Drop Hammer Research Articles

Investigation of dynamic fracture properties of multi-crack tunnel samples under impact loads

Cracks and joints generally exist in various underground structures, and affect the overall mechanical behaviour (strength, elastic modulus, fracture, etc.) of wall rocks significantly. Studying the crack failure behaviour in cracked tunnel samples containing single or multiple cracks can improve our understanding of the dynamic fracture mechanism of cracked tunnels. To investigate the dynamic fracture parameters of cracks in surrounding rock mass, several cracked tunnel model samples having single or multiple cracks were prepared from green sandstone and polymethyl methacrylate. Four groups of impact experiments were conducted using the drop hammer impact testing equipment to determine dynamic fracture properties. Corresponding numerical simulations were performed using a modified version of the finite difference method software and a traditional version of the finite element method software. The effect of single or multiple cracks on crack propagation speed, dynamic fracture initiation time, and fracture initiation toughness under impact loads was analysed and discussed in this study. By comparing the experimental and simulation results, we concluded that multiple cracks have a significant influence on crack propagation speed because the crack propagation speeds of multi-crack samples were larger than those of single-crack samples. Furthermore, the dynamic fracture initiation time decreases with the number of cracks. The dynamic fracture initiation toughness KId and KIId of green sandstone at the crack tip of single-crack samples was 1.94 times greater than that of multi-crack samples.

Theoretical prediction of the impact sensitivities of energetic C-nitro compounds.

In order to design high-energetic and insensitive explosives, the frontier orbital energy gaps, surface electrostatic potentials, nitro group charges, bond dissociation energies (BDEs) of the C-NO2 trigger bonds, and intermolecular interactions obtained by the M06-2X/6-311++G(2d,p) method were quantitatively correlated with the experimental drop hammer potential energies of 10 typical C-nitro explosives. The changes of several information-theoretic quantities (ITQs) in the density functional reactivity theory were discussed upon the formation of complexes. The BDEs in the explosives with six-membered ring are larger than those with five-membered ring. The frontier orbital energy gaps of the compounds with benzene ring are larger than those with N-heterocycle. The models involving the intermolecular interaction energies and the energy gaps could be used to predict the impact sensitivity of the C-nitro explosives, while those involving ΔSS, ΔIF, and ΔSGBP are invalid. With the more and more ITQs, the further studies are needed to seek for a good correlation between impact sensitivity measurements and ITQs for the energetic C-nitro compounds. The origin of sensitivity was revealed by the reduced density gradient method.

Experimental investigation on the characteristics of transient electromagnetic radiation during the dynamic fracturing progress of gas-bearing coal

AbstractThere are extensive studies on electromagnetic radiation (EMR) effects during coal and rock deformation and fracturing processes, but few systematic studies on the EMR features of gas-bearing coal under impact failure circumstances. In order to investigate whether the EMR is affected by the gas (CO2,CH4,N2) sorption, coal type and impact energy, we performed a series of impact loading tests on both briquette coal specimens (BCSs) and raw coal specimens (RCSs) at various pore pressures (0–1.5 MPa). We developed a drop hammer test apparatus to allow impact loading tests on gas-bearing coal, and recorded the EMR signal of the damage of coal simultaneously. The result showed that (i) the amplitude range of the EMR signal generated by the impact damage of the gas-bearing coal is approximately 10–600 mV, with an effective duration time of 3–1500 ms and accumulated energy of 0.1–1000 μJ; (ii) when pore pressure is increased, the maximum amplitude, duration and pulse counts of the EMR decrease accordingly; (iii) the coal powder size and impact energy affect damage features and EMR characteristics, where a higher impact impulse causes more severe damage to coal specimens and (iv) the influence of the presence of adsorptive gas on the EMR signal caused by the destruction of coal bodies has both enhancement and reduction effects. During the early warning assessment of coal and gas outbursts using EMR parameters, not all the waveform parameters are sensitive to coal mine methane, but the significant effect of methane on signal volatility should be considered.

