Split Hopkinson Pressure Bar Method Research Articles

On alternative strain rate analysis for direct impact method

The direct impact method provides a higher sample deformation rate in comparison with the classical Split Hopkinson Pressure Bar method. The principal assumption of strain rate calculation for the direct impact method is constancy of velocity of the impacted face of the specimen during the impact. This assumption delivers reliable results for materials with relatively low yield strength and hardening rate. The present study proposes an alternative procedure for calculation of the specimen strain rate in order to improve accuracy of the direct impact method for wide range of metals and alloys. The proposed procedure is based on the assumption that the transmitted pulse can be splitted into parts, one of which corresponds to stress and the other to strain rate. The procedure has been validated by finite element analysis and the semi-analytical modelling of the direct impact tests of pure copper C101 and additively manufactured Inconel 718 alloy. The proposed method qualitatively changes the shape of the stress-strain curve by adding an unloading area. The numerically estimated accuracy of the method is limited by 4.5% for Inconel 718 and 0.6% for copper C101. The proposed alternative analysis of the strain rate for a direct impact has a number of advantages over the classical methods and can be used in the study of various materials in a wide range of strain rate.

Dynamic Behaviour of Selective Laser Melted 316L Steel - Mechanical Properties and Microstructure Changes

316L steel specimens with three different shear zones made by SLM (Selective Laser Melting) were subjected to dynamic tests using the Split Hopkinson Pressure Bar method. The effect of high-speed deformation on changes in microstructure was analyzed. In addition, the stress-strain relationship was determined from the SHPB results. To visualize the deformation process of the specimens during the tests, a camera with a high frame rate was used. It was shown that as the plastic deformation increases, the hardness of the material increases. Microstructural analysis of dynamically loaded areas revealed numerous defects. Twinning was found to be the main deformation mechanism. Large plastic deformation and many other microstructural changes such as shear bands, cracks and martensite nucleation were also observed.

High strain rate behaviour of different types of chocolate

The mechanical behaviour of five different chocolate types (dark, extra dark, milk, white, and ruby) at high strain rates was studied. The experimental arrangement known as the Split Hopkinson Pressure Bar (SHPB) method was used. This experimental procedure enables to study the mechanical behaviour of materials at strain rates up to 103 1/s. The use of SHPB technique shows that the mechanical properties of tested chocolates significantly increase with the strain rate. This increase is much higher than that observed during the quasi static testing where the strain rate achieves values about 10−3 1/s.

An overview of methods used in dynamic testing of materials

The paper presents an elemental description and methodology of tests that take intoaccount the effect of high strain rate on materials strength parameters. The article describes suchtesting methodology as: Taylor test, split Hopkinson pressure bar method, and expanding ring test todetermine the dynamic properties of materials.Keywords: Taylor test, expanding ring test, split Hopkinson pressure bar, dynamic behaviour of materials

DYNAMIC COMPRESSIVE PROPERTIES OF ALUMINIUM-MATRIX COMPOSITES REINFORCED WITH SiC PARTICLES

The aluminium-matrix composites (AMCs) consisted of (5, 10 and 15) x/% SiC particles (SiCp) in an aluminium alloy 7055 matrix. Specimens were taken from hot-press sintering. High-strain-rate tests were performed using the split-Hopkinson pressure bar (SHPB) method. The microstructures were observed with a scanning electron microscope (SEM) to understand the damage mechanisms of the SiCp/7055 Al composites at high strain rate. The SHPB test results show that the SiCp-reinforced composites are more sensitive to strain rate than the unreinforced material. The strain-rate sensitivity of the flow stress of these composites increases substantially with the increase of the strain rate. The flow stress of SiCp/7055Al composites with 10 x/% and 15 x/% SiCp at 3000 s–1 first increases and then decreases with the increase of the plastic strains, which was caused by the heat generated during adiabatic compression. Microstructure-characterization results show that SiCp cracking and SiCp/7055Al interface debonding are the main damage mechanisms of the composites. The SiCp volume fraction and strain rate affect the damage of composites during the dynamic compressive deformation of the SiCp /7055Al composites.

