2020 Vol. 40, No. 3
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2020, 40(3): 031101.
doi: 10.11883/bzycj-2020-0024
Abstract:
The researchers in solid mechanics are interested in studying the mechanical response of deformed solids with stress-strain constitutive relationships (referred to as deformation-type constitutive relations), while the researchers in fluid mechanics are interested in studying the mechanical responses of fluids with stress-strain rate constitutive relationships (referred to as flow-type constitutive relations). When the dynamic plasticity of structures and materials is concerned, should it be in terms of plastic deformation or plastic flow? This paper discusses this problem from the macroscopic plastic constitutive theory and the microscopic dislocation dynamic mechanism, respectively, and points out that the plastic constitutive relation belongs to the flow-type viscoplastic rate-dependent constitutive relation, which is suitable for both loading and unloading processes. Therefore, the stress-strain diagram should not be used to describe the plastic loading and unloading processes. The elastic-plastic constitutive relation is the coupling of the deformation-type and flow-type constitutive relations.
The researchers in solid mechanics are interested in studying the mechanical response of deformed solids with stress-strain constitutive relationships (referred to as deformation-type constitutive relations), while the researchers in fluid mechanics are interested in studying the mechanical responses of fluids with stress-strain rate constitutive relationships (referred to as flow-type constitutive relations). When the dynamic plasticity of structures and materials is concerned, should it be in terms of plastic deformation or plastic flow? This paper discusses this problem from the macroscopic plastic constitutive theory and the microscopic dislocation dynamic mechanism, respectively, and points out that the plastic constitutive relation belongs to the flow-type viscoplastic rate-dependent constitutive relation, which is suitable for both loading and unloading processes. Therefore, the stress-strain diagram should not be used to describe the plastic loading and unloading processes. The elastic-plastic constitutive relation is the coupling of the deformation-type and flow-type constitutive relations.
2020, 40(3): 032101.
doi: 10.11883/bzycj-2019-0090
Abstract:
In order to establish the link between macroscopic explosion suppression effect and the mechanism of microscopic explosion suppression during the methane explosion suppression process, we used the 20 L spherical explosion test devices and grating spectrometer to collect data on the pressure and flame emission spectrum, analyzed changes of some key free radicals or molecules such as NO, CN, CHO, HNO and OH with the methods of spectrum analysis and data synchronization analysis during the suppression of methane explosion, and obtained the coupling relationship between the development of methane explosion pressure and related free radicals’ content. The research suggests that adding urea can effectively reduce the explosion pressure of methane and extend the explosion induction period of methane. Under urea conditions, the contents of NO and HNO increase while those of CN, CHO and OH are reduced, which lead to the suppression of methane explosion; NO radicals are mainly produced during the stage of pressure increase. CN, CHO, and HNO radicals are mainly produced and peaked during the explosion induction period before pressure rises; OH radicals have always existed throughout the methane explosion and are high in content. The interference on the free radicals above can suppress the methane explosion in relevant reaction stages.
In order to establish the link between macroscopic explosion suppression effect and the mechanism of microscopic explosion suppression during the methane explosion suppression process, we used the 20 L spherical explosion test devices and grating spectrometer to collect data on the pressure and flame emission spectrum, analyzed changes of some key free radicals or molecules such as NO, CN, CHO, HNO and OH with the methods of spectrum analysis and data synchronization analysis during the suppression of methane explosion, and obtained the coupling relationship between the development of methane explosion pressure and related free radicals’ content. The research suggests that adding urea can effectively reduce the explosion pressure of methane and extend the explosion induction period of methane. Under urea conditions, the contents of NO and HNO increase while those of CN, CHO and OH are reduced, which lead to the suppression of methane explosion; NO radicals are mainly produced during the stage of pressure increase. CN, CHO, and HNO radicals are mainly produced and peaked during the explosion induction period before pressure rises; OH radicals have always existed throughout the methane explosion and are high in content. The interference on the free radicals above can suppress the methane explosion in relevant reaction stages.
