Numerical simulation of drop weight impact sensitivity evaluation criteria for pressed PBXs

A thorough understanding of drop-weight impacted responses for polymer-bonded explosives(PBXs)is significant to evaluate their impact sensitivity.The characteristics of the drop-weight impacted pressed PBXs including deforming,fracturing,forming a local high-temperature region and igniting,were simulated using a coupled mechanical-thermo-chemical model integrating micro-defects evolution.A novel evaluation method for impact sensitivity is established using the relation between the input kinetic energy and the output energy due to deformation,crushing energy,local hot spot energy and ignition.The effects of impact velocity on sensitivity were analyzed and the critical local ignition impact velocity is determined as 4.0-4.5 m/s.The simulated results show that shear-crack friction heating is the dominant ignition mechanism.The region along the boundary of PBXs sample is the most hazardous regions where ignition first occur.The propagation of stress wave in PBXs causes shear-crack hotspot and bulk temperature exhibiting an approximate 45°direction evolution path,which is the main reason that dominated damage-ignition region transits from the boundary to the central of sample.

Effect of interface behaviour on damage and instability of PBX under combined tension-shear loading

To study the effect of interface behaviour on the mechanical properties and damage evolution of PBX under combined tension-shear loading, the present work establishes the numerical model of a PBX three-phase hybrid system, which introduces a nonlinear plastic damage cohesion model to study the mechanical response and damage process. The parameters in the model were fitted and calibrated.Taking the crack growth rate as the feature, the damage state in each stage was determined, and the damage instability criterion was given. The effects of interfacial tensile strength and shear strength on the damage process of PBX were studied. On this basis, serrated and hemispherical structures interface of PBX has been developed, which affects the damage process and instability during the loading process.The results indicate that damage state response of PBX experiences the process of stable load bearing,unstable propagation, and complete failure. At the critical moment of instability, the overall equivalent effective strain of material reaches 3024 με and instability loading displacement reaches 0.39 mm. The increase of interfacial tensile strength and shear strength significantly inhibits the damage of PBX. The effect of interfacial shear strength on critical instability of PBX is approximately 1.7 times that of the interfacial tensile strength. Further, interface opening along the normal direction is the main damage form at the interface. Serrated and hemispherical rough interfaces can significantly inhibit propagation of cracks, and the load bearing capacity is improved by 22% and 9.7%, respectively. Appropriate improvement of the roughness of the interface structure can effectively improve the mechanical properties. It is significantly important to have a better understanding of deformation, damage and failure mechanisms of PBX and to improve our predictive ability.

Modification of SDOF model for reinforced concrete beams under close-in explosion

In this paper,a modified single-degree-of-freedom(SDOF)model of reinforced concrete(RC)beams under close-in explosion is proposed by developing the specific impulse equivalent method and flexural resistance calculation method.The equivalent uniform specific impulse was obtained based on the local conservation of momentum and global conservation of kinetic energy.Additionally,the influence of load uniformity,boundary condition and complex material behaviors(e.g.strain rate effect,hardening/softening and hoop-confined effect)was considered in the resistance calculation process by establishing a novel relationship between external force,bending moment,curvature and deflection successively.The accuracy of the proposed model was verified by carrying out field explosion tests on four RC beams with the scaled distances of 0.5 m/kg~(1/3)and 0.75 m/kg~(1/3).The test data in other literatures were also used for validation.As a result,the equivalent load implies that the blast load near the mid-span of beams would contribute more to the maximum displacement,which was also observed in the tests.Moreover,both the resistance model and test results declare that when the blast load becomes more concentrated,the ultimate resistance would become lower,and the compressive concrete would be more prone to softening and crushing.Finally,based on the modified SDOF model,the calculated maximum displacements agreed well with the test data in this paper and other literatures.This work fully proves the rationality of the modified SDOF method,which will contribute to a more accurate damage assessment of RC structures under close-in explosion.

