Drop Hammer Research Articles

The effect of CFRP-shear strengthening on existing circular RC columns under impact loads

In recent decades, accidents involving derailed train collisions have increased, owing to the rapid development of railway construction worldwide. Reinforced concrete (RC) columns in train stations are likely to be subjected to derailed train collisions, resulting in a brittle shear failure of the columns and catastrophic damage to the entire structure. In view of these problems, this study investigated the effectiveness of carbon fiber-reinforced polymer (CFRP) shear strengthening in improving the failure modes of circular RC columns. A drop hammer impact test was performed on a plain circular RC specimen and a CFRP-shear-strengthened circular RC specimen with fixed supports. The tested specimens were scaled from a regular train station column in China, and the impact height was selected as 2/9 of the effective span length, in accordance with the dimensions of a high-speed train locomotive. The test results revealed that wrapping a layer of CFRP sheets can effectively mitigate the shear damage of the concrete, and transform the failure mode of the column specimen from brittle-shear to ductile-flexural. In addition, finite element (FE) models were developed and verified based on the experimental results. Using the verified FE models, the structural design-related parameters (e.g., the longitudinal reinforcement ratio, transverse reinforcement ratio, impact location, concrete compressive strength, and axial load ratio) were extensively analyzed, aiming to further investigate the effectiveness of CFRP strengthening on RC columns. The results demonstrated that wrapping existing RC columns with CFRP can achieve a consistent good strengthening effect, despite variations in these basic design parameters.

Deformation behavior of D-Shaped shallow tunnels under dynamic loading conditions

Underground tunnels have offered a quick and cost - effective alternative for storage, traffic, and transportation in many countries. Due to intensive and more frequent occurrences of worldwide terrorism attacks on urban tunnels, there is a need to develop a methodology for designing underground structures that can withstand such attacks [14]. Therefore, in the present study, the physical modeling of D-Shaped shallow tunnels is carried out to understand tunnel behavior when subjected to a high rate of impact loading (projectile). To understand the tunnel deformation behavior, an extensive experimental investigation is performed on the physical model of the tunnel in synthetic rockmass subjected to impact loads. For rock-tunnel modeling, three different cover depths are considered. The impact loads of varying magnitude are applied at the center using drop hammer equipment. The deformation of the tunnel crown is measured along the tunnel length by means of a linear variable differential transducer (LVDT).Further, the finite element method based numerical investigation is carried out by using Abaqus/CAE numerical tool. The validation of the numerical model is performed by using the experimental results. It is found that the cover depth plays a vital role in determining the overall stresses and deformation when the tunnel is subjected to either static or dynamic loading. From the experimental and numerical investigations, it has been observed that the tunnel with the lowest cover depth experience excessive deformation, and the zone of influence, i.e., area through which crack propagates, is less. The maximum deformation along the tunnel length is observed in the D-Shaped shallow tunnel as compared to the Circular Shape tunnel.

Thermo-mechanical forging of 708M40 steel ring samples: experiments and modelling

A series of ring compression tests using BS970:708M40 alloy steel samples were studied. These tests were conducted using a 2-factor soak-temperature variable, namely 1030 °C and 1300 °C, and a 4-factor lubricant variable consisting of unlubricated samples, synthetic water-based, graphite water-based, and graphite and molybdenum disulphide viscous grease. The lubricant agents were all applied to the tool/billet interface. Process variables such as blow force and heating were controlled with the use of a gravitationally operated drop hammer and an automated programmable induction-heating unit. This matrix of the experimental parameters offered a sound base for exploring dominant factors impacting upon bulk deformation. This deformation was measured using fully calibrated equipment and then systematically recorded. A finite element modelling framework was developed to further improve the thermo-mechanical deformation process understanding, with finite element (FE) predictions validated through experimental measurement. Through the combined experimental and FE work, it was shown that temperature variation in the experimental parameter matrix played a larger role in determining deformation than the lubrication agent. Additionally, the use of synthetic and graphite water-based lubricants does not necessarily produce greater deformation when used in high-temperature forgings due to the lubricants breaking down, evaporating, or inducing rapid billet cooling as a result of the carrier used (water). Graphite-molybdenum disulphate grease far outperforms the other lubricants used in this trial in reducing friction and allowing deformation to occur across a die-face.