2020, 40(3): 032102.
doi: 10.11883/bzycj-2019-0011
Abstract:
The shock compression and chemical reaction behaviors of CL-20/HMX energetic co-crystal explosives were simulated by nonequilibrium molecular dynamics. The spatio-temporal distributions of density, particle velocity, shock hugoniots, shock initiation pressure, and detonation pressure were obtained. The distribution of main intermediate products and the stable products were also investigated. The simulation results show that the initial reaction pathway is N—NO2 cleavage to form NO2 from CL-20 in co-crystal, with N2, CO2 and H2O as the main products. The decomposition rate of CL-20 and HMX increases with the increase of shock wave velocity gradually, but the attenuation rate of CL-20 is higher than that of HMX under each shock condition.
The shock compression and chemical reaction behaviors of CL-20/HMX energetic co-crystal explosives were simulated by nonequilibrium molecular dynamics. The spatio-temporal distributions of density, particle velocity, shock hugoniots, shock initiation pressure, and detonation pressure were obtained. The distribution of main intermediate products and the stable products were also investigated. The simulation results show that the initial reaction pathway is N—NO2 cleavage to form NO2 from CL-20 in co-crystal, with N2, CO2 and H2O as the main products. The decomposition rate of CL-20 and HMX increases with the increase of shock wave velocity gradually, but the attenuation rate of CL-20 is higher than that of HMX under each shock condition.
2020, 40(3): 032201.
doi: 10.11883/bzycj-2019-0128
Abstract:
In order to improve the standard testing method of droplet explosion, the droplet formation process and explosion characteristics of methanol were experimentally studied in the 20 L spherical spray testing system, under different ambient pressures, injection pressures and concentrations. The results show that the increasing of injection pressure is more likely to cause the methanol to break into tiny droplets, leading the explosion limit range of methanol droplets broadened. The increasing of ambient pressure would change the explosion limit range of methanol droplet, and can effectively inhibit the explosion accident caused by methanol leakage. When the ambient pressure is 0.1 MPa, and the injection pressure is 2.1 MPa, with the spray concentration of methanol is 356.4 g/m3, the droplet size of the methanol is 2.5 μm. The maximum explosive pressure, the maximum explosion pressure rising rate and the laminar burning rate are the highest at this inflection point. Small droplets (1−15 μm) are more easily ignited by external energy, and the transient physical and chemical reactions are more rapid and violent during explosion. Larger droplets (more than 22 μm) will cause ignition difficult. However, after the ignition is successful, the explosion characteristics increase with the increasing of methanol droplet concentration, showing an approximate linear rule. At this time, the influence of droplet size of methanol on the above explosion characteristics can be neglected. The results could be helpful to understand the law of droplet explosion, improve the testing method and safety design.
In order to improve the standard testing method of droplet explosion, the droplet formation process and explosion characteristics of methanol were experimentally studied in the 20 L spherical spray testing system, under different ambient pressures, injection pressures and concentrations. The results show that the increasing of injection pressure is more likely to cause the methanol to break into tiny droplets, leading the explosion limit range of methanol droplets broadened. The increasing of ambient pressure would change the explosion limit range of methanol droplet, and can effectively inhibit the explosion accident caused by methanol leakage. When the ambient pressure is 0.1 MPa, and the injection pressure is 2.1 MPa, with the spray concentration of methanol is 356.4 g/m3, the droplet size of the methanol is 2.5 μm. The maximum explosive pressure, the maximum explosion pressure rising rate and the laminar burning rate are the highest at this inflection point. Small droplets (1−15 μm) are more easily ignited by external energy, and the transient physical and chemical reactions are more rapid and violent during explosion. Larger droplets (more than 22 μm) will cause ignition difficult. However, after the ignition is successful, the explosion characteristics increase with the increasing of methanol droplet concentration, showing an approximate linear rule. At this time, the influence of droplet size of methanol on the above explosion characteristics can be neglected. The results could be helpful to understand the law of droplet explosion, improve the testing method and safety design.
