Split Hopkinson Pressure Bar Technique Research Articles

On the Development of Horizontal Impact Test System (HITS): Estimation of Impact Loads and Deceleration

A Horizontal Impact Test System (HITS) will be developed to characterize several energy absorption mechanisms, i.e. axial splitting, tube inversion, deformation/expansion, axial folding, and flattening. There were two alternatives that has been developed for launching mechanism, i.e. spring and Hopkinson pressure bar technique which will be chose further. The HITS mainly consists of a striker bar (impactor), a specimen as in a direct split-Hopkinson pressure bar technique and a die. This paper examines the possibility of using the 70 – 100 mm diameter of the striker bar to be implemented as the impactor for the developed HITS. Afterwards, the size of the striker bar will be a base of determining the launching mechanism. This study was conducted by numerical simulation using LS-DYNA® as the solver. Mild steel was used as a material of specimen. The Cowper-Symonds was used as the constitutive material model for the material of specimen, while the elastic material model was used for the striker bar and the rigid material model was used for the die. Six mechanisms with total of 96 cases of appropriate dimensions were studied. The main purpose of the simulation was to select the range of load cell and accelerometer which would be implemented in the real experimental setup. Based on the result of simulation, the range of load cell was 48 kN to 238 kN, while the range of accelerometer was 358 to 1250 g. Later on, the load vs. displacement can be used for predicting energy absorption for each mechanism.

A novel method in dynamic shear testing of bulk materials using the traditional SHPB technique

This paper aims to propose a novel experimental method to study dynamic shear properties and constitutive behaviors of bulk materials using the traditional split Hopkinson pressure bar (SHPB) technique. In this method, a new kind of double-shear specimen (DSS) and fixture is specially designed for the SHPB system. With this setup configuration, the plastic properties of materials can be obtained conveniently at high strain rates under the state of simple shear. The deformation and failure process of the shear zone can be observed directly by using a high-speed photography. Another advantage of this method is that reloading on the specimen can be prevented for the purpose of post-test observation, with the application of the stress reversal Hopkinson technique (Nemat-Nasser etal., 1991). Using this method, the 603 steel was tested at strain rates from 2500s−1 to 48000s−1. With the aid of finite element analysis (FEA), it's found that the shear stress and shear strain play dominant roles in the stress/strain fields. The distribution of the maximum shear strains in the shear zone showed good consistency with the fracture patterns of the tested samples. Through comparison between the tested and simulated shear stress-shear strain curves, very good agreement is noted for all the tested cases.

Experimental Characterization of Crack Growth Behavior in Adhesive Interface under Impact Loading

A novel experimental method has been developed to evaluate the mode I crack growth behavior of adhesively bonded joints under impact loading. The split Hopkinson pressure bar (SHPB) technique and the digital image correlation (DIC) technique was employed to evaluate the crack growth behavior. To reduce the dynamic effects by controlling loading input of the SHPB apparatus, the fracture toughness was determined precisely based on static evaluation formula. To contrive the testing set-up, high loading rate was kept until the arrest of crack. The fracture toughness of titanium alloy/epoxy adhesively bonded joints during crack propagation was obtained successfully by using present method.

Experimental study for effects of strain rates and joint angles on dynamic responses of simulated rock materials

By using a split Hopkinson pressure bar (SHPB) technique, impact experiments were carried out on the jointed rock specimens simulated by cement-based mortar specimens. The dynamic responses of the simulated jointed rock material with different joint angles at different strain rates were analyzed including stress-strain curve characteristics, failure modes, energy transmission and dissipation. The experimental results show that, with the increase of strain rate, the dynamic elastic moduli increase, and the specimens become more fragile. The peak intensity decreases with the increase of the joint angles whereas when the strain rate increases to a certain extent, the influence of the joint angles on the rock damage formation is no longer obvious. The incident energy, the reflective energy, the transmission energy, and the dissipation energy of the different specimens nonlinearly increase with the increase of the strain rate. The energy dissipation rates of the specimens with inclination joint angles are higher than those of the intact specimens with the increase of the strain rate.

