Dynamic Impact Loading Research Articles

Hybrid manufacturing of AlSi10Mg metamaterials: Process, static and impact response attributes

The work investigates the microstructural, static and impact mechanical attributes of hybrid-cast aluminum AlSi10Mg metamaterials. For the analysis, different metamaterial topologies, namely BCC, IWP and gyroid-based architectures, are considered. The microstructural characteristics of hybrid-cast metamaterials are thoroughly investigated, assessing their attributes through scanning electron microscopy (SEM) and CT-scanning methods. Moreover, their static and impact attributes are experimentally characterized, quantifying elastic and post-elastic properties, while associating the performance of hybrid-cast and as-built, powder bed fusion (PBF) based metamaterials. PBF samples yield overall superior Young's moduli and higher peak stresses, though upon a brittle post-elastic response. Contrariwise, hybrid-cast metamaterials result in a ductile post-elastic, continuum-type plastification performance with a considerable energy absorption capacity that depends on the metamaterial topology, aspects both experimentally and numerically elaborated. Under dynamic impact loading, substantial peak stress and toughness enhancements are recorded for the hybrid-cast specimens. The analysis furnishes process-structure-property benchmark data on the mechanical performance of advanced, hybrid-cast metamaterial topologies for the first time. We aspire that the provided results foster novel pathways in the engineering of advanced media for a variety of base materials beyond the aluminum alloy here investigated.

Numerical Modelling and Investigation of the Impact Behaviour of Single Guardrail Posts

Vehicle restraint systems are vital hardware elements in road safety engineering. The certification process of a vehicle restraint system includes full-scale crash tests, component testing and numerical simulation of these tests. To achieve reliable crash test simulation results, the soil–post interaction must be modelled to capture the behaviour realistically. There is no standardised approach for modelling the soil–post interaction in the praxis. In this study, the finite element method is utilised to investigate the soil–post response under quasi-static and dynamic impact loading. Two different modelling techniques are applied for this purpose. The first technique is the finite element continuum method, with the soil modelled using the advanced hypoplastic constitutive relation and calibrated using laboratory test data. The second technique is a lumped-parameter model, for which a systematic parameters calibration routine using basic soil properties is introduced. The numerical models are validated using a series of full-scale field tests performed by the authors on single posts in standard road shoulder materials. The performance comparison of the investigated modelling techniques shows that the hypoplastic constitutive relation can capture the post behaviour realistically under different loading conditions using the same parameter set. The introduced lumped-parameter model adequately simulates the post behaviour with high computational efficiency, which is very important when simulating several posts. The conducted parametric study elucidates that the soil’s relative density, the post’s embedment length, and the post-section modulus govern the single post’s lateral load-bearing behaviour and energy dissipation capacity.

Review of SHPB Dynamic Load Impact Test Characteristics and Energy Analysis Methods

Since the split-Hopkinson pressure bar (SHPB) test technology was proposed, it has played an important role in the study of dynamic mechanical properties of materials under the impact of dynamic load. It is a major test technology for the study of dynamic mechanical properties of materials. The expansion of the range of materials studied has also posed a challenge to the SHPB test technique, requiring some improvements to the conventional SHPB test apparatus and analysis methods to meet the test conditions and ensure the accuracy of its results. Based on a systematic review of the development of the SHPB test technique and the test principles, the main factors that influence the test’s ability to meet the two basic assumptions at this stage are analyzed, and the ways to handle them are summarized. The stress wave dispersion phenomenon caused by the transverse inertia effect of the pressure bar means that the test no longer satisfies the one-dimensional stress wave assumption, while the pulse-shaping technique effectively reduces the wave dispersion phenomenon and also has the effect of achieving constant strain rate loading and promoting the dynamic stress equilibrium of the specimen. Impedance matching between the pressure bar and specimen effectively solves the problem of the test’s difficulty because the transmitted signal is weak, and the assumption that the stress/strain is uniformly distributed along the length of the specimen is not satisfied when studying low-wave impedance material with the conventional SHPB test device. The appropriate pressure bar material can be selected according to the value of the wave impedance of the test material. According to the wave impedance values of different materials, the corresponding suggestions for the selection of pressure bar materials are given. Moreover, a new pressure bar material (modified gypsum) for materials with very-low-wave impedance is proposed. Finally, for some materials (foamed concrete, aluminum honeycomb, porous titanium, etc.) that cannot meet the two basic assumptions of the test, the Lagrangian analysis method can be combined with SHPB test technology application. Based on the analysis and calculation of the energy conservation equation, the dynamic constitutive relationship of the materials can be obtained without assuming the constitutive relationship of the experimental materials.

