Drop Hammer Research Articles

Mechanical characterization of Mg-B4C nanocomposite fabricated at different strain rates

Magnesium has wide application in industry. The main purpose of this investigation was to improve the properties of magnesium by reinforcing it using B4C nanoparticles. The reinforced nanocomposites were fabricated using a powder compaction technique for 0, 1.5vol%, 3vol%, 5vol%, and 10vol% of B4C. Powder compaction was conducted using a split Hopkinson bar (SHB), drop hammer (DH), and Instron to reach different compaction loading rates. The compressive stress-strain curves of the samples were captured from quasi-static and dynamic tests carried out using an Instron and split Hopkinson pressure bar, respectively. Results revealed that, to achieve the highest improvement in ultimate strength, the contents of B4C were 1.5vol%, 3vol%, and 3vol% for Instron, DH, and SHB, respectively. These results also indicated that the effect of compaction type on the quasi-static strength of the samples was not as significant, although its effect on the dynamic strength of the samples was remarkable. The improvement in ultimate strength obtained from the quasi-static stress-strain curves of the samples (compared to pure Mg) varied from 9.9% for DH to 24% for SHB. The dynamic strength of the samples was improved (with respect to pure Mg) by 73%, 116%, and 141% for the specimens compacted by Instron, DH, and SHB, respectively. The improvement in strength was believed to be due to strengthening mechanisms, friction, adiabatic heating, and shock waves.

A novel UHPFRC-based protective structure for bridge columns against vehicle collisions: Experiment, simulation, and optimization

The paper aims to develop a new protective structure based on ultra-high performance fiber reinforced concrete (UHPFRC) to protect bridge columns against vehicle collisions and to reduce vehicle damage and casualties. The drop-hammer impact tests were performed to investigate the response of the composite structure composed of UHPFRC panels and the energy-absorbing member of corrugated steel tubes. For all test specimens, the expected damage modes were observed during impact testing. Specifically, the energy-absorbing member experienced large deformation to dissipate the kinetic energy of drop hammer, while slight damages occurred in the UHPFRC panels directly contacted with a drop hammer. Also, the impact tests showed that the impact force was more sensitive to the number of corrugated tubes than the tube thickness. On the contrary, increasing the tube thickness more effectively improved the energy dissipation capacity of the structure than adding the number of corrugated steel tubes. A finite element (FE) modeling method considering manufacturing process was proposed and demonstrated to be capable of capturing the impact-induced response of UHPFRC-based composite structures. Comparisons between the experimental data and the numerical results highlighted the importance of including the influence of the manufacturing process in modeling corrugated steel tubes. Using the validated FE modeling method, two types of UHPFRC-based protective structures were investigated and compared. Results showed that the protective structure with disconnection details between inner and outer panels was superior to that with connection details. The advantages of the former one included more effective reductions of the impact force and damage in UHPFRC panels for the reuse to improve the economy. Finally, a multi-objective optimization design procedure was presented to find the optimum configuration of the proposed protective structures under vehicle collisions.

Transverse impact behavior of high-strength concrete filled normal-/high-strength square steel tube columns

High strength materials and dynamic loadings are two important research topics for building structures. In this study, high-strength concrete filled square steel tube columns (HSCFST) using S690 along with Q355 structural steel subjected to transverse impact loadings were experimentally tested by a drop hammer tester, and the impact force, deformation, and energy absorption of such specimens were obtained. Results showed that HSCFST has great impact resistance, showing high impact force plateau value and small deflection. The influence of steel strength, concrete strength, steel ratio, and impact energy on impact resistance was deeply analyzed. Compared with normal-strength materials, using high strength steel can improve its impact resistance but the contribution is not as great as the increment of yield stress, whilst using the high-strength concrete has limited effect. Besides, a refined finite element (FE) method by ABAQUS/Explicit employing the rate-dependent constitutive model for S690 was adopted to simulate the dynamic responses of HSCFST under impact loadings, performing a good agreement with tests. This study provides the basic experimental data of HSCFST subjected to transverse impact and develops a reasonable FE model to predict its impact resistant performance, contributing to the related studies of HSCFST components.

