Split Hopkinson Tension Bar Research Articles

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.

Dynamic tensile properties of ROP/OCC natural hybrid fibers reinforced composites

The dynamic tensile properties of natural hybrid fibers reinforced composites with components of recycled-office-paper (ROP) fibers and old corrugated cardboard (OCC) fibers at different proportions are investigated by using an aluminum split Hopkinson tension bar (SHTB) at the strain rates ranging from 300/s to 1500/s. The influence on strain–stress relationship and strength by changing its component proportions are tested under quasi-static and dynamic loading. It is observed that the strain-rate sensitivities of the composites are varied by changing component proportions. The type of composites with 100% OCC fibers has a negative response to strain-rate effect. Nonetheless, that of 100% ROP fibers is not sensitive to the strain-rate effect. The type of composites with 75% ROP/25% OCC fibers is subject to strain-rate effect when the stress rates are less than 1000/s, but it has a negative response to the strain-rate effect when the strain rates are greater than 1000/s. The failure modes and their corresponding microscopic and macroscopic failure mechanism are also discussed. The composites with different component proportions are failed by combining with fibers break, fibers pull-out, fibers curvature shape changing and scattered, which are fibers failure and fiber–matrix bonding failure.

Deformation mechanism of highly textured AZ31 sheet under high strain rates

AbstractIn this study, in‐plane tension behavior of AZ31 magnesium sheet was measured in rolling direction, transverse direction and 45° direction under high strain rates of 600 s‐1, 1200 s‐1 and 2400 s‐1 at room temperature using a Split Hopkinson Tension Bar (SHTB). The microstructures after high strain rates deformation were observed by optical micrography. The anisotropic behavior was negligible and marginally positive strain rate effects were observed in all deformation directions. Texture analysis of the sheet demonstrated that three possible deformation mechanisms could apply to AZ31 magnesium alloy sheet under high strain rates tension, those are contraction twinning and/or ‐ double twinning, prismatic slip and <c+a> pyramidal slip. The volume fraction of twinning was decreased with strain rate increasing. Dynamic recrystallization took place upon the strain rate of 2400 s‐1 and the dynamic recrystallization mechanism was attributed to the twinning‐induced dynamic recrystallization.

Dynamic properties and strain rate effect of 3D angle-interlock carbon/epoxy woven composites

A series of experiments on 3D angle-interlock carbon/epoxy woven composites were carried out using the split Hopkinson pressure bar and split Hopkinson tension bar. These tests were accomplished to investigate the strain rate effect of the dynamic compressive and tensile properties. The strain rate range is about 500/s to 1500/s for through-thickness compressive tests, 150/s to 600/s for in-plane compressive tests, and 600/s to 1000/s for in-plane tensile tests. The corresponding quasi-static properties are given for reference. The tested specimens were observed using optical microscope and scanning electron microscope. The main failure modes of the specimens include fiber breakage, fiber pull out and matrix crack. The compressive modulus of through-thickness direction is positively related to the strain rate. The compressive strength and modulus of in-plane direction shows a light fluctuation within the studied strain rate. It is found that the tensile strength and modulus of in-plane direction are negatively related to the strain rate.

The influence of different pre-formed holes on the dynamic response of square plates under air-blast loading

Abstract Studying the blast response of plates with pre-formed holes under blast loading serves as a significant method to decouple dynamic response of plates with pre-formed holes under combined blast and fragment loading. Based on material test system (MTS) and Split Hopkinson tension bar (SHTB), the mechanical properties of experimental steel under different strain rates were obtained. Then, the experiments on the responses of plates with square, diamond and circular holes under air-blast loading were carried out and the failure modes of plates with diamond holes were derived. Later, the deformation curves of target plate were obtained by laser reverse forming technique. Next, a constitutive model program considering strain rate, stress status and temperature effect was adopted to carry out numerical simulation calculation, which verified the accuracy of this numerical simulation. Finally, discussions were carried out on the damage level at places where cracks might form, the dynamic stress concentration coefficient and stress status change, and then the influence which pre-formed holes have on the failure mode was analyzed. The results of this study show that: (1) A fortran vectorized user-material subroutine (VUMAT) program considering strain rate, stress status and temperature effect can effectively predict the dynamic response of metallic materials; (2) the shape of pre-formed holes affects the damage level, dynamic stress concentration coefficient and stress status. Besides, failure occurs more easily in plates with diamond pre-formed holes; (3) pre-formed holes have an obvious influence on the failure modes of plates under different blast distance.