Identification of Dynamic Behavior Models of Concrete B22.5

We discuss experimental and numerical studies of the deformation and destruction of fine-grained concrete B22.5 under dynamic loading. The experiments were carried out using the Kolsky (or split-Hopkinson pressure bar) method, and its modifications in the strain rate range from 400 to 2000 s-1. The rate dependences of ultimate stresses and fracture energy in tension and compression are obtained. Based on experimental data, the identification of the dynamic component of two models from the LS-DYNA computational complex was carried out: *MAT_CONCRETE_DAMAGE and *MAT_CSCM. The results of a comparative analysis of the identified models based on single-element modeling and comparison with experimental data are presented. It is shown that the obtained experimental strain rate dependences of the fracture characteristics can significantly improve the predictive ability of the model compared to the default parameter set. Information about the rate dependence of the fracture energy in *MAT_CSCM model makes it possible to more realistically simulate the behavior of the material beyond the ultimate stress.

Dynamic strain field for granite specimen under SHPB impact tests based on stress wave propagation

The full-field strain of rock material under dynamic compression load was studied using the high-speed three dimensional digital image correlation (3D-DIC) method. The dynamic test was conducted on Laizhou granite using a split Hopkinson pressure bar (SHPB) method. Wave propagation, dispersion and radial inertial effect on the specimen were found by DIC results. A recovery of strain in the post-peak stage was detected on the specimen by DIC, which was unrevealed in the traditional one-dimensional theory method. It can be found that the strain measured by strain gauge was a calculated average one, whereas the strain measured by 3D-DIC could reflect more variation details. Specifically, the testing principle with impact loads and rock dynamic behavior was re-examined using stress wave propagation theory. The theoretical results showed that the specimen reached equilibrium after a series of wave reflections and transmissions and its stress was infinitely close to the initial value of 109.2 MPa. Moreover, the specimen had the calculated maximum strain of 0.52% and strain rate of 15.11 s−1, improving the reasonable agreement with the experimental results and requirements of rock mechanical properties measured by SHPB technology.

Study on the response of the Steel / Poly-ether-ether-ketone layered protective structure for a fluidic DTH hammer

The impedance-graded material (IGM) systems can be used as a protective structure to attenuate the stress. Due to the fluidic oscillator of the down-the-hole (DTH) hammer made by cemented carbide was broken during the test, it is urgent to design a protective structure to decay the stress transferred to the fluidic oscillator. This paper studied the stress attenuation effect of Steel-Poly-ether-ether-ketone (PEEK) -layered-protective-structure (SPLPS) subjected to low-velocity impact loadings. The contact state of samples is measured by pressure-sensitive test paper. The experiments were carried out by the split Hopkinson pressure bar (SHPB) method. The stress attenuation effect of 5-layer SPLPS was tested at the different impact velocities. A three-dimensional numerical simulation model is established to simulate the SHPB experiments with the same boundary conditions. The stress attenuation effect of different layers of SPLPS is compared by numerical simulation. Besides, the influence of the thickness of steel-sheet and PEEK-sheet is investigated. And the SPLPS made for the DTH hammer has been tested in the hot dry rock drilling field. The results show that the data of numerical simulation are consistent with the SHPB experimental results. It is found that the different impact velocities do not change the contact state of the sample's end face. The stress attenuation ratio of 5-layer SPLPS in numerical simulation and experiment is 77%. The attenuation ratio increases rapidly when the number of SPLPS layers changes from 1 to 4. When the number of SPLPS layers exceed 4, the attenuation ratio does not increase significantly. Additionally, the thickness of 6 mm and 8 mm are suitable parameters for the SPLPS. The field test has proved that the SPLPS could protect the fluidic oscillator effectively. • A Steel-Poly-ether-ether-ketone (PEEK) layered protective structure (SPLPS) is designed for a fluidic DTH hammer. • The stress attenuation ratio of the 5-layer SPLPS is up to 77% under the low-velocity impact. • The SPLPS have the advantages of simple structure, high service life and low cost for engineering application.