On the effect of compaction velocity, size, and content of reinforcing particles on corrosion resistance of Mg–B4C composites

In this study, the effect of the compaction velocity as well as the size and volume fraction of the reinforcing particles on the corrosion properties of Mg–B4C composites were investigated. Micron/nano particles of boron carbide at different volume fractions (0, 5, and 10%) were added to the matrix (magnesium powder). Powder mixtures were milled in a planetary ball mill and compacted at 450 °C and different strain rates, using Split-Hopkinson bar (SHB), drop hammer, and Instron devices. Microstructural analysis and corrosion tests were carried out on the produced samples. The results indicated that the samples produced at higher compaction velocities had the lower corrosion rate mainly due to the lower porosity and higher relative density of the samples. For example, the measured corrosion rates for the magnesium samples reinforced with 5 vol% of micron B4C particles and compacted by the SHB method were 54 and 76% lower than the similar samples produced by the drop hammer and Instron device, respectively. It was also shown that decreasing the size and content of the reinforcing particles resulted in a reduction in the corrosion rate values. The recorded corrosion rate values for the samples reinforced with 5 and 10 vol% of B4C nanoparticles and compacted by the drop hammer device were 18.3 and 29.18 mm/year, respectively. These valuses were approximately 16 and 13% lower compared to the similar composites reinforced with micron-sized particles. The lowest corrosion rate (1.84 mm/year) was observed in the pure magnesium sample compacted by the SHB which was 29.42% lower than that of the magnesium sample reinforced with 10 vol% of micron-sized B4C.

Experimental study on dynamic and static performance of a new type of pressure-absorbing energy bolt system

Rock burst support of deep rock mass has always been a difficult problem in geotechnical engineering. Aiming at the problem of rock burst in deep rock mass, a new type of pressure yielding and energy-absorbing bolt system is designed in this paper. The energy absorption characteristics of laminated spring members with different wall thicknesses were studied by the static axial compression test and drop hammer dynamic impact test. The experimental results show that under static axial compression, the laminated spring experiences three stages: elastic, yielding, and plastic strengthening; with the increase in the wall thickness, the energy absorbed by the laminated spring increases gradually. Under the action of impact kinetic energy, with the increase in the wall thickness and impact height, the attenuation time of pressure absorption energy increases, while the tray and nut do not change, and the bolt does not extend and bend. The ANSYS static numerical simulation analysis shows that the center of the top circle of the laminated spring component is the center, and the plastic zone radiates outward; the LS-DYNA numerical simulation shows that most of the impact kinetic energy of the drop hammer is absorbed by the laminated spring component, and the rest is small. The partial energy is borne by the bolt body. Therefore, the laminated spring component can effectively absorb the impact kinetic energy, achieve the effect of active pressure and energy absorption, and avoid the bolt being broken so as to avoid the bolt body bearing all the impact kinetic energy and damage.

Research on the reaction temperature of PTFE/Al reactive materials

In order to learn about the energy release characteristics of PTFE/Al (Teflon/aluminum), samples with a mass ratio of 73.5: 26.5 (PTFE: Al) were designed by the molding-sintering process. The drop hammer device and spectrometer system were selected for the experiment. By analyzing the spectrum of the reaction flame, the reaction temperature was calculated. At different drop heights, the reaction temperature changes little, and the standard deviation of reaction temperature of PTFE/Al is 14.90. We believe that the reaction temperature of PTFE/Al does not change with the height of the hammer drop, and the average reaction temperature of PTFE/Al under different drop hammer heights is 2733.41±14.9K.