Crack initiation at high loading rates applying the four-point bending split Hopkinson pressure bar technique

Dynamic crack initiation with crack-tip loading rates of $$\dot{K} \approx 2 \cdot 10^6\,\mathrm {MPa\,m^{0.5}\,s^{-1}}$$ in a high strength G42CrMoS4 steel was investigated. To this end, a previously developed split Hopkinson pressure bar with four-point bending was utilized. V-notched and pre-cracked Charpy specimens were tested. The detection of dynamic crack initiation was performed by analyzing the dynamic force equilibrium between the incident and the transmission bar. High-speed photography of the tests and analysis of the dynamic stress intensity factor revealed that the vibration of the specimen had to be considered. The dynamic and static analyses of the tests lead to nearly the same results when a force equilibrium was achieved. Fracture-surface analysis revealed that elongated MnS inclusions strongly affected both the dynamic crack initiation and growth. Blunting of the precrack did not take place when a group of MnS inclusions was located directly at the precrack tip. Due to the direction of the elongated MnS inclusions perpendicular to the direction of crack growth, the crack could be deflected. The comparison with a 42CrMo4 steel without elongated MnS inclusions revealed the detrimental effect in terms of resistance to crack initiation.

Statistical Analysis of Dynamic Splitting Tensile Strength of Concrete Using Different Types of Jaws

AbstractThe indirect Brazilian test is widely used in engineering practice to indirectly determine the tensile strength of concrete. The Brazilian test (splitting tension test) is performed by applying a concentrated compressive load across the diameter of a disc-specimen. A series of dynamic Brazilian disc tests for concrete specimens were conducted using the split Hopkinson pressure bar (SHPB) technique. The objective of this paper is to investigate the standard Brazilian test by comparing the results obtained using the test with those obtained using a loaded arc, together with a comparison of the experimental results and the statistical analysis. The effect of loading angle on the dynamic splitting tensile strength was studied by one-way analysis of variance. By using linear regression method, the Weibull statistical model was introduced to obtain the relationship between the loading angle and dynamic splitting tensile strength of concrete. It was concluded that dynamic tensile strength of concrete is ...

Influence of Water Content on Mechanical Properties of Rock in Both Saturation and Drying Processes

Water content has a pronounced influence on the properties of rock materials, which is responsible for many rock engineering hazards, such as landslides and karst collapse. Meanwhile, water injection is also used for the prevention of some engineering disasters like rock-bursts. To comprehensively investigate the effect of water content on mechanical properties of rocks, laboratory tests were carried out on sandstone specimens with different water contents in both saturation and drying processes. The Nuclear Magnetic Resonance technique was applied to study the water distribution in specimens with variation of water contents. The servo-controlled rock mechanics testing machine and Split Hopkinson Pressure Bar technique were used to conduct both compressive and tensile tests on sandstone specimens with different water contents. From the laboratory tests, reductions of the compressive and tensile strength of sandstone under static and dynamic states in different saturation processes were observed. In the drying process, all of the saturated specimens could basically regain their mechanical properties and recover its strength as in the dry state. However, for partially saturated specimens in the saturation and drying processes, the tensile strength of specimens with the same water content was different, which could be related to different water distributions in specimens.

Wave dispersion analysis and simulation method for concrete SHPB test in peridynamics

The Split Hopkinson Pressure Bar (SHPB) technique is a widely used method for measuring mechanical properties of materials subjected to high-strain-rate loads, while it is difficult to simulate the whole testing process including high-rate deformation, local damage and failure of materials by the common numerical methods. In this paper, an improved numerical approach based on the non-local peridynamic (PD) theory is employed to study the elastic wave dispersion and propagation and the impact failure of concrete Brazilian discs in SHPB test. Meanwhile, an improved PMB (Prototype Microelastic Brittle) model and an implementation method of the contact-impact process are introduced. In PD, the nonlocal long-range force controls the numerical dispersion of wave through different material point sizes and horizon sizes. The numerical dispersion can result in a slight distortion of the wave speed and crack propagation speed, which is not conductive to failure analysis of solids. The improved PMB model can effectively lessen the numerical dispersion compared with the original PMB model. Furthermore, the PD simulation of concrete Brazilian disc SHPB test can reproduce damage accumulation and progressive failure of a specimen, and produce typical final failure pattern. The PD simulation method for SHPB test can be used to analyze the dynamic response of solids suffering impact load.

