Pressure Bar Experiments Research Articles

Experimental Study on SHPB Dynamic Mechanical Response in Different Damage Zones of Rock under Blast Load

In this study, the damage characteristics of the slit charge pack along the rock slitting and the vertical slitting directions were investigated using green sandstone as the subject. Split-Hopkinson pressure bar (SHPB) experiments were performed on green sandstone, and the dynamic mechanical properties of rock specimen were analyzed in the slitting and vertical slitting directions. Based on the SHPB experiments of the rock specimens in different damage zones along the rock slitting and the vertical slitting directions combined with the fractal theory, the distribution pattern of rock fracture fragmentation was investigated at different positions in the kerf direction and vertical kerf direction. The results of this study show that the dynamic strength of the rock corresponds to the damage degree of the rock, which is greater in the direction of the slit than that in the perpendicular direction of the slit. In addition, the core at 150 mm distance from the blast hole perpendicular to the slit direction could easily bear the blast loads. The lumpiness distribution of rock specimens under impact load shows good statistical self-similarity. The evolution law of fractal dimension shows that with increasing distance from the core to the blast hole, the fractal dimension of the specimen after failure increases. The fractal dimension of the specimen perpendicular to the slit direction is greater than that of the specimen along the slit direction at the same distance.

Study on dynamic mechanical properties and constitutive model description of Inconel718

In this study, the effects of strain rate and temperature on the flow stress of Inconel718 were analyzed by Split Hopkinson Pressure Bar (SHPB) experiment and quasi-static compression experiment. The classical JC constitutive model was established by combining the quasi-static compression experiment with the SHPB experiment. According to the effects of different grain sizes and [Formula: see text] phase on dislocation pile-up, the dislocation pile-up theory was introduced to modify the JC constitutive model. The modified constitutive model was compiled in FORTRAN language, and VUMAT user material subroutine was called and secondary development was carried out to establish the polycrystalline simulation model with different grain sizes. The uniaxial tensile and compression simulation process of polycrystal with different grain sizes was performed. Through comparing the simulation results with the experimental data. The correlation coefficient R, between the simulation and experimental values, is 0.97,981, and the average relative error is only 3.72%. The accuracy of the modified constitutive model was verified.

Discharge characteristics of pre‐polarized doped lead zirconate titanate under impact at room and high temperature

AbstractPiezoelectric materials have more and more applications in modern smart fuzes due to their multiple uses such as energy storage and sensing. The electrical output characteristics of piezoelectric ceramics under high temperature and high overload environments are critical to the reliability of the device. In this paper, the mechanical and electrical response of pre‐polarized doped lead zirconate titanate (PZT‐5H) under impact at room temperature to 250°C, that is, above the Curie temperature, was investigated through a split‐Hopkinson pressure bar experiment with an additional electrical output measurement system. The depolarization effect caused by temperature and the mechanical load was analyzed. A thermoviscoelastic constitutive model considering temperature and the strain rate was built based on the experimental data. The model can successfully predict the mechanical and electrical response of PZT‐5H under impact at different temperatures. The discharge characteristics of PZT‐5H under impact in the cooling stage after high‐temperature depolarization were also investigated, and the apparent flexoelectric coefficient of the PZT after complete depolarization was calculated. The research results show that the dynamic piezoelectric coefficient of PZT‐5H has a nonlinear relationship with temperature. During the high‐temperature cooling process, PZT has a shock‐induced polarization under the impact, and the output voltage is less than the polarized piezoelectric ceramic but higher than the flexural polarization at the same temperature. After complete depolarization, the apparent flexoelectric coefficient of PZT‐5H is 127 μC/m.

Stress Wave Propagation and Energy Absorption Properties of Heterogeneous Lattice Materials under Impact Load

A heterogeneous lattice material composed of different cells is proposed to improve the energy absorption capacity. The heterogeneous structure is formed by setting layers of body‐centered XY rods (BCCxy) cells as the reinforcement in the body‐centered cubic (GBCC) uniform lattice material. The heterogeneous lattice samples are designed and processed by additive manufacturing technology. The stress wave propagation and energy absorption properties of heterogeneous lattice materials under impact load are analyzed by finite element simulation (FES) and Hopkinson pressure bar (SHPB) experiments. The results show that, compared with the GBCC uniform lattice material, the spreading velocity of the stress of the (GBCC)3(BCCxy)2 heterogeneous lattice material is reduced by 18.1%, the impact time is prolonged 27.9%, the stress peak of the transmitted bar is reduced by 34.8%, and the strain energy peak is reduced by 29.7%. It indicates that the heterogeneous lattice materials are able to reduce the spreading velocity of stress and improve the energy absorption capacity. In addition, the number of layers of reinforcement is an important factor affecting the stress wave propagation and energy absorption properties.

