Split Hopkinson Bar Research Articles

Response of gyroid lattice structures to impact loads

This paper reports on a comprehensive investigation of gyroid lattice structures subject to impact loading. AlSi10Mg samples were manufactured using selective laser melting (SLM) and mechanically characterized using Digital Image Correlation (DIC). Universal testing machines, drop weight rig, and a split pressure Hopkinson bar were used to mechanically characterize the aluminium alloy at strain rates ranging from, ε˙=10−3s−1 up to 5500 s−1. Gyroid samples were tested in a drop-weight rig at different impact conditions, where tests results were validated by a FE analysis. These tests, measurements, and analysis enabled the development of a modified gyroid isosurface equation, providing an opportunity to investigate their impact response, where the gyroid wall thickness, unit cell size, and modified isosurface curvature were studied at low-velocity impact conditions. The results indicate that changes in gyroid isosurface topology substantially affect its impact performance.

Mechanics of Distortion of Incident Signals Due to Screw Threads in a Tensile Split-Hopkinson Bar

Abstract A common method of generating a tensile wave in Split-Hopkinson bar experiments is to impact a hollow striker on an anvil/collar attached to the incident bar. When the anvil/collar is attached to the incident bar using screw threads, the incident signals usually deviate from the classical trapezoidal form. In order to minimize this deviation, it is a common practice to sufficiently tighten these threads. However, the fundamental reasons for (i) the signal distortion when the threads are not tightened and (ii) how the distortion is eliminated or minimized on tightening are lacking. For a given diameter of the incident bar, the parameters of the thread that govern the incident signals are its pitch and backlash between the mating threads. The effect of these two parameters is investigated using the finite element method. The finite element results show that in the absence of a backlash, the incident signals obtained from a collar that is integral to the incident bar and a collar that is attached by screw threads are practically identical. When threaded collars are used, the pitch of the screw threads does not influence the incident wave. However, the backlash leads to a dramatic distortion of the incident signals under certain conditions. Insights from the finite element analysis are used to develop a one-dimensional analytical model that explains the distortion of the incident signals. The model is also validated against experiments.

A phase field model for high-speed impact based on the updated Lagrangian formulation

High-speed impact problems usually undergo a form of highly localized plastic deformation under high strain rate with intense shock waves. In this paper, we present a coupled phase field model to simulate fracture in these situations. Our model considers the dynamic finite deformation with both strain hardening and strain rate hardening by the Johnson–Cook plasticity model. In particular, the formulation is posed in an updated Lagrangian framework and the constitutive update is realized with a return-mapping algorithm. Considering the tension–compression asymmetry, a new history variable is developed to enforce the irreversibility condition of crack propagation. The effectiveness of the proposed method is validated with a numerical example of 45 steel impact experiment along with two numerical simulations of split Hopkinson bar experiments on concrete and aluminum alloy specimens, where strong shock waves are generated. Besides, the effect of a few parameters is studied, namely, the regularization length parameter, the energy release rate, the initial impact velocity, and parameters of the Johnson–Cook model. These show the correct trend in terms of the time and extent of crack propagation.

Comparative study on microstructure and properties of different designed PTFE/Cu materials

Polytetrafluoroethylene (PTFE)/Cu is an ideal low-density candidate preferred for energetic charge warheads, which can penetrate reactive armour without explosion. In this study, six types of modified PTFE with different Cu contents were produced using cold isostatic pressing (CIPing), followed by high-temperature sintering (CIPing + HTSing). The crystallinity and phase of the samples were analysed using X-ray Different phase analysis. The stress–strain curves of PTFE/Cu under dynamic impact were evaluated using split Hopkinson bar testing. Scanning electron microscope was used to characterise the microstructure of the PTFE/Cu, as well as to analyse the dynamic failure of the six samples. According to the results, sintering produces a new material (Cu2O) and increases the crystallinity of PTFE, thus strengthening the strength and plasticity of the PTFE/Cu composites. The strength of the CIPed + HTSed PTFE/Cu composites increased sharply at first, and then increased slightly with an increase in density, while the intensity of the CIPed PTFE/Cu samples increased with an increase in density. Therefore, sintering can improve the bonding strength of the PTFE and Cu interface. In addition, the thermal mismatch between PTFE and Cu indirectly enhances the properties of the composites. CIPed + HTSed PTFE/Cu composites enhance their properties by forming molecular chains internally, while the CIPed PTFE/Cu composite changes its properties by increasing its density. Therefore, the effect of the preparation technology on the mechanical properties is direct and significant than the density.

