Split Hopkinson Bar Research Articles

Numerical Simulation on the specimen dynamic plastic deformation behaviour in the torsional split Hopkinson bar test

The torsional split Hopkinson bar (SHB) is an important method to study the dynamic shear behaviour and shear localization of materials under high strain rates. Different specimen sizes were used in literatures, and the size of the specimen might have an effect on the experimental results. Numerical simulation on torsional SHB tests was carried out with LS-DYNA. The strain signal on the incident and transmitted bars were obtained from the simulation just as the experiment. Then the numerical strain-stress relationship of the material was derived from the numerical strain signal using the experiments data process of torsional SHB. The agreement between numerically derived strain-stress results and the specimen material properties specified in numerical modelling indicates that the torsional SHB is applicable to study the dynamic shear behaviour of materials under high strain rates. The specimen gauge diameter has no significant effect on the dynamic torsional test result. However, higher adhesive strength is required to fix the larger gauge diameter specimen on the bars. The specimen gauge thickness has little effect on the experimental results with a modified formula to calculate the specimen stress. Still, the increase of specimen gauge thickness will lead to the increase of non-uniformity of specimen stress and strain (strain rate). Based on the simulation analysis, suggestions on the specimen size design are given as well.

A novel method for severe plastic deformation at high strain rate

Severe plastic deformation (SPD) processing is defined as any method of forming under an extensive hydrostatic pressure that may be used to impart a very high strain to a bulk solid without any significant change in dimensions of the sample, producing exceptional grain refinement. Most of the SPD techniques employ very low processing speeds, however increased deformation rates are known to have a significant effect on the final microstructure. Most of the SPD processes operating at high rates do not impose hydrostatic pressures to the material and can therefore only be used for very ductile materials, while in others, the microstructural changes are limited to the surface layers of the material. To circumvent these restrictions a novel facility has been designed and developed where high hydrostatic pressures are maintained while a high shear deformation is imposed at high strain rates. The device combines the features of a high pressure torsion (HPT) unit with the principle of a torsional split Hopkinson bar (SHB) setup. A small ring-like sample, placed between two molds, is first subjected to a high, static pressure and subsequently to a high speed shear deformation upon release of torsional energy stored in a long bar. Although, the principle is rather straightforward, the design of the setup was extremely critical because of the high forces and energies involved. Tests have been performed on commercially pure aluminum. The material hardness increased in accordance with the microstructure and processing conditions; viz. annealed, only compressed and applied shear strain. Deformed grains departed from equiaxed shape and showed morphological texture in the direction of the shear even at very low strains indicating the presence of shear strains in the material. Further the material, or more specifically its mechanical properties and microstructure evolution is compared with conventional, statically deformed HPT samples.

Predicting the high strain rate response of plasticised poly(vinyl chloride) using a fractional derivative model

Polymers are frequently used in fields as diverse as aerospace, biomedicine, automotive and in-dustrial vibration damping, where they are often subjected to high strain rate or impact loading. Poly(vinyl chloride) (PVC), and its plasticised variants (PPVC), are just two examples of this broad category of materi-als. Since many polymers exhibit strong rate and temperature dependence, including a low temperature brittle transition, it is extremely important to understand their mechanical responses over a wide range of loading con-ditions.PVC with 60 wt% plasticiser is used in this study, as its highly rubbery nature lends itself well to being used in various load mitigation and energy absorption applications. It is challenging to obtain high strain rate data on rubbery materials using conventional techniques such as the split-Hopkinson (Kolsky) bar. Therefore, alternative approaches are required. Based on previous work developing a framework to predict high rate re-sponseusing a fractional derivative model, Dynamic Mechanical Analysis (DMA) experiments are conducted on the PPVC to construct a master curve of storage modulus. These data are used to part-calibrate a modified Mulliken-Boyce model which also takes into account specimen heating to derive stress-strain relationships at strain rates varying from 0.001 s_1 to 13 500 s_1. This model is further calibrated against experiments conducted in a previous study and shown to provide an excellent description of the behaviour at these rates.

