JOURNAL OF BEIJING INSTITUTE OF TECHNOLOGY

Influences of Stress Wave Propagation upon Studying Dynamic Response of Materials at High Strain Rates

WANG Li-li

2004, 13(3): 225-235.

[Abstract](155)

Abstract:
How the wave propagation analysis plays a key role in the studies of dynamic response of materials at high strain rates is analyzed. For the wave propagation technique, the followings are important: the loading and unloading constitutive relation presumed, the positions of the sensors embedded, the interactions between loading waves and unloading waves. For the split Hopkinson pressure bar (SHPB) technique, the assumption of one-dimensional stress wave propagation and the assumption of stress uniformity along the specimen should be satisfied. When the larger diameter bars are employed, the wave dispersion effects should be considered, including the high frequency oscillations, non-uniform stress distribution across the bar section, increase of rise time, and amplitude attenuation. The stress uniformity along the specimen is influenced by the reflection times in specimen, the wave impedance ratio of the specimen and the bar, and the waveform.

Numerical Simulations on Buckling Failure of Preloaded Cylindrical Shell Irradiated by High Power Laser Beam

WANG Ji, WANG Xiao-jun, WANG Feng, ZHAO Kai

2004, 13(3): 236-241.

[Abstract](95)

Abstract:
With finite-element software ANSYS 7.0 and simple thermal-mechanical coupling constitutive relations,the buckling failure of preloaded cylindrical shell irradiated by high power laser beam was studied by numerical simulations. The buckling mode and buckling critical loading were analysed for different preloading conditions. The influence of laser intensity, beam irradiation time, preloading conditions and geometric parameters of cylindrical shell on the buckling mode were discussed. The numerical results show that: ① the buckling deformation of the cylindrical shell was concentrated in the area of laser spot and the radial buckling was the main buckling mode, ② a linear relationship between the buckling eigenvalue and the maximum temperature at the center of laser spot was approached, ③ the buckling failure of cylindrical shell was attributed to the coupling effect of the material softening and the radial deformation in the laser spot, and hence to raise the stiffness of the material would enhance the ability for anti-irradiation of structure substantially.

Ultrasonic Evaluation of the Impact Damage of Polymer Bonded Explosives

CHEN Peng-wan, DAI Kai-da, HUANG Feng-lei, DING Yan-sheng

2004, 13(3): 242-246.

[Abstract](183) [PDF 190KB](2)

Abstract:
The damage properties of polymer bonded explosives under dynamic loading were studied by using ultrasonic evaluation. Explosive samples were damaged by a low-velocity gas gun at different impact velocities. Ultrasonic examination was carried out with a pulse through-transmission method. Spectra analyses were carried out by using fast Fourier transform. Characteristic ultrasonic parameters, including ultrasonic velocities, attenuation coefficients, spectra area and master frequency, were obtained. The correlation between the impact damage and ultrasonic parameters was analyzed. A damage coefficient D was defined by considering a combination of ultrasonic velocity and amplitude. The results show that ultrasonic parameters can be used to quantitatively assess the damage extent in impacted plastic bonded explosives.

Stress Wave Dispersion in Large-Diameter SHPB and Its Manifold Manifestations

WANG Yong-gang, WANG Li-li

2004, 13(3): 247-253.

[Abstract](187) [PDF 251KB](11)

Abstract:
The wave dispersion due to the lateral inertia in the split Hopkinson pressure bar(SHPB) with large-(diameter) bar is numerically analyzed by means of the LS-DYNA3D code. The results show that, ① the stress distribution across the bar section is non-uniform along the radius direction and such non-uniformity depends on the material Poisson ratio and propagation distance; ② with increasing the bar diameter, the high frequency oscillations are notably enhanced and the rise time of wave front becomes longer, meanwhile the amplitude of the stress wave attenuates; ③ with decreasing the rise time of wave front, the wave dispersion markedly enhanced, particularly in the large diameter bar. All of those effects should not be neglected in order to obtain accurate results by the SHPB test.