An Experimental Study of the Feasibility of Identifying the Impact Damages of Reinforced Concrete Piers Using a Modal Frequency Method

In this research study, horizontal impact tests were carried out on five reduced scale pier models using China’s most advanced multifunctional ultrahigh heavy drop hammer impact test system and DHDAS dynamic signal acquisition and analysis system. Due to the fact that the traditional measurement method can only be used for local measurement damage, and the volatility is high, this paper proposes a test method for the modal frequency identification of the overall damage of reinforced concrete pier and applies the ultrasonic damage measurement method to verify the results. The tests analyzed the modal frequencies and ultrasonic velocity identifications for the purpose of evaluating the impact damages of bridge piers, as well as the relationship between them. The results showed that the modal frequencies were consistent with the ultrasonic waves in identifying and evaluating the damages to the piers. Also, the modal frequency damage factors were determined to be functions of the ultrasonic wave velocity damage factors. Therefore, the results of this study confirmed that it was feasible to characterize the impact damages of piers using a modal frequency method.

Study on the Influence of Closed‐Cell Aluminum Foam on the Impact Performance of Concrete Pier after Equal Replacement with Stainless Steel Reinforcement

In this study, the impact test of two groups of reinforced concrete piers protected by closed‐cell aluminum foam is carried out by using the ultrahigh drop hammer impact test system. The purpose of this study is to explore the impact resistance and protective performance of closed‐cell aluminum foam under the impact load on the concrete bridge pier after replacing the ordinary reinforcement with stainless steel reinforcement. The study results show that the impact force is related to the overall stiffness of the specimen, as well as to the failure mode. When the impact velocity is less than 1.42 m/s, the closed‐cell aluminum foam is in an elastic or yielding stage. The change rate of impact force (231 and 97.5, respectively), tip displacement (33.5 and 18, respectively), and ultrasonic damage rate of the concrete in the two groups of specimen is relatively small, while the change rate of the two groups of specimen remains approximately consistent. In addition, when the impact is greater than 1.42 m/s and the closed‐cell aluminum foam is in the densification stage, the change rate of the impact force (increase from 231 to 819 and from 97.5 to 984.5), the tip displacement (increase from 33.5 to 67 and from 18 to 62), and ultrasonic damage rate of concrete are larger, which results in an increase in the dynamic response of the structure.

Static and dynamic testing of tendons

Underground support system using tendons has been one of the significant achievements in Civil and Mining engineering endeavours in facing challenges of ground control. However, shear failure of rock bolts is still one of the least monitored phenomenon in underground excavations with respect to seismic events. The understanding of the performance of rock bolts under dynamic loading condition requires a great deal of research. A series of tests were undertaken utilising a drop hammer mass of 600 Kg from a maximum height of 3.7 m over concrete blocks in the double shear box with chemical resin encapsulated a rock bolt to investigate the performance of rock bolts under dynamic shear load. Load cells, displacement laser and high speed camera were used to monitor the test. Results from the data analyses are presented in the form of displacement, hammer mass drop velocity, acceleration and force variation with time for all components involved in each test. The time factor was found to contribute 30 % of the shear load in static testing in comparison with dynamic; In particular, the force-displacement curve and energy absorption for the reinforcement system are presented to examine the performance of rock bolts and conclusion drawn.