A Preliminary Step Towards a Physical Surrogate of the Human Calvarium to Model Fracture.

A surrogate model of the human calvarium can be used to assess skull-fracture-related head injuries without continuously requiring post-mortem human skulls. Skull simulants developed in the literature often require sophisticated manufacturing procedures and/or materials not always practical when factoring in time or expense considerations. This study's objective was to fabricate three exploratory surrogate models (1. pure epoxy prototype, 2. epoxy-chalk mix prototype, and 3. epoxy-chalk three-layered prototype) of the calvarium to mimic the calvarium's mechanical response at fracture using readily available and cost-effective materials, specifically epoxy and chalk. The surrogates and calvaria were subject to quasi-static and dynamic impact 4-point bending and their mechanical responses were compared statistically. Under quasi-static loading, all three surrogates showed a considerable number of differences in mechanical response variables to calvaria that was deemed significant (p < 0.05). Under dynamic impact loading, there was no sufficient evidence to reject that the average mechanical response variables were equal between the epoxy-chalk three-layered prototype and calvaria (p > 0.05). This included force and bending moment at fracture, tensile strain at fracture, tensile and compressive stress at fracture, tensile modulus, and tensile strain rate. Overall, our study illustrates two main remarks: (1) the three exploratory surrogate models are potential candidates for mimicking the mechanical response of the calvarium at fracture during impact loading and (2) employing epoxy and chalk, which are readily available and cost-effective has the potential to mimic the mechanical response of calvaria in impact loading.

Strain rockburst failure characteristics and mechanism of high stress circular hard rock tunnel triggered by dynamic impact load

Strain rockburst poses a great threat to the construction safety of deep engineering, especially induced by dynamic impact loads such as blasting from the adjacent tunnel excavation. Aiming to clarify the failure characteristics and mechanism of strain rockburst induced by the dynamic impact load, a series of dynamic impact tests under different biaxial prestress conditions were conducted on the sandstone cube specimens with prefabricated circular holes by using the new biaxial Hopkinson pressure bar equipment. The real-time rockburst process and fracture characteristics on both sidewalls were captured by the high-speed camera. The test results show that the strain rockburst process can be divided into: the calm stage, slab buckling and spalling stage, rock slabs ejection stage, and V-shaped notch formation stage. Ultimately, a symmetrical V-shaped notch failure zone was formed on the sidewall, similar to the site. The strain field evolution characteristics and displacement deformation characteristics around the circular hole during the rockburst process were further explained by using digital image correlation (DIC). The failure mechanism of sidewall strain rockburst was closely related to spalling failure, characterized by dynamic tensile failure. The static stress provides the initial compressive stress field and governing the stress distribution around the tunnel. Dynamic impact load disrupts the original stress equilibrium of the surrounding rock, inducing significant alterations in internal stresses and strains within the surrounding rock, consequently inducing spalling or rockburst failure. The influences of lateral pressure on rockburst were also analyzed in detail, while the lateral pressure influences the severity of strain rockburst, which determines the dynamic load response characteristics, influence range of strain concentration zone, displacement deformation characteristics of the tunnel surrounding rock, and the severity of strain rockburst decreases with the increase of lateral pressure.