Response of fractional Maxwell viscoelastic oscillator subjected to impact load

ABSTRACT The prediction for the impact behavior of viscoelastic oscillator (VEO) has a great significance. The fractional Maxwell viscoelastic oscillator (FMVEO) model under impact load is derived by integrating the fractional Maxwell constitutive model into the system vibration equation and further involving the geometric factors of the isolators. The numerical algorithm based on Chebyshev operational matrix is derived. The Split Hopkinson Pressure Bar (SHPB) experiment is conducted to validate the proposed model. The results indicate that FMVEO can fully describe impact properties of the viscoelastic oscillator. The fractional orders α and β are critical to the elasticity-viscosity component of FMVEO and the first amplitude of the impact response, respectively. The response of FMVEO displays an under-damped state for α ≤ 0.5 and over-damped state for α > 0.5. FMVEO degenerates into a pure-elastic system for α = 0 and evolves into a pure-viscous system for α = 1. In frequency domain, there are resonant peaks for the amplitude-frequency response for α ≤ 0.5, while no pronounced peaks exist for α > 0.5. The fractional order β displays a positive correlation with the first amplitude and a weakly negative correlation with the damping. The geometric factor κ should be taken into full consideration in two ways, the relatively small amplitude and the limited installation space, which shows the significant effect of the geometric element on the system response.

NUMERICAL MODELLING OF WAVE SHAPES DURING SHPB MEASUREMENT

The paper aims at the numerical simulation of the wave propagation in compressive Split Hopkinson Pressure Bar (SHPB) experiment. The paper deals with principles of SHPB measurement, optimisation of a numerical model and techniques of pulse shaping. The parametric model of the typical SHPB configuration developed for LS-DYNA environment is introduced and optimised (in terms of element size and distribution) using the sensitivity study. Then, a parametric analysis of a geometric properties of the pulse shaper is carried out to reveal their influence on a shape of the incident pulse. The analysis is algorithmized including the pre- and post-processing routines to enable automated processing of numerical results and comparison with the experimental data. Results of the parametric analysis and the influence of geometric properties of the pulse shaper (diameter, length) on the incident wave are demonstrated.

Dynamic Mechanical Properties of EPS Concrete Under Impact Loading

The finite element method (FEM) models of expanded polystyrene (EPS) concrete were established to study the relationships among dynamic damage, energy absorption and efficiency of buffering materials with varied flexible aggregate contents and impact speeds based on the existing split Hopkinson pressure bar (SHPB) experiments. Applied material indexes including damage degrees, crack rates and energy absorption capacities, and the dynamic responses of EPS concrete under impact loading were investigated. The results show that the failure types of these materials evolve from quasi-brittle destruction to flow-plastic destruction, the damage degree of EPS concrete increases with the enhancement of flexible aggregate content, and the peak of crack rate exists around the extreme point of shock velocities. The energy-absorption efficiency of EPS concrete increases nonlinearly when the EPS beads increase and the shock speed enhances, respectively.

Effect of post cure temperature cycle on dynamic compression behavior of glass filled epoxy composites

Split Hopkinson’s pressure bar experiments are conducted to analyze dynamic compression behavior of high temperature exposed glass filled epoxy composites. Two filler variants, spherical and slender, up to 10% volume fraction, are uniformly dispersed into the matrix. Room temperature curing is followed by exposing the composites to a temperature up to 180 °C. Slender fillers affect overall compression characteristics whereas the influence of slender fillers is observed only in post-yield regime. Computational analysis in combination with experimental observations reveals two competing deformation mechanisms, constraints provided by fillers to polymeric chain movement and complex interaction of stress fields around the fillers.

Theoretical analysis of JMC effect on stress wave transmission and reflection

Taking the joint matching coefficient (JMC) which represents the contact area ratio of the joint in rock masses as the key parameter, a one-dimensional contacted interface model (CIM-JMC) was established in this study to describe the wave propagation across a single joint. According to this model, the reflected and transmitted waves at the joint were obtained, and the energy coefficients of reflection and transmission were calculated. Compared with the modified Split Hopkinson pressure bar (SHPB) experiment, it was validated by taking the incident wave of the SHPB test as the input condition in the CIM-JMC, and the reflected and transmitted waves across the joint were calculated by the model. The effects of four sets of JMCs (0.81, 0.64, 0.49, and 0.36) on the transmission and reflection of the stress wave propagation across the joint were analyzed and compared with the experimental results. It demonstrated that the values of CIM-JMC could represent both the transmission and reflection of the stress wave accurately when JMC > 0.5, but could relatively accurately represent the reflection rather than the transmission when JMC 0.5.