Dynamic tests for a Constant-Resistance-Large-Deformation bolt using a modified SHTB system

Rockbursts frequently occur in deep underground excavations in a sudden or violent ejection of blocks of rocks from excavation walls. In addition to understanding rockburst mechanisms, rockburst control is an important issue for the safety of mining operations. Bolts and anchors are efficient measures to control rockbursts and permit the efficient exploration of underground excavations in mines. As mining depths increase, anchor and bolts with larger extension and higher loading capacity are needed. This paper introduces a Constant Resistant Large Deformation (CRLD) bolt which has been developed at the State Key Laboratory for Geomechanics and Deep Underground Engineering (GDUE), in Beijing, China. Static and dynamic tests were developed for this type of bolt. Advancements in Hopkinson tests were recently developed for CRLD bolts and Split-Hopkinson Tension Bar (SHTB) experiments for these bolts were performed. This paper presents the SHTB equipment and the results obtained with one and two CRLD bolts, as well a deep analysis of the obtained results. To better understand the complex nature of the problem, numerical simulations using several softwares were done for the case of one bolt, which had the dual goal of verifying the experimental and numerical results. In addition, the numerical predictions permit one conducting analysis the behavior of the bolt while targeting the development of large deformations. Finally some relevant conclusions are drawn.

Effect of Strain Rate on Tension Response of Filled Silicone Rubber

Uniaxial tension tests for filled silicone rubber were performed at high strain rates using a split Hopkinson tension bar system. Quasi-static tension tests were carried out using an Instron-E3000 material test system. The grip fixture was designed to reliably connect the tensile specimen with the incident/transmitted bars. The method to increase the signal-noise ratio of the stress pulse in the transmitted bar was proposed. The effect of specimen gage length-to-width ratio on the stress-strain responses was experimentally studied. The suitable specimen geometry was determined by means of experimental investigation and finite element analysis. The automated grid method was used to capture the deformation information of the tensile specimen. Experiments indicate that the tension responses of silicone rubber exhibit the apparent hyper-elastic and rate-dependent characteristics. The values of tensile modulus increase with the increase of strain rate. The stress at a given elongation increases with the increase of strain rate.

Mechanical Behaviour of Al2024-T3 Sheet Metal at Different Strain-rates and Temperatures

Aluminium alloys are widely used in the design of structural parts in the automotive and aircraft manufacturing because of their good specific mechanical properties. Many of the critical situations in these types of industries such as crash or impact phenomena as well as forming processes are related to high strain-rate deformations and adiabatic heating. Therefore, knowing flow and failure behaviour of materials under dynamic loading and applying reliable constitutive models is essential for designing the structural components safely. Johnson-Cook model is one of the most long-established constitutive relations in the dynamic field due to its simplicity and easy calibration. Nevertheless, empirical models do not collect and give information about the reason why materials behave in the way that they do, so micromechanical motivated continuum models are interesting for that. In the present work, authors analyse the mechanical dynamic behaviour of the aeronautical alloy Al2024-T3. An experimental study has been planned performing tests on sheet specimens at different strain-rates and temperatures to analyse the effect of these parameters. Low strain-rate experiments were conducted using a servo-hydraulic machine while high strain-rate tests were carried out using a split-Hopkinson tension bar. Testing temperatures were set at room and 373K by using a furnace made of resistors. The material behaviour is modelled using a micromechanical-based constitutive and a phenomenological failure model. Plastic flow of the material is defined by means of Orowan relation, where both temperature and strain-rate effect are included in the flow rule and the strain hardening law is just a function of equivalent plastic strain; while failure criterion is based on Cockcroft-Latham model. Strain fields on sample surfaces, which were used for obtaining the true stress-strain curves and performing a hybrid experimental-numerical failure calibration, were obtained by means of Digital Image Correlation technique.