Crashworthiness Performance of Aluminium, GFRP and Hybrid Aluminium/GFRP Circular Tubes under Quasi-Static and Dynamic Axial Loading Conditions: A Comparative Experimental Study

Offshore structures are exposed to risks of vessel collisions and impacts from dropped objects. Tubular members are extensively used in offshore construction, and thus, there is scope to investigate their crashworthiness behaviour. Aluminium, glass fibre reinforced polymer (GFRP) and hybrid aluminium/GFRP circular tube specimens were fabricated and then tested under quasi-static and dynamic axial loading conditions. Two hybrid configurations were examined: external and internal layers from respectively aluminium and GFRP, and vice versa. The material impregnated with epoxy resin woven glass fabric was allowed to cure attached to the aluminium layer to ensure interlayer bonding. The quasi-static and dynamic tests were conducted using respectively a universal testing machine at a prescribed crosshead speed of 10 mm/min, and a 78 kg drop hammer released from 2.5 m. The non-hybrid configurations (aluminium and GFRP specimens) outperformed their hybrid counterparts in terms of crashworthiness characteristics.

Research on the Ignition Height and Reaction Flame Temperature of PTFE/Al/Si/CuO with Different Mass Ratios of PTFE/Si

Recent studies have shown that the energy release capacity of Polytetrafluoroethylene (PTFE)/Al with Si, and CuO, respectively, is higher than that of PTFE/Al. PTFE/Al/Si/CuO reactive materials with four proportions of PTFE/Si were designed by the molding–sintering process to study the influence of different PTFE/Si mass ratios on energy release. A drop hammer was selected for igniting the specimens, and the high-speed camera and spectrometer systems were used to record the energy release process and the flame spectrum, respectively. The ignition height of the reactive material was obtained by fitting the relationship between the flame duration and the drop height. It was found that the ignition height of PTFE/Al/Si/CuO containing 20% PTFE/Si is 48.27 cm, which is the lowest compared to the ignition height of other Si/PTFE ratios of PTFE/Al/Si/CuO; the flame temperature was calculated from the flame spectrum. It was found that flame temperature changes little for the same reactive material at different drop heights. Compared with the flame temperature of PTFE/Al/Si/CuO with four mass ratios, it was found that the flame temperature of PTFE/Al/Si/CuO with 20% PTFE/Si is the highest, which is 2589 K. The results show that PTFE/Al/Si/CuO containing 20% PTFE/Si is easier to be ignited and has a stronger temperature destruction effect.

Dynamic characteristics and fractal representations of crack propagation of rock with different fissures under multiple impact loadings

To investigate the influence of the fissure morphology on the dynamic mechanical properties of the rock and the crack propagation, a drop hammer impact test device was used to conduct impact failure tests on sandstones with different fissure numbers and fissure dips, simultaneously recorded the crack growth after each impact. The box fractal dimension is used to quantitatively analyze the dynamic change in the sandstone cracks and a fractal model of crack growth over time is established based on fractal theory. The results demonstrate that under impact test conditions of the same mass and different heights, the energy absorbed by sandstone accounts for about 26.7% of the gravitational potential energy. But at the same height and different mass, the energy absorbed by the sandstone accounts for about 68.6% of the total energy. As the fissure dip increases and the number of fissures increases, the dynamic peak stress and dynamic elastic modulus of the fractured sandstone gradually decrease. The fractal dimensions of crack evolution tend to increase with time as a whole and assume as a parabolic. Except for one fissure, 60° and 90° specimens, with the extension of time, the increase rate of fractal dimension is decreasing correspondingly.

Study the effect of circular hole on dynamic fracture properties of cracked PMMA specimen under impact loads

Hole defects and crack defects usually coexist in a real engineering rock masses, they together affect the structural security of rock masses. With the purpose of investigating the effect of circular hole on crack initiation and propagation process under dynamic loads, a circular opening specimen with straight crack cavity (COSSCC) model was proposed in this study and polymethyl methacrylate was chosen to make the specimens. A series of dynamic experiments were carried out utilizing a drop hammer impact test system (DHITS), crack speeds and crack initiation time were determined utilizing crack propagation gauge (CPG). The maximum principal stress field evolution in the crack evolution process was simulated using a modified finite element difference method software, and dynamic fracture toughness was calculated by a traditional finite element method software. Some significantly conclusions can be obtained: As the inclination angle θ of the circular hole is less than 10o, crack propagation trajectories will propagate to the surface of the circular hole; As the inclination angle θ of circular hole is 20o and 30o, crack propagation trajectories will turn in the direction of circular hole but not reach the circular hole; As circular hole inclination angle θ is 40o and 50o, the crack shows a pure mode I crack propagation. As crack propagates near the circular hole, crack propagation speeds increase significantly. The dynamic propagation toughness during crack propagation is not constant and is less than dynamic initiation toughness (DIT) and the dynamic propagate toughness (DPT) is inversely proportional to the crack speeds.