Combustion crack-network reaction evolution model for highly-confined explosives

The evolution behavior of combustion crack reaction of highly confined solid explosives after non-shock ignition is governed by multiple dynamic processes,including intrinsic combustion of explosives,crack propagation,and rapid growth of combustion surface area.Here,the pressure increase can accelerate the combustion rate of explosives,and the crack propagation can enlarge the combustion surface area.The coupling between these two effects leads to the self-enhanced combustion of explosive charge system,which is the key mechanism for the reaction development after ignition.In this study,combustion cracknetwork(CCN) model is established to describe the evolution of combustion crack reaction of highly confined solid explosives after non-shock ignition and quantify the reaction violence.The feasibility of the model is verified by comparing the computational and experimental results.The results reveal that an increase in charge structure size causes an increase in the time of crack pressurization and extension of cracks due to the high temperature-generated gas flow and surface combustion during the initial stage of explosive reaction,but when the casing is fractured,the larger the charge structure,the more violent the late reaction and the larger the charge reaction degree.The input pressure has no obvious influence on the final reaction violence.Further,a larger venting hole area leads to better pressure relief effect,which causes slower pressure growth inside casing.Larger reserved ullage volume causes longer lowpressure induction stage,which further restrains the internal pressure growth.Furthermore,the stronger the casing constraint,the more rapid the self-enhanced combustion of the high temperaturegenerated gas,which results in more violent charge reaction and larger charge reaction degree during casing break.Overall,the proposed model can clarify the effects of intrinsic combustion rate of explosives,charge structure size,input pressure,relief area,ullage volume,and constraint strength on the reaction evolution,which can provide theoretical basis for violence evaluation and safety design for ammunition under accident stimulus.

A new Ignition-Growth reaction rate model for shock initiation

Accurately predicting reactive flow is a challenge when characterizing an explosive under external shock stimuli as the shock initiation time is on the order of a microsecond.The present study constructs a new Ignition-Growth reaction rate model,which can describe the shock initiation processes of explosives with different initial densities,particle sizes and loading pressures by only one set of model parameters.Compared with the Lee-Tarver reaction rate model,the new Ignition-Growth reaction rate model describes better the shock initiation process of explosives and requires fewer model parameters.Moreover,the shock initiation of a 2,4-Dinitroanisole(DNAN)-based melt-cast explosive RDA-2(DNAN/HMX(octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazoncine)/aluminum)are investigated both experimentally and numerically.A series of shock initiation experiments is performed with manganin piezoresistive pressure gauges and corresponding numerical simulations are carried out with the new Ignition-Growth reaction rate model.The RDA-2 explosive is found to have higher critical initiation pressure and lower shock sensitivity than traditional explosives(such as the Comp.B explosive).The calibrated reaction rate model parameters of RDA-2 could provide numerical basis for its further application.

Damage-ignition mechanism studies on modified propellant with different crosslinking density under dynamic loading

The study of high-energy and low-vulnerability propellants is important for the power performance and safety of solid propellant rocket motors.The modified split Hopkinson pressure bar(SHPB)tests are performed on two kinds of propellant with different crosslinking density to study the dynamic mechanical responses and damage-ignition mechanism.SHPB apparatus is equipped with a highperformance infrared camera and high-speed camera to capture the deformation,damage-ignition feature and temperature evolution images in the impact process.The results suggested that the mechanical responses and damage-ignition mechanism of the propellants were affected by the strain rates and crosslinking density.The damage-ignition degree is more intense and the reaction occurs earlier with the increase of strain rates.For propellant 1 with higher crosslinking density,the critical ignition strain rate is 4500 s^(-1).Two kinds of propellants show different ignition mechanism,i.e.crack generation,propagation and final fracture for propellant 1 while viscous shear flow for propellant 2.Meanwhile,the SEM images also reveal the difference of damage-ignition mechanism of the two kinds of propellants.Finally,the ignition mechanism under different strain rates and critical ignition strain rate of propellants are further explained by the theoretical calculation of temperature variations.

