Hopkinson Pressure Bar Technique Research Articles

Dynamic response and constitutive model of damaged sandstone after triaxial impact

In deep rock engineering, dynamic hazards such as blasting and earthquakes pose serious threats to the safety of underground rock structures, including mining and construction activities. To explore the dynamic mechanical properties and damage mechanisms of rocks under realistic ground stress conditions, this study utilized dynamic triaxial compression tests under hydrostatic pressure. These tests employed a modified split Hopkinson pressure bar (SHPB) technique at various strain rates. Wave velocities of the specimens were measured before and after the tests to calculate the damage factor based on the P-wave velocity. Additionally, dynamic uniaxial compression tests on the damaged sandstone specimens indicated a significant reduction in dynamic strength as the damage impact air pressure increased. There was also a notable correlation between the dissipated energy ratio and the level of rock damage, with more severe damage resulting in a smaller dissipated energy ratio. Macroscopic examination through sieve grain size results confirmed the formation of more cracks under high impact air pressure, while microscopic analysis using scanning electron microscopy (SEM) images uncovered the damage mechanisms within the rock post-impact. Ultimately, this study developed an improved ZWT model to effectively verify the dynamic response of the damaged sandstone.

Influence of Interface Type on Dynamic Deformation Behavior of 3D-Printed Heterogeneous Titanium Alloy Materials.

Using the Split Hopkinson Pressure Bar technique, strain-limited dynamic compressive loading experiments were performed on TA1/TA15 heterostructure (HS) materials. The plastic deformation mechanisms, fracture forms, and energy absorption properties of an HS material with a metallurgical bonding interface (MB) and an HS material without a metallurgical bonding interface (NMB) are compared and analyzed. The results show that there is no significant difference between the two deformation mechanisms. The fracture forms are all "V-shaped" fractures within the TA1 part. The NMB was carried for 57 μs before failure and absorbed 441 J/cm3 of energy. The MB was carried for 72 μs before failure and absorbed 495 J/cm3 of energy. Microstructure observations show that there is a coordinated deformation effect near the MB interface compared to the NMB, with both TA1 and TA15 near the interface carrying stresses. This causes an enhancement of the MB load-bearing time and a 12% increase in energy absorption.

Dynamic properties and plastic dissipative mechanisms of polymeric adhesive under dynamic compression at various strain rates

This work investigated the dynamic compression behavior of structural vinylester adhesive under dynamic compression tests using the Split Hopkinson Pressure Bars technique. The obtained results showed that the dynamic parameters were strain rate sensitive. No macroscopic damage was observed in the specimen at the different applied strain rates. However, the presence of a characteristic second peak mainly at high strain rates on the strain rate profiles was attributed to the plasticity occurrence in the polymer under impact. The non-linear response that appeared on the stress-strain curves of the adhesive had been attributed to the combined material softening and thermal softening. These results demonstrated that the studied adhesive could be considered a potential material for high-rate applications due to the combination of significant toughness with high strength and extensive plasticity. An energetic study determined the energy balance and quantified energy absorption. It was observed that used vinylester adhesive was suitable for dynamic applications because it could absorb a large amount of energy and undergo large compressive deformations without damage. This work provided a complete range of experimental data for structural adhesive, which could help the designer of an energy-absorbing adhesively bonded composite joint structure for high-strain rate applications.

Frequency Characteristics of High Strain Rate Compressions of Cf-MWCNTs/SiC Composites

The incorporation of ductile reinforcements into ceramics helps restrain crack deflection, which can enhance ceramics’ toughness and overcome the matrix’s brittleness. In this paper, we produced a ceramic composite reinforced by carbon fibers coated by multi-wall carbon nanotubes (shortened by Cf-MWCNT/SiC composites) for enhanced impact resistance at a high strain rate that commonly occurs in composite materials used in astronautics, marine, and other engineering fields. The fabrication process involves growing multi-wall carbon nanotubes (MWCNTs) on a carbon fiber woven fabric (Cf) to create the fibril/fabric hybrid reinforcement. It is then impregnated by polymer solution (precursor of the ceramics), forming composites after the pyrolysis process, known as the liquid polymer infiltration and pyrolysis (PIP) technique. To assess the impact resistance of the Cf-MWCNT/SiC under high-strain rate compressions, the split Hopkinson pressure bar (SHPB) technique is employed. Since the failure behavior of the Cf-MWCNT/SiC composites in the absence of the ductile phase is not well understood, the study employs the Hilbert–Huang transform (HHT) to analyze the stress–time curves obtained from the SHPB experiments. By applying the HHT, we obtained the frequency–time spectrum and the marginal Hilbert spectrum of the stress signals. These analyses reveal the frequency characteristics of the Cf-MWCNT/SiC composite and provide insights into the relationship between transformed signal frequency and fracture behavior. By understanding the dynamic fracture behavior and frequency response of the Cf-MWCNT/SiC, it becomes possible to enhance its impact resistance and tailor its performance for specific protective requirements. Therefore, the findings of this study can guide the future design and optimization of Cf-MWCNT/SiC structures for various protective applications, such as body armor, civil structures, and protections for vehicles and aircraft.