Dynamic Behaviors of Mortar Reinforced with NiTi SMA Fibers under Impact Compressive Loading.

In this study, a compressive impact test was conducted using the split Hopkinson pressure bar (SHPB) method to investigate SMA fiber-reinforced mortar’s impact behavior. A 1.5% fiber volume of crimped fibers and dog-bone-shaped fibers was used, and half of the specimens were heated to induce recovery stress. The results showed that the appearance of SMA fibers, recovery stress, and composite capacity can increase strain rate. For mechanical properties, the SMA fibers reduced dynamic compressive strength and increased the peak strain. The specific energy absorption of the reinforced specimens slightly increased due to the addition of SMA fibers and the recovery stress; however, the effect was not significant. The composite behavior between SMA fibers and the mortar matrix, however, significantly influenced the dynamic compressive properties. The higher composite capacity of the SMA fibers produced lower dynamic compressive strength, higher peak strain, and higher specific energy absorption. The composite behavior of the dog-bone-shaped fiber was less than that of the crimped fiber and was reduced due to heating, while that of the crimped fiber was not. The mechanical properties of the impacted specimen followed a linear function of strain rate ranging from 10 to 17 s−1; at the higher strain rates of about 49–67 s−1, the linear functions disappeared. The elastic modulus of the specimen was independent of the strain rate, but it was dependent on the correlation between the elastic moduli of the SMA fibers and the mortar matrix.

Development of impact small punch test for investigating energy absorption

Energy absorption of steel with Transformation-induced Plasticity (TRIP) under bending deformation needs to be carefully examined at very high strain rates for its new applications in the automotive industries. Energy absorption members made of the steel can suppress severe damages by traffic accidents. However, it is difficult to apply three-point bending test based on the split Hopkinson pressure bar (SHPB) method because a use of a thick pre-cracked specimen is required for such materials with high toughness. In contrast, if the impact small punch test, which dominant deformation mode of a relatively-small specimen is bending, is established, its experimental arrangement might become simpler and the SHPB technique is applicable. In this study, a novel and reliable impact small punch testing method based on SHPB technique with high accuracy is developed for an investigation on impact energy absorption of the steel with TRIP. Besides, its design, feasibility and validity are confirmed by finite element simulation for austenitic stainless steel SUS304, a representative of steel with TRIP. The obtained experimental as well as computational results show that the developed testing apparatus for impact small punch test based on SHPB technique is reliable and reasonable for the investigation on impact energy absorption of the high toughness material including steel with TRIP.

Effect of particle size on dynamic mechanical behaviors of W particles/Zr-based bulk metallic glass composites

The effect of particle size on the dynamic mechanical behavior of W particle /Zr-based bulk metallic glass composites (Wp/BMGCs) was investigated by split Hopkinson pressure bar (SHPB) and finite element method (FEM). The yield strength decreases with the increase of W particles size under quasi-static compression, while the fracture strain increases. The BMGC with 60% Wp shows the highest fracture strain, and the BMG matrix shows a uniform rheological characteristic which is different from the brittle fracture characteristic. Under dynamic compression, the yield strength of the BMGCs does not show obvious strain rate sensitivity, and the dynamic fracture strain is significantly lower than the quasi-static fracture strain. With the increase of the W particles size, the cladding on the BMG matrix weakened, and the fracture mode changed from fragmentation to shear fracture. We consider that the combination of interface debonding and cleavage fracture was the very reason for the changed fracture mode. It is found that the Ansys/LS-DYNA code is able to predict the response of the Wp/BMGCs in the element mesh if special care is taken, and good agreement is in general obtained between the numerical simulations and experimental results.