Shock response of cyclotetramethylene tetranitramine(HMX)single crystal at elevated temperatures

To investigate the shock response of cyclotetramethylene tetranitramine(HMX)single crystals at elevated temperatures(below the phase transition point),plate impact experiments at elevated temperatures were designed and conducted.The HMX/window interface particle velocities at temperatures of 300 K,373 K,and 423 K were measured by the velocity interferometry system for any reflector(VISAR)technique.To further analyze the related mesoscale deformation mechanisms,a nonlinear thermoelastic-viscoplastic model was developed,which considers thermal activation and phonon drag dislocation slip mechanisms.The proposed model could well reproduce the measured thermal hardening behavior of Hugoniot elastic limit(HEL)of HMX single crystals.At elevated temperatures,the reduced dislocation mobility was observed,which stems from both phonon scattering and radiative damping effects.Comparatively speaking,radiative damping contributes less than phonon scattering to thermal hardening behavior.The calibrated model was further used to predict shock response of HMX single crystals with different thicknesses at different initial temperatures.Both the stress relaxation and elastic precursor decrease with thickness are mainly due to the rapid dislocation generation.These insights shed light on the interplay between dislocation motion and dislocation generation in thermal hardening behavior,stress relaxation,and elastic precursor decay,which serves to reveal the mesoscale deformation mechanisms at elevated temperatures.

Behavior of reinforced concrete beams and columns subjected to blast loading

In this study, the blast performance of steel reinforced concrete(RC) beams was experimentally and analytically investigated. The experiment consists of a total of 10 one-half-scale beams subjected to different levels of blast loading using live explosives. The reflected pressure-time histories were recorded and different damage levels and modes were observed. The blast resilience of the damaged beams was quantified by measuring the time-dependent displacements. Experiment results show that the damage in steel reinforced concrete beams with higher explosive mass is enhanced compared with that of the beams with smaller explosive mass at the same scaled distance. Based on the experiment data, an empirical expression is developed via dimensional analysis to correct the relationship between the midspan displacement and scaled distance. Besides, a complex single degree of freedom model(SDOF)incorporating complex features of the material behavior, high strain-rate effect and the column geometry was proposed and validated by test results.

Ballistic performance of tungsten particle/metallic glass matrix composite long rod

In the present manuscript numerical analysis on the ballistic performance of a tungsten particle/metallic glass matrix(WP/MG) composite rod is conducted by integrating with related experimental investigations. In the corresponding finite element method(FEM) simulations a modified coupled thermomechanical constitutive model is employed to describe the mechanical properties of metallic glass(MG)matrix, and geometrical models of the WP/MG composite rod are established based on its inner structure. The deformation and failure characteristics of the rod and target materials are analyzed in detail,and the influences of various factors on the ballistic performance of the WP/MG composite long rod are discussed. Related analysis demonstrates that the penetrating performance of the WP/MG rod is similar to that of the tungsten fiber/metallic glass matrix(WF/MG) composite long rod, i.e., a "self-sharpening" behavior also occurs during the penetration process, and correspondingly its penetrating capability is better than that of the tungsten heavy alloy(WHA) rod. However, the mass erosion manner of the WP/MG rod is different and the erosion is relatively severe, thus its penetrating capability is a little lower compared with that of the WF/MG one. Moreover, the impact velocity and the target strength have significant influences on the ballistic performance of the WP/MG composite rod, whereas the effect of initial nose shape is very little.

Experimental investigation of p-section concrete beams under contact explosion and close-in explosion conditions

This paper presents the results of an experimental investigation on explosive breaching of p-section concrete beams. Twenty three p-section concrete beams with a 100 cm length were tested. TNT charges were placed at three positions: contact detonation in the center, contact detonation above the web and close-in detonation in the center. The external and internal breach parameters of the panels were evaluated by measuring the diameter of the ejection crater, spalling crater and breach hole created by the charge detonation. The experimental results were compared to predict values obtained by the analytical models proposed by McVay, Morishita and Remennikov. A modified breach with crater limit line and breach without crater limit line were put forward based on the experimental results. The maximum cross-sectional destruction area ratio(MCDAR) values were used to evaluate the damage degree. The maximum value of MCDAR reached 0.331 corresponding to the C5 experimental condition, of which explosion occurred above the web.