Revealing the internal short circuit mechanisms in lithium-ion batteries upon dynamic loading based on multiphysics simulation

Internal short circuit (ISC) of lithium-ion batteries (LIBs) would be triggered due.to inevitable electric vehicle collision, which pose serious threats to the safety and stability of the battery system. However, there is a lack of research on the ISC mechanism of LIBs under dynamic impact loadings. In this work, a coupled multi-physics model to describe the mechanical, electrical, and thermal response of LIBs under dynamic loading is established. The model can well predict the ISC and thermal runaway evolution process of LIBs with various SOCs under different impact energies. Four ISC modes are revealed through disassembling the LIBs after dynamic loading. Further, a strain-based ISC criterion is proposed to describe the triggering and voltage drop characteristics of each ISC mode. Subsequently, the mechanical-electrical-thermal behavior of LIBs in quasi-static and dynamic loading is compared and analyzed. Afterward, the triggering impact energy of four ISC modes for LIBs with different SOCs is concluded. The model proposed in this paper is currently based on small-sized batteries, and further study is required to extend its application to large-sized batteries.

Numerical Simulation Study on Vibration Reduction Effect of Flexible Cutting-Tooth Unit

Under the conditions of drilling in gravel-bearing and heterogeneous stratas, the movement and force of the PDC bit during drilling are highly unstable. Irregular impact loads often cause fatigue failures such as tooth fracture, tooth breakage and delamination of the composite sheet. Dynamic impact load is the main cause of fatigue failure of cutting-tooth, which seriously affects the rock-breaking performance of PDC bits. This paper proposes a flexible cutting tooth unit consisting of a central tooth, an elastic element, and a base. The technical concept of flexible-cutting rock breaking is to reduce the impact load amplitude suffered during the cutting process to a certain threshold range by setting elastic elements or reducing the support stiffness of the cutting tooth, so as to inhibit the expansion of micro defects caused by the impact dynamic load of cutting teeth and prolong the service life of drill bits. The finite-element models of flexible cutting teeth for rock cutting were established. The influence of cutting depth, front rake angle and stiffness of elastic elements on the cushioning and vibration-absorption effect of flexible cutting was analysed. The results show that flexible cutting can reduce the peak and average value of von Mises stress at the cutting edge. Under different cutting-depth conditions, the decline rates were 21.45–49.02% and 9.42–17.8%, respectively. Then, under different front-rake-angle conditions, the decline rates were 20.51–24.02% and 9.41–17.8%, respectively. There is a suitable stiffness of the elastic element, which makes the peak and average value of von Mises stress at the cutting edge of the flexible cutting-tooth unit perform better and the effect of improving the uneven stress distribution at the cutting edge better. Flexible cutting technology can effectively improve the adaptability of the PDC bit in heterogeneous formations and reduce the occurrence of abnormal failure of cutting teeth. The research results of this paper can provide theoretical support for the drilling speed of PDC bits in hard formations.

Experimental investigation of the response of guardrail posts embedded in road shoulder material under quasi-static and dynamic impact loading

The main functions of vehicle restraint systems (VRS) are to redirect errant vehicles and prevent them from entering an opposing lane or carriageway, as well as preventing them from colliding with structures along the road [1]. In the case of steel guardrail systems, fulfilment of these requirements greatly depends on the soil-post interaction, which influences the deformation and energy dissipations of the VRS during impact.To date, there is no unified method of modelling the behaviour of guardrail posts embedded in road shoulder material in crash tests simulations. For this reason, the influence of soil type and state as well as post characteristics on the observed motion of vehicle and VRS cannot be adequately evaluated, which is, however, necessary to improve and optimise a VRS without increasing the number of crash tests.This paper presents the findings of a series of static and dynamic full-scale loading tests conducted with different road shoulder materials and post types, the aim being to create an experimental database for modelling the soil-post interaction in crash test simulations. The novel dynamic and static testing techniques used ensures repeatability and enables the reliable evaluation and interpretation of the results.We found that not only the static and dynamic post response but also the failure mode are significantly influenced by the relative soil density and post embedment length. As long as there is no yielding of the post section, the dynamic post resistance varies with increasing impact energy until a maximum value, which depends on the dynamic horizontal bearing capacity of the post. Any further increase in impact energy has no influence on the post resistance.