A Tensile Origin for Fault Rock Pulverization

AbstractThe origin of highly fragmented, but weakly strained rocks found along major strike‐slip faults has been enigmatic since their first recognition. These so‐called pulverized rocks occur up to 100 m away from the principal slip zone of seismogenic faults around the world. Previous dynamic compression experiments have suggested that rock pulverization occurs at strain rates on the order of 102 s−1, pointing to a coseismic origin; however, strain rates during earthquake rupture 100 m from faults is expected to be 4 orders of magnitude smaller. We present evidence from new modified Split‐Hopkinson Pressure Bar experiments that instead supports a tensile origin for coseismic rock pulverization. In the new experimental configuration, the axial compressive load from the Split‐Hopkinson Pressure Bar induces radially isotropic tension in a Westerly Granite disk bonded between two lead cylinders. The isotropic tensile state of stress results in the formation of polygonal fracture arrays that bound axis‐parallel columnar fragments. The tensile strength of Westerly Granite measured at strain rates between ~5 and 50 s−1 bridges the gap between low strain rate and shock strengths reported previously, supporting an interpretation that highly fragmented rocks may form in a state of isotropic tension. The resulting fragment size is independent of strain rate and instead appears to be controlled by elastic strain energy, a strong function of material strength, and fracture toughness. Our results provide a solution to the strain rate‐distance scaling problem between laboratory experiments and field observations of pulverized rocks and also explain the asymmetric distribution of pulverized fault rocks about strike‐slip faults.

Design and development of a novel strain gauge automatic pasting device for mini split Hopkinson pressure bar.

In a split Hopkinson pressure bar (SHPB) experiment, the pasting quality of strain gauges will directly affect the accuracy of the measurement results. The traditional method of pasting the strain gauges is done manually by the experimenter. In the process of pasting, it is easy to shift or twist the strain gauge, and the experimental results are greatly affected by human factors. In this paper, a novel type automatic pasting device for strain gauges is designed and developed, which can be used to accurately and rapidly paste the strain gauges. The paste quality is reliable, and it can guarantee the consistency of SHPB experimental measurement. We found that a clamping force of 74 N achieved a success rate of 97%, whilst ensuring good adhesion.

X‐Ray Microtomographic Characterization and Quantification of the Strain Rate Dependent Failure Mechanism in Cenosphere Epoxy Syntactic Foams

This work investigates the failure mechanism in cenosphere epoxy syntactic foams at the quasi‐static and dynamic strain rates. Split‐Hopkinson pressure bar experiments are controlled to stop dynamic deformation of the foams at various strain stages. The internal microstructure at each strain is characterized in the x‐ray microtomography and compared to the microstructure in the foams deformed quasi‐statically. The microscopic observations reveal that the failure process in syntactic foams at the low and high rates is dominated by the crushing of cenospheres and the cracking of the epoxy matrix. However, the mechanism of failure in the foam is significantly affected by the strain rate. Compared to quasi‐static compression, macro‐cracks form earlier in the matrix at dynamic rates and can propagate to split cenospheres. The volume of the damage as defined by the failure of both cenospheres and the matrix is calculated from the x‐ray microtomographic images. It is found that the damage can be quantitatively related to the strain and the strain rate using an empirical equation.

Influences of high-frequency vibration on tool wear in rotary ultrasonic machining of glass BK7

This investigation represented a fundamental research on the potential effects of the ultrasonic on the tool wear involved in rotary ultrasonic machining (RUM) of glass BK7 process. Comparative examinations of the profile deviations and abrasive morphologies of the two diamond tools produced with and without ultrasonic were conducted with optical microscopy, 3-D optical profiler, and scanning electron microscopy (SEM), and their resultant effects on the dimension accuracy and the surface quality of the machined component were also explored. Giving consideration to the strain rate effects of the material provoked by the ultrasonic superposition, the split Hopkinson pressure bar (SHPB) experiments were employed to validate their effects on the dynamic mechanical properties of glass BK7. Afterward, the active mechanisms of abrasive splitting were investigated theoretically. It was found that the ultrasonic superposition by means of suppressing the abrasive dislodgments reduced the profile deviation of the diamond tool, hereby improving the dimensional accuracy of the component. Furthermore, superimposing an ultrasonic vibration would prolong the service life of the abrasives, and this wear-resistant capacity would increase the amount of the working abrasives, thus improving the specimen surface quality. The morphological observations of the abrasives revealed that ultrasonic superposition led to the splitting characteristics converted from transgranular cracking (macrosplintering) to conchoidal fracture (microsplintering). The dynamic mechanical properties of glass BK7 would increase its Young’s modulus, which would reduce the crack nucleation depth in the abrasive, leading the conversion of the splitting appearances.