The Effect of Specimen Dimension on the Results of the Split-hopkinson Tension Bar Testing

The split-Hopkinson tension bar (SHTB) has been used widely to determine the dynamic characteristics of materials. Despite of its application, the tensile test specimen has not been standardised yet, whereas its dimension could affect the test results. Therefore, it is necessary to study the consistency of SHTB results by varying the dimension of its specimen. This paper examines whether consistent results can be achieved from three case studies of SHTB in a direct method. The study was conducted by numerical simulation using LS-DYNA®. A dumbbell-shaped specimen was designed similar to the standard specimen on ASTM A370. Pressure bars were made of Maraging steel with 14mm in diameter and a flange with thickness of 5mm was used, while the specimen was made of 1006 mild steel. The simplified Johnson-Cook was used as the constitutive material model. Three case studies were carried out. The first case study used a group of specimens with thesame gage length of 6mm and varied diameters; the second case study used a group of specimens with the same diameter of 8mm and varied lengths; and the third case study used a group of specimens with the same gage length of 8mm and varied diameters. Based on the results of all case studies, the specimen's length-to-diameter ratio (L/D) of 0.75 generates a good stress-strain behavior.

A Simplified Design for a Split-Hopkinson Tension Bar with Long Pulse Duration

This paper presents a simplified design for a Split-Hopkinson Tension Bar (SHTB) which allows for pulse durations exceeding one millisecond. This testing apparatus is intended for material characterization requiring large amounts of elongation, necessitated either by large specimen dimensions, or large strains to failure. The design proposed here can be assembled from off the shelve components without the need for specialized machining tools. The simple mechanical design is accompanied by a comparatively low-cost, yet high-speed, means of independently measuring specimen strain using a fast line scan camera. The performance of this SHTB implementation is demonstrated by obtaining intermediate strain rate results for tensile yield strength of Polycarbonate in an unprecedented quality.

Deformation and fracture of explosion-welded Ti/Al plates: A synchrotron-based study

Explosion-welded Ti/Al plates are characterized with energy dispersive spectroscopy and x-ray computed tomography, and exhibit smooth, well-jointed, interface. We perform dynamic and quasi-static uniaxial tension experiments on Ti/Al with the loading direction either perpendicular or parallel to the Ti/Al interface, using a mini split Hopkinson tension bar and a material testing system in conjunction with time-resolved synchrotron x-ray imaging. X-ray imaging and strain-field mapping reveal different deformation mechanisms responsible for anisotropic bulk-scale responses, including yield strength, ductility and rate sensitivity. Deformation and fracture are achieved predominantly in Al layer for perpendicular loading, but both Ti and Al layers as well as the interface play a role for parallel loading. The rate sensitivity of Ti/Al follows those of the constituent metals. For perpendicular loading, single deformation band develops in Al layer under quasi-static loading, while multiple deformation bands nucleate simultaneously under dynamic loading, leading to a higher dynamic fracture strain. For parallel loading, the interface impedes the growth of deformation and results in increased ductility of Ti/Al under quasi-static loading, while interface fracture occurs under dynamic loading due to the disparity in Poisson's contraction.

Dynamic tensile deformation behavior of Zr-based amorphous alloy matrix composites reinforced with tungsten or tantalum fibers

Zr-based amorphous alloy matrix composites reinforced with tungsten (W) or tantalum (Ta) continuous fibers were fabricated by liquid pressing process. Their dynamic tensile properties were investigated in relation with microstructures and deformation mechanisms by using a split Hopkinson tension bar. The dynamic tensile test results indicated that the maximum strength of the W-fiber-reinforced composite (757 MPa) was much lower than the quasi-statically measured strength, whereas the Ta-fiber-reinforced composite showed very high maximum strength (2129 MPa). In the W-fiber-reinforced composite, the fracture abruptly occurred in perpendicular to the tensile direction because W fibers did not play a role in blocking cracks propagated from the amorphous matrix, thereby resulting in abrupt fracture within elastic range and consequent low tensile strength. The very high dynamic tensile strength of the Ta-fiber-reinforced composite could be explained by the presence of ductile Ta fibers in terms of mechanisms such as (1) interrupted propagation of cracks initiated in the amorphous matrix, (2) formation of lots of cracks in the amorphous matrix, and (3) sharing of loads and severe deformation (necking) of Ta fibers in cracked regions.

