Narrow Results

A 12-mm caliber gas gun was used to launch a high-strength steel cylindrical projectile to impact a polyurea-coated B₄C ceramic target plate to investigate the effects of the polyurea coating position and the thickness of the polyurea coating on the protective ability of the ceramic composite target plate, reveal the penetration resistance mechanism of the polyurea-coated B₄C ceramic plate and protection mechanism of polyurea coating. The result showed that the ceramic cone top diameter of the composite target plate coated with polyurea on the front side was 52.4–60.5% lower than that of the uncoated ceramic target plate, and the ceramic cone bottom diameter was 0.2–4% lower than that of the uncoated ceramic target plate. For the same area density, the ceramic target plate coated with polyurea on both sides had the largest percentage of mass remaining and the smallest mass of dislodged ceramic fragments. In addition, composite target plates coated with polyurea on the back side and both sides significantly reduced the head velocity of the post-target debris cloud. The polyurea coating on the front of the target plate utilizes the strain rate effect to dissipate the kinetic energy of the projectile and inhibit the spallation of ceramic fragments toward the front of the target plate. The polyurea coating on the back of the target plate utilizes bulge deformation to dissipate the kinetic energy of the projectile and intercepts the scattering of the post-target debris cloud. The polyurea coating effectively reduces ceramic target plate damage and improves composite target plate integrity level and resistance penetration.

The tail drive shaft is easily hit by the ground antiaircraft machine gun in low-altitude helicopter combat, resulting in projectile penetration damage and directly jeopardizing the helicopter’s safety during flight. The projectile penetration damage will cause elastic–plastic deformation and failure damage to the tail drive shaft, and excite high-frequency stress waves suitable for projectile impact monitoring. To examine the effects of different impact parameters on the stress wave characteristics and damage from the tail drive shaft during projectile impact, the LS-DYNA is used to establish a dynamic model of projectile impact on the tail drive shaft. The passive monitoring experiment of projectile impact on the tail drive shaft is carried out based on the PZT piezoelectric sensor. It is concluded that both methods can obtain reliable stress wave signals and damage conditions through the comparison of experimental and simulation results. Then, the wavelet transform and short-time Fourier transform three-dimensional time–frequency are used to select the best frequency band for extracting the stress wave eigenvalues, along with the statistics and analysis of the stress wave characteristic values and damage degree under different impact parameters. The results show that the impact stress waveforms and high-frequency components are significantly different with the change in projectile impact parameters. There are significant differences among different impact parameters on the first trough amplitude, the two peak time intervals, the kurtosis factor, and the bullet hole damage generation laws. The functional relationship between the projectile impact parameters and the characteristic parameters of the stress wave, as well as the damage degree, are established. The research conclusion can provide theoretical reference and data support for projectile impact monitoring and damage assessment of the helicopter tail drive shaft.

The total stress in a structure changes dynamically by the stress multiplication phenomenon in the case of a fixed boundary condition. Therefore, it is important to evaluate the reflected stress waves under an impact loading in structures. In many problems, the method of the classical position-time diagram of wave fronts is effective to analyze the maximum stress in the structure under the impact loading and for a simple arrangement of members along the axis of the one-dimensional structure. In this study, stress analyses in one-dimensional structures based on the position-time diagram of stress wave fronts were realized as a computational method with and without attenuation of stress wave. This method was applied to the problem of stress analyses in a bone under an impact loading, and the stress states were compared with the experimental results.

Drillstring is the most important tool in petroleum drilling engineering. Alternating stress has been found to be responsible for the premature failure of drillstring. Propagation of stress wave, induced by collision between tool-joints of drillstring and borehole wall, is studied in this paper. The condition that all the tool-joints of drill pipes (DPs) strike borehole wall at the same time has been considered. Because of symmetry, the middle cross section of the DP is simplified as fixed end, and mechanical model is established as the beam with both ends fixed. Propagation of lateral displacement wave and stress wave in the DP is investigated by means of Eigen-frequency method and the Finite Element Analysis software ANSYS. The theoretic results coincide with those obtained from numerical modeling very well and also explain the drillstring accidents in gas fields.

This paper investigates the prebuckling dynamics of transversely isotropic thin cylinder shells in the context of propagation and reflection of axial stress waves. By constructing the Hamiltonian system of the governing equation, the symplectic eigenvalues and eigenfunctions are obtained directly and rationally without the need for any trial shape functions, such as the classical semi-inverse method. The critical loads and buckling models are reduced to the problem of eigenvalues and eigensolutions, in which zero-eigenvalue solutions and nonzero-eigenvalue solutions correspond to axisymmetric buckling and nonaxisymmetric buckling, respectively. Numerical results reveal that energy is concentrated at the unconstrained free ends of the shell and the buckling modes have bigger bell-mouthed shapes at these positions.