Determination of dynamic elastic modulus of polymeric materials using vertical split Hopkinson pressure bar

This paper is focused on the determination of dynamic elastic modulus of polymer materials under high strain rate loading using the split Hopkinson pressure bar (SHPB) technique. Experiments conducted on the epoxy specimen by the traditional SHPB and the proposed vertical SHPB equipment demonstrates that the vertical SHPB can give more accurate measurements. The related factors, namely, the effect of stress inequilibrium in specimen, indentation in bars due to specimen and the tilt between specimen and bars, are extensively studied. It is concluded through theoretical analysis and numerical calculations that the influence of stress inequilibrium becomes negligible after two characteristic times. The numerical study on the indentation effect shows that the bar to specimen specific elastic modulus ratio and specific diameter ratio are critical to the level of influence on indentation. However, polymers with low elastic modulus values can still be accurately measured regardless of the indentation displayed. The numerical investigation on tilt effect indicates that the imperfect contact condition severely affects the accuracy of measured elastic modulus. This issue can be rectified by the newly proposed vertical SHPB. It can improve the contact conditions between bars and specimen significantly and offer acceptable accurate measurements for the dynamic elastic modulus of polymeric materials.

炭素繊維強化軟質エポキシ樹脂の衝撃曲げ特性評価

In the present study, quasi-static and dynamic bending behavior of CFRP (carbon fiber reinforced plastics) with soft epoxy matrix was investigated. CFRPs were fabricated with the soft epoxy matrix and a standard carbon woven fabrics. The carbon woven fabrics of 5 layers were laminated to be quasi-isotropic. The impact three-point bend testing machine based on the split Hopkinson pressure-bar technique was developed. The bending tests were performed by a universal testing machine for low deflection velocities, a cam plastometer for middle ones and the impact testing machine for high ones. The experiments reveal that the bending modulus of the CFRPs strongly depends on the deflection velocity.

High strain rate sensitivity of epoxy/clay nanocomposites using non-contact strain measurement

As polymers are rate sensitive to mechanical properties, dynamic tensile tests are carried out on drop mass setup, to obtain medium strain rate stress-strain response which fills the gap between quasi-static strain rate (<10 s−1) and split Hopkinson pressure bar (SHPB) technique (>1000 s−1). The present research work is to study the effect of medium strain rate on tensile behavior of epoxy/clay nanocomposites. Neat epoxy and nanocomposites containing 1.5, 3 and 5 wt% clay content are fabricated using mechanical stirrer. The dispersion of nanoclay in epoxy is examined using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The digital image correlation (DIC) technique is employed for evaluating full-field strain and strain rate using high speed CCD camera, which captures about 100,000 frames per second. Stress-strain measurements are reported for epoxy/clay nanocomposites for strain rates ranging from 0.008 to 450 s−1. The results reveal that the tensile strength and modulus increase with increase in strain rate for epoxy and its clay nanocomposites. The fracture surfaces of tensile specimens are investigated using scanning electron microscopy (SEM).

High strain rate radial compression of Norway spruce earlywood and latewood

The mechanical properties of Norway spruce were studied and a compression model for mechanical pulping was developed. The split-Hopkinson pressure bar technique was combined with high-speed photography to analyse local radial compression. Data analysis focussed on the differences between mechanical properties of earlywood and latewood. Measurements were conducted at both room temperature and 135 °C. The effect of pre-fatigue treatment was also studied. A simple material model was defined linearly in parts and fitted to the measurement data to quantify the differences. New results were found on the differences in inelastic behaviour of earlywood and latewood at large deformations. In addition, other results were in line with previously published results.

