Drop Hammer Research Articles

Roughness surface of raspberry-shaped silica nanoparticles effect on shear thickening colloidal suspensions

Control over the surface roughness of silica nanoparticles can offer exciting opportunities to increase the rheological properties of the silica-based shear thickening fluids. Designing surface roughness as a significant tool indicates the possibility of connecting distinct rheological phenomena and surface chemical functionalized materials. We have experimentally synthesized two kinds of silica nanoparticles with different particle size and different functional groups at the end, which can be combined via chemical covalent coupling to generate raspberry-shaped silica nanoparticles. The dynamic viscosity of the shear thickening fluids was evaluated by rheological experiments. And the maximum impact force of puncture and drop hammer experiments were performed for composite shear thickening materials with different factors. The results showed that the essential contribution of silica nanoparticles surface roughness and covalent force illustrate that stress-activated frictional contacts can play a key role in this shear thickening phenomena. We tried to propose a damage mechanism of the shear thickening materials. The results indicate that the roughness, morphology and diameter of particles, the thickening effect of shear thickening fluids with higher volume fraction, the increase of both the number of layers and the impregnation content can significantly improve the mechanical properties of the materials.

Experimental study on the impact resistance of single-layer spherical reticulated shell roof panels

As a landmark of the city, the impact damage of long-span spatial structures caused by terrorist attacks will cause great social panic. Therefore, the dynamic response and failure modes of large-span spatial structures under impact are significant. In previous research, the effect of roof panels on the impact response was considered less. In this study, to investigate the effect of the roof panel on the impact response of a single-layer spherical reticulated shell structure, a drop hammer impact test of a two-scaled single-layer spherical reticulated shell was conducted. The test is divided into elastic impact test and destructive impact test. In the two scaled models, Model-2 arranged the roof panels. By comparing the experiment and parallel finite element simulation, the accuracy of the simulation method is verified, and it is found that the laying of the roof panels has a great effect on the dynamic response of the reticulated shell structure. In addition, the effects of roof panels on overall displacement, main rod displacement, and strain of the structure were compared and analyzed. Subsequently, the effect of the roof panel on the failure mode of the reticulated shell structure under impact was proved by the destructive impact test and corresponding finite element simulation.

Influence of height and weight of drop hammer on impact strength and fracture toughness of two-stage fibrous concrete comprising nano carbon tubes

Concrete is the most extensively utilized and cheapest building material. The cracking and fracturing of concrete may cause catastrophic harm to the structures. Advancements in fibrous concretes have recently been emerged to mitigate these issues. Two-Stage Fibrous Concrete (TSFC) is a new cement composite made of fibres with surpassing mechanical properties and exceptional toughness values. The production of TSFC involves packing aggregates and fibres in the molding and filling the gaps between the aggregates with high flowability via the injection of cement grouts. Nevertheless, the influence of TSFC comprising multi-walled carbon nanotubes (MWCNT) on impact strength and fracture toughness requires particular attention since it is yet unexamined. The study was divided into two phases to address this knowledge gap. In the first phase, the influence of drop hammer weight and height on the impact resistance of TSFC were examined. Three different heights 600, 750 and 900 mm and three different hammer weights 9, 7.5 and 6 kg were used to assess the TSFC’s impact strength. The fracture toughness under mode (I and III) and mixed-mode (I/III) of TSFC were evaluated in the second phase. For this, many notched specimens of circular disc shape were cast and tested. TSFC was produced with three different fibres; polypropylene fibre, short steel fibre, and long steel fibre with a dosage of 2.5 % by volume. Additionally, composites were created by adding 0.2 % MWCNT by weight of cement. Findings indicated that the failure impact numbers values of mixtures incorporating polypropylene fibre, short steel fibre and long steel fibre were higher than those of the reference plain mixture by 1444 to 1700 %, 3156 to 3786 % and 4322 to 5357 %, respectively. Adding fibre has a more significant impact on mode I and III crack development, with improvements ranging from 46.1 to 92.2 % for mode I and from 16.4 to 30.0 % for mode III, proving that fibres can substantially increase the TSFC's fracture resistance.