Influence of lateral impact on reinforced concrete piers under drying-wetting cycle and chloride ion corrosion environment

In order to study the influence of lateral impact on reinforced concrete piers in marine environment, drop hammer impact tests were carried out on piers with different corrosion rates obtained from drying-wetting cycle and chloride ion corrosion experiment to study the crack propagation process and failure modes of piers. Then by numerical simulation, the influences of impact velocity, impact mass, compressive strength of concrete and impact number on the performance of corroded piers were studied. The results showed that the failure modes of piers with different corrosion rates under lateral impact were different. The non-corrosive and low corrosion rate piers were destroyed by the bending shear which was jointly controlled by the transverse bending crack and oblique shear crack. The medium corrosion rate pier was the bending shear failure caused by oblique shear crack. The high corrosion rate pier was the joint action of bending shear crack and rust expansion crack. The increase of impact velocity, impact mass and impact number will increase the maximum deflection and the damage of the corroded piers, but the increase degrees were different. The increase was largest when the impact number was increased. Increasing impact number from 1 to 5, the maximum deflection increased by 26.3 times and the number of damage element increased by 4.3 times. Increasing the compressive strength of concrete will decrease the damage of pier, but with less degree. Increasing the compressive strength from 25 to 45 MPa, the maximum deflection and number of damage element were decreased by 10.7% and 9.4% respectively.

Research on fracture behavior of lightly reinforced concrete beam under different loading rates

Under impact loads, the crack resistance of reinforcement in concrete is the focus of impact dynamics research. In this study, the three-point bending tests of lightly reinforce concrete beams with notch were conducted with a drop hammer testing machine. The high speed camera and Digital Image Correlation (DIC) technology were applied to catch the fracture process and analyse the displacement field. The shape and geometry of specimen followed the RILEM recommendation, i.e., 150 mm × 150 mm in cross section, 800 mm in length, notch-depth ratio was around 1/3 and span was kept constant 600 mm. And the impact force, mid-span deflection, steel strain beneath the loading point and concrete strain at the notch tip were measured over four loading rates. The experiment results showed that the crack initiation strain rate increased linearlywith the loading rate, and the growth trend weakened when loading rate was 1.771×103 mm/s. Based on the test results, the empirical formula of crack initiation strain rate with respect to loading rate wasgiven, which was meaningful to simulate the crack initiation in concrete under dynamic loading. In addition, with the increase of loading rate, the steel yielded gradually, which resulted in the response time difference between impact force and steel strain decreased. With the recovery of elastic deformation of steel bars, the crack started to close and the macro crack became visible clearly. Meanwhile, the crack mouth opening displacement (CMOD) declined to a constant value after reaching the maximum value. Then, the crack mouth opening displacement rate was obtained from the fitting curve, and a linear growth relationship between CMOD rate and loading rate was founded. The failure process of the lightly reinforced beams was analyzed based on the CMOD rate, which provided an idea for comparing the fracture process of the beam under dynamic and static loads.

쉬트와 와이어 형태를 가진 형상기억합금 강화 케블라 고분자 복합체와 일반 케블라 고분자 복합체의 비교 연구

The main objective of this work is to investigate the effect of shape memory alloy reinforcement in Kevlar polymer composites on mechanical and low-velocity impact properties. The sheet and wire forms of shape memory alloy (SMA-nickel-titanium alloy) are used as potential reinforcements in Kevlar-reinforced polymer composite. The developed composites were analyzed for tensile and flexural behavior. The effective bonding area, referred to as core density, contributes to incremental flexural strength in the SMA sheet reinforced composites. The developed composites exhibited better impact resistance during low-velocity impact tests at different velocities, 2.80, 3.96, and 4.85 m/sec, and simulated using a CEAST drop hammer testing machine. The impact surface of the specimen was studied on a micro-level using SEM analysis and macro-level using Image J software. The impact force, the energy absorbed, and the deformation of the specimen during impact were recorded and analyzed. The experimental results reveal that the SMA sheet reinforced Kevlar composites exhibited appreciable impact resistance with maximum energy absorbing capacity comparatively with plain Kevlar and SMA wire reinforced composites.