DYNAMIC PROPERTIES OF LOW-CARBON STEEL AFTER LONG-TERM STORAGE

Dynamic properties of low-carbon steel are investigated using the Kolsky method (the split Hopkinson pressure bar method). Changes in the properties of the steel after 50 years of storage are determined, and the destroyed samples are subjected to metallographic studies. The fractal dimension of the fracture contours is determined. It is shown that, the long-term storage of steel introduces no significant changes in its properties.

Mechanical properties and failure behavior of rock with different flaw inclinations under coupled static and dynamic loads

The deep fissured rock mass is affected by coupled effects of initial ground stress and external dynamic disturbance. In order to study the effect of internal flaw on pre-stressed rock mechanical responses and failure behavior under impact loading, intact granite specimens and specimens with different flaw inclinations are tested by a modified split Hopkinson pressure bar (SHPB) and digital image correlation (DIC) method. The results show that peak strain and dynamic strength of intact specimens and specimens with different flaw angles (α) decrease with the increase of axial static pressure. The 90° flaw has weak reduction effect on peak strain, dynamic strength and combined strength, while 45° and 0° flaws have remarkable reduction effect. Specimens with 90° flaw are suffered combined shear and tensile failure under middle and low axial static pre-stresses, and suffered shear failure under high axial static pre-stresses. Specimens with 45° and 0° flaws are suffered oblique shear failure caused by pre-existing flaw under different axial static pre-stresses. Besides, based on digital image correlation method, it is found that micro-cracks before formation of macro fractures (include shear and tensile fractures) belong to tensile cracks. Tensile and shear strain localizations at pre-existing flaw tip for specimen with 45° and 0° flaws are produced much earlier than that at other positions.

Dynamic High Strain Rate Characterization of Lithium-Ion Nickel–Cobalt–Aluminum (NCA) Battery Using Split Hopkinson Tensile/Pressure Bar Methodology

The dynamic behavior of the lithium-ion battery is evaluated by simulating the full battery system and each corresponding component, including the jellyroll and thin-foil electrodes. The thin-foil electrodes were evaluated using a novel design of split Hopkinson tensile bar (SHTB), while the jellyroll was evaluated using the split Hopkinson pressure bar (SHPB). A new stacking method was employed to strengthen the stress wave signal of the thin-foil electrodes in the SHTB simulation. The characteristic of the stress–strain curve should remain the same regardless of the amount of stacking. The jellyroll dynamic properties were characterized by using the SHPB method. The jellyroll was modeled with Fu-Chang foam and modified crushable foam and compared with experimental results at the loading speeds of 20 and 30 m/s. The dynamic behavior compared very well when it was modeled with Fu-Chang foam. These studies show that the dynamic characterization of Li-ion battery components can be evaluated using tensile loading of stacked layers of thin foil aluminum and copper with SHTB methodology as well as the compressive loading of jellyroll using SHPB methodology. Finally, the dynamic performance of the full system battery can be simulated by using the dynamic properties of each component, which were evaluated using the SHTB and SHPB methodologies.

Dynamic Compression Strength of Ceramics: What was Learned from an Interlaboratory Round Robin Exercise?

The dynamic compressive strength of a commercial alumina was determined by five participants using the Split-Hopkinson pressure bar (SHPB) method with a dumbbell-shaped specimen. Each participant used their own SHPB test apparatus and imaging set-up to conduct the tests and capture the fracture process. The dumbbell-shaped specimen was designed to increase the likelihood of fracture commencing in the specimen gage section rather than at the interface of the specimen and the SHPB bars. The participants had a good success rate (between 40 and 80%) of tests being deemed valid, even though this was the first time several of the participants had used this specimen geometry. The average dynamic compression strength from the five participants were in excellent agreement with each other and ranged from 4.40 to 4.92 GPa with a grand average of 4.61 GPa ± 0.25 GPa (the average of the laboratory averages). The high success rate and remarkable consistency of the results show that the dumbbell-shaped specimen is the most optimum specimen geometry investigated to-date to determine the dynamic compression strength of advanced ceramics using the SHPB test methodology.