Experimental study on the crashworthiness of bio-inspired aluminum foam-filled tubes under axial compression loading

As a lightweight and efficient energy-absorbing structure, thin-walled filled structure is widely used in various fields. This paper proposed four types of bio-inspired foam-filled thin-walled structures based on two typical straw structures (Cornstalk and Reed): bionic foam core with a central hole, 4 square holes, 4 taper holes and 2 nodes, respectively. The energy absorption capacity and deformation mode of bionic structures were studied by dynamic drop hammer and quasi-static compression. The results showed that 9 of 12 bionic samples had higher energy absorption capacity than the corresponding fully-filled samples. Different bionic features have different effects on the deformation of thin-walled tube, and the bionic features with the best overall performance are the 4 Square Holes and 4 Taper Holes design. It is noted that the bionic method can improve energy absorption capacity and affect the deformation mode of the thin-walled tubes under certain conditions.

Laboratory examination of ballast deformation and degradation under impact loads with synthetic inclusions

This paper presents a laboratory study on alleviating the deformation and degradation (breakage) of ballast subjected to impact loads using geogrids and rubber mats. A series of drop hammer impact tests are carried out to determine how well the geogrid, under-ballast mat (UBM) or under-sleeper pad (USP) can attenuate impact loads and mitigate ballast degradation. Geogrids to be placed at different locations in a ballast assembly, in combination either with a UBM or a USP are tested. Laboratory test results prove that the inclusion of rubber mats and geogrids decrease the dynamic impact loads transferred to the ballast aggregates and subsequently decrease the degradation (breakage) and deformation of ballast. The tensile strength of geogrids and subgrade stiffness are found to considerably influence the performance of geogrid-reinforced ballast under impact loading conditions. The measured impact forces and accelerations of ballast with and without an artificial inclusion show that rubber mats definitely reduce track vibration (acceleration) and the subsequent deformation and breakage of ballast. These inclusions will not only increase safety and passenger comfort they will also lead to more economical and efficient track designs.

Comparative Shear Tests of Bolt Rods Under Static and Dynamic Loading

Abstract This article presents the methodology and results of single shear tests of bolt rods under dynamic impact loading generated by means of a drop hammer. Comparative analysis was also performed for bolt rod load capacity, stress and shear work under static and dynamic (impact) loading. The developed method of single shear testing of bolt rods under impact loading makes it possible to obtain repeatable test results concerning maximum bolt rod shearing force, shear stress and shear work values. Comparative shear tests of four types of bolt rods under static and impact loading showed that the APB-type bolt rods made of AP770 steel, which was characterised by having the highest strength, exhibited the greatest shear work. AM22-type bolt rods exhibited a very similar work value. Though the AM22-type bolt rods made of A500sh steel demonstrated lower strength than the APB-type bolts, as well as a smaller diameter and cross section, they dissipated the impact energy better thanks to their higher plasticity. This could indicate the direction of optimisation for bolt rods in order to increase their impact strength. Mathematical relationships were also formulated for selected tests, describing the real single shear courses F d =f(t) of bolts under impact loading. The obtained relationships could be applied in the load assessment process of bolt rods intended for use under roof caving, tremor and rock burst conditions.

Experimental investigation of impact loading effects on rectangular flat panels of fiber self-compacting cementations composite with expanded steel sheet

