High Strain Rate Loading Research Articles

Manufacturing Process Effect on the Mechanical Properties of Glass Fiber/Polypropylene Composite Under High Strain Rate Loading: Woven (W-GF-PP) and Compressed GF50-PP

Manufacturing Process Effect on the Mechanical Properties of Glass Fiber/Polypropylene Composite Under High Strain Rate Loading: Woven (W-GF-PP) and Compressed GF50-PP

Twinning transfer in a near β-Ti alloy under high strain rate dynamic loading

The microstructure evolution of near β-Ti alloys with initially lamellar and equiaxed microstructures subjected to dynamic loading was investigated. Results showed that during high strain rate compression, shear failure is more likely to occur in equiaxed microstructure. Owing to temperature rise and stress concentration, the lamellar α phase in adiabatic shear band (ASB) zone was recrystallized and re-melted, while the equiaxed α phase transformed in strip-shaped following the propagating direction of adiabatic shear band. During high strain rate loading, the twinning transfer was observed between β matrix and α phase, which demonstrates that twinning is the dominant deformation mode to accommodate plastic deformation.

Modelling and characterisation of the dynamic behaviours of silicone-based composite skin simulant with short polyethylene fibres and bioactive glass particles

The field of medical injury prediction and impact biomechanics in relation to skin and its simulant materials has traditionally focused on quasi-static properties while overlooking the time-dependent behaviour strain rate sensitivity due to the challenges associated with dynamic events. In the present study, silicone-based composites reinforced with short polyethylene fibres and bioglass particles, previously identified as promising bio-integrative skin simulants, were characterised under the intermediate and high strain rates for short durations and viscoelastic behaviours for extended durations. The integration of reinforcements led to an enhancement in the compressive modulus, which elevated as the strain rate increased. Specifically, the composite with 3% reinforcement subjected to high strain rate loading exhibited a compressive modulus up to 9 times greater than that of the same material under quasi-static loading. It was found that the composites with 3% reinforcement exhibited similar dynamic behaviours to those measured in human and animal skins. Furthermore, the finite element modelling of a representative volume element of the developed composite was conducted to explore the necessity of incorporating blood vessels and pressure in numerical modelling. The results indicated that the influence of blood vessels on the viscoelastic responses of the skin simulant was more significant at high oscillated frequencies. However, the impact of blood vessels was relatively small, as evidenced by less than a 15% difference in response between models with and without blood channels under both quasi-static and harmonic loadings. Systematically characterised for the first time in both experiment and modelling, these composite skin simulants successfully emulated the dynamic properties of the human skin, offering a wide range of potential applications.

Experimental study on dynamic behaviour of High Strength Low Alloy Steels at cryogenic temperatures

The dynamic behaviour of the High Strength Low Alloy (HSLA) Steel before and after heat treatment was examined experimentally by performing quasi-static and high strain rate testing at room temperature and cryogenic temperatures. The high strain rate tensile testing of the material was performed on a Split Hopkinson Tensile Bar (SHTB) setup. A customized liquid nitrogen chamber equipped with UTM and SHTB was used to perform the cryogenic temperature testing. The stress–strain response of the material under different loading conditions was recorded and a modified Johnson–Cook material model was derived. It was observed that the flow stress of the material increased when tested at a high strain rate and it further increased at a cryogenic high strain rate loading environment. The microstructure of the material before and after heat treatment was also observed to identify the dominant phase change in microstructure contributing towards improvement in strength after heat treatment. The failure analysis of each category of the samples was examined under the Scanning Electron Microscope (SEM) after the testing. The formation of small and deep dimples for as-received samples and bigger and shallow dimples for heat-treated samples distinguished the type of failure between the quasi-static, high strain rate and cryogenic high strain rate material testing.