2020, 40(3): 032202.
doi: 10.11883/bzycj-2019-0175
Abstract:
In order to avoid damages caused by the explosion of combustible premixed gas in confined space, it is vital to make accurate explosion overpressure prediction in anti-explosion design or daily safety management. Based on the experimental data in literatures, this paper firstly constructd the prediction model of explosion overpressure based on the smooth and laminar flame propagation theory, and then points out it failed to accurately predict the explosion of large-volume confined space. Subsequently we analyzed the instability of flame propagation in large-volume confined space and its resulting frontal wrinkles and turbulent combustion, which greatly increases the surface of the flame front and exhibits self-similar fractal characteristics during flame propagation. Based on the fractal combustion theory and relevant empirical data, we further construct the explosion overpressure prediction model for flammable premixed gas explosion with considering flame wrinkling and turbulent combustion. At the same time, the experimental results are compared. The results demonstrate that the relative errors of experimental and theoretical calculation are 10.4% and 11.1% respectively when the volume of confined space is large, and the peak pressure is estimated by using the explosion overpressure model based on the flame propagation theory of wrinkling and turbulent. The errors are reduced 72.3% and 50.6% than that of the smooth and laminar flame propagation theory explosion overpressure model. The theoretical model established in this paper is in good agreement with the experimental results, and it can meet the needs of structural explosion-resistant design or daily safety management to a certain extent.
In order to avoid damages caused by the explosion of combustible premixed gas in confined space, it is vital to make accurate explosion overpressure prediction in anti-explosion design or daily safety management. Based on the experimental data in literatures, this paper firstly constructd the prediction model of explosion overpressure based on the smooth and laminar flame propagation theory, and then points out it failed to accurately predict the explosion of large-volume confined space. Subsequently we analyzed the instability of flame propagation in large-volume confined space and its resulting frontal wrinkles and turbulent combustion, which greatly increases the surface of the flame front and exhibits self-similar fractal characteristics during flame propagation. Based on the fractal combustion theory and relevant empirical data, we further construct the explosion overpressure prediction model for flammable premixed gas explosion with considering flame wrinkling and turbulent combustion. At the same time, the experimental results are compared. The results demonstrate that the relative errors of experimental and theoretical calculation are 10.4% and 11.1% respectively when the volume of confined space is large, and the peak pressure is estimated by using the explosion overpressure model based on the flame propagation theory of wrinkling and turbulent. The errors are reduced 72.3% and 50.6% than that of the smooth and laminar flame propagation theory explosion overpressure model. The theoretical model established in this paper is in good agreement with the experimental results, and it can meet the needs of structural explosion-resistant design or daily safety management to a certain extent.
2020, 40(3): 033301.
doi: 10.11883/bzycj-2018-0389
Abstract:
In this study, the resistance model of rigid flat-nosed projectile penetrating reinforced concrete target was established, in which the failure mode of reinforcing bar in reinforced concrete was simplified as bending shear failure on the foundation of plain concrete penetration theory. The calculation results of penetration depth agreed well with Young's formula. The results indicated that the model established in this study could reasonably predict the penetration depth. The results show that the penetration depth of the projectile impacting the mesh center is maximum when the ratio of the projectile diameter to the mesh size is less than 1, and the most unfavorable target position depends on the ratio when it exceeds 1. In view of protection, an engineering calculation formula of penetration depth was proposed under the most unfavorable working condition of.
In this study, the resistance model of rigid flat-nosed projectile penetrating reinforced concrete target was established, in which the failure mode of reinforcing bar in reinforced concrete was simplified as bending shear failure on the foundation of plain concrete penetration theory. The calculation results of penetration depth agreed well with Young's formula. The results indicated that the model established in this study could reasonably predict the penetration depth. The results show that the penetration depth of the projectile impacting the mesh center is maximum when the ratio of the projectile diameter to the mesh size is less than 1, and the most unfavorable target position depends on the ratio when it exceeds 1. In view of protection, an engineering calculation formula of penetration depth was proposed under the most unfavorable working condition of.