Application of the split Hopkinson pressure bar technique to study the influence of inertial confinement on the dynamic mechanical response of concrete

Application of the split Hopkinson pressure bar technique to study the influence of inertial confinement on the dynamic mechanical response of concrete

Effects of hydrogen on the dynamic mechanical properties and microstructure of 7055 and 7A52 aluminum alloys

In the present work, the effect of hydrogen on the impact resistance of the 7055 and 7A52 aluminum alloys have been experimentally investigated by using split Hopkinson pressure bar (SHPB) and electrochemical hydrogen charging technique. It was first found that hydrogen induced stress chase after yield deformation in 7055 and 7A52 alloys. Meanwhile, hydrogen reduced the yield strengths of the two alloys. Besides, the evolution of microstructures including cracks, particles and dislocations has been quantitatively analyzed by polarizing microscope (PM), scanning electron microscope (SEM) and transmission electron microscopy (TEM). It has been found that both two aluminum alloys are sensitive to hydrogen embrittlement under high-speed deformation conditions. The hydrogen causes hydrogen compression cracks at grain boundaries, pores, and Al2Cu particles in the material, and the hydrogen cracking of both aluminum alloys after impact appeared to be layered, and the lower layer cracking was flat, which indicated that the hydrogen made the alloy embrittle to some extent. Besides, hydrogen reduces the hindering effect of copper-containing phase and large η phase on dislocation movement, which makes the dislocations distribution inside the 7055 alloy grains more uniform. The results show that 7000-series aluminum alloys are sensitive to hydrogen embrittlement under high-speed deformation conditions.

Mechanical Response of Stress Wave Attenuator With Layered Structure Under Impact Load: Experimental and Numerical Studies

Abstract The fluidic down-the-hole (DTH) hammer drilling is generally regarded as one of the best means for hot dry rock (HDR) drilling. The fluid oscillator, as the core part of the fluidic DTH hammer, is prone to fracture when subjected to impact loads due to its brittle characteristics. Therefore, a steel-layered structure with different contact areas is developed as a stress wave attenuator to protect the fluid oscillator for the DTH hammer under high-temperature drilling conditions. In this paper, the stress wave attenuation performance with different steel-layered structures is analyzed based on the split Hopkinson pressure bar (SHPB) technique. The effects of contact area ratio, the orientation of contact surfaces, and number of layers on the stress wave attenuation are investigated by numerical simulations and laboratory tests. It is found that the attenuation ratios in stress amplitude and impact energy gradually increased with the increase of the contact area ratio. Besides, the orientation of contact surfaces has a significant influence on the attenuation effect. For the layered structure with a two-layer object part, the maximum attenuation ratios of stress amplitude and impact energy are 62.4% and 79.6%, respectively, when the included angle between the two convex structures is increased to 90 deg. Additionally, the attenuation ratio of the layered structure can be improved by increasing the number of layers. The results demonstrate that the stress wave attenuator with layered structures has great potential for brittle materials protection against impact loads.