Investigation of the Hygrothermal Effect on the Nanocomposite Material under High Strain Rate Torsion

This study aims to explain the effect of high strain rate torsion on unidirectional fiber glass / epoxy composite with and without adding nano particles (carbon and alumina) with weight percentage 2% and study the effect of the temperature and humidity on the samples under torsion dynamic test. Using the torsional Hopkinson split bar to testing under angle of the twist θ=10°. It can noticed that the maximum of the shear (stress, strain) of the composite increased with adding nano particles by rate of (24.8%, 3.2%) with added 2% nano carbon and (26%, 8.5%) with added 2% nano alumina, where the shear stress, shear strain and shear strain rate of all samples decreased with increasing (temperature and humidity) with different percentages. Finally, it was modifying Johnson-Cook constitutive model by adding parameter represented the humidity effect to simulate the experimental results, and then it was found the error percentages between the experimental results and theoretical results which range (0.0044% and 15.5%).

Strain rate jump tests on an austenitic stainless steel with a modified tensile Hopkinson split bar

This paper presents an improved experimental setup for high strain rate testing based on the modified Tensile Hopkinson Split Bar device developed previously at TUT. The test setup can be used to study the effects of a sudden large change in the strain rate on the stress flow of the material. The setup allows deforming the sample at a low rate and at isothermal conditions before the high rate loading. During the strain rate jump, the deformation rate is rapidly increased by approximately six orders of magnitude. In this work, the low and high rate deformation of the specimen was recorded with a combination of low and high-speed digital cameras and analyzed using the Digital Image Correlation technique. The measurement provides information about the effects of the strain rate jump on the macroscopic response of the material and allows accurate observation of the deformation of the sample just before, during, and immediately after the strain rate jump, when the conditions change from isothermal to adiabatic. In this paper, we present the results for a metastable austenitic stainless steel and discuss the effects of the strain rate jump on the strain-hardening rate, compare the experimental results with numerical results from a thermomechanical model, and evaluate the effects of the preceding deformation at a low strain rate on the strain localization. We conclude that the strain rate jump results in a clear decrease in the strain-hardening rate, the deformation following the jump is uniform along the gauge section, and that the strain localization is not significantly affected by the strain rate or the amount of pre-strain in the studied conditions.

Split Hopkinson Pressure Bar Apparatus for Compression Testing: A Review

For a long time tests have been created to decide the quality of materials under static stacking conditions. Prior, material properties were measured utilizing water driven or screw sort testing machines that were just equipped for acquiring a greatest strain rate in the range of 0.1 every second. From there onwards other test installations have been created that can accomplish a strain of around 100 every second. The most well-known and generally utilized strategy to decide dynamic material properties is the Split Hopkinson Pressure bar (SPHB) device and strain rates are in the extent of 10 2 – 10 4 every second. Notwithstanding high strain rates, it has been utilized for testing materials with lower strain rates, for example, concrete. In some tests, it has been utilized to describe high temperature conduct. In the recent times, the SPHB mechanical assembly discovers its utilization in property assessment of composite materials. In the split Hopkinson bar test, a short barrel shaped specimen is sandwiched between two long bars. The bars are by and large made of gentle steel with distances across 20mm and a length 500mm.The finishes of the weight bars and specimen are machined level to implement recommended limit conditions. Huge headways executed from the ranges of testing strategies, numerical techniques, and sign preparing have enhanced the exactness and repeatability of high strain rate testing. A few elements impact the precision of the outcomes including longitudinal wave scattering, impedance befuddle of the bars with the specimen and transducer properties among others. Dispersive nature of the bars is to be known with a specific end goal to foresee the weight beats accurately. The paper gives an extensive survey of the SPHB systems used for material property assessment.

Dynamic fracture of a dual phase automotive steel

Dynamic testing of sheet metals has become more important due to the need for more reliable vehicle crashworthiness assessments in the automotive industry. The study presents a comprehensive set of experimental results that covers a wide range of stress states on a dual phase automotive sheet steel. Split Hopkinson bar tensile (SHBT) tests are performed on dogbone shaped samples to obtain the plastic hardening properties at high strain rates. A set of purpose designed sample geometries comprising of three notched dogbone tension samples is tested at high strain rates to characterise the dynamic damage and fracture properties under well controlled stress states. The geometry of the samples is optimised with the aid of finite element analysis. During the tests, high speed photography together with digital image correlation are implemented to acquire full field measurements and to gain more insight into the localisation of strains at high strain rates. An experimental-numerical approach is proposed to effectively determine the fracture characteristics of the dual phase steel under extreme conditions. A modified Bai-Wierzbicki model is implemented to assess the damage initiation and subsequent failure. Additionally, the fracture mechanisms are studied utilizing scanning electron microscopy.

