phases through rapid plastic deformation. By virtue of energy dissipation strategy via the formation of nanocrystallines and dislocation behavior induced by the dissolution of T1 phases, the "positive-positive" effect is observed. The PM 2195 Al-Li alloy undergoes no shear failure even at the strain rate of 9000 s−1, showing great prospects in future applications in dynamic-loading conditions.

Dynamic fracture characterization of glass fiber composites reinforced with through-thickness nylon fibers

ABSTRACT A detailed experimental study is performed to investigate the mode-I dynamic fracture initiation toughness of through-thickness Nylon fibers reinforced glass/epoxy composites using a wet electrostatic flocking method. Composites of a double cantilever beam specimen configuration of two different deniers 1.5D and 3D of 0.75 mm long Nylon fibers with two different fiber densities 150 and 300 fiber/mm2 and composites without Nylon fibers (control) are fabricated. The dynamic fracture initiation toughness is investigated by using a modified split Hopkinson pressure bar setup with a novel fixture in conjunction with a high-speed video camera. In addition, rate sensitivity on dynamic fracture initiation toughness is also investigated for control and composites of 1.5D with fiber density of 300 fibers/mm2 as a function of three different rates of dynamic fracture toughness ( G ˙ = 1500, 2010, and 2,150 kJ/m2/s). Results show that for the composite of 3D–0.75 mm Nylon fiber, G IC values reach 0.78 kJ/m2 (110% increase) and 1.3 kJ/m2 (250% increase) at flock densities of 150 and 300 fibers/mm2 compared to the control sample with the lowest G IC value at 0.37 kJ/m2. The same pattern is observed for a 1.5D–0.75 mm configuration at the flock fiber density of 150 fiber/mm2 where G IC value demonstrates a 75% increase compared to control conditions. The dynamic fracture initiation toughness of flocked composites demonstrates significant rate sensitivity.

Mechanical properties and JC constitutive modification of tungsten alloys under high strain rates and high/low temperatures

The original Johnson-Cook model's poor prediction of W90 alloy stress at high strain rate and high and low temperatures restricts research and utilization of this alloy. This study used a Split Hopkinson Pressure Bar (SHPB) device to conduct dynamic compression tests on W90 alloy over a temperature range of [-90℃,700℃] and analyze its mechanical behavior. We examined the material's different plastic phase characteristics at high and low temperatures and conducted microstructure research on impact test specimens. Then, we analyzed the stress-strain data from SHPB experiments and quasi-static compression tests on the classical Johnson-Cook model. The original Johnson-Cook model was calibrated using the stress-strain data from SHPB and quasi-static compression tests, and the modified Johnson-Cook model was established and verified. The results show that: low temperature significantly strengthens the material's stress, while increasing temperature thermal softening reduces stress; increasing strain rate increases the strain generated by the workpiece, causing faster strain hardening and thermal softening effects, resulting in higher and more stable stress; low temperature makes thicker grain boundaries, increasing plastic stress, and increasing strain rate also increases plastic stress, as well as making the material's grain boundaries thicker. The increase of strain rate makes the grain more dense, which will also show up in the increase of stress on the macroscopic level; the modified Johnson-Cook model in this paper has high agreement with experimental data, with a mean value of MAPE of only 1.85 %,which is 90.20 % lower than the traditional model, and the average value of the RMSE is 85.56 % lower than the traditional model. The modified model significantly outperforms the traditional model in predicting stress and better describes stress changes in W90 at different temperatures.