查看更多>>摘要：To solve the problem of the low interference success rate of air defense missile radio fuzes due to the unified interference form of the traditional fuze interference system,an interference decision method based Q-learning algorithm is proposed.First,dividing the distance between the missile and the target into multiple states to increase the quantity of state spaces.Second,a multidimensional motion space is utilized,and the search range of which changes with the distance of the projectile,to select parameters and minimize the amount of ineffective interference parameters.The interference effect is determined by detecting whether the fuze signal disappears.Finally,a weighted reward function is used to determine the reward value based on the range state,output power,and parameter quantity information of the interference form.The effectiveness of the proposed method in selecting the range of motion space parameters and designing the discrimination degree of the reward function has been verified through offline experiments involving full-range missile rendezvous.The optimal interference form for each distance state has been obtained.Compared with the single-interference decision method,the proposed decision method can effectively improve the success rate of interference.
查看更多>>摘要：By both the Charpy V-notched impact and the projectile tests,we here investigated the dynamic fracture behavior of a recently developed ultrastrong lightweight steel comprising a hierarchical martensitic matrix,dispersed ultra-fine-retained austenite grains and oriented δ-ferrite lamellas,the latter being due to high Al and Si contents employed for low-density design.This steel shows a superior combination of specific ultimate tensile strength and impact toughness to other ultrastrong steels and has successfully arrested a real steel-cored bullet shot.These are attributed to the densely textured δ-ferrite lamellas that can deflect the propagating cracks until they are trapped and enclosed besides austenite-to-martensite transformation crack closure,leading to more energy consumed before failure.These results suggest a new pathway for toughening ultrastrong lightweight steels.
查看更多>>摘要：As the basic protective element,steel plate had attracted world-wide attention because of frequent threats of explosive loads.This paper reports the relationships between microscopic defects of Q345 steel plate under the explosive load and its macroscopic dynamics simulation.Firstly,the defect characteristics of the steel plate were investigated by stereoscopic microscope(SM)and scanning electron microscope(SEM).At the macroscopic level,the defect was the formation of cave which was concentrated in the range of 0-3.0 cm from the explosion center,while at the microscopic level,the cavity and void for-mation were the typical damage characteristics.It also explains that the difference in defect morphology at different positions was the combining results of high temperature and high pressure.Secondly,the variation rules of mechanical properties of steel plate under explosive load were studied.The Arbitrary Lagrange-Euler(ALE)algorithm and multi-material fluid-structure coupling method were used to simulate the explosion process of steel plate.The accuracy of the method was verified by comparing the deformation of the simulation results with the experimental results,the pressure and stress at different positions on the surface of the steel plate were obtained.The simulation results indicated that the critical pressure causing the plate defects may be approximately 2.01 GPa.On this basis,it was found that the variation rules of surface pressure and microscopic defect area of the Q345 steel plate were strikingly similar,and the corresponding mathematical relationship between them was established.Compared with Monomolecular growth fitting models(MGFM)and Logistic fitting models(LFM),the relationship can be better expressed by cubic polynomial fitting model(CPFM).This paper illustrated that the explosive defect characteristics of metal plate at the microscopic level can be explored by analyzing its macroscopic dynamic mechanical response.
查看更多>>摘要：The existing indoor fusion positioning methods based on Pedestrian Dead Reckoning(PDR)and geomagnetic technology have the problems of large initial position error,low sensor accuracy,and geomagnetic mismatch.In this study,a novel indoor fusion positioning approach based on the improved particle filter algorithm by geomagnetic iterative matching is proposed,where Wi-Fi,PDR,and geomagnetic signals are integrated to improve indoor positioning performances.One important contribution is that geomagnetic iterative matching is firstly proposed based on the particle filter al-gorithm.During the positioning process,an iterative window and a constraint window are introduced to limit the particle generation range and the geomagnetic matching range respectively.The position is corrected several times based on geomagnetic iterative matching in the location correction stage when the pedestrian movement is detected,which made up for the shortage of only one time of geomagnetic correction in the existing particle filter algorithm.In addition,this study also proposes a real-time step detection algorithm based on multi-threshold constraints to judge whether pedestrians are moving,which satisfies the real-time requirement of our fusion positioning approach.Through experimental verification,the average positioning accuracy of the proposed approach reaches 1.59 m,which improves 33.2％compared with the existing particle filter fusion positioning algorithms.
查看更多>>摘要：In this study,a three dimensional(3D)numerical model of six-degrees-of-freedom(6DOF)is applied to simulate the water entries of twin spheres side-by-side at different lateral distances and time intervals.The turbulence structure is described using the shear-stress transport k-ω(SST k-ω)model,and the volume of fluid(VOF)method is used to track the complex air-liquid interface.The motion of spheres during water entry is simulated using an independent overset grid.The numerical model is verified by comparing the cavity evolution results from simulations and experiments.Numerical results reveal that the time interval between the twin water entries evidently affects cavity expansion and contraction behaviors in the radial direction.However,this influence is significantly weakened by increasing the lateral distance between the two spheres.In synchronous water entries,pressure is reduced on the midline of two cavities during surface closure,which is directly related to the cavity volume.The evo-lution of vortexes inside the two cavities is analyzed using a velocity vector field,which is affected by the lateral distance and time interval of water entries.