Flow and fracture characteristics of near alpha titanium alloy

To understand the flow and fracture behavior of near-α titanium alloy, the influence of stress triaxiality and strain rate on the failure behavior of this alloy was considered. The combined effect of strain rate, temperature and stress triaxiality on the behavior was studied by testing both smooth and notched specimens. Johnson-Cook (J-C) constitutive and fracture models were established based on high strain rate tensile data obtained from Split hopkinson tension bar (SHTB) and quasi-static tests. The Johnson Cook constitutive and fracture models have been calibrated. A modified Johnson–Cook model was established and proved to have high accuracy. The calibrated model has been validated by simulating the tension tests at various triaxialities using ANSYS autodyn software. Finite element simulations were used to predict the effective plastic strain versus triaxiality history within the deforming specimens. The fracture surfaces of specimens tested under various strain rates and temperatures were studied under scanning electron microscopy (SEM).

Effect of strain rate and stress triaxiality on tensile behavior of Titanium alloy Ti-10-2-3 at elevated temperatures

In this study, Split hopkinson tension bar (SHTB) has been employed to investigate the dynamic tensile flow behavior of Ti-10-2-3 alloy at high strain rates and elevated temperatures. The combined effect of stress triaxiality, strain rate and temperature and on the tensile behavior of the alloy was evaluated. Johnson-Cook (J-C) constitutive and fracture models were developed based on high strain rate tensile data. A modified Johnson–Cook model was established and proved to have high accuracy. A comparative assessment has been done to confirm the accuracy of modified J–C model based on finite element method (FEM). The improved model provides better description on the influence of equivalent plastic strain rate and temperature on the plastic flow. The simulation results proved to be in good agreement with the experimental data. The fracture surfaces of specimens tested under various strain rates and temperatures were studied under scanning electron microscopy (SEM).

Dynamic tensile deformation and damage of B4C-reinforced Al composites: Time-resolved imaging with synchrotron x-rays

Dynamic tensile experiments are conducted on 15% and 30% in weight percentage B4C/Al composites with a split Hopkinson tension bar, along with high-speed synchrotron x-ray digital image correlation (XDIC) to map strain fields at μm and μs scales. As manifested by bulk-scale stress–strain curves, a higher particle content leads to a higher yield strength but lower ductility. Strain field mapping by XDIC demonstrates that tension deformation and tensile fracture, as opposed to shear and shear failure, dominate deformation and failure of the composites. The fractographs of recovered samples show consistent features. The particle-matrix interfaces are nucleation sites for strain localizations, and their propagation and coalescence are diffused by the Al matrix. The reduced spacing between strain localization sites with increasing particle content, facilitates their coalescence and leads to decreased ductility. Designing a particle-reinforced, metallic-matrix composite with balanced strength and ductility should consider optimizing the inter-particle distance as a key parameter.

Experimentation and Modeling of the Tension Behavior of Polycarbonate at High Strain Rates.

A comprehensive understanding of the mechanical behavior of polycarbonate (PC) under high-rate loadings is essential for better design of PC products. In this work, the mechanical behavior of PC is studied during tensile loading at high strain rates, using a split Hopkinson tension bar (SHTB). A modified experimental technique based on the SHTB is proposed to perform the tension testing on PC at rates exceeding 1000 s−1. The effect of strain rates on the tension stress–strain law of PC is investigated over a wide range of strain rates (0.0005–4500 s−1). Based on the experiments, a physically based constitutive model is developed to describe the strain rate dependent tensile stress–strain law. The high rate tensile deformation mechanics of PC are further studied via finite element simulations using the LSDYNA code together with the developed constitutive model.