Dynamic compressive behaviour of Mg foams manufactured by the direct foaming process

Increasing interest in lightweight Mg foams for the potential impact energy dissipation application in transport industry requires a good understanding of the dynamic deformation and failure process of the foam. The split-Hopkinson pressure bar technique, combined with high speed photography and post-test scanning electron microscopic examination, was applied to investigate the dynamic behaviour of closed-cell Mg foams manufactured by the direct foaming process. The deformation and failure behaviour at dynamic strain rates was then compared to the quasi-static response. It was found that the fracture of Mg cell walls is brittle and strain rate sensitive. Compared to quasi-static compression, a large quantity of short macro-cracks evolves in cell walls at dynamic strain rates, causing more fragments and thus dissipating more impact energy. The rate dependent fracture mechanisms in cell walls significantly affect the strain rate sensitivity of bulk properties of the foam such as peak stress, plateau stress and energy dissipation capacity.

Characterisation of dynamic behaviour of alumina ceramics: evaluation of stress uniformity

Accurate characterisation of dynamic behaviour of ceramics requires the reliable split-Hopkinson pressure bar (SHPB) technique and the condition of uniaxial homogeneous specimen deformation. In this study, an experimentally validated 3D finite element model of the full scale SHPB experiment was developed to quantitatively evaluate the wave propagation in the bars and the stress distribution/evolution in the alumina specimen. Wave signals in both the SHPB experiments and the finite element model were analysed to characterise the dynamic behaviour of alumina. It was found that the equilibrium of both stresses within the specimen and forces at the specimen ends can be established in the intermediate stage of deformation. The validity of stress uniformity in the alumina specimen supports the assumption of uniaxial homogeneous specimen deformation in the SHPB and validates the characterisation of dynamic behaviour of alumina ceramics.

Dynamic compressive and splitting tensile tests on mortar using split Hopkinson pressure bar technique

Dynamic compressive and tensile properties of mortar under impact loading were investigated experimentally by using a split Hopkinson pressure bar (SHPB) apparatus with pulse shaping technique. Firstly, the basic principle, experimental limitations and some feasible improvements/modifications of SHPB technique used for dynamic tests on concrete-like materials were summarized briefly. And then the dynamic compressive strength, stress versus strain response, and failure modes of mortar were discussed and analyzed. Finally, a dynamic Brazilian disc test was conducted to obtain the splitting tensile property of mortar, and some typical experimental results were presented. Both compressive and splitting tensile results show that mortar is a strain-rate sensitive material. Either compressive or tensile strength enhances with the increase of strain rate, especially when the strain rate is greater than the transition strain rate, which is around 20 s-1 for the dynamic compression and 2.0 s-1 for the splitting tension, respectively. These findings are helpful to guide the design and application of concrete structures.

Mechanical behavior and damage kinetics of woven E-glass/vinylester laminate composites under high strain rate dynamic compressive loading: Experimental and numerical investigation

Split Hopkinson pressure bar (SHPB) is one of the most important and recognized apparatus used for characterizing the dynamic behavior of materials. In the first part of this study, the results from a series of SHPB tests on the woven composites are presented in this paper. The compressive material properties are determined by testing the [0°/90°]26 laminate systems from low to high strain rates. Samples of cubic geometry are tested in in-plane and out-of-plane direction. Preliminary compressive stress–strain vs. strain rates data obtained show that the dynamic material strength increases with increasing strain rates. The tests show a strong material sensitivity to dynamic loading. For in-plane tests, there is a transitional strain rate, reflecting the dependencies on strain rate observed in experiments. The results indicate that the stress–strain curves, maximum compressive stresses and strains evolve as strain rate changes. During the experiments, a high speed camera was used to determine the kinetics of damage. The specimens are mainly damaged in a crushing and shear failure mode under out-of-plane loading, as for in-plane test, the failure was dominated by fiber buckling and delamination. In the second part of this study, numerical models without damage are developed to investigate the validity of assumptions of compression Split-Hopkinson Pressure Bar technique. Abaqus software was used for the numerical simulation. The results obtained by numerical investigation of SHPB are compared with the in-plane and out-of-plane compression test of a woven composite. A good correlation was noted between the experimental and numerical results, which allows validate the numerical approach.

