Transmission Bar Research Articles

Technique for Combined Dynamic Compression–Shear Testing of PBXs

We modify the split Hopkinson pressure bar and propose a compression–shear experimental method to investigate the dynamic behavior of polymer-bonded explosives (PBXs). The main apparatuses used include an incident bar with a wedge-shaped end and two transmission bars. We employ Y-cut quartzes with a rotation angle of 17.7° to measure the shear force and an optical system for shear strain measurement. A PBX with a density of 1.7 g/cm3 is investigated using the proposed method. Experimental results show that the specimen endures both compression and shear failure. Compression failure stress rises, and shear failure stress decreases as the strain rate increases. The sequences of shear and compression failure times are various at different strain rates. Based on the maximum shear failure criterion, we conclude that these phenomena are related to the experimental loading path.

Normal and Transverse Displacement Interferometers Applied to Small Diameter Kolsky Bars

For Kolsky bar testing beyond strain-rates of 10,000/s, it is useful to employ bars with diameters of only a few millimeters or less. Furthermore, very small (sub-millimeter) systems are compatible with micron-sized specimens, to be used, for example, for the determination of mesoscale properties. However, at these sizes, traditional strain-gage measurements of the longitudinal waves within the bars become impractical. In this paper we describe the application of optical measurement techniques to two Kolsky bars, with 3.2 and 1.6 mm diameters. A transverse displacement interferometer is used to measure the displacement of the mid-point of the incident bar and provide measurements of the incident and reflected pulses. Similarly, a normal displacement interferometer is used to measure the displacement of the free-end of the transmitter bar and provide a measurement of the transmitted pulse. The new methods are used to characterize the behavior of 6061-T6 aluminum at rates greater than 100,000/s. The feasibility of application to smaller bars is also discussed.

Interferometry-based Kolsky bar apparatus

A new experimental approach of the Kolsky bar system using optical interferometry is presented for determination of dynamic behavior of materials. Conventional measurements in the Kolsky bar system are based on recording the strain histories on the incident and transmitter bars with two strain gauges, and require good adhesion between the gauge and the bar. We suggest an alternative approach, based on measuring the actual velocities of the bars by using fiber-based velocity interferometry. Two fiber focusers illuminate the bars at a small angle and collect reflected Doppler-shifted light, which is interfered with a reference beam. Velocities are calculated from short-time Fourier transform and phase-based analysis, and the dynamic stress-strain curve is derived directly from the measured velocity traces. We demonstrate that the results coincide with those obtained by conventional strain gauge measurements. The new method is non-intervening and thus not affected by bar impacts, making it more robust and reliable than strain gauges.

Tension and Compression Behavior of Pre-Stressed Steel Strands at High Strain Rate

In this paper is described the mechanical characterization at high strain rate of the high strength steel usually adopted for strands. The experimental set-up used for high strain rates testing: in tension and compression was the Split Hopkinson Pressure Bar installed in the Laboratory of Dynamic Investigation of Materials in Nizhny Novgorod. The high strain rate data in tension was obtained with dog-bone shaped specimens of 3mm in diameter and 5mm of gauge length. The specimens were screwed between incident and transmitter bars. The specimens used in compression was a cylinder of 3mm in diameter and 5mm in length. The enhancement of the mechanical properties is quite limited compared the usual reinforcing steels.

High Strain Rate Tensile Behavior of Aluminum Alloy 7075 T651 and IS 2062 Mild Steel