Attitude deflection of oblique perforation of concrete targets by a rigid projectile

A perforation model is developed to predict the attitude deflection in the oblique perforation of concrete targets by a rigid projectile,in which the inertial moment of the projectile is introduced,together with taking the attitude deflection during the shear plugging sub-stage into account,and the shape of the plug formed on the rear surface of target is also re-investigated.Moreover,a new classification of concrete targets is proposed based on the target thickness,with which the attitude deflections in different kinds of concrete targets are analyzed.It is found that the numerical results by using the new perforation model are in good agreement with the previous experimental data and simulated results.Furthermore,the variations of the attitude deflection with the initial conditions(the initial attitude angle and the initial impact velocity) are investigated.

Deformation treatment and microstructure of graphene-reinforced metal matrix nanocomposites: A review of graphene post-dispersion

Graphene/aluminum(Gr/Al)composites have attracted the attention of researchers all over the world due to their excellent properties.However,graphene agglomerates easily because of the van der Waals force between graphite sheets,thereby affecting the performance of the composites.Decreasing the agglomeration of graphene and dispersing it uniformly in the Al matrix is a key challenge.In the preparation process,predispersion treatment and deformation treatment can play important roles in graphene dispersion.Researchers have conducted a series of research and literature reviews of the graphene predispersion and consolidation of composites.However,they paid less attention to post-deformation processing.This review summarizes different deformation treatments involved in the preparation process of Gr/Al composites and the evolution of the microstructure during the process.Research on deformation parameters is expected to further improve the properties of Gr/Al composites and would provide a deep understanding of the strengthening effect of graphene.

Experimental investigations of mechanical and reaction responses for drop-weight impacted energetic particles

Low-velocity drop-weight impact experiments on individual and multiple Cyclotetramethylene tetranitramine (HMX) energetic particles were performed using a modified drop-weight machine equipped with high-speed photography components. Multiple particles experienced more severe burning reactions than an individual particle. Comparisons between impacted salt and HMX particle show that jetting in HMX is mainly due to the motion of fragmented particles driven by gaseous reaction products. Velocity of jetting, flame propagation, and area expansion were measured via image processing, making it possible to quantify the chemical reaction or mechanical deformation violence at different stages.

Thermal hazard assessment of TKX-50-based melt-cast explosive

In the present study, thermal hazards of TNT and DNAN used as the molten binder in TKX-50-based meltcast explosives were comparatively studied through accelerating rate calorimeter(ARC) and Cook-off experiments. Two kinds of ARC operation modes were performed to investigate the thermal safety performance under adiabatic conditions(HWS mode) and constant heating(CHR mode). The obtained results demonstrated that at both heating modes, DNAN/TKX-50 outperformed TNT/TKX-50 from the thermal safety point of view. However, the sensitivity to heat of the samples was reverse because of the different heating modes. In addition, the results of thermal hazard assessment obtained from the cookoff experiment complied with ARC analysis which indicated the molten binder TNT replaced by DNAN would reduce the hazard of the TKX-50 melt cast explosive. Furthermore, the results of cook-off experiments also showed that DNAN/TKX-50 outperformed TNT/TKX-50 from the aspect of thermal stability, which was consistent with the result of CHR mode because of the similar heating process.

Application of split Hopkinson tension bar technique to the study of dynamic fracture properties of materials

A novel approach is proposed in determining dy- namic fracture toughness （DFT） of high strength steel, using the split Hopkinson tension bar （SHTB） apparatus, com- bined with a hybrid experimental-numerical method. The center-cracked tension specimen is connected between the bars with a specially designed fixture device. The fracture initiation time is measured by the strain gage method, and dynamic stress intensity factors （DSIF） are obtained with the aid of 3D finite element analysis （FEA）. In this approach, the dimensions of the specimen are not restricted by the connec- tion strength or the stress-state equilibrium conditions, and hence plane strain state can be attained conveniently at the crack tip. Through comparison between the obtained results and those in open publication, it is concluded that the ex- perimental data are valid, and the method proposed here is reliable. The validity of the obtained DFT is checked with the ASTM criteria, and fracture surfaces are examined at the end of paper.