Study on dynamic fracture properties of sandstone under the effect of high-temperature using large-scale sample

High temperatures are a crucial factor in the drilling of rock and the extraction of geothermal resources, to investigate the change rules of the dynamic fracture mechanism and physical parameters of sandstone under high temperature environment, this study conducted a series of physical parameter measurements on the sandstone treated in a range of temperatures (room temperature to 700 °C) using large single cleavage semicircle compression (LSCSC) sample. According to the experimental results, it can be observed that 400 °C is the threshold for the variation of the physical parameters of sandstone. The mass, P-wave velocity, S-wave velocity, elastic modulus and Poisson’ ratio have a significant downward trend with the temperature. The main mineral components of sandstone undergo thermochemical reactions and the variation of quartz is a fundamental cause for the deterioration of sandstone. The dynamic loading test was conducted using a drop hammer impact device and the fractured sample, the dynamic fracture test results indicate that the dynamic crack initiation time (CIT) of the samples is advanced with the temperature, but the crack speed and dynamic fracture toughness decreased with the temperature. Further discussions about SEM images and fractal box dimension of fracture surfaces can be found that with the temperature increases, the proportion of intergranular fractures of sandstone gradually increases, the effect of dynamic loads changes from destroying the grain structure of sandstone to orderly coalescing the micro-cracks in the sandstone.

A 3D-Printed Sole Design Bioinspired by Cat Paw Pad and Triply Periodic Minimal Surface for Improving Paratrooper Landing Protection.

Paratroopers are highly susceptible to lower extremity impact injuries during landing. To reduce the ground reaction force (GRF), inspired by the cat paw pad and triply periodic minimal surface (TPMS), a novel type of bionic cushion sole for paratrooper boots was designed and fabricated by additive manufacturing. A shear thickening fluid (STF) was used to mimic the unique adipose tissue with viscoelastic behavior found in cat paw pads, which is formed by a dermal layer encompassing a subcutaneous layer and acts as the primary energy dissipation mechanism for attenuating ground impact. Based on uniaxial compression tests using four typical types of cubic TPMS specimens, TPMSs with Gyroid and Diamond topologies were chosen to fill the midsole. The quasi-static and dynamic mechanical behaviors of the bionic sole were investigated by quasi-static compression tests and drop hammer tests, respectively. Then, drop landing tests at heights of 40 cm and 80 cm were performed on five kinds of soles to assess the cushioning capacity and compare them with standard paratrooper boots and sports shoes. The results showed that sports shoes had the highest cushioning capacity at a height of 40 cm, whereas at a height of 80 cm, the sole with a 1.5 mm thick Gyroid configuration and STF filling could reduce the maximum peak GRF by 15.5% when compared to standard paratrooper boots. The present work has implications for the design of novel bioinspired soles for reducing impact force.

Experimental study on compression mechanical characteristics of filled rock joints after multiple pre-impacts

The prefabricated artificial filled jointed rock specimens are impacted by a self-made drop hammer impact device for many times, and the specimens with different degrees of cumulative damage characteristics are obtained. Then, the static and dynamic compression mechanical properties are studied by using universal testing machine and SHPB device. Through the static compression test, the strength and deformation characteristics of jointed rock specimens after multiple impacts are obtained, and the influence of the damage degree of jointed rock specimens characterized by wave velocity on the compressive strength of filled joints is analysed. Based on the results of SHPB impact test, the dynamic strength and deformation evolution, wave propagation law and energy dissipation law of filled joints after multiple impacts are analysed. During the SHPB test, the impact failure process of rock specimens is recorded by a high-speed camera. The experimental results show that the damage degree of jointed rock samples increases nonlinearly after multiple impacts. The attenuation laws of static strength and dynamic strength of rock samples under the same damage evolution conditions are different. With the increase of impact times, the failure mode of jointed rock samples after damage is simpler and tends to compression failure.