Experimental Study on Common and Steel Fiber Reinforced Concrete Under Dynamic Tensile Stress

DONG Xin-long, CHEN Jiang-ying, GAO Pei-zheng, QI Zhen-lin, WANG Yong-zhong, WANG Yong-gang, WANG Li-li

2004, 13(3): 254-259.

[Abstract](153) [PDF 233KB](3)

Abstract:
Split Hopkinson technique has been developed to test the strength of common concrete and steel fiber reinforced concrete under dynamic tensile stress. Two types of test methods are considered, the splitting tensile test and a modified spalling test in which a specimen is loaded under uniaxial stress. The result shows that the dynamic strength enhancement of concrete is remarkable by using the reinforcing fiber. But for the common concrete, the base of compressive strength seems to show little effect on the tensile strength under dynamic loading. The experimental results also show that the resistance to tensile fracture of the steel fiber reinforced concrete for C100-mix is higher than those of C40-mix.

Studies on the Coefficient of Friction During the Oblique Impact of a Hard Sphere Against a Ductile Solid Target

CHEN Da-nian, Al-Hassani S T S, YU Yu-ying, CHEN Jian-ping, WANG Huan-ran, YIN Zhi-hua

2004, 13(3): 260-265.

[Abstract](130)

Abstract:
Following the original approach of Bowden and Tabor and introducing state variables, an effective friction coefficient μ_efor solid particle erosion is defined as a combination of shearing term and ploughing term. In the case of continuous sliding, based on considering the interaction between asperities under certain condition, it is indicated that during the oblique impact of a hardened steel sphere against a mild steel target, a possible value of μ_eis 0.05, which was chosen in all of the calculations by Hutchings for consistency with both experiments and calculations. In the case of continuous ploughing, it is shown that the value of μ_eis a function of the impact process and the initial impact angle and is greater than 0.05 on an average for Hutchings' experiments. It is suggested that the variation of sliding, rolling and ploughing state at each instant in the impact process makes “the coefficient of friction” equal to 0.05 for Hutchings' experiments, and in general, makes the effective friction coefficient during particle impact on metal far less than the friction coefficient during simple continuous sliding on an average.

SPH Simulation of Hypervelocity Impacts

LI Jin-zhu, ZHANG Qing-ming, LONG Ren-rong

2004, 13(3): 266-269.

[Abstract](106)

Abstract:
The smooth particle hydrodynamics (SPH) method is a very important tool to resolve hypervelocity problems. The basic principle of SPH method and how to generate a proper SPH mesh is described. The results of SPH simulations of hypervelocity impacts on thin or thick aluminum plates, performed by using the LS-DYNA 3D computer code, are also reported. The forming process and composition of the debris clouds simulated are identical with the experiment results. It can be concluded that the simulation is reasonable and SPH method is an ideal method for hypervelocity impact simulation.

Study on the Dynamic Behavior of Tungsten Alloy at Strain Rates up to 5 000 s^-1

ZHANG Bao-ping, ZHENG Yu-liu, WU Hai-jun

2004, 13(3): 270-273.

[Abstract](129)

Abstract:
The dynamic stress-strain curves of 93% tungsten （W） alloy in the forged state at strain rates up to (5 000 s^-1) and in the temperature range from 223 K to 473 K were measured with the split Hopkinson pressure bar （SHPB） technique. Based on the above experimental data a dynamic constitutive equation considering the effects of strain rate, temperature and the special microstructure of such a kind of W-alloy was proposed. The numerical simulation for the experimental process with this constitutive equation was also carried out, the results show that the constitutive relationship constructed in this paper is very satisfactory for representing the dynamic responsive behavior of material.

Experimental Study of Hypervelocity Impact on Multi-Shock Structure

ZHANG Qing-ming, CHEN Yan-hai, HUANG Feng-lei

2004, 13(3): 274-279.