Effect of CFRP Shear Strengthening on the Flexural Performance of the RC Specimen under Unequal Impact Loading

Strengthening with externally bonded CFRP reinforcement is widely used in structural reinforcement and attractive to stakeholders and engineers because of ease and speed of construction, corrosion resistance, lightweight, high strength, and versatility stiffness which can be oriented according to the need. Numerous research studies were carried out to explore RC beams’ flexural and shear performance when subjected to dynamic impact loading. The results were auspicious in using such a technique of strengthening. Regular square section reinforced concrete frame members strengthened by CFRP material is taken as the research object. However, little attention to the impact behavior of CFRP‐shear‐strengthened square reinforced concrete (RC) specimens has been paid. The dynamic response of CFRP to reinforced concrete members under unequal cross‐impact is discussed. This paper investigates the effectiveness of CFRP strengthening on the square RC specimen in preventing shear failure and evaluation of the flexural performance of the strengthened specimen under the impact load. The drop hammer impact test is firstly conducted on RC specimens with and without CFRP strengthening. The results show that using CFRP to strengthen the RC specimen in shear is very effective at preventing shear failure and leading the specimen’s response to flexural domination. This result is also the motivation for developing a numerical model supported by experimental tests to study the flexural performance of strengthened RC specimens. It is found that the strengthened specimen is prone to exhibit pure bending deformation under the impact load in terms of dynamic amplification factor (DAF) for section moment. Then, an extensive parameter study is carried out to evaluate further the influence of impact velocity, reinforcement ratio, and concrete strength on the flexural performance of the strengthened specimen and CFRP layers. Such a holistic study may provide preliminary research regarding the use of CFRP to strengthen RC specimens in shear under impact loads and will enhance the current state of knowledge in this area; also, the optimal value of the CFRP reinforcement layer was proposed.

Experimental and Theoretical Analysis for Isolation Performance of New Combined Isolation Devices under Blast Loading

The strong shock and vibration effect caused by explosion may pose a serious threat to the surrounding environment and the safety of personnel and equipment. This also makes the problem of vibration isolation and absorption of the structures subjected to blast loading increasingly prominent. In this paper, three kinds of new combined isolation devices with high resistance are designed and manufactured, and the characteristic parameters such as natural vibration period, frequency, and damping ratio are obtained through drop hammer impact test. Based on the Duhamel integral principle, analytical solutions of dynamic response of the combined isolation devices under rectangular pulse blast loading are derived, and the calculation expressions of transmissibility and vibration isolation rate are proposed. Combined with the test results, the isolation performance of three kinds of combined isolation devices under blast loading is obtained by using the theoretical calculation formula, and the influencing factors of isolation performance are further analyzed parametrically. The research results provide a reference for the application of combined isolation devices in isolation and shock absorption of structures under blast loading.

Impact Response of Tubular Structure with Internally Stacked Circular Rings

During collision, occupants and structures experience serious injuries and damages due to high impulsive force and poor impact energy absorption. Performance of energy absorption can be improved by selecting a suitable structure and material. A good crashworthiness structure should have low Initial Peak Force (IPF), high Crush Force Efficiency (CFE) and high Specific Energy Absorption (SEA). This paper introduces a new design that consists of stacked circular tubes within a square column which will be subjected to axial impact loading. The impact results were compared to static analysis of previous study in terms of IPF, CFE, SEA and DAF. Simulation results were validated by experiment before on embarking simulation parametric study. The experiment was conducted using the Drop Hammer Machine, Instron Dynatup 8250. Simulation results show good agreement with the experimental results. The increase in IPF and SEA were influenced by inertia compared to static at the same crush distance. Critical parameters were determined to give the best crushing performance among stacked tubes designs. From this study, it can be concluded that the best crushing performance depends on the dominant structure, amount of plastic hinge and structure stiffness.

Experimental study on the behaviour of sand compaction columns in sandy strata

ABSTRACT In the present study, laboratory plate load tests were carried out on sand compaction columns (SCC) installed in dry sandbed to assess the behavior of the SCC improved strata under static loading. The drop hammer compaction method was used to install the columns in the sandbed prepared for a particular density in the test tank. Column depth and diameter employed were 500 mm and 60 mm respectively throughout the study. A series of tests were conducted on single SCCs, and column groups to investigate the pressure-settlement behavior of the SCC improved sandbed. Tests were carried out on the columns with the spacing to diameter ratio (S/D) of 1.5 to 3.5 installed at sandbed prepared at an initial relative density (RD) of 30 to 60%. It is found from the study that for the loose sand strata of 30% RD, the maximum improvement achieved by adopting 1.5D spacing doubles the RD to 57%.