Stress-wave propagation in multilayered and density-graded viscoelastic medium

Uncovering the dynamic behaviors of viscoelastic medium with complicated internal geometric structures, i.e., graded density and multilayers, is of great significance in the fields of impact energy absorption and structure protection. Herein, a theoretical solution for the stress wave propagating in density-graded viscoelastic medium is first obtained by combining the Laplace transform method and the Eulerian governing equation. Effects of graded density (e.g., materials with density increasing or decreasing) and multi-layers on wave dispersions in viscoelastic medium are then discussed. The propagation of shear stress in striped density-graded medium is deduced and analyzed while subjected to oblique impacting. The wave propagation rate of shear stress across the interfaces of multi-layered medium is discussed. Dynamic mechanical parameters of density-graded TiB2-TiB-Ti ceramics are experimentally studied by the split Hopkinson pressure bar (SHPB) technique. The dynamic laser irradiating technique combined with the advanced velocity interferometer method is employed to detect the mechanical parameters of shell with layered-structure (composed of nacre and chalk). The effects of internal geometric structures and contents on dynamic behaviors of viscoelastic medium are revealed, and the theoretical results are preliminarily validated by experimental results. This work provides a useful method to calculate and analyze the dynamic viscoelastic properties of complicated mediums, which could be also used as a guideline for designing anti-impacting composites with density-graded and layered structures.

Dynamic Characterization of Concrete using Split Hopkinson Pressure Bar

Concrete exhibits brittle behaviour and is weak under tensile and flexural loading. The response of concrete to dynamic loading is of interest in a variety of civilian and military applications. Understanding the response of concrete to impact or explosive loading is important for effective protection of defence and civil structures. The split Hopkinson pressure bar (SHPB) technique has been used widely to measure the dynamic strength enhancement of materials at high strain rates. Although, SHPB technique has been verified for metallic materials, but validity and accuracy of SHPB results for non-metallic, e.g. concrete materials have not been thoroughly studied so far. The present study examines the application of SHPB to determine the dynamic strength of concrete under compressive loading. The aim of this study is to understand the strain rate effect on the ultimate uniaxial compressive strength of concrete in SHPB tests for two different grades of concrete. The behaviour of concrete at strain rates of the order of 200 - 600 per second and pressures up to 0.38 MPa are studied experimentally. The strength of concrete is found to be increased with the increase in strain rates. Further, it is observed that due to the composite microstructure of concrete, deformation and stresses are non-uniform in the concrete specimens.

Dynamic Tensile Mechanical Properties of Outburst Coal Considering Bedding Effect and Evolution Characteristics of Strain Energy Density

The evolution of strain energy density of outburst-prone coal is of great significance for analyzing the characteristics of energy accumulation and release in coal and rock masses. The dynamic mechanical properties of coal samples were tested by using the split Hopkinson pressure bar (SHPB) technique. Dynamic tensile mechanical properties, layered effect and density evolution characteristics of strain energy for coal were studied. The dynamic failure and crack propagation process of the specimen were recorded with a high-speed camera. In addition, a digital image correlation (DIC) method was used to analyze the evolution characteristics of the strain field during the deformation process of the specimen. The distribution characteristics of the particle fragments were statistically analyzed. The results show that the bedding orientation of the coal has a significant effect on its deformation and damage features. The presence of weak planes, microcracks and laminae causes its shear damage zone to behave more complex. If the crack plane coincides with the high shear stress plane, the developed shear cracks extend along the weak laminae and the shear damage zones in BD specimens are not symmetrically distributed. When the laminated surface of the coal sample is at a certain angle with the impact loading direction, the damage mode is coupled with tensile and shear damage. The percentage mass distribution of particles and fines increases with increasing bedding orientation. The effect of water on the dynamic damage of coal samples is significant. Based on the principle of pressure expansion of wing-shaped cracks, the formula for calculating the dynamic strength of water-saturated coal samples under dynamic loading was derived.

Mechanical behaviors of the carbon fiber reinforced X-core sandwich structure under the dynamic compression

A split Hopkinson pressure bar technique was applied to investigate the dynamic response of carbon fiber composite sandwich structures with the X-cores. The compressive mechanical behaviors of the composite X-core unit cells and its foam reinforced unit cells at the strain rates ranging from 3•10−4 s−1 to 600 s−1 were obtained. During the dynamic tests, the dynamic stress equilibrium state in the unit cells was achieved by adopting the pulse sharpers and discussed comprehensively. The measured structural strength increased by 81.9 % on average at the high strain rates than that under the quasi-static compression. Under the dynamic compression, the failure modes captured by a high-speed camera were found similar with that of the quasi-static tests. Finite element analysis was utilized to study the variation history of stress and strain rate fields of the core. Because of the material failure strain enhancement effect, the experimental and simulated compressive strength of the composite unit cells both increased significantly at the high strain rates. At the same high-speed compressive strain rate, the dynamic reinforcement effect of the composite sandwich unit cell was smaller than that of the parent material.