Within the framework of linear two-dimensional elastodynamics, stress wave intensity attenuators are studied under material and boundary condition discontinuities collectively. The influence of various parameters on the efficiency of stress wave attenuators is investigated thoroughly and a comprehensive understanding of the response is developed under dynamic loadings for a wide range of frequencies. In particular, the effect of in-plane and out-of-plane dimensions, incident wave frequencies (wavelength), rigidity of the host structure, and impedance mismatch between different layers have been examined. The dependence of stress wave attenuator efficiency and robustness are found to be a complex function of all relevant parameters, and performance is observed to vary significantly for various combinations. To illustrate the significance of combined effects of various parameters on the potential efficiency of the stress wave intensity attenuators, an optimization problem is solved. An optimal material set-up of a 12-layered structure, subjected to transient loadings with varying durations and wide range of frequency contents, is presented. A coupled genetic algorithm-finite element methodology is developed specifically for the optimal design of layered structures. This methodology is highly suitable for investigating the solution space that is too large to be explored by an exhaustive parametric study. The results of the optimal designs evidently show that the efficiency of the stress wave attenuators depends significantly on the duration of transient loading, and high efficiency can be attained for short durations.

Reducing the damage of structures under stress wave is one of the popular and challenging issues in the field of protection engineering. In this study, the effect of plastic modified concrete as an attenuation layer on stress wave was investigated through large-scale explosion tests. The volume ratios of plastic particles used as replacement material were 2%, 4%, and 8%. Two explosions were used in each group to analyze the change of pressure under the plastic modified concrete as the attenuation layer. Experimental results showed that as low-impedance material, plastic particles can reduce the stress wave transmitted through the plastic modified concrete, and the elastic–plastic deformation of plastic increases energy consumption. Plastic modified concrete is more difficult to be compacted than loess attenuation layer. Thus, the clipping performance of the plastic modified concrete under the second explosion is better than that of the loess attenuation layer. The effect of plastic modified concrete on shortening the action time of stress wave is not obvious. Plastic modified concrete reduces damage by weakening the peak value of stress wave. The effects of explosive quantity, detonation distance, and plastic particle content on stress wave were studied through experiments and simulation. The propagation process of waves in the plastic modified concrete was studied via numerical simulation, and a fitting formula of wave cutting performance of plastic modified concrete was presented.

According to the scientific research needs of deep multi-field coupling true triaxial compression testing machines, it is necessary to find a suitable boundary material to be placed between the rock and the transmission bar to achieve the effects of wave elimination and energy absorption. This enables the conditions of the test requirements to be met under the interaction of true triaxial and strong disturbance. Therefore, the improved split Hopkinson pressure bar (SHPB) device was used to carry out a series of experiments on the wave elimination and energy absorption characteristics of different boundary materials, and to study the energy evolution characteristics and stress wave propagation laws of different materials under static–dynamic coupling loading. The results show that under single impact loading, owing to the difference in wave impedance between the material and the rock rod, the porous material will not only reflect the tensile wave that has a greater impact on the rock rod, but also its weakening effect on the reflection stress is weaker than that of the wave-absorbing metal plate. In terms of overall energy conversion, the energy absorption rate of the porous material PM-1 is superior, and the conversion rate of the reflection and transmission energies is also greatly reduced. Under the impact of cyclic loading, the reflection stress of the wave-absorbing metal plate increased slightly with an increase in the number of impacts. However, with the increase of impact times, the reflection peak stress of porous material PM-1 decreases at first and then increases, and the reflected wave changes from tensile wave to compression wave, which is not as good as that of wave dissipation metal plate in terms of repeatability and fatigue resistance.

In this research, thermoelastic effect was investigated for biostimulation without damage on the biological medium using laser. Thermoelastic effect was generated and mechanical stress was induced by laser irradiation on the collagen, the main protein in the human body, under various conditions with short pulsed laser. The threshold laser energy to induce stress wave in each medium thickness was examined with a piezo sensor. Based on the test, the stimulation strength can be controlled through the adjustment of medium thickness, laser energy and beam diameter. The result implies that precise stimulation with various strengths of stress waves can be generated at the target depth without direct contact with the biological medium. This research can be used valuably in various fields such as contactless biostimulation, low power laser treatment and laser haptic applications.