The effect of particle size on microstructure, relative density and indentation load of Mg-B4C composites fabricated at different loading rates

The effect of reinforcing particle size on microstructure, relative density and indentation of Mg reinforced by 0, 1.5, 3, 5 and 10% volume fractions of nano- and micro-sized B4C was investigated. The composites were fabricated through powder compaction technique at strain rates of 1.6 × 103 s−1, 8 × 102 s−1 and 8×103 s−1 using split Hopkinson bar, drop hammer and Instron, respectively. The results indicated that the size of B4C and loading rate had significant effect on relative density. For example, the relative density of Mg-10 vol.% B4C nanocomposite was around 2.5% higher than that of its corresponding microcomposites. The relative density of the samples produced at high rate of loading was in average 1.2% higher than that of the samples fabricated quasi-statically. The results of indentation tests on the produced nanocomposite and microcomposite samples also revealed that loading rate and B4C particle size had significant effect on strength of specimens. For example, for Mg-5 vol.% B4C, the maximum load in load–depth curve of the specimens produced by split Hopkinson bar increased from 530 N for micron-sized B4C to 780 N for nano-sized B4C, around 45% improvement. Moreover, nanocomposites had better indentation resistance compared to similar micro composites fabricated using the three methods.

Experimental Study of Mechanical Properties and Impact-Induced Reaction Characteristics of PTFE/Al/CuO Reactive Materials

Metal/fluoropolymer materials are typical reactive materials. Polytetrafluoroethylene (PTFE)/Al/CuO reactive materials were studied in this research. Scanning electron microscopy (SEM), quasi-static compression, the Split Hopkinson pressure bar test, and the drop hammer test were used to study the mechanical properties and induced reaction characteristics of the reactive materials with different Al/CuO thermite contents and different particle sizes. SEM images of the samples demonstrate that the reactive materials were mixed evenly. The compression test results show that, if the particle size of PTFE was too small, the strength of reactive materials after sintering was reduced. After sintering, with the increase in the content of Al/CuO thermite, the strength of the micro-sized PTFE/Al/CuO firstly increased and then decreased. The Johnson–Cook constitutive model can be used to characterize the reactive materials, and the parameters of the Johnson–Cook constitutive model of No. 3 reactive materials (PTFE/Al:Al/CuO = 3:1) were obtained. The reliability of the parameters was verified by numerical simulation. Drop hammer tests show that the addition of Al/CuO aluminothermic materials or the use of nano-sized PTFE/Al reactive materials can significantly improve the sensitivity of the material. The research in this paper can provide a reference for the preparation, transportation, storage, and application of reactive materials.

The effect of compaction loading rate on hardness and wear resistance of Mg–B4C nanocomposite

Hardness and wear resistance of Mg–B4C nanocomposites produced using dynamic and quasi-static compaction method were investigated. Micron-sized Mg particles were mixed with 1.5%, 3%, 5% and 10% volume fractions of nano-sized B4C. High rate compactions were conducted at 723 °K and at strain rates of and using Split Hopkinson Bar (SHB) and Drop Hammer (DH), respectively and low rate were conducted at strain rate of The results revealed that the hardness was increased by 25% at high rate loading compactions with respect to the low rate compactions. The results also indicated that the hardness of Mg-10 vol%B4C nanocomposites fabricated by SHB was around 50% higher than that of the as-received Mg. The same trend was observed in wear resistance of the high and low rate compacted specimens so that the wear resistance of Mg-10 vol%B4C nanocomposites fabricated by SHB was around 70% higher than that of the as-received Mg. Finally, the friction coefficient of the samples was 43% lower than the dynamically produced samples.

Research on the Energy Release Characteristics of Six Kinds of Reactive Materials.