High strain rate deformation behavior of Al0.65CoCrFe2Ni dual-phase high entropy alloy

An industrially-cast high entropy alloy with a nominal composition of Al0.65CoCrFe2Ni, having FCC and BCC phases, was compressed at high strain rates using a split Hopkinson pressure bar under a range of test temperatures. The strain rate sensitivity and strain hardening behaviour of the alloy were analyzed. The alloy exhibited high strain hardening and excellent strain rate sensitivity owing to its large solid solution strengthening and low athermal strength contributors. The alloy also displayed outstanding dynamic strain rate sensitivity compared to similar dual-phase high entropy alloys of the AlCoCrFeNi alloy system. The alloy exhibited excellent resistance to thermal softening at high strain rate loading due to higher phonon drag associated with an increase in the operating temperature. The substructures formed in the alloy during high strain rate deformation at room temperature comprised of Lomer-Cottrell (L-C) locks, deformation twins, planar deformation bands, and high dislocation pile-ups at interphase boundaries and subgrain boundaries. The high strain rate deformation at high temperatures resulted in the formation of dislocation cells, kinks and jogs, enabling the alloy to retain strain hardening behavior. The Johnson-Cook (J-C) model accurately predicted the plasticity behavior of the alloy under high strain rate loading.

Dynamic behavior of vitrimers under high–strain rate loadings

Vitrimers, which combine the permanent structure of thermosets with the recyclability of thermoplastics, are expected to be the next generation of thermosets used in the composites. The dynamic behavior of vitrimers under high–strain rate loading is becoming increasingly important as they will be used in composite structure subjected to impact loading, however, these studies have not been reported. Herein, we investigate the dynamic behavior of epoxy–based vitrimers under high–strain rate loading by split Hopkinson pressure bar (SHPB) system. The widely studied epoxy–based vitrimers is prepared by adding very small amount of special catalyst into the conventional epoxy. The true stress – strain curves of epoxy–based vitrimers as well as conventional epoxy under the strain rates of 800/s, 2000/s and 4000/s are obtained. The effect of strain rate and the content of catalyst on the deformation behaviors are discussed in details. Moreover, a strain rate–dependent phenomenological constitutive model is developed. Finally, the absorbed energy density is obtained, and the reasons for the energy density improvement from traditional epoxy to epoxy–based vitrimers is discussed. This work explores the great potential of vitrimers in improving the impact performance of thermosets.

Preparation and penetration behavior of the reactive fine-grained tungsten heavy alloy

A new reactive fine-grained tungsten heavy alloy (RF-WHA) with self-sharpening ability was developed, which is a high-density and -strength material for armor-piercing projectiles with excellent penetration performance and reactive energy-releasing properties. This paper introduces the preparation method, microstructure and mechanical properties of the RF-WHA. Comparative experiments were conducted between RF-WHA and the conventional tungsten heavy alloy (WHA) long rod penetrating semi-infinite medium‑carbon steel target at impact velocities ranging from 1000 m/s to 1700 m/s using a 25 mm ballistic gun. The depth of penetration (DOP), crater channels and residual rods of the RF-WHA and WHA at different velocities were obtained. The experiment results show that the RF-WHA rod has reactive energy-releasing properties, and the DOP of the RF-WHA rod is 10%–15% higher than that of the WHA rod. The RF-WHA rod has an excellent self-sharpening behavior during penetrating. The failure mode of the RF-WHA rod was described by systematically analyzing the macroscopic morphological and microscopic metallurgical of crater channels and residual rods. It is noted that the self-sharpening behavior is related to the adiabatic shear sensitivity and brittle behavior of the RF-WHA under high and medium strain rate loading.

PIEZO1 gain-of-function gene variant is associated with elevated tendon stiffness in humans.

Prolonged periods of increased physical demands can elicit anabolic tendon adaptations that increase stiffness and mechanical resilience or conversely can lead to pathological processes that deteriorate tendon structural quality with ensuing pain and potential rupture. Although the mechanisms by which tendon mechanical loads regulate tissue adaptation are largely unknown, the ion channel PIEZO1 has been implicated in tendon mechanotransduction, with human carriers of the PIEZO1 gain-of-function variant E756del displaying improved dynamic vertical jump performance compared to noncarriers. Here, we sought to examine whether increased tendon stiffness in humans could explain this increased performance. We assessed tendon morphological and mechanical properties with ultrasound-based techniques in 77 participants of Middle- and West-African descent, and we measured their vertical jumping performance to assess potential functional consequences in the context of high tendon strain-rate loading. Carrying the E756del gene variant (n = 30) was associated with 46.3 ± 68.3% (p = 0.002) and 45.6 ± 69.2% (p < 0.001) higher patellar tendon stiffness and Young's modulus compared to non-carrying controls, respectively. While these tissue level measures strongly corroborate the initial postulate that PIEZO1 plays an integral part in regulating tendon material properties and stiffness in humans, we found no detectable correlation between tendon stiffness and jumping performance in the tested population that comprised individuals of highly diverse physical fitness level, dexterity, and jumping ability.