Effect of impact loading on the dynamic response of steel beams with hexagonal web opening

Owing to the limited literature on the impact performance of steel beams with hexagonal web opening (SBHWOs), this study conducted drop hammer tests and used finite element methods to investigate the dynamic response of SBHWOs subjected to impact loads. The impact force, displacements and accelerations of the beam, and the strains around a hexagonal web opening were collected during the tests. The impact process was captured by a high-speed camera. Furthermore, a finite element model was developed to analyze the post-impact internal forces of SBHWO, development and distribution of stresses in its web, the effect of impact position, and differences with solid webbed steel beam (SWSB). Results showed that the impact force, displacements, and web-post buckling of the SBHWO were significantly affected by impact energy, spacing between openings, and opening height. The average energy absorption ratio of the SBHWOs was found to be 51.56%. Additionally, the midspan acceleration decayed rapidly after impact and spread from the midspan towards the supports. The finite element analysis proved that numerical models can be used to accurately predict the dynamic responses of a SBHWO subjected to impact loads. The axial forces, shear forces, and bending moments of the SBHWO redistribute themselves during the impact, and the magnitude of the internal forces increased very rapidly before decaying gradually. The pattern through which stress developed in the SBHWO web was obtained using finite element modeling. A position close to the support approximately 1/4 of the calculated length of the SBHWO was the most unfavorable impact position. The SWSB was more resistant to impact load than the SBHWO.

Dynamic fracture process of T-shaped beam-column specimens with prefabricated cracks under offset impact

Using the digital dynamic caustics experimental system and numerical simulation software, the drop hammer impact tests of T beam-column specimens with different locations of precast cracks were carried out, and the change rules of the track, velocity, dynamic stress intensity factor, and the distribution law of stress field were obtained under dynamic pressure. The results show that under the action of the reflected tensile wave, the beam-column joint is most vulnerable to damage due to the stress concentration at the beam-column joint. Because of the interaction between the two precast beams with more free cracks, it is easier to form cracks between the two precast beams. The maximum value of crack propagation velocity was about 300 m / s, showing the change law of rapid increased at first and then decreased vibration. The stress intensity factor was between 1.7 MPa·m1/2 and 1.8 MPa·m1/2, the specimen with two prefabricated cracks was easier to form stress concentration at the intersection of beam and column, and the effect of stress wave is stronger. Taking the neutral axial symmetry of the beam as the symmetry center, the strain amplitude is approximately equal and the force is opposite. Due to the attenuation of the stress wave, the strain on the upper and lower sides of the beam decreases gradually with the increase of the distance from the impact loading position. Numerical simulation software was used to verify the experimental results, and MAXS failure judgment was added to calculate the strain change in the fracture process.

Experimental Study on Damage Evolution Characteristics of Coal Samples under Impact Load under Different Surrounding Pressures

Abstract To study the response characteristics of coal samples to impact load under different surrounding pressures, the self-developed magnetic drop hammer impact device was used to measure the change of ultrasonic wave velocity before and after drop hammer impact, the internal damage of coal samples was quantitatively characterized, and then, the damage factor, damage difference, and crack development direction of briquette samples were analyzed. Based on the Griffith strength theory, the relationship between stress intensity factor, crack development angle, and surrounding pressure ratio of coal samples with composite cracks was deduced. It was found that a certain surrounding pressure would slow crack expansion and reduce the amount of damage increase. The damage degree of briquette samples caused by impact load under no surrounding pressure is the largest, followed by that at the surrounding pressure ratio of 0 and 2 and that at the surrounding pressure ratio of 1 is the smallest. When the amount of damage difference of the sample is positively correlated with the number of impacts, the crack development is directional; when the amount of damage difference of the sample is nonpositively correlated with the number of impacts, the direction of crack development is random. It is therefore reasonable to predict the direction of crack expansion by the trend in the amount of damage difference. Besides, the difference in impact load between briquette and raw coal samples under different surrounding pressure ratios was discussed, and the fractal dimension of the sample surface under different surrounding pressure ratios was compared and analyzed. It is found that the fractal dimension of raw coal is larger than that of briquette, indicating that the crack development of raw coal is higher after impact; the damage amount of raw coal is larger than that of briquette, indicating that raw coal is easier to be damaged after impact.

