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A nearly fully dense grade 300 maraging steel was fabricated by selective laser melting (SLM) additive manufacturing with optimum laser parameters. Different heat treatments were elaborately applied based on the detected phase transformation temperatures. Microstructures, precipitation characteristics, residual stress and properties of the as-fabricated and heat-treated SLM parts were systematically characterized and analyzed. The observed submicron grain size (0.31 μm on average) suggests an extremely high cooling rate up to 10 7  K/s. Massive needle-shaped nanoprecipitates Ni 3 X (X = Ti, Al, Mo) are clearly present in the martensitic matrix, which accounts for the age hardening. The interfacial relations between the precipitate and matrix are revealed by electron microscopy and illustrated in detail. Strengthening mechanism is explained by Orowan bowing mechanism and coherency strain hardening. Building orientation-based mechanical anisotropy, caused by 'layer-wise effect', is also investigated in as-fabricated and heat-treated specimens. The findings reveal that heat treatments not only induce strengthening, but also significantly relieve the residual stress and slightly eliminate the mechanical anisotropy. In addition, comprehensive performance in terms of Charpy impact test, tribological performance, as well as corrosion resistance of the as-fabricated and heat-treated parts are characterized and systematically investigated in comparison with traditionally produced maraging steels as guidance for industry applications.

Improving the balance of strength and toughness in structural materials is an ongoing challenge. Delamination and grain refinement are some of the methods used to do this. In this paper, two different steels, 0.15% C–0.3% Si–1.5% Mn–Fe and 0.4% C–2% Si–1% Cr–1% Mo–Fe (mass %), were prepared. Two steel bars with an ultrafine elongated grain (UFEG) structure were fabricated via multipass warm caliber rolling. The UFEG steels were characterized by a strong //rolling-direction fiber texture. The transverse grain size, dt, was 1.0 µm for the low-carbon steel and 0.26 µm for the medium-carbon steel. For comparison, conventional heat-treated steels were also fabricated. An instrumented Charpy impact test was performed, and the impact load (P) and deflection (u) during the test were recorded. The P–u relations at the test temperature at which delamination fracture occurred exhibited a unique curve. Delamination effectively enhances the low-temperature toughness, and this was characterized by a plateau region of constant load in the P–u curve. Assuming no delamination, two routes in the P–u curves, the ductile route and the brittle route, were proposed. The results showed that the proposed methods can be predicted by an energy curve for ultrafine grained steels. Delamination is a more effective method of enhancing toughness for ultra-high-strength steels.

Natural lignocellulosic fibers (NFLs) possess several economic, technical, environmental and social advantages, making them an ideal alternative to synthetic fibers in composite materials. Caranan fiber is an NFL extract from the leafstalk of the Mauritiella armata palm tree, endemic to South America. The present work investigates the addition of 10, 20 and 30 vol% caranan fiber in epoxy resin, regarding the properties associated with Izod notch tough and ballistic performance. Following ASTM D256 standards, ten impact specimens for each fiber reinforcement condition (vol%) were investigated. For the ballistic test, a composite plate with 30 vol%, which has the best result, was tested with ten shots, using 0.22 ammunition to verify the energy absorption. The results showed that when compared to the average values obtained for the epoxy resin, the effect of incorporating 30 vol% caranan fibers as reinforcement in composites was evident in the Izod impact test, producing an increase of around 640% in absorption energy. Absorbed ballistic energy and velocity limit results provided values similar to those already reported in the literature: around 56 J and 186 J, respectively. All results obtained were ANOVA statistically analyzed based on a confidence level of 95%. Tukey’s test revealed, as expected, that the best performance among the studied impact resistance was 30 vol%, reaching the highest values of energy absorption. For ballistic performance, the Weibull analysis showed a high R2 correlation value above 0.9, confirming the reliability of the tested samples. These results illustrate the possibilities of caranan fiber to be used as a reinforcement for epoxy composites and its promising application in ballistic armor.