Effect of Strain Rate on Compressive Properties of High-Speed Tool Steel

In the present study, the effect of strain rate on compressive properties of high-speed tool steel was experimentally investigated. It is not easy to obtain the stress-strain relationship under high-speed deformation since high-speed tool steel has very high strength. In order to carry out the impact compressive test of high-strength materials, we developed the split Hopkinson bar (SHB) compression test apparatus using cemented carbide for the elastic bar. The impact test was obtained at the strain rate of 103 s-1. The quasi-static test was carried out at strain rates of 10-3 to 10-1 s-1. Both tests were conducted under three temperature conditions, 298 K, 201 K, and 77 K. Within the set of experiments, almost all specimens showed an increase in flow stress with increasing strain rate (strain rate dependence of material strength) was confirmed. However, the impact test at 77 K showed that the effect of work softening was greater than the strain rate dependence of strength.

Identification and formulation of stress-strain relationship for pure nickel and Inconel 718 in a wide strain rate range （10<sup>−3</sup>/ss~10<sup>4</sup>/ss）

Cold spraying is a surface coating technique in which a powder material is impacted on a base material at high speed without melting to form a dense coating layer. However, the stress-strain relationship in the strain rate range of 5 × 103 to 2 × 104/ss , which is required for Cold spraying, has not yet been sufficiently obtained. In this study, stress-strain relationships of pure nickel and Inconel 718 as target materials in a wide range of strain rates from quasi-static strain rate (10−3/ss) to ultra-high strain rate up to 104/ss were investigated by means of a Split Hopkinson bar method and a high-speed material testing machine (TS2000 Saginomiya co. Ltd.). From the experimental results, the stress-strain relationships of pure nickel and Inconel 718 in the wide strain rate range were clarified and a strain rate sensitive constitutive model was formulated by referring the Tanimura-Mimura (2009) model.

Evaluation of impact characteristics of reduced alloy duplex stainless steel sheet

SUS821L1 has been effectively used on bridges and land rocks. However, it has been anticipated that these structures may experience impact loading from earthquakes and tsunamis. It is required to evaluate the mechanical properties of SUS821L1 by impact testing because impact loading yields different mechanical properties than static loading. In this study, a falling drop impact tester with the split-Hopkinson bar method is used to evaluate the impact parameters of SUS821L1. The impact tensile and the static tensile test have demonstrated that the maximum stress is 100MPa greater than the static tensile test and the maximum strain is 3% greater than the impact tensile test.

Investigation on the dynamic compressive behavior of waste tires rubber-modified self-compacting concrete under multiple impacts loading

The rubberized self-compacting concrete (RSCC) combines the better ductility and toughness of rubber aggregates and the better working performance of self-compacting concrete (SCC), and has gained popularity. In practice, concrete structures often suffer multiple impacts loading during their service life. Therefore, this work investigates the dynamic compressive properties of RSCC under multiple impacts loading by the split Hopkinson bar technique (SHPB) and takes the influences of rubber content, impact velocity, and thermally treated temperature into consideration. The results indicate that the incorporation of rubber aggregate gives the SCC better ductility and toughness, especially by a rubber content of 20%. The resistance to multiple impacts loading, the resistance to deformation, and the ability to absorb energy are all enhanced by the rubber aggregates. Furthermore, the thermal treatment shows a slight strengthening effect on the dynamic resistance of SCC but SCC still maintain its characteristic of brittleness. The thermal treatment shows a significant attenuation effect on the dynamic response of RSCC.