Impact loadings essentially differ from static loadings due to a huge amount of the force applied to milliseconds under these loadings. Energy absorption of composites is proper criteria to examine the function against impact loading. Energy absorbers are widely used in the industry. At the same time, due to their unique properties, the use of strong self-compacting composites has been considered by the researchers. High tensile, compressive, and flexural strengths have made these concrete composites more eminent. In a comprehensive experimental work, using four basic mix designs, 64 rectangular composite panels were made with 100 mm2 of area and 30, 45, 60, and 75 mm thickness and tested by impact loading. Compressive, tensile, and flexural strength tests were performed on all the four max designs. Steel fibers with percentages of 0, 0.25, 0.5, and 0.75 with 25 m of length were used to make the concrete composites. A hammer with 180 kg weight and 7500 J power was used as the impact loading with drop hammer test machine (DH-TM). The specimens were dynamically loaded by drop test from a 60 cm height. The use of steel fibers and expanded sheets in combination with each other significantly increases the energy absorption. Moreover, the initial peak force increases, while crushing length and deformation of the specimens reduce.

Investigation of Residual Bearing Capacity of Corroded Reinforced Concrete Short Columns under Impact Load Based on Nondestructive Testing

This paper investigates the damage and residual bearing capacity of corroded reinforced concrete (RC) short columns after impact and presents a method for evaluating the residual bearing capacity of RC short columns by nondestructive testing. Firstly, accelerated corrosion test and drop hammer impact test were carried out to obtain specimens under different impact loads and corrosion rates. Then, the damage caused by corrosion and impact loads was evaluated by supersonic wave aided nondestructive test. Through the damage factor, the influence of corrosion rate and impact loads on the specimen was revealed, and the calculation method of corrosion rate and impact velocity was proposed. Finally, according to the bearing capacity test results, the influence factors of reinforcement, concrete, and impact were introduced into the bearing capacity calculation equation. Considering the relationship between the residual bearing capacity of RC short columns and the damage factor, an improved formula for calculating the residual bearing capacity of corroded RC short columns under impact loads was proposed. This work lays the foundation for further research on mechanical properties of corroded RC structures under impact loads.

An Experimental Study of the Effects of Different Reinforcement Ratios on the Impact Resistance Behaviors of Reinforced Concrete Beams

Abstract In this experimental study, for the purpose of furthering the understanding of the effects of different reinforcement ratios on the impact resistance behaviors of reinforced concrete (RC) beams, a series of vertical impact tests were conducted on five RC beams with different reinforcement ratios using a domestically advanced ultrahigh heavy drop hammer impact testing machine system. The impact velocities and force of the drop hammer during the impacts, displacements, and strain of the test points, as well as the crack propagation process, were collected and analyzed. The experimental test results revealed the following. (1) When the reinforcement ratios ranged between 0.58 and 1.63 %, the RC beams had mainly suffered bending failures. However, when the reinforcement ratios were increased, it was observed that the overall stiffness of the beams had been effectively improved; displacements, steel strain, and damages were reduced; and the impact resistance abilities of the beams had been enhanced. (2) When the reinforcement ratios had ranged from 1.63 to 2.57 %, shear failures were found to be the main failure mode of the RC beams. It was found that increases in the reinforcement ratios could not effectively reduce the damages and were not conducive to the beams’ resistance to impact loads. (3) It was determined that because of the shearing effects of the impact loads, the shear force of the RC beams had increased.

Experimental on Impact Mechanical Behavior of the Carbon Fibre Reinforced Plastic-Reinforced Stainless Steel Reinforced Concrete Piers

To study the impact resistance of the stainless steel reinforced concrete after reinforced with CFRP (Carbon Fibre Reinforced Plastic), the multifunction ultra-high heavy drop hammer test system was adopted to conduct multiple horizontal impact test research on three stainless steel reinforced concrete piers before and after they are reinforced. The test results showed that with equal impact energy, the maximum impact force of the stainless steel reinforced concrete piers was larger than that of the stainless steel reinforced concrete piers that were reinforced with CFRP, while after the concrete piers were reinforced, the peak displacement of the piers was obviously smaller than that before they were reinforced and the residual deformation also became smaller, which improved the flexural rigidity of the section. And the local anti-damage capacity can be improved so as to lengthen the life of structures by reinforcing the stainless steel reinforced concrete pier with carbon fiber.