查看更多>>摘要：In this paper,to study the mechanical responses of a solid propellant subjected to ultrahigh acceleration overload during the gun-launch process,specifically designed projectile flight tests with an onboard measurement system were performed.Two projectiles containing dummy HTPB propellant grains were successfully recovered after the flight tests with an ultrahigh acceleration overload value of 8100 g.The onboard-measured time-resolved axial displacement,contact stress and overload values were success-fully obtained and analysed.Uniaxial compression tests of the dummy HTPB propellant used in the gun-launched tests were carried out at low and intermediate strain rates to characterize the propellant's dynamic properties.A linear viscoelastic constitutive model was employed and applied in finite-element simulations of the projectile-launching process.During the launch process,the dummy propellant grain exhibited large deformation due to the high acceleration overload,possibly leading to friction between the motor case and propellant grain.The calculated contact stress showed good agreement with the experimental results,though discrepancies in the overall displacement of the dummy propellant grain were observed.The dynamic mechanical response process of the dummy propellant grain was analysed in detail.The results can be used to estimate the structural integrity of the analysed dummy propellant grain during the gun-launch process.
查看更多>>摘要：Experimental investigations on dynamic in-plane compressive behavior of a plain weave composite were performed using the split Hopkinson pressure bar.A quantitative criterion for calculating the constant strain rate of composites was established.Then the upper limit of strain rate,restricted by stress equi-librium and constant loading rate,was rationally estimated and confirmed by tests.Within the achiev-able range of 0.001/s-895/s,it was found that the strength increased first and subsequently decreased as the strain rate increased.This feature was also reflected by the turning point(579/s)of the bilinear model for strength prediction.The transition in failure mechanism,from local opening damage to completely splitting destruction,was mainly responsible for such strain rate effects.And three major failure modes were summarized under microscopic observations:fiber fracture,inter-fiber fracture,and interface delamination.Finally,by introducing a nonlinear damage variable,a simplified ZWT model was devel-oped to characterize the dynamic mechanical response.Excellent agreement was shown between the experimental and simulated results.
查看更多>>摘要：The recent increase in blast/bombing incidents all over the world has pushed the development of effective strengthening approaches to enhance the blast resistance of existing civil infrastructures.Engineered geopolymer composite(EGC)is a promising material featured by eco-friendly,fast-setting and strain-hardening characteristics for emergent strengthening and construction.However,the fiber optimization for preparing EGC and its protective effect on structural elements under blast scenarios are uncertain.In this study,laboratory tests were firstly conducted to evaluate the effects of fiber types on the properties of EGC in terms of workability,dry shrinkage,and mechanical properties in compression,tension and flexure.The experimental results showed that EGC containing PE fiber exhibited suitable workability,acceptable dry shrinkage and superior mechanical properties compared with other types of fibers.After that,a series of field tests were carried out to evaluate the effectiveness of EGC retrofitting layer on the enhancement of blast performance of typical elements.The tests include autoclaved aerated concrete(AAC)masonry walls subjected to vented gas explosion,reinforced AAC panels subjected to TNT explosion and plain concrete slabs subjected to contact explosion.It was found that EGC could effectively enhance the blast resistance of structural elements in different scenarios.For AAC masonry walls and panels,with the existence of EGC,the integrity of specimens could be maintained,and their deflections and damage were significantly reduced.For plain concrete slabs,the EGC overlay could reduce the diameter and depth of the crater and spallation of specimens.
查看更多>>摘要：As one of the most widely used personal protective equipment(PPE),body armors play an important role in protecting the human body from the high-velocity impact of bullets or projectiles.The body torso and critical organs of the wear may suffer severe behind-armor blunt trauma(BABT)even though the impactor is stopped by the body armor.A type of novel composite material through incorporating shear stiffening gel(STG)into ethylene-vinyl acetate(EVA)foam is developed and used as buffer layers to reduce BABT.In this paper,the protective performance of body armors composed of fabric bulletproof layers and a buffer layer made of foam material is investigated both experimentally and numerically.The effectiveness of STG-modified EVA in damage relief is verified by ballistic tests.In parallel with the experimental study,numerical simulations are conducted by LS-DYNA® to investigate the dynamic response of each component and capture the key mechanical parameters,which are hardly obtained from field tests.To fully describe the material behavior under the transient impact,the selected constitutive models take the failure and strain rate effect into consideration.A good agreement between the experimental observations and numerical results is achieved to prove the validity of the modelling method.The tests and simulations show that the impact-induced deformation on the human body is significantly reduced by using STG-modified EVA as the buffering material.The improvement of pro-tective performance is attributed to better dynamic properties and more outstanding energy absorption capability of the composite foam.
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