Currently, PTFE/Al is widely used in the reactive fragmentation warhead. However, for the same explosive yield, the reactive fragments usually have a smaller damage-radius than the inert fragments because PTFE/Al has a poor penetration ability and needs an impact-speed up to 1000 m/s to stimulate its chemical reaction. To enhance the damage power of reactive fragments, six kinds of reactive materials (PTFE/Al, PTFE/B, PTFE/Si, PTFE/Al/B, PTFE/Al/Si, and PTFE/Al/CuO) based on PTFE were designed and studied. Through the drop weight system and the self-designed energy release test device, qualitative and quantitative analysis of the energy release ability of six kinds of reactive materials were carried out. The qualitative analysis results indicate that the reactions of PTFE/B and PTFE/Si are weak under the impact of drop hammer with only a very weak fire light produced, while the reactions of PTFE/Al, PTFE/Al/B, PTFE/Al/Si, and PTFE/Al/CuO are relatively intense, and the reaction of PTFE/Al/Si is the most intense. Through the self-designed energy release test device, the energy release ability of the reactive material was quantitatively compared and analyzed. The results show that the energy release ability of the four formulations were as follows: PTFE/Al/Si > PTFE/Al/CuO > PTFE/Al/B > PTFE/Al. Therefore, it can be concluded that the PTFE/Al/Si formulation is a new reactive material with strong energy release ability, which can be a new choice for reactive fragment.

Impact resistances of RC beams at/after high temperature: Meso-scale analysis

Impact (or blast) and fire often take place simultaneous and threaten the service life of buildings. The coupling effect of high temperature and high strain rate on the reinforced concrete (RC) structures has aroused wide concern of researchers. Based on the heterogeneity of concrete, a meso-scale numerical model was established to study the impact resistance of RC beams at/after high temperature. The strain rate effect of both concrete and steel bars and their performance degradation affected by fire and bond-slip behavior between them were taken into account. The effectiveness of numerical model was validated by comparing the results observed from the simulation and drop hammer test at room temperature. Then the impact responses of RC beams at/after high temperature was compared and analyzed. It is indicated that the proposed numerical method can effectively describe the failure mode of RC beams under the combined effect of fire and impact. RC beams at high temperature exhibit more serious damage than those specimens after cooling down at specified fire duration. The difference is widening along with the increase of fire duration.

Experimental and numerical study on impact resistance of RC bridge piers under lateral impact loading

Vehicle collisions with piers are main threat to bridge structures, thus the impact resistance of pier is significant for bridge structure safety. This paper presents the experimental and numerical work undertaken for investigating the impact process, damage and failure mode, dynamic behavior, and impact resistance evaluation method of reinforced concrete (RC) piers under lateral impact loading. By using a horizontal impact system, a series of simplified truck model collision test on the square sectional RC piers are performed, in which two main designing parameters, i.e., impact velocity and longitudinal reinforcement ratio, are especially concerned. Moreover, the detailed finite element models are established by the commercial program LS-DYNA, which are verified against the test results. It indicates that, the shape of impact force time-history does not exhibit the platform stage compared with that of the conventional drop hammer impact tests attributed to the shear failure mode occurred for the present columns; the damage level, impact force, displacement at impact position and energy dissipation rise with increasing the impact velocity; increasing the longitudinal reinforcement ratio improves the impact resistance of RC piers effectively; a plastic hinge forms with the hoop reinforcement yielding before the shear failure of column; the impact force causes a considerable change of the axial force of RC piers during the impact process. Finally, based on the energy conservation law, the residual displacement at the impact position is adopted and validated as the reasonable criterion for evaluating the impact resistance of RC piers. The present work can provide helpful references for the anti-collision design and impact resistance evaluation of the urban and highway bridge piers.