Research on low-velocity impact characteristics of Z-pin reinforced quartz/phenolic laminates

The problem of in-plane damage caused by dynamic impact loading of composite laminates can be enhanced by implanting Z-pins. The effect of Z-pin implantation on the impact resistance of composite laminates is investigated experimentally, and the damage process is analyzed by establishing a low-velocity impact finite element model of Z-pin reinforced laminates. The test results show that the maximum impact contact force of Z-pin reinforced laminates can be increased by 8.23%, and the maximum contact force and absorbed energy of Z-pin reinforced samples both increase gradually with the increase of impact energy. The simulation results show that with the increase of impact energy, the expansion of impact damage is hindered by the Z-pin, and it reduces in-plane matrix damage. The expansion path of the interlaminar crack changes when it reaches the Z-pin enhancement area, and the shape of the interlaminar damage changes from circular to elliptical.

Dynamic impact behavior and notched fatigue life of GLARE laminates

As a member of the fiber metal laminates (FMLs) family, GLARE laminates have been widely used in aircraft skin structure due to the excellent resistance to dynamic impact and fatigue loadings. The low-velocity impact of GLARE laminates are conducted to explore the mechanical response on the impact energy and punch shape. The impact indentations are characterized by the damage morphology and quantitative dimension analysis. The mechanism of impact damage resistance is illuminated by the contact force-time curve, intra-laminar and inter-laminar damage modes. The lagrangian SPH modelling is employed to numerically analyze the bird-strike behavior of GLARE laminates, revealing the variation of bird particle flow and impact energy with different impact attitudes. The tension-tension fatigue of GLARE laminates is experimentally performed subjected to different stress levels. The influence of the circular open-hole on the fatigue residual life of GLARE laminates is studied. Considering the uncertainty of S-N curves, the probabilistic fatigue life P-S-N curves are developed under different survival probabilities. These would be more positive for the fatigue life estimation and reliability design for GLARE laminates in aircraft engineering application.

Loading rate effect on mixed-mode I + II fracture of V-notched Brazilian disc rock specimen under impact loads

Dynamic failure processes of rock masses containing numerous discontinuities including pores, cracks and notches have been the major issues in structural safeties of underground buildings subjected to impact loads, such as earthquake, blast and vehicle vibration. At present, most of the studies are focused on the dynamic fracture behaviors of rocks containing crack-like defects, while the studies on rocks containing notch-type defects are very rare, especially under mixed-mode loading conditions. This paper aims to study the effects of loading rate on mixed-mode I + II fracture characteristics of flattened V-notched Brazilian disc rock specimen under impact loads by means of an split Hopkinson pressure bar (SHPB) system combined with a high-speed camera. Dynamic fracture tests were performed on a total of 125 groups of V-notched rock specimens with different notch angles (0°, 30°, 45°, 60°, and 90°) and loading angles (0°, 30°, 45°, 60°, and 90°) at different air pressures (0.16 MPa, 0.18 MPa, 0.20 MPa, 0.22 MPa, and 0.24 MPa). Based on dimensional analysis, a numerical method was proposed to establish the relationship between mixed-mode notch stress intensity factor (NSIF) at the V-notch tip and external axial load. The dynamic notch fracture toughness values of flattened V-notched Brazilian disc rock specimens were calculated from the maximum values of dynamic force in the SHPB tests according to the proposed numerical method. The effects of loading rate, notch angle, and loading angle on the mixed-mode I + II notch fracture toughness value and crack initiation angle were quantitatively analyzed, which were verified using a modified maximum tangential stress (MTS) criterion by considering the loading rate effect. It is found that the loading rate has a significantly positive effect on the notch fracture toughness value, but has a negative effect on the crack initiation angle due to the inertial effect under dynamic impact loads. Generally, pure mode I fracture occurs for the loading angle of 0° or 90°, pure mode II fracture occurs for the loading angle varying from 35° to 45°, and mixed-mode fracture occurs for other loading angles. Good consistencies of the mixed-mode I + II notch fracture toughness values and crack initiation angles of the V-notched rock specimens are observed between the modified MTS criterion prediction and experimental results. This work may improve the understanding of the dynamic fracture behaviors of rock masses containing notch-type defects during tunnel excavation and underground construction process.