On impact behaviors of 3D concave structures with negative Poisson’s ratio

Lattice structures with high mechanical properties and lightweight performance attracted increasing attention to the application of automotive, railway vehicle, and aerospace industries. This work developed a novel three-dimensional concave lattice structure using a hyper-elastic soft photopolymer via the digital light processing technique. The axial low-velocity impact behaviors of the proposed structures were investigated by a drop hammer machine and numerical simulation. The results showed that the novel 3D concave structures could present continuous auxetic behaviors with large negative Poisson’s ratios. The structural design parameters such as concave angle and thickness had significant effects on Poisson’s ratio and energy absorption. Specifically, the structures with concave angles from 10° to 20° and with thicknesses from 0.5 mm to 1.0 mm presented the higher negative Poisson’s ratio. The highest negative Poisson’s ratio of −0.86 was obtained. This work offered a new insight into designing novel meta-lattice structures with improved impact resistance.

Evaluation and degradation mechanism of residual performance of RC beam after drop hammer impact

The residual load-carrying performance after impact is a key index to evaluate the impact-resistant capacity of engineering-used structure member. A multi-functional experimental facility was developed to conduct both the impact test and post-impact static loading test on two types of flexural-failure-dominated RC beam specimens with different tensile reinforcement ratios. Three-dimensional nonlinear finite element(FE) analysis was conducted to reproduce the impact process and to investigate the plastic damage of RC beam in depth. The degradation mechanism of load-carrying performance of RC beam after impact was investigated through fiber-section analysis with uniaxial damaged material models of concrete and reinforcement. The experimental and numerical results demonstrate that the resistance and stiffness of impact-damaged RC beam decrease in different levels. The residual flexural resistance and stiffness ratios are greatly affected by the longitudinal reinforcements. The yield state of compressive reinforcements plays critical effect on the residual performance of RC beam after impact.

Feasibility of UHPC shields in spent fuel vertical concrete cask to resist accidental drop impact

Ultra-high performance concrete (UHPC) has been widely utilized in military and civil protective structures to resist intensive loadings attributed to its excellent properties, e.g., high tensile/compressive strength, high dynamic toughness and impact resistance. At present, aiming to improve the defects of the traditional vertical concrete cask (VCC), i.e., the external storage facility of spent fuel, with normal strength concrete (NSC) shield, e.g., heavy weight and difficult to fabricate/transform, the feasibility of UHPC applied in the shield of VCC is numerically examined considering its high radiation and corrosion resistance. Firstly, the finite element (FE) analyses approach and material model parameters of NSC and UHPC are verified based on the 1/3 scaled VCC tip-over test and drop hammer test on UHPC members, respectively. Then, the refined FE model of prototypical VCC is established and utilized to examine its dynamic behaviors and damage distribution in accidental tip-over and end-drop events, in which the various influential factors, e.g., UHPC shield thickness, concrete ground thickness, and sealing methods of steel container are considered. In conclusion, by quantitatively evaluating the safety of VCC in terms of the shield damage and vibrations, it is found that adopting the 300 mm-thick UHPC shield instead of the conventional 650 mm-thick NSC shield can reduce about 1/3 of the total weight of VCC, i.e., about 50 t, and 37% floor space, as well as guarantee the structural integrity of VCC during the accidental drop simultaneously. Besides, based on the parametric analyses, the thickness of concrete ground in the VCC storage site is recommended as less than 500 mm, and the welded connection is recommended for the sealing method of steel containers.

Study of the coupling effect of elliptical cavities and cracks on tunnel stability under dynamic loads

This study to explore the coupling effect of elliptical cavities and cracks on the dynamic stability and failure modes of tunnels. A series of dynamic numerical simulations and model tests were conducted by using AUTODYN code and a modified drop hammer impact test device. A cracked tunnel model with an elliptical cavity was proposed in this study. The crack coalescence behaviour, crack initiation time and stress field around the elliptical cavity and the crack tips were calculated by using numerical simulation, and the dynamic initiation fracture toughness (DIFT) was also determined according to experimental and numerical results. According to these results, we conclude that the long and short axes of the elliptical cavity have a great influence on the crack coalescence behaviour and the stress field, and the long axis of the elliptical cavity has a greater influence than the short axis. When the inclination angle θ of the elliptical cavity is 90°, the stability of the crack is lowest, and when the inclination angle θ of the elliptical cavity is 45°, the effect of the elliptical cavity on the crack initiation behaviour is largest. DIFT was calculated and indicated that when the inclination angle of the elliptical cavity shows its smallest value, 90°, the cracked tunnel stability is lowest.