Effects of fibrous Cr phase on the adiabatic shearing anisotropic behavior of the Cu-15Cr in-situ composite

The effects of fibrous Cr phase on the adiabatic shearing anisotropic behavior of the Cu-15Cr in-situ composite model alloy were firstly studied by a split Hopkinson pressure bar (SHPB), Optical Microscope (OM), and X-ray diffraction (XRD) technique. Specimens were loaded along the rolling direction (Cu-15Cr-0°) and perpendicular to the rolling direction (Cu-15Cr-90°). The experimental and analysis results showed that there were differences in the number of phase interfaces and phase interface distance that the shear stress crossed when sheared along with different directions due to the directional arrangement of the fibrous Cr phases, which caused the strain hardening, strain rate hardening and thermal softening of the Cu-15Cr in-situ composite were anisotropic. The Cu-15Cr-90° specimen possessed lower strain rate hardening and higher thermal softening at the same strain rate, and the strain hardening rate was similar to that of the Cu-15Cr-0° specimen as the shearing process proceeded, so the Cu-15Cr-90° specimen was more prone to adiabatic shearing than the Cu-15Cr-0° specimen. In addition, a splitting crack was produced at the Cr phase and matrix interface in the Cu-15Cr-0° specimen due to the additional tensile stress produced in the Cr phase during shearing. The perpendicular Cr phase in the Cu-15Cr-90° specimen was deflected parallel to the shear stress during shearing, so it was not easy to produce splitting cracks. However, the tip of the Cr phase without deflection was more likely to possess high additional tensile stress, so the crack in the Cu-15Cr-90° specimen was extended along the edge of the adiabatic shear region at the tip of the Cr phase.

Experimental Investigation on the Energy Consumption Difference between the Dynamic Impact and the Drilling Tests of Rocks

The split Hopkinson pressure bar (SHPB) technique and various drilling tests were performed on three types of lithologies (granite and cyan and red sandstone) to understand the variation in energy consumption (EC) of the different methods used for rock crushing. The dynamic behavior of rocks was analyzed based on the dynamic stress-strain curve, the peak stress, the energy, and the failure mode for the SHPB test, while the drilling curve, the influence of the drilling pressure (DP) and the revolution speed (RS) on the drilling speed (DS) were analyzed for the drilling test. Additionally, the EC difference was compared based on the EC required to break a single unit volume of rock. The obtained results indicate that the sensitivity of the dynamic strength of rocks with different lithologies to strain rate is different and that the higher the uniaxial compressive strength is, the more sensitive it is. Additionally, the strengthening of the peak stress is more pronounced with the increase in the strain rate. The energy utilization efficiency (EUE) of the SHPB test sample has a positive correlation with the strain rate. Moreover, the typical drilling process can be divided into the initial drilling and stable drilling stages, with the DS during the stable drilling stage being lower than that during the initial drilling stage. The DP and the RS have a linear positive correlation with the DS, with the influence of the DP on the DS being more obvious. Finally, the EC during the drilling process is higher than the EC during the SHPB technique.

Numerical investigation of the dynamic behavior of a Ti/TiB functionally graded material using Split Hopkinson Pressure Bar test

This work focuses on the mechanical behavior, at high strain rates, of metal-ceramic functionally graded materials (FGMs) using a Split Hopkinson Pressure Bar (SHPB) technique. The validity and the accuracy of the SHPB test results for these composite materials have not been meticulously studied due to their complex mechanical response under dynamic loadings. In this paper, numerical simulations of SHPB tests were performed to determine the dynamic response of specific FGMs at various strain rates. The effects of the compositional gradient exponent and the striker velocity, as well as increasing/decreasing stiffness through the thickness of the functionally graded specimen, were investigated. The considered material is composed of Titanium mono-boride ceramic (TiB) and Titanium metal (Ti) phases; the volume fraction (Vc) of the ceramic constituent varies through-thickness following a power-law distribution. The locally effective material properties were evaluated using a homogenization method based on the self-consistent method (SCM). The elastoplastic behavior of the FGMs under dynamic loading is described using dynamic Tamura-Tomota-Ozawa model (DTTO). The numerical simulations indicated that the compositional gradient exponent and striker velocity have significant effects on the dynamic response of the FGMs. On the other hand, the increasing/decreasing stiffness has a minor effect due to the low thickness of the experimental functionally graded specimen. Finally, the effect of the high strain rates the multiple interface reflection/transmission of the stress wave is very considerable for the design and the optimization of FGMs behavior under dynamic loading.