A Review of the Torsional Split Hopkinson Bar

Mechanical behavior of materials at medium and high strain rates (101∼104 s−1) is the foundation of developing mechanical theories, building material models, and promoting engineering design and construction. The torsional split Hopkinson bar (TSHB) is an effective experimental technique for measuring the pure shear mechanical properties of materials at high strain rates. In this study, the state‐of‐the‐art in TSHB experimental technique is presented. Five typical types of TSHB loading mechanisms, i.e., prestored energy loading, explosive loading, direct impact loading, flywheel loading, and electromagnetic loading, were systematically reviewed. The TSHB fundamentals were outlined, which include elementary components, basic assumptions, working principles, the pulse shaping technique, specimen design, and the single‐pulse loading technique. In addition, the combined loading and high/low temperature experimental techniques, which were developed based on TSHB, were also discussed in detail. Nearly all necessary elements for conducting a TSHB experiment and analyzing the experimental data were provided. Some research directions should be further pursued, such as extending the range of applicable materials and developing the combined loading techniques.

Numerical Study on Impact Testing of Low-Strength Materials Using Flanged Split-Hopkinson Bar Method

In this study, we investigated the effect of flange part on stress wave propagation in the compression test of low strength material using flanged Split Hopkinson Bar method. Due to the flange with a larger radius than the input/output bar, a pair of reflected waves of compression and tension with apparently different shapes from the one-dimensional reflected wave depending on mechanical impedance mismatch was observed. Depending on the relationship between the elastic wave propagation speed in the input/output bar and the gauge position, the precursive reflected wave of compression corresponds to the rising time of the stress wave, and the tensile reflected wave propagated subsequently corresponds to the falling time. It was found that when the flange is used for impact compression test of low strength material, the effect of the flanges on the reflected wave becomes small. However, it was suggested that the transmitted wave may be affected in the impact compression test of low strength material.

Challenges related to testing of composite materials at high strain rates using the split Hopkinson bar technique

The design of sample geometries and the measurement of small strains are considered the main challenges when testing composite materials at high strain rates using the split Hopkinson bar technique. The aim of this paper is to assess two types of tensile sample geometries, namely dog-bone and straight strip, in order to study the tensile behaviour of basalt fibre reinforced composites at high strain rates using the split Hopkinson bar technique. 2D Digital image correlation technique was used to study the distribution of the strain fields within the gauge section at quasi-static and dynamic strain rates. Results showed that for the current experiments and the proposed clamping techniques, both sample geometries fulfilled the requirements of a valid split Hopkinson test, and achieved uniform strain fields within the gauge section. However, classical Hopkinson analysis tends to overestimate the actual strains in the gauge section for both geometries. It is, therefore, important to use a local deformation measurement when using these 2 geometries with the proposed clamping technique.

High Rate Characterization of Three DP980 Steels

Advanced high strength steels (AHSS) are used extensively in the automotive industry in the ongoing effort to reduce vehicle weight. Their increased strength allows for the reduction of sheet thickness, and thus a reduction in mass, while offering formability and cost advantages when compared to other metal alloys typically considered for lightweight applications. DP980 steels are AHSS being considered for structural energy absorbing components; however, there is a lack of published information on their high rate behaviour. This paper presents the results of an experimental program that characterized three production DP980 steels from three different manufacturers at strain rates of 0.001, 1, 10, 100 and 1,000 s-1. An electro-mechanical frame was used for the quasi-static tests, the 1, 10, and 100 s-1 tests were carried out using a fast hydraulic apparatus and the 1,000 s-1 experiments were carried out using a tensile split Hopkinson bar. The quasi-static hardening response at strains higher than the uniform elongation of about 7% was obtained by using a shear test, thus avoiding the use of inverse modelling techniques. The results indicate that the DP980 steels are moderately rate sensitive, with one of the materials showing higher sensitivity than the others. One of the materials exhibited a yield point phenomenon that appears to affect the behaviour of the material at 100 and 1,000 s-1, however, the reasons for this behaviour remain an open question. The data was fit to modified Johnson-Cook and Cowper-Symonds model to account for rate sensitivity. The results presented in this paper provide a tool for modelling the dynamic behaviour of DP980 steels.