A quasi-isentropic model of a cylinder driven by aluminized explosives based on characteristic line analysis

A quasi-isentropic study on the process of driving a cylinder with aluminized explosives was carried out to examine the influence of the aluminum(Al) reaction rate on cylinder expansion and the physical parameters of the detonation products. Based on the proposed quasi-isentropic hypothesis and relevant isentropic theories, the characteristic lines of aluminized explosives driving a cylinder were analyzed,and a quasi-isentropic model was established. This model includes the variation of the cylinder wall velocity and the physical parameters of the detonation products with the Al reaction degree. Using previously reported experimental results, the quasi-isentropic model was verified to be applicative and accurate. This model was used to calculate the physical parameters for cylinder experiments with aluminized cyclotrimethylenetrinitramine explosives with 15.0 % and 30.0 % Al content. The results show that this quasi-isentropic model can be used not only to calculate the cylinder expansion rule or Al reaction degree, but also to calculate the physical parameters of the detonation products in the process of cylinder expansion. For explosives with 15.0 % and 30.0 % Al, 24.3 % and 18.5 % of the Al was found to have reacted at 33.9 μs and 34.0 μs, respectively. The difference in Al content results in different reaction intensity, occurrence time, and duration of two forms of reaction(diffusion and kinetic) between the Al powder and the detonation products;the post-detonation burning reaction between the Al powder and the detonation products prolongs the positive pressure action time, resulting in a continuous rise in temperature after detonation.

A melt-cast Duan-Zhang-Kim mesoscopic reaction rate model and experiment for shock initiation of melt-cast explosives

A melt-cast Duan-Zhang-Kim(DZK)mesoscopic reaction rate model is developed for the shock initiation of melt-cast explosives based on the pore collapse hot-spot ignition mechanism.A series of shock initiation experiments was performed for the Comp B melt-cast explosive to estimate effects of the loading pressure and the particle size of granular explosive component,and the mesoscopic model is validated against the experimental data.Further numerical simulations indicate that the initial density and formula proportion greatly affect the hot-spot ignition of melt-cast explosives.

Progress in improving thermodynamics and kinetics of new hydrogen storage materials

Hydrogen storage material has been much developed recently because of its potential for proton exchange membrane （PEM） fuel cell applications. A successful solid-state reversible storage material should meet the requirements of high storage capacity, suitable thermodynamic properties, and fast adsorption and desorption kinetics. Complex hydrides, including boron hydride and alanate, ammonia borane, metal organic frameworks （MOFs）, covalent organic frameworks （COFs） and zeolitic imidazolate frameworks （ZIFs）, are remarkable hydrogen storage materials because of their advantages of high energy density and safety. This feature article focuses mainly on the thermodynamics and kinetics of these hydrogen storage materials in the past few years.

Ubiquitiformal Crack Extension in Quasi-Brittle Materials

Based on the concept of ubiquitiform,a ubiquitiformal crack extension model is developed for quasi-brittle materials.Numerical simulations are carried out using the ABAQUS software with the XFEM-based cohesive segments method to determine the ubiquitiformal crack extension path or fracture surface profile of the material under quasi-static loading.Such a ubiquitiformal crack model removes the singularity of a fractal crack;for the latter,the boundary value problem cannot be uniquely defined.In the simulation,the material properties,e.g.,the tensile strength,are assumed to obey the Weibull distribution.The meso-element equivalent method is used to determine the correlation between the Weibull distribution parameters and the aggregate gradation of concrete materials.The numerical results show that the complexities of the ubiquitiformal crack configurations are in good agreement with the previous experimental data.Through the numerical simulation,it is further demonstrated that the complexity of a ubiquitiformal crack is insensitive to the random spatial distribution of the aggregates,but more dependent on the Weibull distribution parameters which reflect the heterogeneity of the concrete.