Dynamic Characterization of Additively Manufactured Polylactide (PLA)

Additively manufactured materials have excellent properties with wide applications in many industries. For designing components exposed to extreme loading situations, it is essential to characterize the high strain rate response of 3D printed (fused deposition modelling) materials. In this study, uniaxial quasi-static and dynamic compressive tests were carried out at various strain rates (10−2 s−1 and 200 s−1 to 1800 s−1) for 3D printed PLA. Strain rate dependent compressive response of Polylactide acid (PLA) disk specimens 3D printed at 0°, 45° and 90° orientations was obtained using the Split Hopkinson bar technique. The results show that the compressive strength increases with corresponding strain rates for 0° and 45° print orientations. PLA printed at 0° has higher compressive strength compared to 45° and 90° orientations under quasi-static as well as high strain rate loading. Toughness was observed to increase with strain rate in all three orientations. A simple modification to the Johnson-Cook model is proposed, which accounts for the effects of print orientation, porosity and strain softening behavior.

Effects of Acidic Environment on Dynamic Mechanical Properties and Porosity Evolution Characteristics of Coal

Coal pillars left in coal mines are often subjected to long-term submersion by groundwater and chemical solutions and are susceptible to deterioration and even destabilization damage under dynamic load disturbance. In order to investigate the effects of acidic environment on dynamic mechanical properties and porosity evolution characteristics of coal, a split Hopkinson bar (SHPB) was used to test the dynamic compressive strength and tensile strength of coal samples under different acid environment. The results showed that the sample density gradually decreased with the increase of the number of wet and dry cycles, but the decrease was significantly related to the pH value. Longitudinal wave velocity of coal sample decreases gradually with the increase of drying and wetting cycles, and the decreasing speed is first fast and then slow. The stronger the acidity of the solution, the more times the dry-wet cycle, and the higher the water absorption of the sample. In the early stage of dry-wet cycle, the coal is significantly affected, and the average deterioration degree is large. After that, the influence of cyclic action is reduced, and the average degradation degree is small. Porosity of coal increases continuously under the action of dry-wet cycle. The stronger the acidity, the greater the change in initial porosity. In the 20th cycle, the porosity of the acidic environment increases significantly at once and then decreases continuously.

On the Development of a Release Mechanism for a Split Hopkinson Tension and Compression Bar.

Split Hopkinson bars are used for the dynamic mechanical characterisation of materials under high strain rates. Many of these test benches are designed in such a way that they can either be used for compressive or tensile loading. The goal of the present work is to develop a release mechanism for an elastically pre-stressed Split Hopkinson bar that can be universally used for tensile or compressive loading. The paper describes the design and dimensioning of the release mechanism, including the brittle failing wear parts from ultra-high strength steel. Additionally, a numerical study on the effect of the time-to-full-release on the pulse-shape and pulse-rising time was conducted. The results of the analytical dimensioning approaches for the release mechanism, including the wear parts, were validated against experimental tests. It can be demonstrated that the designed release concept leads to sufficiently short and reproducible pulse rising times of roughly 0.11 ms to 0.21 ms, depending on the pre-loading level for both the tension and compression wave. According to literature, the usual pulse rising times can range from 0.01 ms to 0.35 ms, which leads to the conclusion that a good average pulse rising time was achieved with the present release system.

Study on Dynamic Mechanical Properties and Failure Mechanism of Sandstones under Real-Time High Temperature

The research on dynamic mechanical properties of rocks under high temperature is the basis for safe and efficient implementation of deep coal mining and underground coal gasification engineering. In this paper, the split Hopkinson bar (SHPB) with real-time high-temperature function was adopted to systematically study dynamic mechanical properties of sandstones. The research showed that under the condition of a fixed temperature, with the increase of strain rate, the dynamic compressive strength and dynamic peak strain of sandstone increased gradually, and the variation of dynamic elastic modulus with strain rate was not obvious. With the increase of temperature, the dynamic compressive strength of sandstone increased first and then decreased, the dynamic peak strain increased gradually, and the dynamic elastic modulus decreased overall. The variation law of macroscopic failure mode and energy dissipation density with temperature was revealed, and the change mechanism was explained considering the influence of high temperature on the internal structure of sandstone. Based on the principle of component combination and the theory of micro-element strength distribution, the dynamic statistical damage constitutive model was established, and its parameters had certain physical significance. Compared with the experimental results, the established model can well describe the dynamic stress-strain relationship of sandstone under real-time high temperature.