Energy Absorption Mechanism of Thermoplastic Fiber-Reinforced Plastics under Impact Loading Using Split-Hopkinson Pressure-Bar Method

The energy absorbing performance in the progressive failure of glass long-fiber-reinforced polyamide was evaluated by using the split Hopkinson pressure-bar method. An impact compression test of glass long-fiber-reinforced polyamide was performed from –30 °C to 90 °C, and the temperature-independent energy absorbing performance was confirmed only for the progressive failure mode. To clarify this phenomenon, compression tests, interlaminar compressive shear tests and mode-I fracture-toughness tests were conducted under static and impact conditions. The compression strength and the shear strength of all specimens decreased with an increase in temperature. The toughness improved with temperature. In addition to the mechanical tests, failure-mode analysis was performed by using a three-dimensional X-ray microscope to clarify the absorbing mechanism. From the above, it was concluded that the temperature-independent energy absorbing performance results from a balance of these mechanical properties against the temperature change.

Review of Intermediate Strain Rate Testing Devices

Materials undergo various loading conditions during different manufacturing processes, including varying strain rates and temperatures. Research has shown that the deformation of metals and alloys during manufacturing processes such as metal forming, machining, and friction stir welding (FSW), can reach a strain rate ranging from 10−1 to 106 s−1. Hence, studying the flow behavior of materials at different strain rates is important to understanding the material response during manufacturing processes. Experimental data for a low strain rate of <101 s−1 and a high strain rate of >103 s−1 are readily available by using traditional testing devices such as a servo-hydraulic testing machine and the split Hopkinson pressure bar method, respectively. However, for the intermediate strain rate (101 to 103 s−1), very few testing devices are available. Testing the intermediate strain rate requires a demanding test regime, in which researchers have expanded the use of special instruments. This review paper describes the development and evolution of the existing intermediate strain rate testing devices. They are divided based on the loading mechanism; it includes the high-speed servo-hydraulic testing machines, hybrid testing apparatus, the drop tower, and the flywheel machine. A general description of the testing device is systematically reviewed; which includes the working principles, some critical theories, technological innovation in load measurement techniques, components of the device, basic technical assumption, and measuring techniques. In addition, some research direction on future implementation and development of an intermediate strain rate apparatus is also discussed in detail.

Multi-scale numerical simulation of impact failure for cylindrical CFRP

In this study, we aimed to predict various impact failure behavior of cylindrical CFRPs with multiscale analyses using individual properties of fiber, interface, and resin including strain-rate dependence. We conducted compressive impact tests for the cylindrical CFRPs using split Hopkinson pressure bar (SHPB) method. Tests were carried out under four different compressive condition; radial direction and axial direction compressive tests for CFRP-H in which the fiber was aligned in the hoop direction and CFRP-L in which the fiber was aligned in the axial direction. Strengths in Hashin damage theory used in the macroscale analyses were determined by microscale analyses. Failure envelope in transverse direction and fiber-axial shear strength were obtained by 3D periodical unit cell analyses with considering the strain-rate dependence of resin strength. In addition, tensile and compressive strengths in fiber-axial direction were determined using the simultaneous fiber failure model and the fiber microbuckling model, respectively. Results of macroscale SHPB analyses were compared to test results. In terms of fracture process, analysis results did not match to test results because we conducted simulations without considering fracture energy. In terms of failure strength, failure load and failure occurred point, we could confirm rather good agreement between analysis results and test results.

Shear band widening mechanism in Ti–6Al–4V under high strain rate deformation

Abstract

Determination of the mechanical properties of concrete using the split Hopkinson pressure bar method

Fine-grained concrete was subjected to tests with and without a pulse shaping techniques. Procedure of dispersion shift of pulses in the measuring bars was used in processing the experimental data. The time histories of forces acting on the sample from the side of measuring bars and the strength characteristics of the material obtained on their basis are compared when using a pulse shaper and correcting the shape of pulses taking into account dispersion as well as with traditional testing and data processing.