Intralaminar fracture toughness of UD glass fiber composite under high rate fiber tension and fiber compression loading

Automotive and aeronautical composite structures can be subjected to dynamic loading scenarios (e.g. crash, bird strike). In both industrial areas, energy-based damage models for composite materials, able to predict initiation and evolution of damage of a composite ply under various loading conditions, are increasingly used. These models require the specification of fracture toughness parameters for the main failure modes. In previous works [1],[2], the authors have successfully enhanced a methodology suggested by Catalanotti et al. [3],[4] to measure the fiber failure crack resistance curves of a UD carbon fiber composite under dynamic loading. This methodology uses the relation between the energy release rate, the crack resistance curve and the size-effect law. For the determination of the size-effect law, double-edge-notched specimens of different sizes are tested. In this work, the developed approach is used for the measurement of the fracture toughness for fiber tension and compression failure of a UD glass-epoxy composite under high rate loading. Static tests are performed on an electromechanical test machine. For the high rate tests, split-Hopkinson bars for tensile (SHTB) and compressive (SHPB) loading are used. The results show, that the fracture toughness of the UD glass-epoxy composite, associated with the fiber failure modes, increases with increasing loading rate for tension as well as for compression. The observed strain rate effect on the fracture toughness of the glass-epoxy composite is more pronounced than for the carbon-epoxy composite investigated in [1],[2], in particular for fiber tensile failure.

A Wedge-DCB Test Methodology to Characterise High Rate Mode-I Interlaminar Fracture Properties of Fibre Composites

A combined numerical-experimental methodology is presented to measure dynamic Mode-I fracture properties of fiber reinforced composites. A modified wedge-DCB test using a Split-Hopkinson Bar technique along with cohesive zone modelling is utilised for this purpose. Three different comparison metrics, namely, strain-displacement response, crack propagation history and crack opening history are employed in order to extract unique values for the cohesive fracture properties of the delaminating interface. More importantly, the complexity of dealing with the frictional effects between the wedge and the DCB specimen is effectively circumvented by utilising right acquisition techniques combined with an inverse numerical modelling procedure. The proposed methodology is applied to extract the high rate interlaminar fracture properties of carbon fiber reinforced epoxy composites and it is further shown that a high level of confidence in the calibrated data can be established by adopting the proposed methodology.

Fracture analysis of high strain rate torsion samples of aluminium zinc magnesium copper alloy

Aluminium alloy T was subjected to Torsional Split Hopkinson Bar A Hollow thin wall cylindrical samples were twisted with an instant release of bar with stored toque generating strain rate in the range of s to s Few samples failed around highest strain rate in the range of s A hollow thin wall cylindrical sample failing under high strain torsion revealed a range of failure from brittle to ductile owing to material behaving differently at different strain rates along the circumference of fracture Brittle failure at high strain rates were typically flat with sheared precipitates and intermetallic inclusions Ductile failure showed elongated oval dimples typical of shear fracture and bypassed precipitates and intermetallic inclusions Sample bending crack splitting and crack diversion was also observed

Experimental and numerical investigation on the dynamic increase factor of tensile strength in concrete

A unique design for a vertical tension split Hopkinson bar (TSHB) with steel bars 100 mm in diameter and an aluminum hammer is presented. This facility was used to investigate the behavior of concrete under dynamic tension. Experiments conducted with specially shaped concrete samples demonstrate that the testing system is capable of accurately depicting the dynamic tensile strength of concrete. The experimental data were used to calibrate three concrete material models implemented in the numerical code LS-DYNA: (1) The Karagozian & Case (K&C) concrete damage model (*MAT_072R3), (2) the 072-BGU version of the K&C damage model, and (3) the continuous smooth cap model (*MAT_CSCM, *MAT_159). It was found that the automatic generation option included in these models does not provide satisfactory results. However, by readjusting only a few parameters of each of these material models, very good agreement between experimental and numerical results was achieved. This paper outlines the necessary calibration process and its theoretical background.