[Abstract](186)

Abstract:
Hypervelocity impact tests on multi-shock shields are carried out in order to develop space structures (against) space debris impacts. Sheets of LY12 aluminum were used as bumpers. The total thickness of shield structure, which consists of several sheets with various thickness, is 3.0 mm or 2.0 mm. Results of the tests show that the type 0.5 mm+0.5 mm+0.5 mm+0.5 mm is a better choice of spacecraft shield structure.

Experimental Study and Numerical Simulation of Hypervelocity Projectile Impact on Double-Wall Structure

CHEN Yan-hai, ZHANG Qing-ming, HUANG Feng-lei

2004, 13(3): 280-284.

[Abstract](117)

Abstract:
Tests of hypervelocity projectile impact on double-wall structure were performed with the front wall ranging from 0.5 mm to 2.0 mm thick and different impact velocities. Smooth particle hydrodynamics (SPH) code in LS-DYNA was employed for the simulation of hypervelocity impact on the double-wall structure. By using elementary shock wave theory, the experimental results above are analyzed. The analysis can provide an explanation for the penetration mechanism of hypervelocity projectile impact on double-wall structure about the effect of front wall thickness and impact velocity.

Model for Long-Rod Penetration into Semi-Infinite Targets

ZHANG Lian-sheng, HUANG Feng-lei

2004, 13(3): 285-289.

[Abstract](233) [PDF 269KB](9)

Abstract:
Based on the equation of momentum conservation, an improved equation for the quisi-steady penetration of a long rod into homogeneous semi-infinite targets has been derived, assuming that the flow interface between the rod material and the target material is hemispherical and that the normal pressure on the interface is defined by the dynamic spherical cavity expansion. The equation has a form similar to the Tate equation, and the parameters in this equation have definite physical senses and practical values.

Experimental Investigation and Numerical Simulation of Air-Breathing Mode for Laser Propulsion

GONG Ping, TANG Zhi-ping

2004, 13(3): 290-294.

[Abstract](185)

Abstract:
Laser propulsion is a new concept technique of propulsion and will have important application in future space technology. There are two main driving types: the air-breathing mode and the rocket ablation mode. Vertical flight experiments have been carried out with a simple paraboloid type lightcraft in the air-breathing mode by TEA-CO₂laser. In simulation a new model is used for LSD/LSC wave, the result shows that the momentum coupling coefficient increases with the increase of the pulse energy.

Dynamic Response of Stiffened Plates with Holes Subjected to Shock Waves and Fragments

LIU Yan, ZHANG Qing-ming, HUANG Feng-lei

2004, 13(3): 295-298.

[Abstract](135)

Abstract:
The power field of shock waves and fragments is analyzed and set up, and the damage modes of stiffened plates are put forward. According to the structural characters of the stiffened plates investigated and the properties of the shock waves and fragments, the experiments on the shock waves acting on the stiffened plates (penetrated and non-penetrated by fragments) are mainly conducted. The dynamic response rules of stiffened plates with holes under shock waves and fragments loading are obtained. The results show that the penetration of fragments into stiffened plates hardly affects their deformation produced by shock waves.

Research on Protection Capability of Flying-Whip Multifunctional Explosive Reactive Armor

ZHANG Hu-sheng, ZHANG Bao-ping, ZHANG Qing-ming, LIU Chang-lin

2004, 13(3): 299-304.

[Abstract](87)

Abstract:
The experimental research on protection capability of the flying-whip multifunctional explosive reactive armor (ERA) was performed, in which the comparison experiment was made on the damage effect of the flying-whip's geometrical figuration, material property and driven velocity on the long-rod armor-piercing-projectile. The moving velocity of the flying-whip driven by different explosives and the pressure attenuation law of shock wave travelling in the back plate were measured respectively with the electric probe method and the manganin piezoresistive gauge technique. The following conclusions based on a great quantity of experimental data were drawn: compared with the sandwich ERA the flying-whip multifunctional ERA has very good protection function against the long-rod armor-piercing-projectile, the shaped charge warhead and the anti-armor tandem warhead. In addition, the composite plate made of the armor-steel and rubber plate can lessen the vibration and shock of the main armor caused by the explosion of the charge.