Tension testing of silicone rubber at high strain rates

The effect of strain rate on the tension behavior of filled silicone rubber was investigated over a wide range of strain rates. A non-contact optical measurement technique was used to accurately capture the deformation information of the tensile specimen under quasi-static loading conditions. Experimental methodology to investigate the tension response of rubber-like materials at high strain rates is presented. Dynamic tensile tests were performed using a split Hopkinson tension bar (SHTB) system. Specific specimen geometry and grip connection were designed to meet the requirements of one-dimensional experimental principle in the Hopkinson bar tests. The dynamic tensile stress-strain response of silicone rubber at high strain rates up to 1400 s−1was obtained. Experiments reveal that the adopted experimental technique is reasonable for testing the tension response of soft materials at high strain rates, and the tensile behavior of silicone rubber is greatly dependent on the strain rate. The values of stiffness and nominal stress at a given elongation all increase with increased strain rate. According to the incompressible assumption, a phenomenological-based visco-hyperelastic constitutive model is proposed to describe the tensile behavior of silicone rubber over a wide range of strain rates.

Tensile behaviour of aluminium 7017 alloy at various temperatures and strain rates

The objective of the present study is to carry out high strain rate tensile tests on 7017 aluminium alloy under different strain rates ranging from 0.01, 500, 1000 and 1500s−1 and at temperatures of 25, 100, 200 and 300°C. Quasi-Static tensile stress–strain curves were generated using INSTRON 8500 machine. Johnson-Cook (J-C) constitutive model was developed for 7017 aluminium alloy based on high strain rate tensile data generated from split Hopkinson tension bar (SHTB) at various temperatures. This study evidently showed an improvement in dynamic strength as the strain rate increases. The predictions of J-C model are observed to be in consistence with the experimental data for all strain rates and temperatures. The fracture surfaces of specimens tested were studied under SEM. The change in fracture mode has been observed at different strain rates. The shear mode of fracture is dominant at lower strain rates (0.01 and 500s−1); whereas cup- and cone-like surface representing dimple structure is found at the higher strain rates (1000 and 1500s−1). The numbers of dimples at high strain rates are more than the quasi-static and intermediate strain rates. It is also observed that the flow stress decreases with increase in temperature. The 7017 aluminium alloy demonstrates thermal softening at higher temperatures. So when the temperature is more than 200°C at these strain rates, thermal softening is predominant mode of deformation mechanism. It is found that when the temperature increases to 200°C, the number of dimples rises and the dimple size of 7017 aluminium alloy is larger than at lower temperatures.

Effects of strain rate on mechanical properties in tension of a commercial aluminium alloy used in armour applications

The use of aluminium alloys in the construction of defense vehicles is strongly increasing in these last decades because they require new lightweight armours for improving their survivability without sacrificing efficiency and performance. In this paper the effects of strain rate on the mechanical properties in tension of a commercial aluminium alloy AA7081 is analyzed. The tests have been carried out by means of three different set-ups for quasi-statics, medium and high strain rates. For the high and medium strain rate tests have been used respectively a Split Hopkinson Tension Bar device and a Hydro-pneumatic machine installed in the DynaMat Laboratory of the University of Applied Sciences of Southern Switzerland-Lugano. The results show: an increase of the stress at a given strain when increasing the strain-rate from 10-3 to 103 s-1, a strain-rate sensitivity of the uniform and fracture strain, a moderate reduction of the cross-sectional area at fracture with increasing the strain-rate. Based on these experimental results the parameters required by the Johnson-Cook constitutive law have been determined.

Mechanical Properties of a Human Eardrum at High Strain Rates After Exposure to Blast Waves

The mechanical properties of a human tympanic membrane (TM) or eardrum were characterized at high strain rates after multiple exposures to blast waves. Human cadaveric temporal bones were subjected to blast waves at first, then TM strip specimens were prepared either along the radial or the circumferential direction. A highly sensitive miniature split Hopkinson tension bar was used for tensile experiments on the human eardrum strip specimens at high strain rates. The mechanical properties of the human TMs before and after exposure to blast waves were compared and discussed. The mechanical properties in the time-domain were subsequently converted to the corresponding properties in the frequency domain to investigate the effect of blast waves on the viscoelastic properties. The results indicate that the blast waves have different effects on the mechanical properties in the radial and circumferential directions. After exposure to the overpressure induced by the blast waves, the mechanical behavior in the radial direction in general becomes stiffened, while it is weakened in the circumferential direction. The results could be analyzed further in an ear simulation model to develop understanding of the effect of blast waves on hearing loss.