Salts of Picramic Acid – Nearly Forgotten Temperature‐Resistant Energetic Materials

AbstractThermally stable explosives are becoming more and more important nowadays due to their important role in the oil and mining industry. The requirements of these explosives are constantly changing. Picramate‐based compounds are poorly investigated towards their energetic properties as well as sensitivities. In this work, 13 different salts of picramic acid were synthesized as potential energetic materials with high thermal stability in a simple one‐step reaction and compared with commercially used lead picramate. The obtained compounds were extensively characterized by e. g. XRD, IR, EA, DTA, and TGA. In addition, the sensitivities towards impact and friction were determined with the BAM drop hammer and the BAM friction tester. Also, the electrostatic discharge sensitivity was explored. Calculations of the energetic performance of selected compounds were carried out with the current version of EXPLO5 code. Therefore, heats of formation were computed and X‐ray densities were converted to room temperature. Some of the synthesized salts show promising characteristics with high exothermic decomposition temperatures. Especially, the water‐free rubidium, cesium, and barium salts 5, 6 and 10 with decomposition temperatures of almost 300 °C could be promising candidates for future applications.

3D-printed flexible, Ag-coated PNN-PZT ceramic-polymer grid-composite for electromechanical energy conversion

Abstract Piezoelectric ceramic materials such as Pb(Zr,Ti)O3 (PZT) based ceramics have excellent electromechanical energy conversion ability, however, their rigid and undeformed characters are not suitable for flexible electronics application. Here, we report the design of flexible (0–3) connection ceramic-polymer composite made of polydimethylsiloxane (PDMS) elastomeric matrix doped with Ag-coated PNN-PZT (0.55Pb(Ni1/3Nb2/3)O3-0.135PbZrO3-0.315PbTiO3) ceramic heterojunction particles, and the 3D printing method for fabricating complex three-dimensional grid architectures. It was found that the 3D-printing, non-stereolithographic grid-composite exhibits a greater flexible character after doping ceramic particles and also excellent electromechanical coupling with a piezoelectric voltage coefficient g33 as high as 400 × 10−3 m V N−1, which is one order of magnitude higher than that of PZT based ceramics. Under drop hammer (~20 N) impact, it can instantly drive over 20 commercial red-LEDs lighting directly without using a charge storage capacitor. This work shows that 3D-printed flexible ceramic-polymer composite has potential to replace brittle piezoceramics for electromechanical energy conversion and touching force sensor applications, such as soft robotics, artificial muscles and biology signal identification.

Performance and damage of carbon fibre reinforced polymer tubes under low-velocity transverse impact

Abstract The performance and damage of carbon fibre reinforced polymer (CFRP) tubes under low-velocity transverse impact are investigated. Drop hammer tests were performed to determine the mechanical behaviour and failure modes of different specimens. Further, numerical simulations were performed to analyse the damage evolution during the impact process, and the effect of lamination sequence on the transverse impact performance was analysed. The results show that under impact energies of 5 and 10 J, the tubes tend to generate circumferential cracks. When the impact energy is between 15 and 30 J, the tubes absorb energy by generating circumferential and longitudinal cracks. The damage evolution of the tubes can be better understood through the simulation results. When the ply angle increases from 0° to 75°, the fracture mode of the tube changes from a longitudinal to a circumferential crack. Tubes with complex ply angles have better energy absorption performance.

