查看更多>>摘要：Polypropylene(PP)fibres have primarily used to control shrinkage cracks or mitigate explosive spalling in concrete structures exposed to fire or subjected to impact/blast loads,with limited investigations on capacity improvement.This study unveils the possibility of using PP micro-fibres to improve the impact behaviour of fibre-reinforced ultra-high-performance concrete(FRUHPC)columns.Results show that the addition of fibres significantly improves the impact behaviour of FRUHPC columns by shifting the failure mechanism from brittle shear to favourable flexural failure.The addition of steel or PP fibres affected the impact responses differently.Steel fibres considerably increased the peak impact force(up to 18％)while PP micro-fibres slightly increased the peak(3％-4％).FRUHPC significantly reduced the maximum mid-height displacement by up to 30％(under 20° impact)and substantially improved the displacement recovery by up to 100％(under 20° impact).FRUHPC with steel fibres significantly improved the energy absorption while those with PP micro-fibres reduced the energy absorption,which is different from the effect of PP-macro fibre reported in the literature.The optimal fibre content for micro-PP fibres is 1％due to its minimal fibre usage and low peak and residual displacement.This study highlights the potential of FRUHPC as a promising material for impact-resistant structures by creating a more favourable flexural failure mechanism,enhancing ductility and toughness under impact loading,and advancing the un-derstanding of the role of fibres in structural performance.
查看更多>>摘要：The infrared conformal window is one of the most critical components in aircraft.Conformal windows with high performance bring low aberrations,high aerodynamic performance,reliability in extreme working environments,and added value for aircraft.Through the past decades,remarkable advances have been achieved in manufacturing technologies for conformal windows,where the machining ac-curacy approaches the nanometer level,and the surface form becomes more complex.These advances are critical to aircraft development,and these manufacturing technologies also have significant reference values for other directions of the ultra-precision machining field.In this review,the infrared materials suitable for manufacturing conformal windows are introduced and compared with insights into their performances.The remarkable advances and concrete work accomplished by researchers are reviewed.The challenges in manufacturing conformal windows that should be faced in the future are discussed.
查看更多>>摘要：The ballistic resistance and failure pattern of a bi-layer alumina 99.5％-aluminium alloy 1100-H12 target against steel 4340 ogival nosed projectile has been explored in the present experimental cum numerical study.In the experimental investigation,damage induced in the ceramic layer has been quantified in terms of number of cracks developed and failure zone dimensions.The resultant damage in the backing layer has been studied with variation in the bulge and perforation hole in the backing layer with the varying incidence velocity.The discussion of the experimental results has been further followed by three dimensional finite element computations using ABAQUS/Explicit finite code to investigate the behaviour of different types of bi-layer targets under multi-hit projectile impact.The JH-2 constitutive model has been used to reproduce the behaviour of alumina 99.5％and JC constitutive model has been used for steel 4340 and aluminium alloy 1100-H12.The total energy dissipation has been noted to be of lesser magnitude in case of sub-sequential impact in comparison to simultaneous impact of two projectiles.The distance between the impact points of two projectiles also effected the ballistic resistance of bi-layer target.The ballistic resistance of single tile ceramic front layer and four tile ceramic of equivalent area found to be dependent upon the boundary conditions provided to the target.
查看更多>>摘要：The stratification phenomenon resulting from differences in the physical properties of solid-liquid components seriously affect the final combustion and explosion characteristics of mixed fuel under the action of oscillation.The effects of oscillation on the physical stability of mixed fuel with two solid-liquid ratios and three liquid component distribution ratios have been investigated using a self-designed experimental system at oscillation frequencies of 60-300 r/min.The explosion characteristics of mixed fuel before and after oscillation are gained from a 20 L spherical explosion container system.When the mass ratio of liquid components is controlled at 66.9％,64.7％,62.6％the final explosion characteristics are stable,with a maximum difference of only 0.71％.The volume of liquid fuel precipi-tation increases with increasing oscillation frequency when the mass ratio of liquid components reaches 71.7％,69.6％,67.7％.The fuel explosion overpressure after oscillation decreases with increasing liquid precipitation volume,and the repeatability is poor,with a maximum standard deviation of 82.736,which is much higher than the ratio without stratification.Properly controlling the mass ratio of liquid com-ponents of the mixed fuel can effectively combat the impact of oscillation on the physical state and maintain the stability of the final explosion characteristics.
查看更多>>摘要：In this paper,high cis-1,4 content hydroxyl-terminated polybutadiene(cis-HTPB)with different molec-ular weights was prepared through the oxidative cracking process using cis-butadiene rubber as raw material.Firstly,this article comprehensively compared the differences between cis-HTPB and conven-tional I-HTPB in terms of molecular weight distribution,functionality,viscosity,molecular polarity,and other physicochemical properties,which provided effective data support for its subsequent application.In addition,the reaction kinetics study showed that cis-HTPB with isocyanate curing agent has high reactivity,allowing it to be rapidly cured at low temperatures,and the cured elastomers had excellent mechanical properties,with tensile strength and elongation up to 1.89 MPa and 1100％,respectively.It was also found that cis-HTPB has extremely excellent low-temperature resistance,and the glass tran-sition temperature(Tg)of its cured elastomer is as low as-101 ℃.Based on the above studies,cis-HTPB is applied as a binder in composite solid propellants for the first time to investigate its practical perfor-mance,and the results indicated that cis-HTPB-based propellants have excellent process and mechanical properties.