In this paper, the object of our study is two coupled partial differential equation which have a rich physical background. With a reasonable change to the initial value, we use central difference method and compact finite difference method to get the numerical results of the governing equations. In the analysis of the latter, we use the Hermitian and skew-Hermitian splitting (HSS) method to solve a large sparse non-Hermitian positive definite system of linear equations. In this way, the computational efficiency can be improved and the convergence is guaranteed which is proved by Bai [Bai et al. [2003] “Hermitian and skew-Hermitian splitting methods for non-Hermitian positive definite linear systems,” SIAM J. Matrix Anal. Appl.24, 603–626] and Chen [Chen et al. [2014] “Convergence analysis of the modified Newton-HSS method under the Hölder continuous condition,” J. Comput. Appl. Math.264, 115–130]. And we also pay attention to the effect of initial conditions, on which we add a small perturbation to observe its influence by comparing numerical results.

A symplectic system is developed for dynamic buckling of cylindrical shells subjected to the combined action of axial impact load, torsion and pressure. By introducing the dual variables, higher-order stability governing equations are transformed into the lower-order Hamiltonian canonical equations. Critical loads and buckling modes are converted to solving for the symplectic eigenvalues and eigensolutions, respectively. Analytical solutions are presented under various combinations of the in-plane and transverse boundary conditions. The results indicated that in-plane boundary conditions have a significant influence on this problem, especially for the simply supported shells. For the shell with a free impact end, buckling loads should become much lower than others. And the corresponding buckling modes appear as a "bell" shape at the free end. In addition, it is much easier to lose stability for the external pressurized shell. The effect of the shell thickness on buckling results is also discussed in detail.

Considering the effects of first-order shear deformation theory (FSDT) and stress wave, the dynamic buckling governing equations of cylindrical shells under axial step load are derived. Based on the Ritz method and Variable Separation method, the analytical solution of the critical load on the dynamic buckling can be obtained. The influences of first-order shear deformation effect, boundary conditions, the number of circumferential waves, etc. on dynamic buckling load are discussed by using MATLAB software and the results show that dynamic buckling of cylindrical shells occuresmore easily when considering shear effect.

Considering the effects of shear deformation and stress wave, the dynamic buckling governing equations of rectangular plates under axial step load are established. Based on the Rayleigh-Ritz method, the expression of the critical load is got. The relation curve between the critical load and critical length is described by using MATLAB software. In this paper, the influences of thickness, first-order shear deformation (FSD), and the number of modes are discussed.

Considering the effect of stress wave, the dynamic buckling governing equations and boundary conditions of composite cylindrical shells under axial step load are derived based on the Hamilton principle. The expression of radial displacement function along the circumferential direction is assumed since the cylindrical shell is closed. The solutions of the governing equations are obtained by the state-space technique. The determinant of the coefficient matrix must be equal to zero if the linear equations have a non-trivial solution. The relationship between the critical load and length and the influences of boundary conditions, modes, etc. on critical load are obtained by programming with MATLAB software before and after the reflection of stress wave.

The equation of pressure and stress wave coupling propagation in pipeline was derived and it reveals that the wave energy interact each other due to water hammer, generating serious destruction continuously. The nonlinear mechanism of energy transfer between different modes of the fluid was explained, and the analysis shows that the propagation process of forward and backward pressure wave is not a linear superposition, but a nonlinear one. Thus the transmitted energy increases dramatically, which leads to the amplified nonlinear damage effects. The water hammer can be monitored by the modal shape and the measured signal. The influence of different parameters on water hammer has been studied. Results show that pipe length, diameter and velocity have significant effects on water hammer. When a lower stiffness pipe (LSP) and a multi-band LSP are used to connect the pipeline, the water hammer is not eliminated. However, increasing pipe diameter, reducing the flow rate and the length are beneficial to control the damage of hammer.

Drillstring is the most important tool in petroleum drilling engineering. Alternating stress has been found to be responsible for the premature failure of drillstring. Propagation of stress wave, induced by collision between tool-joints of drillstring and borehole wall, is studied in this paper. The condition that all the tool-joints of drill pipes (DPs) strike borehole wall at the same time has been considered. Because of symmetry, the middle cross section of the DP is simplified as fixed end, and mechanical model is established as the beam with both ends fixed. Propagation of lateral displacement wave and stress wave in the DP is investigated by means of Eigen-frequency method and the Finite Element Analysis software ANSYS. The theoretic results coincide with those obtained from numerical modeling very well and also explain the drillstring accidents in gas fields.