Investigations on the effect of strain rate on tensile properties of two materials, namely, aluminum alloy 7075 T651 and IS 2062 mild steel, are presented. Experimental studies were carried out on tensile split Hopkinson pressure bar (SHPB) apparatus in the strain rate range of 54–164/s. Uncertainty analysis for the experimental results is presented. Johnson–Cook material constitutive model was applied to predict the tensile yield strength of the tested materials at different strain rates. It is observed that the tensile yield strength is enhanced compared with that at quasi-static loading. During tensile SHPB testing of the specimens, it was observed that the peak force obtained from the strain gauge mounted on the transmitter bar is lower than the peak force obtained from the strain gauge mounted on the incident bar. An explanation to this is provided based on the increase in dislocation density and necking in the tested specimens during high strain rate loading and the consequent stress wave attenuation as it propagates within the specimen. The fracture behavior and effect of high strain rate testing on microstructure changes are examined. The peak force obtained based on strain gauge mounted on the transmitter bar is lower than the peak force obtained based on strain gauge mounted on the incident bar. There is an increase in tensile yield strength at high strain rate loading compared with that at quasi-static loading for both materials. The enhancement is more for IS 2062 mild steel than that for aluminum alloy 7075 T651. In the range of parameters considered, the strength enhancement factor was up to 1.3 for aluminum alloy 7075 T651 and it was up to 1.8 for IS 2062 mild steel. Generally, there was a good match between the experimental values and the Johnson–Cook model predictions.

Improved Kolsky tension bar for high-rate tensile characterization of materials

A new Kolsky tension bar has been re-designed and developed at Sandia National Laboratories, CA. The new design uses the concept that a solid striker is fired to impact an end cap attached to the open end of the gun barrel to generate dynamic tensile loading. The gun barrel here serves as part of the loading device. The incident bar that is connected to the gun barrel and the transmission bar follow the design similar to the Kolsky compression bar. The bar supporting and aligning systems are the same as those in the Kolsky compression bar design described by Song et al (2009 Meas. Sci. Technol. 20 115701). Due to the connection complication among the gun barrel, bars and specimen, stress-wave propagation in the new Kolsky tension bar system is comprehensively analyzed. Based on the stress-wave analysis, the strain gage location on the incident bar needs to be carefully determined. A highly precise laser-beam measurement system is recommended to directly measure the displacement of the incident bar end. Dynamic tensile characterization of a 4330-V steel using this new Kolsky tension bar is presented as an example.

Shear properties of acrylic under high strain rate loading

AbstractAcrylic is being used in structural applications because of its higher resistance to projectile impacts. High strain rate shear loading is one of the critical conditions. In the present study, properties of typical acrylic under high strain rate shear loading are presented. Torsional Split Hopkinson Bar apparatus was used for the studies in the shear strain rate range of 290 per sec to 791 per sec. Thin‐walled tubular specimens with hexagonal flanges were used for the experimental studies. Details of specimen configuration, data acquisition, and processing are presented. Shear strength is presented as a function of shear strain rate. It is observed that the shear strength at high strain rate is enhanced up to 25% compared with that at quasi‐static loading in the range of parameters considered. Comparison of torque versus time behavior derived from signals obtained from strain gauges mounted on incident bar and transmitter bar is also presented. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

A technique for combined dynamic compression-shear test

It is critically important to study the dynamic response of materials under a combined compression-shear loading for developing constitutive laws more accurately and fully. We present a novel technique to achieve the combined compression and shear loadings at high strain rates. The main apparatus consists of a strike bar, an incident bar, and two transmission bars. The close-to-specimen end of the incident bar is wedge-shaped with 90°. In each experiment, there are two identical specimens, respectively, agglutinated between one side of the wedge and one of transmission bars. When a loading impulse travels to specimens along the incident bar, because of the special geometrical shape, the specimen-incident bar interface gets an axial and a transverse velocity. Specimens endure a combined compression-shear loading at high strain rates. The compression stress and strain of the specimens are deduced from signals recorded by strain gages mounted on the bars. The shear stress is measured by two piezoelectric transducers of quartz (Y-cut with rotation angle 17.7°) embedded at the close-to-specimen end of transmission bars; the shear strain is measured with a novel optical technique, which is based on the luminous flux method. An analytic model was proposed and validated by the numerical simulations. The simulation results yield good agreement with the analytic results. The proposed technique was then validated through experiments carried out on lead specimens, by comparing experimental results with that of the split Hopkinson pressure bar experiments.