Experimental and numerical studies on pulse shaping techniques used in split Hopkinson pressure bar for testing concrete material

Split Hopkinson pressure bar (SHPB) is commonly used to characterise materials under high strain rates. However, conventional SHPB tests on brittle materials have encountered several experimental challenges for high strain rate loading. In relatively brittle materials like concrete, the deformation of the specimen is very small when subjected to impact loading; hence, it is very difficult to obtain the prerequisites of valid SHPB tests such as dynamic equilibrium and constant strain rate in the specimen. To overcome these issues, the current study presents the importance of the pulse shaper approach in SHPB application for the dynamic characterisation of concrete material. Selection of the appropriate dimension of pulse shaper assists in facilitating dynamic stress equilibrium and constant strain rate in the specimen. In the present study, copper pulse shapers are used for the evaluation of concrete under high strain rate loading using an SHPB set-up. Parameters such as the effect of dimensions (diameter and thickness) of pulse shapers on loading pulses, dynamic equilibrium, constant strain rate and material responses are studied. Experimental results reveal the prediction of suitable pulse shapers for 50–200/s strain rates. In addition, numerical simulation is also performed and results are validated with the experimental data.

Effect of strain rate on mechanical response and failure characteristics of horizontal bedded coal under quasi-static loading

Strain rock burst is one of the main types of rock bursts. Studying the mechanical response and acoustic emission characteristics of coal under quasi-static loading is significant to control and prevent strain rock bursts. In this paper, coal’s strength, deformation, energy evolution, and failure characteristics were analyzed with different strain rates under quasi-static loading. The strength characteristics of coal show a strain rate effect to a certain extent and the elastic modulus decreases first and then increases with stain rate increasing. Moreover, the elastic strain energy of coal samples always accounts for a high proportion before failure and the failure of coal presents a combined failure mode of tensile and shear under the dominance of tensile failure. The contribution of the shear failure to coal failure increases correspondingly when strain rate increases. Under quasi-static loading, There is a range where the strain rate effect does not appear, named as strain rate effect invisible area. The high static loading stress, and direct action of high strain rate loading should be avoided to reduce the risk of rock burst, especially for isolated coal pillars. The research achievements deepen the understanding of strain rock burst and provides critical support for the prevention of strain rock burst induced by high static loading.

Discontinuous Yielding of Fe E420 under High Strain Rate Loading

The discontinuous yielding behaviour of Fe E420 steel was experimentally investigated through tensile tests conducted on laminated sheets. In order to assess the response under both quasi-static and dynamic loading conditions, uniaxial tensile tests were performed according to ASTM E8 standards, and dynamic tests were conducted on the Hopkinson bar using a specially designed flat specimen. Three nominal strain rates (2×10−4, 500, and 1000) s−1 were considered. The effect of heat treatment on the material was also considered. For this purpose, the proposed experimental campaign was carried out on both the as-is material and on one treated by water quenching. All dynamic tests were performed with the direct tensile split Hopkinson pressure bar and were recorded via a high frame rate camera. The post failure fracture surfaces were investigated using scanning electron microscopy technique.

In-operando spectroscopic interrogation of macromolecular conformational changes in polyurea elastomers under high strain rate loading