Mechanical characterization of additively-manufactured metallic lattice structures with hollow struts under static and dynamic loadings

Inspired by the natural hollow structures, periodic lattice structures composed of hollow struts and spheres were designed and fabricated by selective laser melting (SLM) process with 316L stainless steel. Two architecture configurations with Body-Centered Cubic (BCC) and Face-Centered Cubic (FCC) symmetry were taken into consideration. Finite element (FE) simulations based on representative volume element (RVE) models and cell-assembly models were conducted to investigate the elastic response and large deformation behavior of the hollow lattice materials, respectively. Afterwards, compression experiments were carried out on an electronic universal machine and a drop hammer (DH) system to explore the quasi-static and dynamic mechanical response of the lattice specimens. The complete deformation evolutions of the lattice samples under different loading velocities were captured through high-resolution photography and inspected by the digital imaging correlation (DIC) analysis. Both the experimental research and numerical simulations demonstrated that the hollow beam cross-sections contributed to the stable crushing response of the proposed lattice structures under either quasi-static or dynamic compression. Accordingly, the post-yield behavior of the tested lattice structures was quite smooth without any fluctuations. Meanwhile, the specific strength and energy absorption of the hollow lattice structures were found to be superior to many existed lattice materials with solid struts, and comparable with the reported triply periodic minimal surface (TPMS) lattice structures. Finally, the effect of the geometric characteristic parameters on the specific mechanical properties of the lattice structures was discussed according to the supplemental analysis by numerical simulations.

Intelligent safeguarding Leather with excellent energy absorption via the toughness-flexibility coupling designation

High-performance intelligent body armors with both soft wearability and good energy absorption are increasingly required in modern social life. Leather, a breathable, pliable, and tough natural material has extensive applications in the anti-impact field. In this study, a multifunctional Leather/CNT-SSG/PU sandwich structure with high energy absorption and excellent sensing monitoring properties is designed by integrating tough leather and flexible conductive shear stiffening gel (CNT-SSG). The maximum force dissipation capacity of the Leather/CNT-SSG/PU reaches 84 % in the drop hammer test and the limit penetration velocity up to 142.4 m/s in the ballistic test. Moreover, the Leather/CNT-SSG/PU exhibits excellent stress wave dissipation properties due to the hierarchical structure. By introducing the interdigital electrode array on the non-woven fabric layer, a bulletproof vest (Leather/CNT-SSG/NWF) with intelligent anti-impact performance and in situ monitoring behavior is successfully achieved. This toughness and flexibility coupling designation provides a novel method for next-generation soft body armor.

A Numerical Study on the Crack Propagation of Homogenized Micro-Crack Crushing for Concrete Pavement

Homogenized micro-crack crushing is a new method of concrete pavement rehabilitation that makes full use of the bearing capacity of the original concrete pavement, whereby the treated pavement can be directly overlaid with hot mixed asphalt concrete. In order to solve the problem of longitudinal crack extension of a cracked core in the weak position of local pavement, the crack propagation mode of concrete slabs under low-velocity impact from different numbers and distributions of impact heads was studied using numerical simulation. Moreover, field tests were also carried out to determine the optimal layout of drop hammer impact heads, and the numerical simulation results were compared with the experimental data. The results show that, when a concrete slab is impacted by a single impact head, the bottom of the slab first cracks and then the crack develops upward until the main crack runs through the concrete slab. However, when the concrete slab is impacted by multiple-impact heads, including three or four impact heads, the concrete slab forms a triangular or quadrilateral fracture core, respectively. The numerical simulation results are in good agreement with the laboratory experiments. Through the optimized arrangement of the hammer’s impact heads, the problem of longitudinal crack extension of a cracked core can be effectively solved, and the probability of reflection cracks can be reduced.