Experimental study on the effect of defect curvature on the impact fracture behavior of structures using the real-virtual caustics method.

Defects have significant influence on the impact fracture behavior of structures. In this paper, the real-virtual caustics method is used to study the impact fracture behavior of structures with elliptical arc defects under impact loading of the drop hammer, and the impact loading process is simultaneously analyzed. The research results show that impact loading of the drop hammer in this experiment is a multi-period dynamic loading process, and the fracture of specimens under impact loading of the drop hammer is an energy-controlled process. The running crack initiates under impact loading and propagates toward the elliptical arc defect. After reaching the end of the elliptical arc defect, the running crack arrests and accumulates energy, and then it initiates again and propagates toward the loading position. The greater the end curvature of the elliptical arc defect, the shorter the time for the running crack to stagnate and accumulate energy at the defect end, and the earlier the time for the running crack to initiate again at the defect end, the smaller the impact loading stress of the drop hammer and the dynamic stress intensity factor of running crack tip when the running crack initiates again at the defect end.

Mechanism study of preventing crack propagation of fractured rockunder dynamic loads

To deeply understanding dynamic fracture properties and preventing crack propagation of fractured rock mass under dynamic loads, impact experiments were conducted using TWSRC (tunnel with single radial crack) samples, and sandstone were selected as the raw material to manufacture fractured rock samples. The crack initiation, propagation and obstructing behavior were measured by using a drop hammer impact test device and crack propagation gauge measuring system. The mechanism of preventing crack propagation and failure behavior during dynamic fracturing process was focused, and then the corresponding numerical simulation was conducted by using the finite difference code, which can be used to accurately estimate the experiment result. The results indicate that the whole dynamic fracturing process of fractured rock under dynamic loads is composed of the cyclic process of crack initiation, high-speed crack propagation, slowly deceleration, preventing crack propagation. In addition, the period of crack obstruction was approximate the microsecond level. The ratio of transgranular (TG) fracture at the crack obstruction point of fractured rock was smaller than that of the crack initiation point, and the ratio of TG fracture of green sandstone during the dynamic fracturing process was larger than that of black sandstone. The fracture energy for crack initiation again after crack obstruction was much less than the fracture energy required for the initial initiation of pre-existing crack.

Development of impact resistant high-strength strain-hardening cementitious composites (HS-SHCC) superior to reactive powder concrete (RPC) under flexure

The drop-weight impact behavior of the ductility enhanced high-strength strain-hardening cementitious composites (dHS-SHCC) is compared to that of conventional reactive powder concrete (RPC) to validate its super-impact resistance. All materials have an identical fiber volume fraction of 2%. The compressive strengths of RPC and dHS-SHCC are approximately 210 and 115 MPa, respectively. RPC has the highest ultimate tensile strength of 17.9 MPa, while the developed dHS-SHCC has the highest tensile strain capacity and energy absorption capacity up to the peak (g-value) of 5.9% and 526.0 kJ/m3, respectively. It also absorbs 3.2 times higher energy up to the peak under flexure (toughness) as compared to that of RPC. The RPC beams completely fail upon impact of a drop hammer with a potential energy of 490 J. However, the dHS-SHCC beams are only partially damaged along with multiple microcrack formations and rebounds. Moreover, it produces at least four times the microcracks and is more sensitive to the strain-rate than those of RPC. Consequently, it can be determined that using developed dHS-SHCC is effective in resisting impact loads as compared to RPC.

Research on similar test device of deep roadway impact

With the continuous increase of coal mining depth, the impact of ground pressure disaster is showing more and more serious development trend, which poses a great threat to the safety production of coal mine and the life safety of miners. The design and construction of similar test bench for deep tunnel excavation has become one of the difficult problems of scholars at home and abroad When the impact of high altitude falling hammer on the tunnel is tested, the drop hammer hits the pressure column and will be bounced, causing two shocks and affecting the experimental results. Based on the disaster mechanism of roadway rock burst, and simulating the damage caused by the immediate release of rock burst, we designed a test system that can fall smoothly when the falling hammer falls, and it will be held when the falling hammer starts to fall again. The device prevents the second impact of the falling hammer in the impact test of the roadway, and realizes the dynamic loading of the roadway.