Dynamic response of steel-reinforced concrete-filled circular steel tubular members under lateral impact loads

Abstract In this study, dynamic response of steel-reinforced concrete-filled circular steel tubular (SRCFST) members was experimentally studied under lateral impact loads. Eighteen SRCFST specimens were prepared and tested by the drop hammer impact test system. Various parameters, namely, cross-section of profiled steel, impact velocity and impact direction for I section profiled steel were considered to examine their influences on the failure mechanism, impact force, impact duration, displacement response and energy absorption. Since the beams may be of different constraints at boundary and the columns are axially loaded, boundary condition and axial load level were also considered. The experimental results show that the energy dissipation ratios (Eab/Ek) of all tested specimens except CIFP2-0 are more than 85% and the maximum energy dissipation ratio reaches 97.5%, indicating SRCFST members can absorb large amounts of input energy. Moreover, the inserted profiled steel can effectively enhance impact resistance while reduce energy absorption. It is also found that the peak value (Fmax) of impact force is very sensitive to the variation of impact velocity. The plateau value (Fplateau) of impact force and displacement response are very sensitive to the variation of boundary conditions and axial loads, which can significantly change the lateral flexural stiffness of specimens. Given the same impact energy input, SRCFST members with cruciform section profiled steel under fixed-fixed boundary conditions exhibit the best impact resistance performance with the lowest mid-span displacement.

Experimental Study on the Dynamic Behavior of Running Crack Affected by Defect Shape and Filling Material

Using the caustics method and the experimental system of digital laser dynamic caustics, the model experiment of drop hammer impact loading was carried out, and the effect of the defect shape (circular and rectangular) and the filling material (air, epoxy, and silicone rubber) on the propagation behavior of the running crack was investigated. The experimental results show that, under the impact loading, the running crack initiates at the end of precrack and propagates toward the defect. After the running crack connects to the defect, it accumulates energy within a certain period before initiating again at the upper edge of the defect. Subsequently, only one running crack is formed at the upper edge of the circular defect, but two running cracks are formed at the upper edge of the rectangular defect. The defect shape and the filling material have a significant effect not only on the energy accumulation time of the running crack at the defect but also on the stress intensity factor when initiating at the defect. The effect degree of the defect shape on the running crack propagation behavior is in the following order: circular defect > rectangular defect, whereas the effect degree of the filling material on the running crack propagation behavior follows this order: air > silicone rubber > epoxy.

Effects of Loading Rate Range on Mechanical and Microstructural Properties of Dynamically Compacted Iron Powder

In this paper, an attempt was made to investigate the effects of dynamic loading rate on the properties of iron powder with a focus on the microstructural characteristics of the compacted material. Dynamic compaction tests were performed using an instrumented die equipped with a load cell under the low loading rate ranging from 20 to 44 kN/ms in a drop hammer and the high loading rate ranging from 308 to 665 kN/ms in a one-stage light gas gun. Optical and scanning electron microscopes, diametric compression test, and Vickers hardness measurement were used to investigate microstructural and mechanical properties of the green compacts. The obtained results show that the range of loading rate completely changes the behavior of the powder by changing the number and velocity of the propagating stress waves. Comparison of the properties of specimens with equivalent density compacted under two ranges of loading rates indicates that the high range significantly increases the strength and hardness, distributes stress and porosity more uniformly, and reduces initiation and growth of inter-particle cracks as well as the compaction force required to achieve a specific density.

Low velocity impact performance of curved steel-concrete-steel sandwich shells with bolt connectors

Abstract The low velocity impact performance of curved steel-concrete-steel (CSCS) sandwich shells with bolt connectors subjected to a hemi-spherical head drop hammer was investigated in this paper. Experimental results including impact force history, displacement history and permanent deformation were analyzed to reveal the influences of concrete core and steel plate thickness as well as shear connectors’ spacing on the impact performances of CSCS shells. The impact process consisted of three stages, i.e., inertial stage, loading stage and unloading stage. Meanwhile, three failure types were observed from the nine specimens subjected to impact load. They were plastic deformation of steel plate without fracture (Failure type I), fracture of steel plate (Failure type II) and penetration of steel plate (Failure type III). The presence of bolt connectors was proven to be an effective way to prevent the detachment of steel plates from concrete core. Moreover, finite element (FE) method was used to simulate the CSCS shells subjected to impact load. The accuracy of the FE model was verified by comparing the impact load history, displacement history and failure type from FE simulations with test results. It turned out that concrete core was the main part to dissipate impact energy, followed by top steel plate and bottom steel plate.