Dynamic failure and fracture mechanism in alumina ceramics: Experimental observations and finite element modelling

The application of ceramics in composite armour requires the investigation on the failure and fracture mechanisms of the ceramics under dynamic loading. In this study, the split-Hopkinson pressure bar technique with high speed photography was used to directly observe the dynamic macro-cracking and fragmentation process in alumina and to characterise the stress and strain histories. An experimentally validated 3D finite element model of the full scale test was developed to predict the stress distribution and damage evolution in alumina that were related to the experimental observations. It was found that the damage (cracking) evolution is determined by the multiaxial stress state, in particular the substantial localised tensile stress concentration and the stress direction. The cracking process leads to the longitudinal axial splitting failure on the surface and the internal diagonal damage bands. The dynamic failure process in alumina is dominated by both the intergranular and transgranular fracture mechanisms.

Numerical Simulation for Deformation of Polymer Samples Using the Split Hopkinson Pressure Bar Technique

Results of numerical simulation for deformation of polymer samples using the split Hopkinson pressure bar technique are presented. Computations have been carried out by the finite element method in axisymmetrical statement within the framework of the elastic-plastic medium model. The received numerical results are compared with experimental data. Good quantitative agreement between the numerical results and experimental data on the pulse amplitude was shown.

High strain-rate response of injectable PAA hydrogel

Hydrogel materials have been widely considered as potential soft tissue replacements because of their high permeability, hydrophilicity, and biocompatibility, as well as their low coefficient of friction. Injectable (thermo-responsive) hydrogels can provide support and cushioning at irregularly shaped disease sites, and are thus suitable for use in treating osteoarthritis or degenerative disc disease. However, while some injectable hydrogels have been proven to sustain human body weight during daily activities, their mechanical properties under harsh dynamic conditions have not been well documented. A specified injectable polyacrylic acid (PAA) hydrogel was prepared for this study. To simulate sudden impacts or unexpected shocks to the PAA hydrogel, the split Hopkinson pressure bar technique was utilized. The dynamic responses of various hydrogels at confined high strain rates (100–2590 s−1) were presented. Hydrogel specimens with 3.37, 6.75, and 13.5% acrylic acid (AAc) concentrations were tested in the following three different material conditions: raw, phosphate-buffered saline (PBS) swollen, and PBS swollen with elevated temperature (37 °C). The dynamic bulk moduli of the hydrogels varied from 1.55 to 47.8 MPa depending on the given hydrogel’s AAc concentration and swollen condition.

Fabrication and characterization of composite materials based on porous ceramic preform infiltrated by elastomer

Abstract The paper presents the experimental results of fabrication and characterization of ceramic- elastomer composites. They were obtained using pressure infiltration of porous ceramics by elastomer As a result the composites in which two phases are interpenetrating three-dimensionally and topologically throughout the microstructure were obtained. In order to enhance mechanical properties of preforms a high isostatic pressure method was utilized. The obtained ceramic preforms with porosity gradient within the range of 20-40% as well as composites were characterized by X-ray tomography. The effect of volume fraction of pores on residual porosity of composites was examined. These results are in accordance with SEM images which show the microstructure of composites without any delaminations and voids. Such composites exhibit a high initial strength with the ability to sustain large deformations due to combining the ceramic stiffness and rubbery elasticity of elastomer. Static compression tests for the obtained composites were carried out and the energy dissipated during compression was calculated as the area under the stress-strain curve. The dynamic behavior of the composite was investigated using the split Hopkinson pressure bar technique. It was found that ceramic-elastomer composites effectively dissipate the energy. Moreover, a ballistic test was carried out using armor piercing bullets.