Determining the effects of non-catastrophic nail puncture on the operational performance and service life of small soft case commercial Li-ion prismatic cells

This work focused on the operational effects of Li-ion batteries (LIBs) that had been partially damaged and were allowed to continue operating afterwards. The intent was to simulate the operation of a damaged cell in an abusive environment, such as that found in electric vehicles and hybrid-electric vehicles. A test rig implementing a drop hammer with a nail attached was used to puncture approximately halfway through cycling LIBs to determine how their operation was affected. For all LIBs tested, the puncture caused a rapid spike in temperature. The dynamic impact immediately caused an initial drop in capacity of approximately 11%, and capacity continued to decrease throughout the remainder of the cycles. The cells also reached their end of life within an average of 43 cycles after the puncture, much sooner than the rated lifetime. Postmortem analysis of the punctured LIBs presented impact induced physical damage to the electrodes, oxidation of the electrolyte, and degradation of the interior components of the cells. This caused a decrease in discharge capacity and shortened useful lifetime due to hindering the charge capability of the cells. The degradation mechanism of the cells was further determined using peak tracking of the incremental capacity curves throughout their lifetimes.

Performance Test and Microstructure of Modified PVC Aggregate-Hybrid Fiber Reinforced Engineering Cementitious Composite (ECC).

This study aims to solve the problems of the high cost, heavy pollution and poor performance of traditional engineered cementitious composites (ECC) by adding modified Polyvinyl chloride (PVC) aggregate, Polypropylene (PP)–Polyvinyl alcohol (PVA) hybrid fiber and large amount of fly ash. The PVC aggregate is modified by pre-coating silica fume with a PP fiber volume content of 0.5%, PVA fiber volume contents of 1%, 1.5%, and 2%, PVC aggregate contents of 10%, 20%, and 30%, and fly ash volume content of 69%. Different properties and microstructures were studied by carrying out cube compression tests, splitting tensile tests, water absorption tests, drop hammer impact tests, scanning electron microscopy and nuclear magnetic resonance tests. According to the test results, under the same content of PVC aggregate, the use of modified PVC aggregate can, not only effectively avoid the decrease in strength and increase of water absorption, but also improve brittleness and impact failure energy. Regardless of the kind and content of fiber, the compressive strength and brittleness will decrease, while the splitting tensile strength, water absorption, and impact failure energy will increase. After adding 0.5% PP and 1.5% PVA fiber, the performance is ordinary and a negative mixing effect occurs. As more modified PVC aggregate is added, the strength of the ECC concrete with PP–PVA hybrid fiber and modified PVC aggregate added slowly decrease, while the water absorption and impact failure energy increase. Based on a comprehensive analysis of the test data, the reinforcement method of adding 1.5% PVA-0.5% PP hybrid fiber-30% modified PVC aggregate is superior to adding 1.5% PVA fiber, but slightly inferior to adding 2% PVA fiber. This study argues that the reinforcement method is of great significance for the promotion and application of ECC.

Intralayer interfacial sliding effect on the anti-impact performance of STF/Kevlar composite fabric

The influence of intralayer interfacial sliding among yarns on the enhancement of anti-impact and energy absorption was investigated in this work. Firstly, the rheological test and yarn pull-out test were conducted to analyze the relationship between the state of shear thickening fluid (STF) and the interfacial sliding among yarns. Then, the influence of impact velocity and constraint conditions on the interfacial sliding were studied by drop hammer and ballistic tests. The results showed that the impact speed and constraint conditions would affect the relative interfacial sliding rate among yarns, which would affected the rheological state of STF and the anti-impact performance of fabric. Besides, the mathematical model of the relationship between projectile velocity and fabric kinetic energy under the strong constraints was given by combining with the results of finite element analysis (FEA) of ballistic impact. This work provided a design idea for the body armor with high protective performance.