Dynamic compression mechanical properties of eco-friendly ultra-high performance concrete produced with aeolian sand: Experimental and three-dimensional mesoscopic investigation

The dynamic mechanical properties of ultra-high performance concrete (UHPC) containing aeolian sand was studied via the split Hopkinson pressure bar technique and three-dimensional mesoscopic numerical simulation. A novel method for three-dimensional modeling of end-hook steel fibers was proposed. Employing the 3D mesoscale model, the dynamic compressive responses of aeolian sand-based UHPC were simulated and analyzed. Simulation results exhibited constitutive relationships and failure patterns that agree well with the impact test results, which verified that the established three-dimensional mesoscopic model enables effective analysis of the dynamic mechanical properties of aeolian sand-based UHPC. Then the effects of strain rate, fiber content and matrix strength on the stress-strain relation and energy evolution process were quantitatively discussed, which provide a basis for the application of aeolian sand-based UHPC.

Numerical Simulation of Rock Dynamic NSCB Test Based on a Self-Developed Dynamic Damage Model

Elucidating and understanding the dynamic fracture characteristics of rocks play an essential role in the application of rock engineering and geophysics. In this study, based on a self-developed dynamic damage model, a rock notched semi-circle bend test with the Split Hopkinson Pressure Bar technique is numerically simulated. The study focuses on three aspects including damage evolution, energy evolution, and failure mode of rock under different loading velocities. From the simulated results, the following conclusions can be conducted: 1) the damage range increases gradually with the increase of loading velocity; 2) the crack propagates to the loading point along the symmetry axis of the samples under different loading velocities; 3) the loading velocity has an important influence on the failure mode of straight notch semi-circular marble, whose mechanism can be explained by that the local high strain rate leads to the obvious randomness and uncertainty of crack activation in rock; and 4) the energy evolution of notched semi-circle bend is vitally affected by loading velocity, and the deformation and the failure process of straight notch semicircular marble under dynamic loading can be divided into five stages according to the ratio of internal energy to total energy. The beneficial findings may provide some references in practice design from engineering problems.

Impact strength of thermally aged double lap adhesively bonded joints

In this paper deep investigations and full characterization of SA 70 epoxy's properties as a bulk material and as an adhesive in a bonded assembly, are performed. Those properties are essential for an industrial or a supplier and whose determination is not at all obvious. Double lap bonded steel-SA 70 epoxy assemblies were prepared and then thermally aged within a wide interval of temperatures up to 370 °C. The thickness of the central adherend was kept always equal to twice the thickness of each of the upper and lower adherend. Ethanol and sand blasting were the two surface treatments adopted for the substrates while three different central adherend thicknesses from each surface treatment were prepared. Preliminary investigations were performed through thermogravimetric analysis (TGA) tests applied on cured SA70 epoxy specimens and this for the whole ageing temperatures range; showing that the onset temperature for this adhesive is around 380 °C and a major mass loss occurs at 420 °C. Isothermal TGAs were performed at 300 °C, 350 °C and 370 °C in order to choose ageing duration. The aged specimens were then tested under impact at 10 m/s using the Split Hopkinson Pressure Bar (SHPB) technique. It was found that the sand blasted specimens exhibit higher shear strength. No clear effect of substrates’ thicknesses on the joint shear strength was noticed. In addition, the evolution of the shear strength with the ageing temperature was found to be in line with the TGA results: the strength increases with the ageing temperature up to 200 °C; after 225 °C the strength decreases. From 300 °C and above the strength drops drastically.

