Impact of characteristic length and loading rate upon dynamic constitutive behavior and fracture process in alumina ceramics

Understanding the impact performance of ceramic materials requires accurate corresponding relationship between mechanical response and fracture behavior. In this study, constitutive behaviors of alumina ceramics were successfully determined via split-Hopkinson pressure bar (SHPB) system coupled with high-speed camera to track the deformation and failure process. Failure strength of alumina demonstrated a strong dependency on strain rate beyond a critical value (namely transition strain rate). Inelastic deformation in the dynamic stress-strain curves implied that degradation of modulus does occur. The incorporating such degradation (damage evolution) in modulus enabled a more accurate evaluation of transition strain rate as a function of characteristic length of specimen. On-line observation revealed that longitudinal cracks dominated the failure process of alumina with negligible interfacial friction. However, interfacial friction became significant with the decreased characteristic length, thus the inclined cracks dominated fracture in alumina. It was found that the effect of interfacial friction can be minimized by lowering the impact velocity to maintain the uniaxial loading status in SHPB loads. Finally, it is suggested that an aspect ratio of 1.0 for the specimen should be suitable for alumina due to its insensitivity to interfacial friction within the achievable strain rate.     

Minimum-variance recursive state estimation for complex networks with stochastic switching topologies and random quantization under try-once-discard protocol

This article is concerned with the issue of minimum-variance recursive state estimation (MVRSE) for a class of nonlinear dynamical complex networks (NDCNs) with stochastic switching topologies and random quantization under the try-once-discard (TOD) protocol. Two sequences of Bernoulli distributed random variables with given occurrence probabilities are utilized to characterize the stochastic switching manners of network topologies and the randomly occurring quantized output measurements, where the quantization effects are portrayed by the uniform quantizer. Moreover, the TOD protocol is adopted to arrange the order of the information transmission of network nodes so as to alleviate the communication burden and mitigate the network congestions. The focus of the MVRSE issue is to develop a novel state estimation algorithm such that, for all stochastic switching topologies, random quantization effects and TOD protocol, an optimized upper bound of the estimation error covariance is guaranteed by properly designing the estimator gain. In addition, the theoretical proof is derived, which illustrates that the state estimation error is exponentially mean-square bounded under certain conditions. Meanwhile, we also present the related theoretical analysis, which discusses the impact caused by random quantization. Finally, a numerical experiment is utilized to show the validity of the novel MVRSE approach.     

Organic interlayers boost the activity of MoS2 toward hydrogen evolution by maintaining high 1T/2H phase ratio

1T-MoS₂ shows great promise for hydrogen production due to phenomenal performance in catalyzing hydrogen evolution reaction (HER) from water. However, this phase converts to low-active 2H–MoS₂ on superambient heating. A series of MoS₂ layered compounds (LCs) with guest organic cations was prepared to reveal the effect of cationic organics on the structure stabilization of 1T-MoS₂ and thereby on the catalytic performance of this phase in HER. The results showed that LCs provide significantly higher 1T/2H ratio after heating than non-stabilized 1T phase and some of them exhibit excellent thermal durability in the capacity of HER catalysts. The effect is most pronounced for the organics remaining tightly bound to sulfide sheets in a sulfuric acid electrolyte. Cetyltrimethylammonium and bis(dimethylamino)naphthalene perfectly matched this criterion and provided a long-term maintaining of catalyst activity. The data obtained in this study are hoped to offer new approach for rational design of the efficient non-precious 1T-MoS₂-based electrocatalysts for hydrogen production.     

Propagation characteristics of unstable detonation waves in a round tube with annular perturbation

The detonation propagation characteristics of the mixtures, 2H₂ + O₂+3Ar and CH₄ + 2O₂, were investigated. Accordingly, the mixtures were tested in round tube with inner diameter of D = 80 mm and annular tubes with widths of w = 25 mm, 15 mm, and 5 mm. The two mixtures represent stable mixture with regular cell pattern and unstable mixture with irregular cell pattern, respectively. Smoked foils were utilized to record cellular structure under various initial conditions. Subsequently, the length scale L_dsc was measured, which represents the length from the start of the test section to the position where the cellular structure changes drastically (the cell size obviously increases or the structure disappears). The results reveal that both mixtures can successfully propagate in round tube and annular tubes of 25 mm and 15 mm, but fail in 5 mm annular tube. The L_dsc value of 2H₂ + O₂ + 3Ar is higher than that of CH₄ + 2O₂ in 80 mm and 15 mm tubes, but it is opposite in 25 mm tube. Moreover, the relationship between L_dsc and hydraulic diameter D_H was analysed. For a given tube, the values of L_dsc and L_dsc/D_H increased when the initial pressure increased. And the variation trend of L_dsc and L_dsc/D_H of CH₄ + 2O₂ is steeper. Furthermore, the mixtures 2H₂ + O₂ + 3Ar and CH₄ + 2O₂ resulted in over-driven detonation in 15 mm and 25 mm annular tubes, respectively. The ratio between the total reaction length (sum of the induction length and exothermic length) and the hydraulic diameter (D_H/(Δ_i + Δ_e)) correspond to critical values of 18 for hydrogen-oxygen-argon and 6 for methane-oxygen, below which the detonation will fail.     

Effect of water on the thermal stability of alpha-aluminum hydride

Water can significantly enhance the thermal spontaneous combustion and explosion hazard of alpha-aluminum hydride (α-AlH₃). Thermal analysis suggested that, as the water content increased from 10% to 40%, the total reaction heat was increased by 68.3%. α-AlH₃ with different water contents (10%–40%) underwent hydrolysis reaction and oxidation reaction with the activation energy of 57.9–68.7 kJ/mol and 88.3–107.6 kJ/mol, respectively. As the heating rate increased from 0.2 to 2 °C/min, the onset temperature, the peak temperature, and the maximum heat flow of the exothermic reaction were increased. As the storage temperature increased from 45 to 65 °C, the time to maximum reaction rate decreased from 5.16 to 0.84 h. Combining the results of SEM, XRD, XPS and thermal analysis, the reaction mechanism of α-AlH₃ and water under isothermal and non-isothermal conditions was revealed.     

Automatic terminal guidance for small fixed-wing unmanned aerial vehicles

The research presented in this article focuses on expanding and deepening the prior research of a low-cost terminal guidance system in a previous paper entitled “Design, implementation and verification of a low-cost terminal guidance system for small fixed-wing UAVs.” An automatic terminal guidance workflow is specially designed for an individual in a small fixed-wing unmanned aerial vehicle (SUAV) swarm. The extended work around the proposed workflow primarily involves upgrading onboard hardware modules to improve sensor accuracy and environmental adaptability, the imaging performance of the seeker, as well as the computational capability of the image processor, applying object detection to replace the human-in-the-loop function and adopting the integral proportional guidance law in the vertical direction to reduce the required overload and obtain a larger impact angle. Furthermore, we conducted several field tests on two types of SUAV against a stationary target on the ground in a field scenario. The experiments have generated valuable onboard image data and SUAV status information, all of which are aligned in the time domain. The only remaining data sets that support the findings of this study are available from the corresponding author. Our study into automatic terminal guidance has yielded a solution of the automatic strap-down monocular terminal guidance problem of individual SUAVs. The field trials of a single SUAV demonstrate the robustness and efficiency of the proposed automatic terminal guidance methodology and lays a foundation for the future SUAVs' cooperative attack test.