Aleksandr V.StankevichNikolay A.RasputinAnisa Kh.RudinaGennady L.Rusinov...
210-224页
查看更多>>摘要：In this work,comprehensive studies of 2,4-dinitroanisole(2,4DNAN)were carried out using powder thermorentgenography of the internal standard.The time of the complete polymorphic transition in the solid phase β → α in 2,4DNAN under various combinations of conditions has been determined.It has been established that,regardless of the season of manufacture of the substance,when it is stored for 8-9 months,with a change in ambient temperature from minus 30 ℃ to plus 30 ℃,a complete polymorphic transition β→α occurs.When stored in conditions below minus 5 ℃,polymorphic transition does not occur.When stored in conditions above plus 30 ℃ in a closed container,polymorphic transition occurs within 3 weeks.The polymorphic transition is accompanied by a decrease in density by 1.3％-1.5％and an increase in melting temperature by 10-12 ℃,depending on the degree of purity of the starting sub-stance.The activation energy of the molecular rearrangement was 68-70 kJ/mol(16.5±3 kcal/mol).The mechanism of polymorphic transition has been evaluated,which is presumably based on internal homodiffusion and energy transfer to the surface of the mass of powder particles and the product.The average activation energy of the polymorphic transition process was 110±6.2 kJ/mol(26.2 kcal/mol).In an open container,reactions proceed by a homogeneous mechanism,and in a closed container by a heterogeneous mechanism involving the gas phase.
查看更多>>摘要：Protection of urban critical infrastructures(CIs)from GPS-denied,bomb-carrying kamikaze drones(G-B-KDs)is very challenging.Previous approaches based on drone jamming,spoofing,communication interruption and hijacking cannot be applied in the case under examination,since G-B-KDs are uncon-trolled.On the other hand,drone capturing schemes and electromagnetic pulse(EMP)weapons seem to be effective.However,again,existing approaches present various limitations,while most of them do not examine the case of G-B-KDs.This paper,focuses on the aforementioned under-researched field,where the G-B-KD is confronted by two defensive drones.The first neutralizes and captures the kamikaze drone,while the second captures the bomb.Both defensive drones are equipped with a net-gun and an innovative algorithm,which,among others,estimates the locations of interception,using a real-world trajectory model.Additionally,one of the defensive drones is also equipped with an EMP weapon to damage the electronics equipment of the kamikaze drone and reduce the capturing time and the overall risk.Extensive simulated experiments and comparisons to state-of-art methods,reveal the advantages and limitations of the proposed approach.More specifically,compared to state-of-art,the proposed approach improves:(a)time to neutralize the target by at least 6.89％,(b)maximum number of missions by at least 1.27％and(c)total cost by at least 5.15％.
查看更多>>摘要：Considering their affordability and high strength-to-weight ratio,lightweight aluminium alloys are the subject of intensive research aimed at improving their properties for use in the aerospace industry.This research effort aims to develop novel hybrid composites based on AA 2014 alloy through the use of liquid metallurgy stir casting to reinforce dual ceramic particles of Zirconium Diboride(ZrB2)and Boron Carbide(B4C).The weight percentage(wt％)of ZrB2 was varied(0,5,10,and 15),while a constant 5 wt％of B4C was maintained during this fabrication.The as-cast samples have been assessed using an Optical Mi-croscope(OM)and a Scanning Electron Microscope(SEM)with Energy Dispersive Spectroscopy(EDS).The properties such as hardness,tensile strength,and wear characteristics of stir cast specimens were assessed to examine the impact of varying weight percentages of reinforcements in AA 2014 alloy.In particular,dry sliding wear behaviour was evaluated considering varied loads using a pin-on-disc tri-botester.As the weight％of ZrB2 grew and B4C was incorporated,hybrid composites showed higher hardness,tensile strength,and wear resistance.Notably,the incorporation of a cumulative reinforcement consisting of 15 wt％ZrB2 and 5 wt％B4C resulted in a significant 31.86％increase in hardness and a 44.1％increase in tensile strength compared to AA 2014 alloy.In addition,it has been detected that wear resistance of hybrid composite pin(containing 20 wt％cumulative reinforcement)is higher than that of other stir cast wear test pins during the whole range of applied loads.Fractured surfaces of tensile specimens showed ductile fracture in the AA 2014 matrix and mixed mode for hybrid composites.Worn surfaces obtained employing higher applied load indicated abrasive wear with little plastic deformation for hybrid composites and dominant adhesive wear for matrix alloy.Hence,the superior mechanical and tribological performance of hybrid composites can be attributed to dual reinforcement particles being dispersed well and the effective transmission of load at this specific composition.
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