Composites reinforced with natural lignocellulosic fibers (NLFs) are gaining relevance as the worldwide demand for renewable and sustainable materials increases. To develop novel natural composites with satisfactory properties, less common NLFs should also be investigated. Among these, the Cyperus malaccensis (CM), a type of sedge fiber, is already used in simple items like ropes, furniture, and paper, but has not yet been investigated as composite reinforcement for possible engineering applications. Therefore, the present work evaluated for the first time the properties of novel epoxy composites incorporated with 10, 20, and 30 vol.% of CM sedge fibers. Tensile, Izod-impact, and ballistic impact tests were performed, as well as Fourier transform infrared (FT-IR) spectroscopy and thermal analysis of the composites. Results disclosed a decrease (−55%) in tensile strengths as compared to the neat epoxy. However, the elastic modulus of the 30 vol.% sedge fiber composite increased (+127%). The total strain and absorbed ballistic energy did not show significant variation. The Izod impact energy of the 30 vol.% composite was found to be 181% higher than the values obtained for the neat epoxy as a control sample. An increase in both stiffness and toughness characterized a reinforcement effect of the sedge fiber. The thermal analysis revealed a slight decrease (−15%) in the degradation temperature of the CM sedge fiber composites compared to the neat epoxy. The glass-transition temperatures were determined to be in the range of 67 to 81 °C.

The ACI 544-2R introduced a qualitative test to compare the impact resistance of fibrous concretes under repeated falling-mass impact loads, which is considered to be a low-cost, quick solution for material-scale impact tests owing to the simplified apparatus, test setup and procedure, where none of the usual sophisticated sensors and data acquisition systems are required. However, previous studies showed that the test results are highly scattered with noticeably unacceptable variations, which encouraged researchers to try to use statistical tools to analyze the scattering of results and suggest modifications to reduce this unfavorable disadvantage. The current article introduces a state-of-the-art literature review on the previous and recent research on repeated impact testing of different types of fibrous concrete using the ACI 544-2R test, while focusing on the scattering of results and highlighting the adopted statistical distributions to analyze this scattering. The influence of different mixture parameters on the variation of the cracking and failure impact results is also investigated based on data from the literature. Finally, the article highlights and discusses the literature suggestions to modify the test specimen, apparatus and procedure to reduce the scattering of results in the ACI 544-2R repeated impact test. The conducted analyses showed that material parameters such as binder, aggregate and water contents in addition to the maximum size of aggregate have no effect on the variation of test results, while increasing the fiber content was found to have some positive influence on decreasing this variation. The survey conducted in this study also showed that the test can be modified to lower the unfavorable variations of impact and failure results.

This study represents a novel contribution regarding the behavior of fiber-metal laminates (FMLs) under tensile impact loading. The experimental program included axial tensile-impact tests and quasi-static tensile and flexural tests after impact. The FMLs’ core consisted of an epoxy matrix reinforced with long glass fibers, and their skins had 1050 aluminum plates. Several variables were investigated, including the lay-up methods and the insertion of the [90°] layers within [Al/0°/Al] and [Al/0°/0°/Al] FML specimens. The results showed that the residual tensile load of [Al/0°/90°/0°/Al] sandwich specimens’ lay-up method was 6.65 kN , from the original ultimate load of 17.69 kN . For bending after impact, the residual load was 0.79 kN , from the original one of 1.46 kN . Although inserting the [90°] layers provided low tensile strength, it effectively blunted the propagating crack in the load’s direction under either the quasi-static tensile or the tensile-impact tests. For progressive damage, the [0°] layer showed fiber breakage and fiber-kinking, and the [90°] layer exhibited critical matrix cracking. Meanwhile, the aluminum plates exhibited a transverse crack during the delamination between the composite core and aluminum plates. During the tensile-impact test, two divisions of delamination were observed: partial delamination aside from [0°] and severe delamination with [90°].

Existing portable barriers often sustain severe damage or catastrophic failure when subjected to vehicle collisions at high protection levels. As the number of road reconstruction and expansion projects grows, the safety performance of portable barriers becomes crucial for safeguarding the well-being of personnel in construction work zones. Moreover, numerical analysis has been extensively employed in simulating and evaluating roadside safety hardware over the past decade. However, in contrast to simple material tests, the actual damage modes and deformation mechanisms of barriers can differ substantially from simulated outcomes.​ Therefore, two sets of full-scale impact tests (at 80 km/h and 60 km/h) were initially carried out. In accordance with the design specifications for portable barriers outlined in international standards, the impact resistance of a newly designed portable concrete barrier (NPCB-2) at its most unfavorable installation position was investigated. Finite element (FE) models were established using LS-DYNA and validated against the crash test data to ensure the reliability of barrier safety performance evaluation.​ Subsequently, the validated FE models were utilized to assess the effects of reinforcement spacing, connection thickness, and barrier segment length on actual impact performance. Additionally, additional reinforcing bars were incorporated to further enhance the bonding strength between rectangular steel tubes and steel rebars, leading to the proposal of a modified portable barrier configuration (NPCB-4). Numerical simulations verified that the impact resistance performance of the modified barrier, including the impact process, stress distribution, acceleration, angular displacement, and energy absorption, meets the requirements of relevant specifications. The combined experimental-simulation approach adopted in this barrier design offers a valuable reference for the development of novel barrier configurations in the future.​