Effect of thermo-oxidative ageing on the thermo-mechanical responses of 3D braided carbon fiber/epoxy composites during high-speed impact

By developing a thermo-mechanical coupled model, we quantitatively analyzed the effect of thermo-oxidative ageing on the temperature, thermal strain and thermal stress histories of braided composites during high-speed impact. We also considered temperature dependence of elastic modulus during impact in this model. The proposed model was first applied to the unaged composites, then the results were compared to the impact responses captured by the split Hopkinson bar and the high-speed camera. The comparisons of the experimental and numerical results show that the proposed model could capture the adiabatic temperature rise, stress-strain responses and failure process of braided composites during high-speed impact. The validated model was subsequently employed to thermo-oxidative aged composites. Results show that the temperature rise, thermal strain and thermal stress all depend on the mechanical properties of resin, and they decreased with the increase of ageing temperature since the performance of resin is compromised by thermo-oxidation ageing. The change of resin performance after ageing leads to the change in the temperature rise during impact, so the stiffness of composites will decrease in varying degrees after ageing.

Effect of Strain Rate and Silica Filler Content on the Compressive Behavior of RTM6 Epoxy-Based Nanocomposites.

The aim of this paper is to investigate the effect of strain rate and filler content on the compressive behavior of the aeronautical grade RTM6 epoxy-based nanocomposites. Silica nanoparticles with different sizes, weight concentrations and surface functionalization were used as fillers. Dynamic mechanical analysis was used to study the glass transition temperature and storage modulus of the nanocomposites. Using quasi-static and split Hopkinson bar tests, strain rates of 0.001 s−1 to 1100 s−1 were imposed. Sample deformation was measured using stereo digital image correlation techniques. Results showed a significant increase in the compressive strength with increasing strain rate. The elastic modulus and Poisson’s ratio showed strain rate independency. The addition of silica nanoparticles marginally increased the glass transition temperature of the resin, and improved its storage and elastic moduli and peak yield strength for all filler concentrations. Increasing the weight percentage of the filler slightly improved the peak yield strength. Moreover, the filler’s size and surface functionalization did not affect the resin’s compressive behavior at different strain rates.

On the Development of New Test Techniques to Measure the Tensile Response of Materials at High and Ultra-high Strain Rates

BackgroundThere is a lack of reliable methods to obtain valid measurements of the tensile response of high performance materials such as fibre composites, ceramics and textile products at high rates of strain.ObjectiveWe propose and assess two new test techniques aimed at measuring valid tensile stress versus strain curves at high and ultra-high strain rates.MethodsWe conduct detailed, non-linear explicit Finite Element (FE) simulations of the transient response of the test apparatus and specimen during the tests and we develop simple analytical models to interpret the test measurements. We consider two test techniques: one based on the split Hopkinson bar apparatus, and suitable for strain rates of up to 1000 /s, and a second technique relying on projectile impact and aimed at measurements at strain rates higher than 1000 /s.ResultsThe simulations are successfully validated using test data at strain rates of order 200 /s and then used to predict the test performance at strain rates up to approximately 5500 /s. We find that both techniques can give valid stress versus strain curves across a wide range of strain rates.ConclusionsWe identify the limits of both techniques and recommend optimal measurement strategies for dynamic testing of materials with different ductility.

Performance characteristics of Hopkinson’s set-up pneumatic launcher

The paper presents a performance characteristics of a pneumatic launcher, which is an important element of the split Hopkinson bar set-up (SHPB) at the Department of Military Engineering and Infrastructure (the Military University of Technology in Warsaw) for the purpose of dynamic strength tests of construction materials. The process of experimental calibration of the launcher for selected loading bar-projectiles is shown. Two types of compression during direct impact tests were also used simultaneously to investigate the behaviour of metallic samples with the use of this launcher as well as the Hopkinson measuring bar: the first — a short cylindrical sample, including a miniature (small diameter) sample, and the second — a long cylindrical sample (Taylor test). The relationships describing the stress and strain state as a function of strain rate for the first type of the experiment and engineering empirical formulas for the second type of the research were given.