Temporary and permanent macromolecular conformational changes can accompany the deformation of elastomers under high strain rate loading. Mechanical failure can occur as spallation, volumetric cracking, subsurface morphological changes, and plastic deformations. While high strain rate loading has been extensively reported using various loading mechanisms, where the current state-of-the-art relies on cascading failure and spectroscopic analyses after mechanical loading. In recent years, in-situ spectro-mechanical characterization, entailing concurrent spectroscopic interrogation and mechanical loading, has interested the scientific community in avoiding destructive evaluations in favor of noninvasive characterization, preferably during loading. To overcome the current limitations, this paper reported the first in-operando spectro-mechanical characterization of elastomeric polymers (polyurea is used as a representative material) loaded at high strain rate using bulk terahertz spectroscopy synchronized in real-time with laser-induced shock wave setup. Spectroscopic terahertz signals were collected concurrently with the imposition of shock waves based on the exfoliation of a sacrificial metallic layer using a high-energy laser pulse with nanosecond duration. The shock-loaded samples were also characterized using the scanning electron microscope, revealing signs of plastic deformations and morphological failure throughout the cross-section, including evidence of crazing and vitrification separately. Multifaceted time and frequency domain analyses elucidated the conformational changes, including spectral peak shifting, enhancement, manifestation, and concealment. The time domain analysis leveraged the dynamic time wrapping approach to quantify the temporal disparity between terahertz signals collected from unloaded, during shock, and loaded samples by calculating the Euclidean distances among signal pairs. Microscopy revealed morphological changes that corroborated the terahertz spectral differences at several energy fluences. Finite element analysis was performed to assess the levels of stresses and strains as a function of the energy fluence from focusing the high-energy laser illumination onto the sacrificial energy layer. The stresses at the depths of failure determined using electron microscopy, corresponded to the tensile strength of the material. The present results demonstrate the viability of the spectro-mechanical characterization of polymers using terahertz-based spectroscopy and laser-induced shock wave, contributing to a new experimental paradigm in polymer mechanics under shock loading.

Assessment of a high strength concrete using experimental and numerical methodologies for high strain rate ballistic impacts

The high compressive strength of concrete materials enables it to be implemented for many applications. Understanding its response under high strain rate loadings is especially important for some of these applications. This study investigates the high strain rate response of a high-performance concrete denoted as BBR9 under ballistic impact. This is conducted through both an experimentally and numerically based methodology. BBR9 targets are impacted with a spherical projectile at velocities ranging from 432 to 1459 m/s and three target thicknesses ranging from 25.4 to 58.1 mm. These impacts are studied numerically by formulating a finite element model corresponding to the experimental design in this study. The mechanical response of the concrete target is captured using the Holmquist-Johnson-Cook (HJC) concrete model with its parameters calibrated to confined compressive experiments of the BBR9 concrete. The simulations show good agreement with the kinetic experimental data with an average error across all simulation points of approximately 5%, thereby validating the HJC model for this concrete in these high strain rate loading applications.

Rate-dependent tensile behavior and statistic strength model for fiber tow composites

The mechanical performance of the fiber tow composite is crucial for textile composites. To characterize the tensile properties of the carbon fiber tow composite, a tensile test methodology for high strain rate loading was developed based on the Split Hopkinson tensile bar equipped with an ultra-high-speed camera. The tensile performance of the tow composites is measured under quasi-static and dynamic loading rates, respectively. Furthermore, the effects of gauge length, fiber counts of specimens and the strain rate sensitivity are investigated. The results indicate that the tensile strength of carbon fiber tow composite is related to the length and fiber counts of the tow specimen and affected by the loading rates. In addition, a modified Weibull model was proposed to evaluate the effects of gauge length and strain rate on the tensile strength of the carbon fiber tow composites.

Modelling and optimisation of TPMS-based lattices subjected to high strain-rate impact loadings

Lattices structures show promising applications in aerospace, biomedical and defence sectors, in which high energy absorption and lightweight structures are required. This work studies Triply Periodical Minimal Surfaces (TPMS) with potential for impact engineer applications, focusing on material characterisation, modelling and performance optimisation. For this purpose, stainless steel 316 L lattice samples made by additive manufacturing were tested in a wide range of strain-rates and various building directions using a universal testing machine and Split Hopkinson Pressure Bar, equipped with a Digital Image Correlation system. Then, the obtained properties were implemented in an explicit finite element model and validated against experimental results related to different TMPS topologies and impact scenarios. A theoretical model is also proposed to predict the TPMS-based lattices quasi-static and impact responses up to the densification threshold. Finally, the validated numerical models were used to predict the behaviour of several functionally graded TPMS topologies, indicating the architectures with superior impact performance. The graded topologies were then manufactured and experimentally tested. The results indicate that graded topologies exhibit up to 18% higher energy absorption when compared to their non-graded counterparts. The theoretical and numerical models developed in this paper provide an effective approach for designing and predicting high energy absorption architectures subjected to quasi-static and impact loadings.