In-plane impact behavior of 3D-printed auxetic stainless honeycombs

In order to fully comprehend dynamic responses of metallic auxetic honeycombs, drop hammer impact tests and corresponding finite element (FE) analyses were conducted. The three-dimensional (3D) printing technology was adopted to manufacture stainless steel specimens with varying cell side length-to-thickness ratios and cell configurations. The specimens were crushed under three distinct energy inputs, and the deforming process and relevant mechanical parameters, e.g., the Poisson's ratio, failure mode, plateau stress and energy absorption capacity were studied. The FE results can generally compare well with the experimental ones and facilitate clarifying the impact mechanisms of the experimental results. Improvements of the auxetic specimens in energy absorption were verified through comparison with that of the corresponding convex honeycomb. The results reveal that the specimen with an auxetic honeycomb has higher plateau stress and specific energy absorption with less deformation when the geometric size is the same. A small side length-to-thickness ratio and input impact energy can lead to greatly improved energy absorption efficiency.

Experimental study and statistical analysis of lean iron ore concrete with high impact resistance

ABSTRACT Impact tests were conducted using a specially made drop hammer impact device on concrete specimens (C60–C90) containing lean iron ore from mine waste as coarse aggregate to realize enhanced impact resistance in applications such as pile driving. Impact resistance tests were also conducted on equivalent conventional limestone concrete specimens for comparison. The statistical analysis results show that the number of impacts at the initial and final cracking of lean iron ore concrete conformed to the Weibull distribution of the strength grade. The impact resistance of lean iron ore concrete was ~0.65–2.03 times greater than that of conventional limestone concrete. In this study, as the impact fracture energies of the C80 and C90 specimens were the maximum and the damage to the lean iron ore aggregate was obvious, the properties of the lean iron ore were completely utilized. Consequently, a linear relationship between the failure resistance of lean iron ore concrete and its strength grade was identified. Finally, the use of lean iron ore as a coarse aggregate in concrete was shown to effectively reduce the probability of the precast concrete pile head bursting during driving.

Realization of an ultra-high pressure dynamic calibrate system by drop hammer based on fiber Bragg grating strain sensor.

In this letter, we propose a novel technique for dynamic ultra-high pressure calibration that measured pressure by FBG based strain sensor. Generally, the traditional method of dynamic ultra-high pressure calibration by standard sensor is costly and it is difficult to improve the accuracy. Therefore, we prefer FBG strain sensor to replace the standard sensor to calibrate the ultra-high pressure. In this proposal, the calibration process is that the central wavelength of the FBG attached to the elastic element changes rapidly with the strain of the elastic element during the drop hammer impact, synchronously. This allows the calibration accuracy to be easily increased to 0.02% and the cost to be reduced by 1/100 compared to traditional calibration techniques. The experiment results show that coefficient of linear correlation between the strain waveform and the pressure signal reaches 0.999. The strain calibration based on FBG is of great significance to the measurement and calibration of dynamic ultra-high pressure sensors.

Impact resistance of concrete pavement surface reinforced with fibre mesh

ABSTRACT The surface layer of military airfield pavements is prone to damage, including cracking, abrasion, and spalling. It is proposed to strengthen the pavement mortar layer by laying fiber mesh to solve these issues. In this study, three meshes of carbon fiber, aramid fiber, and basalt fiber were selected for surface reinforcement across six pavement slabs of 1.5m (length) × 1.5m (width) × 0.25m (height). The drop hammer impact test was carried out on the six pavement slabs. The test data were analyzed using impact times, crack development characteristics, and development models on the surface deflection as evaluation indices. The results show that the fiber mesh can effectively improve the impact resistance of concrete pavement slabs. Aramid fiber mesh best strengthened the pavement surface, whereas the basalt fiber mesh had the least strengthening effect. However, the mesh size has a negligible influence on the impact resistance of concrete pavement slabs. The effect of fiber mesh on improving the impact resistance of the concrete pavement slab corner was greater than that of the concrete pavement slab edge midpoint. Considering the performance and cost indicators, within the scope of the study, aramid fiber mesh is the most suitable for surface strengthening of airfield concrete pavement.