Research on dynamic constitutive model of ultra-high performance fiber-reinforced concrete

A dynamic constitutive model for ultra-high performance fiber-reinforced concrete (UHPFRC) was investigated in this paper. In view of the particularity of UHPFRC material, two improvements including the modifications of yield surface and the change of strain rate effect due to the adjunction of steel fibers were made based on the Holmquist-Johnson-Cook model (HJC model). Further, on the purpose of model parameter determination, a series of mechanical tests for UHPFRC material, including uniaxial compression test, bending test, uniaxial cyclic compression test and SHPB test were carried out. And the 22 calculation parameters required in the improved HJC model such as strength parameters, damage parameters and pressure parameters were obtained through data analysis and fitting. Finally, the numerical simulation of the SHPB test performed in this study and the drop hammer impact test in the literature were carried out in order to verify the rationality and practicability of the improved HJC model. The numerical simulation results of SHPB test show that the failure morphology and stress-strain curves of UHPFRC specimens are in good agreement with the test results. When the improved HJC model and the appropriate corrosion strain value (1.0% in this work) were selected, the mid-span deflection-time curve obtained by numerical simulation is consistent with the result of the drop hammer impact test.

OZM Ball Drop Impact Tester (BIT‐132) vs. BAM Standard Method – a Comparative Investigation

AbstractSafety, performance, cost efficient synthesis and toxicity are the most important aspects of modern explosives. Sensitivity measurements are performed in accordance with different protocols all around the world. Sometimes the BAM drop hammer does not accurately reflect the sensitivity of an energetic material, in particular the sensitivity of primary explosives. Therefore, we present here preliminary results obtained using the novel ball drop tester (BIT‐132), manufactured by OZM research, following MIL‐STD‐1751 A (method 1016). The ball drop impact sensitivity tester is a device in which a free‐falling steel ball is dropped onto an unconfined sample, and is expected to produce more realistic results than the currently commonly used BAM method. The results obtained using the probit analysis were compared to those from the BAM drop hammer and friction tester. The following sensitive explosives were investigated: HMTD, TATP, TAT, Tetrazene, MTX‐1, KDNBF, KDNP, K2DNABT, Lead Styphnate Monohydrate, DBX‐1, Nickel(II) Hydrazine Nitrate, Silver Acetylide, AgN3, Pb(N3)2 RD‐1333, AgCNO, and Hg(CNO)2.

Study on Cumulative Damage Law of Stainless Steel-Reinforced Concrete Columns under Step Impact Loading

In this study, an ultrahigh drop hammer impact test system was adopted for multiple horizontal impact tests on stainless steel-reinforced concrete columns and ordinary-reinforced concrete columns with the same longitudinal reinforcement diameter. The damage performance after impact was studied, and the finite element model was established. The test measured the impact force, displacement, cracking of the specimen during the impact, and the concrete damage near the bottom of the specimen. The test results showed that the failure mode of the stainless steel-reinforced concrete specimen under multiple impacts was the same as that of the ordinary reinforced concrete specimen. Under the same impact conditions, the maximum impact force, the maximum displacement, and the damage degree of stainless steel-reinforced concrete column specimen were lower than those of the ordinary reinforced concrete specimen.

Experimental investigation of thermomechanical behaviour of NiTi shape memory alloy under low-velocity impact loading

This paper experimentally investigates the thermomechanical behaviour of NiTi shape memory alloy (SMA) under low-velocity impact loading. The phase transformation temperatures of the NiTi material are measured by means of the Differential Scanning Calorimetry (DSC) method. The impact tests are carried out on a drop hammer impact system, wherein force sensor and infrared camera are used to capture the thermomechanical behaviour of NiTi SMA. Several prime material features of SMA, such as superelastic stress-strain response, stress-induced martensitic phase transformation, residual deformation, heat production and energy dissipation, are observed in the drop hammer test of the superelastic NiTi sheet. Considering the impact test is an approximate adiabatic process, the temperature variation on NiTi sheet is highlighted, due to the latent heat and intrinsic dissipation. This temperature variation will affect the mechanical behaviour of SMA, and therefore bring about complex thermomechanical coupling effect in the low-velocity impact test.