Effect of Mass Density on the Compressive Behavior of Dry Sand Under Confinement at High Strain Rates

Dynamic compressive behavior of dry quartz sand (Quikrete #1961 sand quarried in Pensacola, FL) under confinement was characterized using a modified long split Hopkinson pressure bar (SHPB). Sand grains were confined inside a hollow cylinder of hardened steel and capped by cemented tungsten carbide cylindrical rods. This assembly was subjected to repeated shaking to consolidate sand to attain precise bulk mass densities. It is then sandwiched between incident and transmission bars on SHPB for dynamic compression measurements. Sand specimens of five initial mass densities, namely, 1.51, 1.57, 1.63, 1.69, and 1.75 g/cm3, were characterized at high strain rates near 600 s−1, to determine the volumetric and deviatoric behaviors through measurements of both axial and transverse responses of a cylindrical sand sample under confinement. The stress–strain relationship was found to follow a power law relationship with the sand initial bulk density, with an exponent of 8.25, indicating a behavior highly sensitive to mass density. The energy absorption density and compressibility of sand were determined as a function of axial stress.

High strain rate mechanical behavior of epoxy under compressive loading: Experimental and modeling studies

Investigations on high strain rate behavior of epoxy LY 556 under compressive loading are presented. Compressive Split Hopkinson Pressure Bar (SHPB) apparatus was used for the experimental investigations. The studies are presented in the strain rate range of 683–1890 per second. It was generally observed that the compressive strength is enhanced at high strain rate loading compared with that at quasi-static loading. During SHPB testing of the specimens, it was observed that the peak force obtained from the strain gauge mounted on the transmitter bar is lower than the peak force obtained from the strain gauge mounted on the incident bar. Further, an analytical method is presented based on variable rate power law for the prediction of compressive strength at high strain rate loading for epoxy LY 556. Using the analytical method, high strain rate compressive stress–strain behavior is presented up to strain rate of 10,000 per second.

Effects of tissue preservation temperature on high strain-rate material properties of brain

Postmortem preservation conditions may be one of factors contributing to wide material property variations in brain tissues in literature. The objective of present study was to determine the effects of preservation temperatures on high strain-rate material properties of brain tissues using the split Hopkinson pressure bar (SHPB). Porcine brains were harvested immediately after sacrifice, sliced into 2 mm thickness, preserved in ice cold (group A, 10 samples) and 37 °C (group B, 9 samples) saline solution and warmed to 37 °C just prior to the test. A SHPB with tube aluminum transmission bar and semi-conductor strain gauges were used to enhance transmitted wave signals. Data were gathered using a digital acquisition system and processed to obtain stress–strain curves. All tests were conducted within 4 h postmortem. The mean strain-rate was 2487±72 s −1. A repeated measures model with specimen-level random effects was used to analyze log transformed stress–strain responses through the entire loading range. The mean stress–strain curves with ±95% confidence bands demonstrated typical power relationships with the power value of 2.4519 (standard error, 0.0436) for group A and 2.2657 (standard error, 0.0443) for group B, indicating that responses for the two groups are significantly different. Stresses and tangent moduli rose with increasing strain levels in both groups. These findings indicate that storage temperatures affected brain tissue material properties and preserving tissues at 37 °C produced a stiffer response at high strain-rates. Therefore, it is necessary to incorporate material properties obtained from appropriately preserved tissues to accurately predict the responses of brain using stress analyses models, such as finite element simulations.

An experimental and numerical investigation of the static and dynamic constitutive behaviour of aluminium alloys

A split Hopkinson pressure bar (SHPB) set-up was used to investigate the dynamic constitutive behaviour of commercial aluminium alloys both experimentally and numerically. The study was conducted in a 500–10 000 s×1 strain rate regime. Both regular solid and modified hollow transmission bars were employed in realizing this strain rate regime. Four different aluminium alloys, namely 7075-T4, 2024-T3, 6061-T6, and 5182-O, were considered for investigation. A copper-110 alloy pulse shaper was used to obtain better force equilibrium conditions at the bar–specimen interfaces. A plastic kinematic model was used to model the rate-dependent behaviour of aluminium alloys using commercially available ANSYS LS-DYNA software. Compared with the quasi-static condition, all four alloys showed a slight rate dependency with an increased flow stress ranging from 50 to 100 MPa at much higher strain rates. It was established from the final results that the experimentally determined dynamic constitutive behaviour matches very well with the numerical value in a 2000–5000 s×1 strain rate regime.