Compressive response of PMMA microcellular foams at low and high strain rates

ABSTRACTMicrocellular foams are widely applied in various applications in both civil and military applications for barriers and energy absorption materials. Poly(methyl methacrylate) microcellular foams were fabricated via supercritical foaming method. Field emission scanning electron microscopy, differential scanning calorimetry, and mechanical test machine were used to visualize the foam structure and test the quasi‐static compression properties. Moreover, Split Hopkinson Bar (SHPB) setups were adopted to explore the dynamic compression properties. The experimental results show that the microcellular foams have homogeneous cell size distribution and exhibit superior compressive behavior at both quasi‐static and high strain rates. The mechanical properties depend on both foam density and strain rate. Strain rate effects are clearly observed. At quasi‐static strain rate and 7500 S−1 regime, cell wall bucking and folding are the main failure mechanism. However, at high strain rate regime, softening phenomenon is observed. By roughly calculating the energy absorbed and the temperature rise, the temperature of the foams will rise up to as high as 130 °C after conducting high strain rate compression, and it is postulated that the generated heat will destroy the cell structure of the foams. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46044.

Dynamic bridging mechanisms of through-thickness reinforced composite laminates in mixed mode delamination

Delamination resistance of composite laminates can be improved with through-thickness reinforcement such as Z-pinning. This paper characterises the bridging response of individual carbon fibre/BMI Z-pins in mixed mode delamination at high loading rate using a split Hopkinson bar system. The unstable failure process in quasi-static tests, was also captured with high sampling rate instruments to obtain the complete bridging response. The energy dissipation of the Z-pins were analysed, and it was found that the efficacy of Z-pinning in resisting delamination growth decreased with an increase in mixed mode ratio, with a transition from pull-out to pin rupture occurring. The Z-pin efficacy decreased with loading rate for all mode mix ratios, due to the changing in failure surface with loading rate and rate-dependent frictional sliding.

Simulation of the plastic response of Ti–6Al–4V thin sheet under different loading conditions using the viscoplastic self-consistent model

In this article, the capacity of the Visco-Plastic Self-Consistent model (VPSC) as a constitutive model for the simulation of cold-rolled Ti–6Al–4V under a diverse set of loading conditions is investigated. The model uses information about the material crystallographic texture and grain morphology obtained with EBSD together with a grain constitutive model that is sensitive to strain rate and temperature. The ability of the model to capture the macroscopic response and underlying microstructural phenomena is critically assessed considering various sets of hardening parameters, obtained applying different fitting procedures to tensile experiments in different directions on the sheet plane. In order to validate the fitted parameters, the obtained model is applied to the simulation of experiments in a wide range of testing conditions, including uniaxial, plane strain and simple shear loading modes at strain rates ranging from 10−4s−1 to 103s−1 and temperatures between −10°C and 70°C. High strain rate experiments were performed using a Split Hopkinson Bar setup, with specimens designed to achieve the desired loading mode, while isothermal experiments were performed with the help of a fluid cell to keep a constant temperature. A good agreement between experimental and simulation results is obtained.

Dynamic response of a Q&P steel to high-strain-rate tension

The experimental study on the volume fraction of retained austenite for QP980CR steel under high-strain-rate tension is briefly described. An interrupted tensile split Hopkinson bar (TSHB) is developed to control the elongation of specimens. The QP980CR steel samples recovered from the interrupted TSHB tests are investigated using synchrotron X-ray diffraction (XRD) to analyze the effects of strain and strain rate on the martensitic transformation of retained austenite. A constitutive model of QP980CR steel coupling with the transformation-induced plasticity (TRIP) effect is presented based on Delannay's mean-field modeling. The stress–strain curves of quasi-static and dynamic tensile tests for QP980CR steel are compared with the results predicted by the presented constitutive model. The diffuse necking of QP980CR steel sheet specimens in TSHB tests is analyzed using Batra and Wei's instability criterion and the presented constitutive model. The effects of strain rate and temperature on the dynamic tensile fracture strain of QP980CR steel are also given.

Study of the dynamic fracture characteristics of coal with a bedding structure based on the NSCB impact test

To study the dynamic fracture deformation and failure characteristics of coal samples, the split Hopkinson bar (SHPB) impact loading system is used to test the dynamic fracture toughness of coal samples based on the notched semi-circular bend (NSCB) method under an impact load. The influence of the impact velocity and bedding angle on the dynamic fracture toughness, failure strain and strain rate of coal samples is preliminarily discussed, and a high-speed camera records and observes the dynamic fracture process of the coal sample. Finally, the distinct lattice spring model (DLSM) numerical analysis method is applied to analyze the sample dynamic splitting process and the change in the stress field. Meanwhile, the influences of the impact velocity, bedding angle, elastic modulus of bedding medium, bedding spacing and bedding width on the KIC test results are quantitatively analyzed.