2020, 40(3): 033302.
doi: 10.11883/bzycj-2019-0086
Abstract:
Survivability and reliability assessment of components/key components on high-speed penetrating projectiles is a hot and difficult issue in the field of EPW development. Due to the cost limitation of prototype test, it is feasible to carry out non-proportionally scale experimental research by carrying prototype fuze components on scaled projectiles. Through the analysis of the process mechanism of a projectile penetrating concrete target, the analytic solution of rigid-body deceleration when the projectile penetrating the semi-infinite thick concrete target and the multi-layer thin concrete target are discussed respectively. From the point of view of similarity of rigid-body deceleration, the non-proportionally reduced-scale criterion of projectile is proposed when the traditional scaling scheme can not meet the requirements of similarity. The numerical results show that under the condition of penetrating semi-infinite thick concrete target, the rigid-body deceleration of the non-proportionally reduced-scale projectile can achieve the same conditions as which of the prototype projectile from the point of view of pulse width and amplitude; under the condition of penetrating multi-layered thin target, through reasonably setting the scale factor and adjusting the layout of the target plate and the initial velocity of the projectile. The pulse width and amplitude of the rigid-body deceleration in the reduced scale test can cover them in the prototype test. The rigid body deceleration characteristics obtained from scaled model test can provide reliable overload environment reference for missile projectile design.
Survivability and reliability assessment of components/key components on high-speed penetrating projectiles is a hot and difficult issue in the field of EPW development. Due to the cost limitation of prototype test, it is feasible to carry out non-proportionally scale experimental research by carrying prototype fuze components on scaled projectiles. Through the analysis of the process mechanism of a projectile penetrating concrete target, the analytic solution of rigid-body deceleration when the projectile penetrating the semi-infinite thick concrete target and the multi-layer thin concrete target are discussed respectively. From the point of view of similarity of rigid-body deceleration, the non-proportionally reduced-scale criterion of projectile is proposed when the traditional scaling scheme can not meet the requirements of similarity. The numerical results show that under the condition of penetrating semi-infinite thick concrete target, the rigid-body deceleration of the non-proportionally reduced-scale projectile can achieve the same conditions as which of the prototype projectile from the point of view of pulse width and amplitude; under the condition of penetrating multi-layered thin target, through reasonably setting the scale factor and adjusting the layout of the target plate and the initial velocity of the projectile. The pulse width and amplitude of the rigid-body deceleration in the reduced scale test can cover them in the prototype test. The rigid body deceleration characteristics obtained from scaled model test can provide reliable overload environment reference for missile projectile design.
2020, 40(3): 033303.
doi: 10.11883/bzycj-2019-0094
Abstract:
To investigate the dynamic response characteristics of the water-filled double layer structure when encountering a collision load, a series of collision tests are conducted and the response of the structure filled with water and unfilled with water is compared. The results indicate that the internal water can protect the upper plate during a collision. Finally, the effect of fluid-solid coupling between internal water and double layer structure on the collision response is investigated by analyzing the test data of the acceleration of the striker and the disturbance pressure of the internal water.
To investigate the dynamic response characteristics of the water-filled double layer structure when encountering a collision load, a series of collision tests are conducted and the response of the structure filled with water and unfilled with water is compared. The results indicate that the internal water can protect the upper plate during a collision. Finally, the effect of fluid-solid coupling between internal water and double layer structure on the collision response is investigated by analyzing the test data of the acceleration of the striker and the disturbance pressure of the internal water.
2020, 40(3): 033304.