High-temperature compressive performance of the pre-impacted 2D-C/SiC composites: Coupling effect of pre-impact energy, temperature, temperature-keeping duration, and strain rate

Two-dimensional carbon fiber reinforced silicon carbide (2D-C/SiC) composites are well-known for the high-temperature performance in aeronautical and aerospace fields, but they are susceptible to foreign object damage in service. Therefore, the high-temperature performance of pre-impacted 2D-C/SiC is closely connected with flight security. In this study, the pre-impact testing was conducted using a stress-reversal split Hopkinson bar, and the pre-impact energies were set from 0.1J to 0.9J. Subsequently, the pre-impacted specimens were heated for a period in the temperature-keeping procedure from 20 °C to 1600 °C. Oxidation kinetics of the pre-impacted 2D-C/SiC specimens were obtained based on the weight loss-temperature trends. Finally, the specimens were compressed under high temperatures at the strain rates from 10−4/s to 1000/s, so the residual compressive strength (RCS)-temperature trends were achieved. Hence, the correlation between the weight loss and the RCS was established. Interestingly, the pre-impact strengthening effect on the compressive strength was found. The results revealed that the RCS of 2D-C/SiC is affected by various parameters, providing new sights into the high-temperature damage tolerance of ceramic matrix composites.

Effect of Acid and Alkaline Environment on Dynamic Strength and Porosity Characteristics of Bursting Liability Coal

In order to investigate the influence of acid and alkaline environment on dynamic strength and porosity characteristics of bursting liability coal, scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis were used to compare the microstructures of coal with different bursting liabilities. A split Hopkinson bar (SHPB) was used to test the dynamic compressive strength and tensile strength of coal samples with different bursting liabilities. The results show that the surface micromorphology and structure characteristics of coal samples with different bursting liabilities are representatives, which can be used as an auxiliary basis to determine the bursting liability of coal seam. The microstructure of coal with strong bursting liability is characterized by mylonitic, fragmentary, and brecciated structure, and the microstructure is diverse and complex. However, the microstructure of no bursting liability coal is single and uniform. Coal with strong bursting liability shows tensile, compressive, and shear cracks produced by tectonic action, and the distribution of cracks is complicated. The development of fissures is greatly affected by the degree of coal metamorphism, organic components, minerals, and other factors. Under acidic and alkaline environments, the decrease amplitude of tensile strength of coal is obviously larger than that in neutral solution, which indicates that under the action of acid-based solution soaking, the easily soluble minerals in coal react with hydrogen ions and hydroxyl ions in solution obviously. Porosity increment in acidic environment is much larger than that in alkaline and neutral environments. The strong bursting liability coal is more sensitive to acidic environment, while the no bursting liability coal is more sensitive to alkaline environment.

An experimental study of aggregate shape effect on dynamic compressive behaviours of cementitious mortar

An experimental investigation is conducted to study the effect of aggregate shape on mortar dynamic failure behaviours. Split Hopkinson bar device is employed to compress cylindrical mortar samples containing irregular glass aggregates and rounded glass aggregates under high strain rates from 1000 s−1 to 2500 s−1. A new insight into the aggregate shape effect on the mortar cracking mechanisms is presented at the microscale using micro-CT. The cracking characteristics are found to be highly dependent on the aggregate shape, where more rounded aggregates in mortar are less likely to possess transgranular cracks after the initiation of intergranular cracks in the weak interfacial transition zone. These microscopic cracking mechanisms are validated by the cumulative distribution evolutions of particle size and morphological parameters (elongation and flatness), which are further manifested by the dynamic compressive strength. The results demonstrate that mortar with more regular aggregates exhibits higher dynamic compressive strength and strain rate sensitivity.

Parameter Determination and Model Modification of Sherwood-Frost Constitutive Model

The Sherwood-Frost constitutive model is widely used to predict the mechanical behavior of polymer materials, and it is a very important material model. In this study, the stress-strain curve of high-density polyethylene (HDPE) with a strain rate ranging from 935 to 5450 s−1 was obtained through the split Hopkinson bar test device. Based on the HDPE dynamic mechanical performance test, the method for determining the strain-rate parameters of the Sherwood-Frost constitutive model was analysed and the strain-rate term in the constitutive model revised. Using the experimental results of Sherwood and Frost, the method to determine the parameters of the density and temperature terms in the model was introduced. This study provides a reference for parameter determination and model modification of the Sherwood-Frost constitutive model.