Validation of a Finite Element Modeling Process for Auxetic Structures under Impact

Auxetic materials behave unconventionally under deformation, which enhances material properties such as resistance to indentation and energy absorption. Auxetics, therefore, have the potential to enhance sporting protective equipment. Herein, finite element modeling, additive manufacturing and impact testing of three auxetic lattices, and a conventional equivalent, with a view to advance auxetic implementation within sports equipment, are explored. The lattices are modeled and impacts are simulated between 1 and 5 J, for flat and hemispherical drop hammers. Simulation outputs including peak impact force, impact duration, maximum axial strain, and Poisson's ratio are compared with experimental results from equivalent impact energies on additively manufactured lattices, using an instrumented drop tower and a high‐speed camera. The simulation and experimental results show broad agreement for all lattices and scenarios, demonstrated by comparative force versus time plots and maximum compression images. The benefits of developing and validating finite element models of three auxetic lattices (and the conventional honeycomb lattice) under various impact scenarios, as a process, are discussed, including material characterization of an exemplar thermoplastic polyurethane. Future work can use the models to further investigate auxetic lattices, selecting and tailoring candidates to further explore their potential application to specific personal protective equipment in sport.

Experimental study on the dynamic behavior of T-shaped steel reinforced concrete columns under impact loading

This paper reports an experimental investigation into the behavior of axially loaded steel reinforced concrete (SRC) columns subjected to impact loads. A total of seven SRC columns with T-shaped steel skeleton were prepared and tested using a drop hammer impact system. The testing variables include the impact height, stirrup spacing and the axial compression ratio. The impact force time histories and lateral displacements at mid-span and quarter-span were recorded and analyzed. The experimental results indicated that the decrease of stirrup spacing resulted in a smaller lateral deflection. Axial load was also found to have a strong influence on the residual deflection of columns subjected to impact loading. In addition, the finite element analysis models, which considers the effects of strain rate of steel and concrete as well as the axial loads, were established to simulate the experiment. The FEA results are proved to agree well with that of experiment. The findings obtained from this study can facilitate the application of SRC columns in improving the impact-resistant performance of bridge and building columns.

Synthesis and Properties of 3,6‐Dinitropyrazolo[4,3‐c]‐pyrazole (DNPP) Derivatives

AbstractA series of novel fused‐ring pyrazolo[4,3‐c]pyrazole derivatives featuring N‐dinitromethyl and N‐fluorodinitromethyl energetic groups (compounds 3–7) were synthesized using a nine‐step reaction. All these compounds were characterized using multinuclear nuclear magnetic resonance (NMR) spectroscopy, infra‐red (IR) spectroscopy, and elemental analysis. X‐ray diffraction analysis was performed, and the single‐crystal structures of compounds 3, 4, 6, and 7 were obtained. For these newly prepared energetic materials, the thermal stability was determined using differential scanning calorimetry (DSC), while the sensitivities were evaluated using BAM drop hammer and friction test. The calculated heat of formation values and the measured densities were used to determine the detonation parameters, including detonation velocity and pressure, using the EXPLO5 program. Of all the prepared compounds, dipotassium 1,4‐bis(dinitromethyl)‐3,6‐dinitro‐1,4‐dihydropyrazolo[4,3‐c]pyrazole (3) was crystallized as a three‐dimensional energetic metal‐organic framework (MOF) and showed outstanding detonation performances, which even outperformed the traditional primary explosive lead azide. Compound 7 exhibited a high crystal density of 1.939 g cm−3, the high decomposition temperature of 213 °C and desirable impact and friction sensitivities (IS: 12 J; FS: 240 N). These combined properties and performances make these novel fused‐ring energetic compounds suitable candidates for high‐performance energetic materials.