Influence of tool plunging rate on mechanical properties and microstructure of friction stir welded DMR249A high strength low alloy (HSLA) steel butt joints

Abstract The main aim of this study is to analyse the influence of tool plunging rate on tool wear and stir zone characteristics of DMR249A high strength low alloy (HSLA) steel joints developed using friction stir welding (FSW). The HSLA steel plates were welded at five levels of tool plunging rates varying from 2.0 mm min−1 to 3.0 mm min−1. The tool rotational speed of 600 rpm and welding speed of 30 mm min−1 were kept constant during welding. The wear of FSW tool was analysed by microscopic observations of tool condition after welding, stir zone characteristics, weight loss measurement and pin profile analysis. The characterization techniques of optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to analyse the microstructural characteristics of stir zone of welded joints. Results disclosed that the severe degradation of FSW tool occurred due to the sudden dynamic impact load on tool pin while plunging and more diffusion of work piece material into the tool pin which leads to insufficient plasticized material flow. The tool plunging rate of 2.5 mm min−1 showed minimum tool wear and produced defect free sound joint.

Numerical study on the anti-impact performance of novel concrete-filled steel tubular joint with corrugated web and cover plates

To improve the impact resistance of the traditional concrete-filled steel tubular (CFST) bolt-welded hybrid joint, a novel connection configuration with the corrugated web and the cover plate was proposed in this study. The failure modes and bearing mechanism of the novel connection under vertical dynamic impact load were studied in detail based on the finite element model validated against the previous test results. The falling-debris impact on top of the joint column can be simplified as the drop hammer of the non-deformed discrete shell rigid body. The effect of the design parameters including the dimensions of the cover plate and corrugated web was analyzed numerically. Results show that the existence of the cover plate increases the bending capacity of the steel beam and delays the fracture of the connection. The fracture and local buckling of the web are delayed due to the folding effect of the corrugated web. The energy consumption of the joint with the cover plate and the corrugated web is 20.2%–52.8% higher than that of the traditional joint, in which the corrugated web and bottom flange with cover plate contribute 6.6%–8.3% and 16.3%–32.2% respectively. The time history curves of the internal force of the joints are mainly consisted of the beam mechanism stage, the catenary mechanism stage and the failure stage. Through the parameter analysis, it is suggested that the long diameter of the bolt hole is 0.08–0.09 times of the beam height and the thickness of the cover plate is 0.42–0.50 times of the flange thickness to reach the optimum impact resistance. Furthermore, a simplified calculation method for the impact resistance considering the effect of the corrugated web and the cover plate is proposed. Impact energy can be absorbed by the bending effect and the catenary effect of the joint. The relative errors of the predicted impact resistance and energy absorption by theoretical formulas are within +10% compared with the experimental and numerical results.

Static and dynamic mechanical behavior of self-supporting lattice with symmetric gradients

Self-supporting lattice structures (s-LS) have shown great potential in various applications such as energy absorption. However, conventional s-LS designs solely rely on uniform structural configurations, limiting their mechanical properties. In this study, we present gradient lattice structures (GLS) by incorporating a size-graded design strategy. Both bending- and stretching-dominated GLS are fabricated using the Selective Laser Melting (SLM) technique. A comprehensive investigation of their mechanical properties, failure modes, and energy absorption is conducted under both quasi static and dynamic impact loading. It is observed that the stretching-dominated GLS exhibits significantly higher load-carrying capacity and energy absorption efficiency compared to the bending-dominated GLS. The gradient design enabled GLS to manifest different internal stress distribution, deformation processes, and failure modes in response to loadings. Furthermore, GLS also displays significant strain-rate dependent mechanical behavior, leading to improved energy absorption efficiency while maintaining a high material utilization rate. Our gradient-based design strategy may pave the way for more applications in energy absorption using lattice structures.