Dynamic compressive response of bovine liver tissues

This study aims to experimentally determine the strain rate effects on the compressive stress–strain behavior of bovine liver tissues. Fresh liver tissues were used to make specimens for mechanical loading. Experiments at quasi-static strain rates were conducted at 0.01 and 0.1 s −1. Intermediate-rate experiments were performed at 1, 10, and 100 s −1. High strain rate (1000, 2000, and 3000 s −1) experiments were conducted using a Kolsky bar modified for soft material characterization. A hollow transmission bar with semi-conductor strain gages was used to sense the weak forces from the soft specimens. Quartz-crystal force transducers were used to monitor valid testing conditions on the tissue specimens. The experiment results show that the compressive stress–strain response of the liver tissue is non-linear and highly rate-sensitive, especially when the strain rate is in the Kolsky bar range. The tissue stiffens significantly with increasing strain rate. The responses from liver tissues along and perpendicular to the liver surface were consistent, indicating isotropic behavior.

Development of a dynamic triaxial Kolsky bar

We present a confined Kolsky bar device capable of applying hydrostatic confining pressures and dynamic axial shear loads to right-circular cylindrical samples. The conventional Kolsky bar apparatus is modified by adding a high-pressure hydraulic chamber capable of applying radial confining pressures up to 400 MPa to the test specimen. An additional pressure chamber is added to the free end of the transmission bar and applies the axial portion of the hydrostatic pressure to the specimen. Once confinement is achieved, a striker bar impacts the incident bar to apply a dynamic, axial shear load to the test specimen. Pulse shaping techniques are employed in this device to generate the desired incident pulse necessary to achieve stress equilibrium in the sample and strain the sample at a nearly constant rate. We present data for an Indiana limestone tested at confining pressures up to 200 MPa and strain rates of 400 s−1, and compare results to quasi-static data.

Shear properties of epoxy under high strain rate loading

Shear properties of epoxy LY 556 under high strain rate loading are presented. Torsional Split Hopkinson Bar apparatus was used for the studies in the shear strain rate range of 385–880 per sec. Experimental details, specimen configuration and development, data acquisition, and processing are presented. Shear strength, shear modulus, and ultimate shear strain are presented as a function of shear strain rate. For comparison, studies are presented at quasi-static loading. It is observed that the shear strength at high strain rate is enhanced up to 45% compared with that at quasi-static loading in the range of parameters considered. Further, it is observed that, in the range of parameters considered, the change in shear properties with the change in shear strain rate is not significant. Comparison of torque versus time behavior derived from signals obtained from strain gauges mounted on incident bar and transmitter bar is also presented. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers

A New Compression Intermediate Strain Rate Testing Apparatus

A new apparatus for testing in compression at intermediate strain rates is introduced. The apparatus consists of a loading hydraulic actuator and a very long (40 m) transmitter bar. The specimen is placed with one end touching the end surface of the long bar and the other end free. The specimen is loaded by the actuator that impacts the specimen’s free end directly. Once loaded, the specimen deforms between the actuator and the transmitter bar. As the specimen is loaded and deformed, a compression wave propagates into the transmitter bar. The amplitude of this wave is measured with strain gages that are placed on the transmitter bar at a distance of about five diameters from the specimen. The wave in the transmitter bar propagates all the way to the end of the bar and then reflects back toward the specimen. The experiment can continue until the reflected wave reaches the strain gages (milliseconds). The strain in the specimen (full field) is measured directly on the specimen using Digital Image Correlation with high speed cameras.