doi: 10.11883/bzycj-2019-0103
Abstract:
The numerical simulation of magnetically driven one-sided flyer plate experiment usually does not take into account the influences of cathode motion and ablation width in the thickness direction on the boundary magnetic field. Hence, one-sided computational model was usually applied to simulate magnetically driven one-sided flyer plate experiment. In order to understand the reason why magnetically driven one-sided flyer plate experiment can be simulated by one-sided computational model, magnetically driven one-sided flyer plate experiment (experiment PTS-061 with a 0.972-mm-thick aluminum flyer plate and experiment PTS-064 with a 1.041-mm-thick aluminum flyer plate) were simulated by two-sided computational model. In the experiments with experiment PTS-061 and PTS-064, displacement of thin flyer plate current-loading surface increases with time; displacement of thick cathode current-loading surface does not increases with time, and remains basically unchanged with small displacement in the middle and late stage of experiments. At the end of experiment PTS-061, displacement of thin flyer plate current-loading surface is 4.9 mm, and displacement of thick cathode current-loading surface is only 1.7 mm. At the end of experiment PTS-064, displacement of thin flyer plate current-loading surface is 4.1 mm, and displacement of thick cathode current-loading surface is only 0.9 mm. The reason of one-sided computational model can be adopted in magnetically driven one-sided flyer plate experiment is not that cathode plate position remains unchanged, but instead that cathode current-loading surface has smaller displacement remaining basically unchanged in the middle and later stage of experiments, and the displacement of thin flyer plate current-loading surface has a greater influence on the boundary magnetic field that of thick cathode current-loading surface in the late stage of experiments.
The numerical simulation of magnetically driven one-sided flyer plate experiment usually does not take into account the influences of cathode motion and ablation width in the thickness direction on the boundary magnetic field. Hence, one-sided computational model was usually applied to simulate magnetically driven one-sided flyer plate experiment. In order to understand the reason why magnetically driven one-sided flyer plate experiment can be simulated by one-sided computational model, magnetically driven one-sided flyer plate experiment (experiment PTS-061 with a 0.972-mm-thick aluminum flyer plate and experiment PTS-064 with a 1.041-mm-thick aluminum flyer plate) were simulated by two-sided computational model. In the experiments with experiment PTS-061 and PTS-064, displacement of thin flyer plate current-loading surface increases with time; displacement of thick cathode current-loading surface does not increases with time, and remains basically unchanged with small displacement in the middle and late stage of experiments. At the end of experiment PTS-061, displacement of thin flyer plate current-loading surface is 4.9 mm, and displacement of thick cathode current-loading surface is only 1.7 mm. At the end of experiment PTS-064, displacement of thin flyer plate current-loading surface is 4.1 mm, and displacement of thick cathode current-loading surface is only 0.9 mm. The reason of one-sided computational model can be adopted in magnetically driven one-sided flyer plate experiment is not that cathode plate position remains unchanged, but instead that cathode current-loading surface has smaller displacement remaining basically unchanged in the middle and later stage of experiments, and the displacement of thin flyer plate current-loading surface has a greater influence on the boundary magnetic field that of thick cathode current-loading surface in the late stage of experiments.
2020, 40(3): 034101.
doi: 10.11883/bzycj-2019-0113
Abstract:
Preload selection is one of the key problems for sealing design of closure-flange structure. In this study, a dual spring mass model accounting for the bending deformation of the closure is developed by estimating the equivalent stiffness and mass of the closure flexural vibration. The predicting result of dynamic response is in a great agreement with experiments. Finally, the law of variation of both axial and flexural deformation of the closure with bolt preloading is summarized, and the results show tthe clearance of sealing surface decreases gradually to a stable value with the increase of preloading force. And the bending deformation of the end cap is the main factor affecting this stable value.
Preload selection is one of the key problems for sealing design of closure-flange structure. In this study, a dual spring mass model accounting for the bending deformation of the closure is developed by estimating the equivalent stiffness and mass of the closure flexural vibration. The predicting result of dynamic response is in a great agreement with experiments. Finally, the law of variation of both axial and flexural deformation of the closure with bolt preloading is summarized, and the results show tthe clearance of sealing surface decreases gradually to a stable value with the increase of preloading force. And the bending deformation of the end cap is the main factor affecting this stable value.