Study on Mechanical Behavior and Mechanism of Sandstone under the Coupling Effect of Water Content and Dynamic Load

In the process of underground engineering construction, rock mass often faces the dual influence of dynamic load disturbance and groundwater, it is therefore essential to investigate the mechanical response of the rock mass under the coupling effect of dynamic load disturbance and water content. In this paper, dynamic load impact tests were carried out on sandstone with bullet velocities of 5 m/s, 10 m/s, and 15 m/s and water content of 0, 0.3, 0.6, and 0.9, and the mechanical behavior and mechanism response of water content to sandstone were investigated. The research findings indicate that this study has made significant contributions in quantifying the along grain and trans-grain fractures of microcracks. It has explored the influence of water content and dynamic loading on the strength mechanism of sandstone. It was discovered that the dynamic loading and water content significantly affect the ratio of along grain and trans-grain fractures, thereby influencing the dynamic behavior of sandstone. The findings suggest a negative association between rock strength and water content and that its peak strength rises as the bullet velocity rises. The fracture characteristics of rock are influenced by water content and bullet velocity. The sample’s fracture degree increases with an increase in water content, its particle size distribution map is evident, and there is a positive relation between bullet velocity and fractal dimension. The energy conversion mechanism of the rock is influenced by the water content, as the bullet velocity increases, the absorbed energy density of the rock becomes higher. Furthermore, the correlation between the absorbed energy intensity and density and its fractal dimension is quantified. It is found that energy density and strength are positively correlated. The greater the fractal dimension, the higher the energy density absorbed.

Dynamic Crushing Behavior of Ethylene Vinyl Acetate Copolymer Foam Based on Energy Method.

This paper aimed to experimentally clarify the dynamic crushing mechanism and performance of ethylene vinyl acetate copolymer (EVA) and analyze the influence of density and thickness on its mechanical behavior and energy absorption properties under dynamic impact loadings. Hence, a series of dynamic compression tests were carried out on EVA foams with different densities and thicknesses. When the impact energy is 66.64 J, for foam with a density of 150 kg/m3, the maximum contact force, maximum displacement, maximum strain, absorbed energy, and specific energy absorption (SEA) increased by 20 ± 2%, -38.5 ± 2%, -38.5 ± 2%, 4 ± 2%, and 105 ± 2%, respectively, compared to foam with a density of 70 kg/m3. The ratios of absorbed energy to impact energy for different thickness specimens are almost equal. The specimen density has no effect on the efficiency of energy absorption and has a greater effect on the SEA. Meanwhile, when the impact energy-to-thickness ratio is 1680 J/m, compared to foam with a thickness of 30 mm, the maximum contact force, maximum displacement, maximum strain, absorbed energy, and SEA for foam with a thickness of 60 mm increased by 28.5 ± 2%, 211.3 ± 2%, 56.6 ± 2%, 100.8 ± 2%, and 0.4 ± 0.5%, respectively. When the impact energy is 66.64 J, compared to foam with a thickness of 30 mm, the maximum contact force, maximum displacement, maximum stain, absorbed energy, and SEA for foam with a thickness of 60 mm increased by -42.5 ± 2%, 163.5 ± 2%, 31.7 ± 2%, 4.1 ± 2%, and 4.1 ± 2%, respectively. The SEA of two different-thickness EVA specimens is almost equal, about 2.8 J/g. The ratios of absorbed energy to impact energy for different thickness specimens are almost equal, both at 72%. The specimen thickness has no effect on the efficiency of energy absorption and has a greater effect on the maximum contact force. In the range of impact energy, thickness, and density studied, the absorbed energy and SEA are not affected by the thickness of EVA specimens and are determined by the impact energy. The density has no significant effect on the absorbed energy but has a greater effect on the SEA. However, for EVA foams, the greater the density, the greater the mass, and the higher the cost. Taking into account lightweight and cost factors, when optimizing cushioning design within a safe range, we can choose EVA foams with a smaller density and thickness.

Determining loading patterns in the bearing structure of a railroad flatcar with a floor made from sandwich panels