Effect of High Loading Rate on SCC Behaviour of SUS304 Austenitic Stainless Steel

This study deals with the effect of intermittent impact loading on the SCC growth behaviour of SUS304 austenitic stainless steel. To this end, the Split-Hopkinson pressure bar (SHPB) experiments on SUS304 were performed first to establish the dynamic tensile stress-strain response at strain rates up to 700s-1. The analyses of dynamic stress intensity factors for wedge loading experiments on modified compact tension specimens, designated WLCT, were then performed by the finite element method. The SCC experiments with the intermittent impact loading for WLCT specimens of SUS304 by the use of the SHPB were mentioned and discussed as well.

An Experimental Study on Dynamic Constitutive Relationship of 7075-T651 Aluminum Alloy

7075-T651 aluminum alloy are widely used in aeronautical applications such as wing panels, but there is no corresponding constitutive model for it now. In this paper, the flow behavior of 7050-T651 aluminum alloy was investigated by Split Hopkinson Pressure Bar (SHPB) and quasi-static compression experiment system. The strain hardening parameters were obtained by quasi-static compression experiment data, and the strain rate hardening parameters at various strain rates (1000-3000s-1) and room temperature, and the thermal softening parameter at various temperatures (20-300°C) where strain rate is 3000s-1 were obtained by SHPB experiment data. Then the constitutive equation of 7075-T651 aluminum alloy is obtained based on Johnson-Cook constitutive equation model.

Temperature in high temperature SHPB experiments

As an experimental technique, it’s desired that the temperature in specimen is uniform in high temperature split Hopkinson pressure bar (SHPB) experiments. However, the temperature in specimen decreases and the temperature of bars increases when specimen starts to contact with bars, which induces the nonuniform temperature distribution in specimen, and may result in inaccuracy of experimental results. In this paper, the temperature distributions of specimen and bars in high temperature SHPB experiments were investigated while the specimen was heated alone. Firstly, the temperature history of specimen was measured at different initial temperatures by experiments, then simulation was carried out. Simulation results were consistent with experimental results by adjusting the thermal contact coefficient between specimen and bars. By this way, the thermal contact coefficient and simulation results were validated, and the proper cold contact times of specimen and bars in high temperature SHPB experiments were discussed. Finally, the results were compared with those in references.

Experimental Investigation of the Deformation Behavior of ABS Resin at Elevated Temperature and High Strain Rate

Split Hopkinson Pressure Bar (SHPB) experiments are carried out to study the deformation behavior of acrylonitrile-butadiene-styrene (ABS) resin at elevated temperature and high strain rate. The temperature and strain rate considered are 293K and 343K and 8.0×102s-1, 2.7×103s-1 and 1.0×104s-1, respectively. The curves of engineering stress and strain at different temperatures and different strain rates are experimentally obtained. The effects of temperature, strain rate and the fraction of ABS on the deformation behavior of ABS resin are discussed in detail. Then, a rate and temperature dependent phenomenological constitutive law for ABS resin is developed.

Dynamic Response and Fracture of High Strength Boride/Alumina Ceramic Composite

Dynamic response and fracture of high strength boride/alumina ceramic composite were investigated by split Hopkinson pressure bar (SHPB) experiment in this paper. The compressive stress–strain curves and dynamic compression strength of the composites were tested. The surface’s microstructure of fractured composites were examined by using scanning electron microscope (SEM) to investigate the fracture mechanism. The results show that boride/alumina has high dynamic compressive strength and high Young’s modulus. The main fracture mode of the material is the fracture of the ceramic grains. The micro-voids and flaws, generated during the sintering and manufacturing of material and mechanical process of specimen, decrease the strength of the material because they provide the source of crack expansion when the material undergoes the dynamic loadings.

New formulations for the Hopkinson bar technique to extract a response of the constituent material in composite specimens

During stress wave propagation through a composite material, the wave velocities through the fibre and matrix are different because of the differences in the respective moduli and densities. The stresses and strains generated in the constituent material are accordingly related to the wave velocities. In a typical split Hopkinson pressure bar (SHPB) experiment, the composite coupon is held in between the incidence and transmitter bars and a global response is obtained. While this provides a basis of comparison between various material systems, the information is not sufficient to predict failure of the composite precisely. Knowledge of the individual responses of the fibres and the matrix is needed for accurate failure analysis. However, the existing SHPB formulations are not capable of extracting individual responses from a regular test set-up. In the current investigation, a new set of mathematical formulations has been derived from the momentum concept so that the individual strain and stress responses of the fibres and the matrix can be estimated from regular SHPB tests. Coaxial cylindrical coupons have been designed and fabricated for this purpose. Test results indicate that there is good correlation between the predicted and experimental data, especially at lower strain rates. Details of the mathematical derivations and experimental procedures are described in the paper.