Experimental Investigation of Two-Dimensional Shock Initiation Process of Cast Composition B

HUANG Feng-lei, HU Xiang-yu

2004, 13(3): 305-307.

[Abstract](172) [PDF 247KB](3)

Abstract:
A two-streak high-speed photography measuring system is designed, which can successfully record the reactive shock front and the reaction profile within the run distance of shock initiation under two-dimensional shock initiation. The strong reaction delay time and the shape of reaction shock front are determined in the cast composition B (RDX/TNT/60/40). A low level reaction zone has been found and analyzed.

Numerical Simulation of Rigid Projectile's Normally Penetrating into Granite Targets

ZHANG De-zhi, ZHANG Xiang-rong, LIN Jun-de, TANG Run-di

2004, 13(3): 308-312.

[Abstract](213)

Abstract:
The process of penetrating into granite was simulated by using program LS-DYNA3D. The granite was represented by the isotropic elastic-plastic model with failure criterion and the projectile was modeled by rigid model. The depth of penetration from simulations is identical with experiments. Penetration deceleration vs striking velocity was acquired at the same time, which can assist in the design of penetration weapons with payload and fuse. Through numerical simulation, that material model is considered with straightforward physical meaning, (a few) parameters which can be determined easily are more practical for engineering calculation along with (experiments).

Dynamic Properties of Fiber Reinforced Cement Mortar

TANG Zhi-ping, XU Song-lin, HU Xiao-jun, LIAO Xiang-li, CAI Jian

2004, 13(3): 313-316.

[Abstract](129)

Abstract:
Based on the shear wave tracing(SWT) technique proposed by Tang Z P, particle velocity gauge and the dual internal measurement for pressure and shear waves (IMPS) system are applied to investigate the responses of fiber reinforced cement subjected to impact loading. Series of experiments are conducted. The results show that there exist four critical points, A, B, C, D, in p-V Hugoniot curves. They correspond to the Hugoniot elastic limit (HEL) of the material, the critical point for shear strength limit and transition from damage state to failure state, void collapse, and solid compression, respectively. The critical point B is difficult to be aware of and never reported. However, it can be clearly disclosed with SWT method. Based on the analyses of shear strength, it can be concluded that the transversal wave, especially the unloading transversal wave, is especially important for the dynamic damage investigation of brittle materials.

Experimental Study on Penetration and Perforation of Laminated Kevlar

WANG Yuan-bo, WANG Xiao-jun, HU Xiu-zhang, SUN Yu-xin

2004, 13(3): 317-323.

[Abstract](212)

Abstract:
The penetration behavior and perforation characteristics of Kevlar/Epoxy laminates with various thickness in quasi-static and ballistic perforation penetrated by steel projectiles with different noses are investigated. Quasi-static tests are conducted on MTS810 testing system. The results indicate that global deformation is the major mechanism of energy absorption and woven laminates exhibit larger energy dissipation than that of angle-plied laminates. Therefore, the woven laminates have better quasi-static penetration resistance. Ballistic tests with velocity of 200-700 m/s are executed by using a powder gun with 7.62 mm barrel. Comparing ballistic experimental results with those under quasi-static condition, both the perforation performance and the failure modes are related closely to the speed of penetrator. Quite different from quasi-static tests, ballistic tests indicate that thick angle-plied laminate targets are even better than woven laminates in resisting ballistic impact. It is observed that the damage zone of the laminate is localized highly with the increasing of the impact velocity and correspondingly, the failure modes are more manifold. The shape of projectile noses affects the impact resistance of laminated Kevlar significantly in the range of velocity around the ballistic limit.

Method of Testing the Flyer Sensitivity of Explosives

WANG Gui-ji, ZHAO Jian-heng

2004, 13(3): 324-327.