The object of research is the processes of emergence, perception, and redistribution of loads in the supporting structure of a flatcar with a floor made of sandwich panels. To reduce the impact of dynamic loads on the supporting structure of a flatcar, as well as the safety of goods transported in it, it is proposed to manufacture floors from sandwich panels. Within the framework of the study, mathematical modeling of the dynamic load on a flatcar when it runs in a loaded state was carried out. It was found that, taking into account the proposed solution, the accelerations acting on the supporting structure of a flatcar are reduced by 8.4 % compared to the typical one. At the same time, accelerations acting on cargo placed on a flatbed car are reduced by 11.7 %. The results of calculating the strength of sandwich panels, when arranged on a flatcar, proved the feasibility of the proposed improvement. The main indicators of dynamics of the improved flatcar structure operated in an empty state were determined. It was established that the flatcar movement is assessed as "good". A feature of the results reported here is that the improved supporting structure of a flatcar contributes not only to reducing its dynamic load but also to improving the safety of transported cargoes. The scope of practical application of the results is the engineering industry, in particular, railroad transport. The condition for the practical application of the research results is the use of energy-absorbing material in the structure of sandwich panels. The study could contribute to devising recommendations regarding the design of modern structures of railroad vehicles and increasing the efficiency of the functioning of the transportation industry

Low/intermediate speed impact‐induced ignition and damage of a novel high‐energy solid propellant

AbstractThe damage and ignition response of a novel propellant is investigated using a modified split Hopkinson pressure bar (SHPB). The mechanical response of the propellant exhibits strong strain rate dependency following a power law. The whole process from mechanical damage to onset of ignition, deflagration and potential deflagration to detonation transition (DDT) under different strain rates (1000–5000 s−1) is captured via high‐speed photography and digital image correlation (DIC). To clarify the onset and extent of the resulting reaction in terms of the mechanical damage caused by impact, meso‐scale analysis is used to evaluate the propellant before and after dynamic impact loading. The ignition response under impact loading is mainly caused by shear flow, and ignition after multiple impacts due to the reflection of stress waves. Dense debris clouds produced by the first impact are observed in the case of a strain rate of 5000 s−1 leading to DDT when the second impact initiated ignition.

Dynamic tensile mechanical properties of thermally damaged sandstone under impact loads and the influence mechanism of composition

The stability of surrounding rock in the combustion void is the key to continuous, safe, and efficient gas production in an underground coal gasification project. The key factors influencing the stability of surrounding rock in the combustion void are high temperature and impact dynamic load. At the same time, rock structure failure mainly depends on the rock materials’ tensile strength. Therefore, it is fundamental to study the dynamic tensile mechanical properties of thermally damaged rocks to control the surrounding rock stability of the combustion void in UCG projects. This paper used the coal measures as the research object. The dynamic direct tensile test of thermally damaged sandstone was carried out using a high-temperature load system and split Hopkinson tension bar (SHTB) test system. The test results showed that the dynamic tensile strength of sandstone increased linearly with the increase in strain rate. In contrast, it increased first and then decreased with temperature. In addition, the nonlinear regression method was applied to establish the prediction model of dynamic tensile properties of thermally damaged sandstone. XRD analyzed the composition and mass fractions of sandstone. From the standpoint of composition change, the change mechanism of the mechanical properties of thermally damaged coal measured in sandstone was illustrated.

Mechanical behaviors of cementations matrix composites reinforced with basalt weft-knitted spacer fabric under low-velocity impact

The attention of this paper is focused on the impact resistance behavior and reinforcing mechanism of cement-based composites reinforced with weft-knitted spacer fabrics (FKRCs). Composites were manufactured with basalt spacer fabrics and calcium sulfoaluminate cement, both of which are eco-friendly materials. Three kinds of FKRCs with varying needle span distances of the spacer fabrics were compared with plain concrete and concrete reinforced with basalt fibers via a static-bursting test and low-velocity impact test. In addition, the failure morphologies of damaged specimens were analyzed. The experimental results revealed that the basalt spacer fabric was able to obviously enhance the impact resistance and energy absorption capacity of the concrete, while the reinforcing effect of basalt fibers was relatively restricted. In addition, spacer fabrics played a crucial role in retarding the crack generation and propagation in the cement matrix by the textile bridging effect. Furthermore, FKRCs were more sensitive to dynamic impact load, exhibiting a higher maximum peak load but lower maximum displacement. With the increase of span distances, the impact resistance of the specimens was enhanced with higher maximum impact load and energy absorption, and the brittleness of plain concrete was improved. In addition, the spacer yarn structure exhibited considerable effects on the impact behavior of the composites rather than basalt spacer fabrics. Therefore, the FKRC is suitable for structural components that require higher impact resistance in engineering applications.