Spall response of medium-entropy alloy CrCoNi under plate impact

Dynamic mechanical properties of high-entropy alloys (HEAs) or medium-entropy alloys (MEAs) under high strain rate loading are of increasing interest for their promising applications in dynamic extreme environments. Here, spall damage of a coarse-grained CrCoNi MEA with a face-centered-cubic structure is firstly investigated by combining plate impact experiments, and molecular dynamics (MD) simulations. Yield strength, spall strength and spall-induced re-acceleration at different impact velocities are derived from free-surface velocity history measurements. In addition, soft-recovered samples after spallation are characterized with scanning electron microscopy and electron backscatter diffraction. The CrCoNi MEA exhibits the highest spall strength (around 4 GPa) among the MEAs/HEAs ever reported, with similar or better impact ductility compared to the reported MEAs/HEAs. With increasing impact velocity, the dominant damage mode of the CrCoNi MEA undergoes a gradual transition from intergranular damage, to mixed intergranular and intragranular damage, and then to intragranular damage. MD simulations reveal multiple deformation mechanisms (dislocation slip, stacking faults and twinning) for the CrCoNi MEA under impact loading, and show consistent spall damage mode transition with the experimental observations. The excellent combination of spall strength and ductility for the CrCoNi MEA is mainly attributed to the pronounced intragranular void nucleation followed by ductile void growth and coalescence, ensuring broad application prospects of the CrCoNi MEA in impact scenarios.

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.

Grain scale residual stress response after quasi-static and high strain rate loading in SS316L

Grain scale residual stress response after quasi-static and high strain rate loading in SS316L

Assessment of the ballistic impact response of Cor-Tuf UHPC concrete using the HJC constitutive model

Concrete offers superior strength in compressive loadings and is implemented for many applications. The high compressive strengths enable concrete to resist high strain rate loading scenarios such as ballistic impacts. A variety of concrete denoted as Cor-Tuf, which is classified as ultra-high-performance concrete with a compressive strength of 210 MPa, is evaluated in this study. The response of this concrete is assessed through a finite element analysis under the high strain rate loadings of ballistic impacts. To capture the response of the concrete, a plasticity and damage constitutive model denoted as the HJC model is implemented. The parameters of this model are calibrated to the Cor-Tuf concrete using confined compression experiments, unconfined compression experiments, and shock experiments. The concrete target is impacted at speeds between 610 m/s through 1112 m/s, and the results are compared to existing experimental data. Our results show that the HJC model can predict the response of this impact to the Cor-Tuf concrete targets as an average error of 5.85% is found. The results of this study present parameters which can be implemented with the HJC concrete model for future studies to model the response of the Cor-Tuf UHPC.

Design, development, and calibration of split Hopkinson pressure bar system for Dynamic material characterization of concrete

Split Hopkinson pressure bar (SHPB) system is significantly used for dynamic material characterization in the range of strain rates 102–104 s-1; however, there is no standard design methodology or readily available technique for the development of this apparatus. The objective of this study is to present a detailed design, development and calibration of SHPB apparatus for dynamic material characterization of concrete in compression. The calibration of the loading and bar components has been presented with the help of experimental results and validated following an analytical approach for one-dimensional stress wave propagation. The experimental pulse duration, 124.5 microsecond, and elastic wave speed, 4820 m/s, was measured with 2% deviation from the analytical results. Under three different impact velocities, a minimum 1.09% and maximum 4.14% decrement was observed in the incident wave as compared to analytical formulation. The recorded strain signals were captured in the transmission bar with a decrement of 1, 3, and 3.3% in peak strain when compared to the incident bar, at 4.5, 4.9, and 5.7 m/s impact velocities. The incident and transmission bars had almost identical wave characteristics demonstrating that the bar system has been perfectly and precisely aligned, and almost complete wave transfer is seen to have occurred. Experiments performed on M35 concrete specimens using the developed SHPB setup have been presented and discussed. The results demonstrated that the developed SHPB setup is capable to provide accurate results for the dynamic material characterization of concrete at high strain rate loading.