Comparison of Low Impedance Split-Hopkinson Pressure Bar Techniques in the Characterization of Polyurea

The split-Hopkinson pressure bar (SHPB) technique has been widely employed for over fifty years in characterizing the high strain-rate properties of many common engineering materials. Historically, however, this technique has had limited success in characterizing soft materials, since their low mechanical impedances can increase delays in attaining dynamic equilibrium and result in transmission pulses with extremely low signal-to-noise ratios. Due to interest in improving characterization of soft materials at high strain rates, numerous modifications to the traditional SHPB technique have been proposed. These include: using more sensitive piezoelectric gauges, employing hollow transmission bars, utilizing lower impedance polymeric pressure bars, and the use of pulse shaping techniques. To date, there has been no comparative studies or consensus within the SHPB community as to which approach is most advantageous. The goal of this investigation is to compare a number of these techniques, specifically the use of PMMA pressure bars and a hollow aluminum transmission bar (both with and without pulse shaping), alongside more traditional solid aluminum pressure bars in the characterization of polyurea, a common low impedance polymer. The advantages and disadvantages of each technique in generating high strain-rate stress-strain curves are discussed.

Comparison of quasi-static and dynamic compression behavior of closed-cell aluminum foam

Aluminum foams have the potential to be used as structural material for impact energy absorption applications due to their extended constant stress region during compression. The compressive behavior of closed-cell aluminum foams synthesized by liquid melt route using TiH2 as a blowing agent is studied in the present work. Both quasi-static (ε˙=1×10−3 s−1) and dynamic (ε˙∼750 s−1) compression tests were carried out on foam specimens having relative density ranging from 0.062 to 0.373 and average pore diameter in the range of 2.2mm to 4.5mm. The dynamic tests were conducted using a split Hopkinson pressure bar (SHPB) apparatus with a hollow aluminum transmitter bar. The results of the study indicated that the plateau stress of aluminum foam increases with relative density and strain rate. Additionally, at high strain rates, an increase in the energy absorption capacity was observed. Such higher energy absorption capability during dynamic compression is beneficial when the material is used for impact energy absorption applications.

A measurement of wave propagation in the split Hopkinson pressure bar

This paper presents results from optical and analytical measurements of stress wave propagation in a rod of PMMA. The rod is loaded between the incident and transmitter bars of a split Hopkinson pressure bar. Displacements in the rod are measured using high speed photography with digital image correlation. Independent calculations of strain and displacement in the rod are made from the waves measured at the Hopkinson bar gauge stations, from which all required material parameters (apart from density) can be calculated by applying standard wave propagation theory. The paper finishes with a discussion of how these measurements will be developed in the future.

Dynamic fracture toughness of a high strength armor steel

This paper summarizes the results of a research being carried out to determine fracture behavior both in static and dynamic conditions of high strength armor steel Armox500T. In this research, notched specimens were cut to be tested in three-point bending test. Specimens were pre-cracked by flexural fatigue. Thereafter, some specimens were tested in bending up to rupture to determine the static fracture toughness K IC . To obtain fracture toughness in dynamic conditions, a split Hopkinson bar modified to perform three-point bending tests was used. In this device, displacements and velocities of the specimen were measured, as well as the rupture time by means of fracture detection sensors, glued to the specimens. After that, a numerical simulation of the test was performed by using LS DYNA hydrocode, obtaining stresses and strain histories around the crack tip. From these results, the stress intensity factor history was derived. By using the rupture time, measured by the sensors, the value of the fracture toughness computed was unrealistic. Therefore, the use of a numerical procedure to obtain the rupture time was decided, by comparing experimental results of velocities at the transmission bar with numerical results obtained with several rupture times. With this procedure, the computation of dynamic fracture toughness was possible. The method shows that the measurement of the dynamic fracture toughness is possible without the needs of using crack sensors or strain gauges. It can be observed that fracture toughness of this steel under static and dynamic conditions is quite similar.