[Abstract](167)

Abstract:
By means of Mylar flyer shock explosives driven by electric gun, the method of testing the flyer initiation sensitivity of explosives is studied, and some experiments are done. The experimental results show that the test method established is correct, which is very important and instructive to study and evaluate the safety and reliability of explosives. For the moment, the test should be researched and discussed further.

Calculation of the Distribution Rule of Equivalent Strain Rate near Explosive Welding Interface

LI Xiao-jie, YAN Hong-hao, LI Rui-yong, WANG Jin-xiang

2004, 13(3): 328-330.

[Abstract](194)

Abstract:
The objectives of this study were to analyze the distribution of equivalent strain rate near the stagnation point and probe into the effects of colliding angle on strain rate. An ideal fluid model of symmetrically colliding was used to research them. Calculations showed the equivalent strain rate and the colliding half angle are closely related to each other with the material geometrical size and explosive velocity selected, the equivalent strain has large gradient within several jet thicknesses near the stagnation point, the maximal strain points are lined up along a beeline, but a curve near the stagnation point. With different colliding angles, they can be fitted by using exponential curve. That is, the exponential curve can be regarded as the token curve in explosive welding.

Experimental Study of Effects of Tail Wings on Submunition Aerodynamic Characteristics

WANG Hai-fu, LI Xiang-rong

2004, 13(3): 331-335.

[Abstract](109)

Abstract:
Aimed at the needs of deceleration of submunitions dispensed from the ballistic missile, wind tunnel tests were performed on the submunitions with different tail wing sizes at the Mach number range from 0.7 to 3.0 and the angle of attack range from 0° to 14°. Experimental data about the variance of aerodynamic coefficients with the Mach number and angle of attack were obtained systemically. The effects of the tail wing sizes on the drag coefficients and the center of pressure coefficients were discussed. Analyzed results show the arc tail wings designed are beneficial to both the deceleration effect and static stability. These results are significant to the tail wing design and its applications to the submunitions deceleration.

Study on Property of Desensitized Explosive Film

LI Guo-xin, WANG Xiao-li, JIAO Qing-jie, LIU Shu-zhen

2004, 13(3): 336-340.

[Abstract](118)

Abstract:
The mechanical sensitivity, the critical thickness of detonation wave propagation and detonation velocity of desensitized PETN film were studied by experiments. The relationship between the mass of desensitizer paraffin wax and the friction sensitivity of desensitized PETN film was tested. According to the microstructure of film, the function of desensitizer was explained. It was proved that the explosive film could make explosive element micromation and kept its inherence properties by the result of testing the propagating critical dimension of the desensitized PETN film detonation wave. The explosive velocity of confined desensitized PETN film was tested by the multiplex optical fibre.

Study on the Detonation Danger of Solid Propellants

HUANG Feng-lei, ZHANG Bao-ping

2004, 13(3): 341-345.

[Abstract](186)

Abstract:
A measurement system to study shock initiation behavior of solid propellants was established experimentally. By using this system, the study on shock initiation to the recovered solid propellants with micro damage was performed, especially on the deflagration to denonation transition (DDT) process of solid propellants under both the strong and weak conditions of restriction. The experimental results show that there is a fully compression region in DDT process.

Experimental and Numerical Investigations of the Dynamic Response of Ring-Stiffened Cylinder Subjected to Underwater Explosion

ZHU Xi, ZHANG Zhen-hua, FENG Gang, LI Yu-jie, LIU Jian-hu, HE Bin

2004, 13(3): 346-350.

[Abstract](103)

Abstract:
Experimental and numerical investigations were carried out on the free-free end ring-stiffened cylinder subjected to underwater explosion loading. Numerical analysis was carried out by using the MSC.DYTRAN finite element code and the results were compared with experiment results. General coupling was used to simulate the interaction between fluid and structure. The strain rate effect, geometric nonlinearity and initial abnormity in shape were considered. The effective plastic stress and the strain of shell between ribs on different locations were compared and damage mechanism were analyzed.

2004 Vol. 13, No. 3