Aramid fabric was widely used in bulletproof armor due to its excellent mechanical properties. Previous studies have shown that high and low temperature had a great impact on the mechanical properties of aramid fibers. This work used ZnO particles to modify aramid fabric to improve its ballistic performance under extreme temperature environment via SEM imaging, quasi-static tensile test, yarn pullout test, ballistic impact test and numerical simulation. The tensile strength and failure strain of modified yarn was 13.3% and 25.9% higher than those of neat yarn under extreme temperature. The ballistic limit velocity of ZnO modified aramid fabric was 124.1% and 164.1% higher than that of neat fabrics under high and low temperature environments, and the areal density was only increased by 8.2%. The weakening of the ballistic performance of aramid fabric was due to micro-surface damage caused by irregular crack defects on the fiber surface and changes in molecular orientation. The numerical simulation results were in good agreement with the ballistic impact test. The influence of changes in fabric mechanical parameters on the ballistic impact protection mechanism was elucidated, further demonstrating the significant improvement of ballistic performance of ZnO modified fabric under extreme temperature.

With the increase in depth and complexity of underground construction, rock burst disasters occur frequently, resulting in significant casualties and economic losses. To prevent and control rock burst disaster, this paper introduces three specifications of flexible protection nets, developed using a biaxial warp knitting process with high-strength polyester fiber as the raw material. These flexible protection nets aim to address the shortcomings of traditional metal nets. First, tensile tests and node stripping tests were carried out to determine the strength of the three types of flexible net strips and their nodes, and to analyze the response mechanism of the flexible net under static load. Subsequently, a large impact test platform was designed based on the tunnel net anchor system form of the flexible protection net, this platform utilized a drop ball to simulate rock burst impacts and conducted multiple sets of tests to quantify the capacity of the three types of flexible protection nets in intercepting rock burst impacts. Finally, mechanical analysis of the flexible protection net was performed to determine the dynamic process of intercepting the drop ball under various conditions, and combined with the experimental results, the deformation and damage characteristics of the nets were revealed. This study provides a reliable new method for underground engineering dynamic disaster protection.HighlightsDeveloped high-strength polyester fiber flexible protection nets to address the limitations of traditional metal nets in mitigating rock bursts.Conducted static tests on flexible protection nets to determine the static characteristics of various specifications.Developed a drop ball impact test device for tunnel net-anchor integrated protection systems, accounting for rock burst characteristics.Performed impact tests to reveal the dynamic response of flexible protection nets under rock burst impact loads.

Tungsten powder/polytetrafluoroethylene (W/PTFE) composites have the potential to replace traditional metallic materials as casings for controllable power warheads. Under explosive loading, they generate high-density and relatively uniformly distributed metal powder particles, thereby enhancing close-range impact effects while reducing collateral damage. To characterize the material’s response under impact loading, plate impact tests were conducted to investigate the effects of tungsten content (70 wt%, 80 wt%, and 90 wt%) and tungsten particle size (200 μm, 400 μm, and 600 μm) on the impact behavior of the composites. The free surface velocity histories of the target plates were measured using a 37 mm single-stage light gas gun and a full-fiber laser interferometer (DISAR), enabling the determination of the shock velocity–particle velocity relationship to establish the equation of state. Experimental data show a linear relationship between shock velocity and particle velocity, with the 80 wt% and 90 wt% composites exhibiting similar shock velocities. The fitted slope increases from 2.792 to 2.957 as the tungsten mass fraction rises from 70 wt% to 90 wt%. With particle size increasing from 200 μm to 600 μm, the slope decreases from 3.204 to 2.756, while c0 increases from 224.7 to 633.3. Comparison of the Hugoniot pressure curves of different specimens indicated that tungsten content significantly affects the impact behavior, whereas variations in tungsten particle size have a negligible influence on the Hugoniot pressure. A high tungsten content with small particle size (e.g., 90 wt% with ~200 μm) improves the overall compressive properties of composite materials. Based on the experimental results, a mesoscale finite element model consistent with the tests was developed. The overall error between the numerical simulations and experimental results was less than 5% under various conditions, thereby validating the accuracy of the model. Numerical simulations revealed the coupling mechanism between tungsten particle plastic deformation and matrix flow. The strong rarefaction unloading effect initiated at the composite’s free surface caused matrix spallation and jetting. Multiple wave systems were generated at the composite–copper interface, whose interference and coupling ultimately resulted in a nearly uniform macroscopic pressure field.