Effect of Strain Rate on Mechanical Behavior and Microstructure Evolution of Ti-Based T110 Alloy

Recently developed thermally hardenable medium-level alloyed titanium alloy T110 was studied from the viewpoint of microstructure influence on the mechanical behavior under quasi-static and high-strain rate deformation. The globular microstructures of two types differing in aspect ratio of α-globules were formed under thermomechanical processing conditions with different reductions (εtotal = 2, and 3) and final annealing at 850 °C, 3 h. The thermally hardened state was formed by conventional solid-solution treatment at a temperature of two-phase α+β field (880 °C, 45 min), water quenching, and final aging (550 °C, 5 h). Stress–strain dependencies were estimated on tension with strain rates varied from 8·10−4 to 4·10−2 s−1, as well as compression with a strain rate of 10−3 s−1 (quasi-static) and under high-strain rates varied from 870 s−1 to 3520 s−1; the deformation with high-strain rates has been achieved using split Hopkinson pressure bar (SHPB) technique. The tested material was also assessed using strain energy (SE) parameter. A parallel study of the microstructure and crystallographic texture formed during testing allowed to propose a reliable deformation and fracture mechanisms, depending on the stress state (tension, compression) and strain rate. A special role of the initial microstructure is noted, which determines the plasticity of the tested alloy, the sites of pore nucleation, features of crack growth, as well as the localization of deformation, which is determined by the mode and rate of loading. It is established and explained why the best balance of strength and ductility, and thus, the highest SE values in the all tests carried out were ensured by the alloy in the state with a more uniform microstructure of globular morphology (after the rolling with reduction εtotal =3), which, in turn, provided general superiority over the widely used Ti-6Al-4V alloy.

Strain rate effects on the thermomechanical behavior of glass/polyester composite joints

AbstractFor several decades, composite materials have been used in different applications due to their multiple advantages. It has become necessary to understand the dynamic behavior of these materials under critical loads. In‐plane (IP) and out‐of‐plane (OP) dynamic compression tests were investigated on a cubic specimen of composite joints under low to high strain rates using the Split Hopkinson Pressure Bars (SHPB) technique. The specimens consist of two adherents (glass/polyester composite) assembled by an adhesive with different thicknesses (1 and 2 mm). During the experimental tests, the high‐speed camera was used to check the damage history and the failure mechanisms on adhesively bonded composite joints. The sample surface temperatures were monitored using an infrared camera (IR). For bonding of two composite substrates, it can be noted that the thickness of the adhesive has a significant effect on the dynamic behavior and the kinetics of the damage. It has been concluded that the damage was localized at the level of the layers in the case of in‐plane loading and the level of the adhesive zone in the case of out‐of‐plane loading. The thermomechanical behavior of cubic specimens of adhesively bonded joints subjected to dynamic compression has been studied. The results underline the sensitivity of the dynamic behavior of the adhesively bonded joints to the strain rate, the loading direction and the appearance of heat dissipation linked to the increase in temperature following the initiation of damage. This rise of temperature can dangerously approach the glass transition temperature of the matrix.

SHPB experimental method for ultra-soft materials in solution environment

Biological soft tissues usually perform physiological functions in solutions, and the mechanical properties of the bio-soft matters in air and solution may differ from that in solution, but the measurement technique of the dynamic mechanical properties of ultra-soft materials in solution is still an unexplored realm. In order to develop a feasible method to test ultra-soft materials in solution by the split Hopkinson pressure bar (SHPB) technique, this paper uses the liquid silicone rubber (LSR) whose property is seldom affected by water and the purified water as a testing material and a solution, respectively. The double-striker electromagnetic driving SHPB system, which has been designed for ultra-soft materials, is adopted to carry out dynamic test for LSR in solution, meanwhile the testing process was recorded by a high-speed digital camera. The tests include three cases named as: Specimen in air, No specimen in water, Specimen in water. Specimen in air was set as a reference; No specimen in water was used to learn the mechanism of load transferring of the water and to set the correction of the effect of the water; Specimen in water was the standard test in water whose results must be modified for the characterization of materials. The high-speed images show that the water may be regarded as a steady flow until the constant strain rate loading ended. Based on such phenomenon and the unchanged volume of specimen and by simplifying the description of the flow field, a data correcting method was set up on the basis of the deduction of both the specimen deformation and fluid flow field to modify the experimental result tested in water. According to the comparison, experimental result in air shows good agreement with corrected result in solution, indicating the feasibility of our recommended method for the SHPB tests in solution. In this way, our research is of great significance in laying an important foundation in the new academic field of the dynamic mechanical behaviors of bio-soft matters in solution.