2020, 40(3): 034102.
doi: 10.11883/bzycj-2019-0234
Abstract:
To improve the temperature drift characteristics of piezoresistive pressure sensors, a temperature compensation model for the pressure sensors was constructed based on genetic algorithm and wavelet neural networks. By considering the problems of slow convergence and high probability of the local optimal solutions of the wavelet neural networks, the genetic algorithm was applied to optimize the connection weights, expansion parameters and translation parameters of the wavelet neural networks. Based on the calibration data of the pressure sensors, the BP neural network, wavelet neural network and genetic wavelet neural network were used to study the temperature compensation, respectively. The results show that the genetic wavelet neural network was compatible with the time-frequency local characteristics of the wavelet analysis and the self-learning ability of the neural networks, showing high convergence speed and compensation accuracy. After the compensation, the output values of the sensors were closer to the calibration ones. The maximum error was changed from −17.44 kPa to 0.38 kPa, and the maximum relative error was changed from −14.0% to 0.38%. The constructed model is applied in the temperature compensation of explosion static pressure in finite space, and the practical effect is good.
To improve the temperature drift characteristics of piezoresistive pressure sensors, a temperature compensation model for the pressure sensors was constructed based on genetic algorithm and wavelet neural networks. By considering the problems of slow convergence and high probability of the local optimal solutions of the wavelet neural networks, the genetic algorithm was applied to optimize the connection weights, expansion parameters and translation parameters of the wavelet neural networks. Based on the calibration data of the pressure sensors, the BP neural network, wavelet neural network and genetic wavelet neural network were used to study the temperature compensation, respectively. The results show that the genetic wavelet neural network was compatible with the time-frequency local characteristics of the wavelet analysis and the self-learning ability of the neural networks, showing high convergence speed and compensation accuracy. After the compensation, the output values of the sensors were closer to the calibration ones. The maximum error was changed from −17.44 kPa to 0.38 kPa, and the maximum relative error was changed from −14.0% to 0.38%. The constructed model is applied in the temperature compensation of explosion static pressure in finite space, and the practical effect is good.
2020, 40(3): 035101.
doi: 10.11883/bzycj-2018-0252
Abstract:
Numerical studies were conducted by using the finite element software LS-DYNA to investigate the performances of the retrofitted autoclaved aerated concrete masonry (AAC) walls subjected to gas explosions. A simplified numerical model for the masonry walls was developed and calibrated with the test data. Under the blast loads specified by the design codes, the influences of wall height and thickness on the structural response of the unstrengthened one-way AAC masonry walls were discussed. In addition, the performances of the BFRP strip and spray-on polyurea strengthened one-way AAC masonry walls were compared and the retrofitting suggestions for engineering practice were proposed. It is found that the numerical predications of the mid-span displacements and failure modes are in agreement with the test data. Under the specified blast loads, the unstrengthened masonry walls mainly fail for bending of the structures, and with the increase of wall height, the failure mode changes from flexure failure to shear failure. Using the BFRP strips can improve the stiffness and arching effect of the walls significantly while the spray-on polyurea can enhance the tensile membrane effect of walls effectively. The failure mode of the strengthened masonry walls is a typical flexure failure. The fracture of the BFRP strips generally occurs at the mid-span area of the one-way masonry walls, while the fracture of spray-on polyurea occurs at the boundary of the masonry walls.
Numerical studies were conducted by using the finite element software LS-DYNA to investigate the performances of the retrofitted autoclaved aerated concrete masonry (AAC) walls subjected to gas explosions. A simplified numerical model for the masonry walls was developed and calibrated with the test data. Under the blast loads specified by the design codes, the influences of wall height and thickness on the structural response of the unstrengthened one-way AAC masonry walls were discussed. In addition, the performances of the BFRP strip and spray-on polyurea strengthened one-way AAC masonry walls were compared and the retrofitting suggestions for engineering practice were proposed. It is found that the numerical predications of the mid-span displacements and failure modes are in agreement with the test data. Under the specified blast loads, the unstrengthened masonry walls mainly fail for bending of the structures, and with the increase of wall height, the failure mode changes from flexure failure to shear failure. Using the BFRP strips can improve the stiffness and arching effect of the walls significantly while the spray-on polyurea can enhance the tensile membrane effect of walls effectively. The failure mode of the strengthened masonry walls is a typical flexure failure. The fracture of the BFRP strips generally occurs at the mid-span area of the one-way masonry walls, while the fracture of spray-on polyurea occurs at the boundary of the masonry walls.
