Determination of stress intensity factors in half-plane containing several moving cracks

The dynamic stress intensity factors in a half-plane weakened by several finite moving cracks are investigated by employing the Fourier complex transformation. Stress analysis is performed in a half-plane containing a single dislocation and without dislocation. An exact solution in a closed form to the stress fields and displacement is ob- tained. The Galilean transformation is used to transform between coordinates connected to the cracks. The stress components are of the Cauchy singular kind at the location of dislocation and the point of application of the the influence of crack length and crack running force. Numerical examples demonstrate velocity on the stress intensity factor.     

Conservation of mass for a particle moving with high velocity

I.IntroductionItiswell-knownthatthe"lassofaparticleoranoh.jectisstrictlytoobservethelawofcollservatiollof1llasswbedmovewitlllowvelocity.Butathighvelocity,particularlynearthevelocityoflight,themass(restmass)oftheparticle,accordingtothetheoryofspecialrelativity,increasesrapidlywithitsincrementofthevelocity.Inotherwords,themassisnotinconservatioll\vitllitsvelocity.However.withtheoriesandresearchmethodsbringingforthflewideasandcomplement,andthehumanthoughtdevelopingconstantlywhatthemassisnotconse…     

A closed crack tip model for interface cracks inthermopiezoelectric materials

The interface crack problem of a bimaterial thermopiezoelectric solid was treated byapplying the extended version of Strohs formalism and singular integral equation approach. Theinterface crack considered is subjected to combined thermal, mechanical and electric loads.Under the applied loading, the interface crack is assumed to be partially opened. Formulation ofthe problem results in a set of singular integral equations which are solved numerically. Thestudy shows that the contact zone is extremely small in comparison with the crack length. Basedon the formulation, some physically meaningful quantities of interest such as stress intensityfactors and size of contact zone for a particular material group are analyzed.     

In-fiber doppler velocimeter for velocity measurements of moving surfaces

An optical fiber Doppler velocimeter using a 3×3 coupler with a large path imbalance Mach-Zehnder interferometer for passive demodulation of Doppler wavelength shifts is demonstrated. The particle velocity on the free surface end of the Hopkinson bar is directly obtained using the all-fiber Doppler velocimeter. These data show good agreement with the particle velocity predicted using strain gage data combined with one-dimensional stress wave propagation theory.     

P- and S-wave velocity and anisotropy in saturated rocks with aligned cracks

Rocks in the Earth’s crust contain cracks in variable amounts and on variable scales. Depending on the in situ stress level, cracks are always distributed in a common normal direction (so-called aligned cracks). In this study, we focus on the velocity and anisotropy of perfectly aligned cracks (parallel cracks). The theoretical model is introduced to calculate the elastic properties of cracked rocks based on solid matrix properties, crack parameters, and fluid properties. The theoretical results using high- and low-frequency limits and intermediate frequency for velocities and anisotropy are compared with experimental data measured in the laboratory, where rocks are saturated with water and air. We used four synthetic rocks with different crack densities (0.0, 0.0243, 0.0486, and 0.0729) in this study. The comparison shows that the results for high, low, and intermediate frequencies provide good predictions of the shear velocities and anisotropy for saturated cracked rocks. Quantitatively, the results of the P-wave velocity and anisotropy for intermediate frequency fit the measured data better than that for the high- and low-frequency limits. Compared with water saturation, dry cracks with high compliance have a strong influence on the total compliance of rock. The water saturation results in high P-velocities and a low P-anisotropy, while dry cracks induce much lower P-velocities and a high P-anisotropy. The shear wave anisotropy is fluid-independent, theoretical results fit the measured data well in both water saturation and the dry case.     

Dynamics of impacts with a table moving with piecewise constant velocity

Dynamics of a ball moving in gravitational field and colliding with a moving table is considered. The motion of the limiter is assumed as periodic with piecewise constant velocity. It is assumed that the table moves up with a constant velocity and then goes down with another constant velocity. The Poincaré map describing evolution from an impact to the next impact is derived. Several classes of solutions are computed in analytical form.     

Uniformly moving screw dislocation interacting with interface cracks in anisotropic bimaterials

This paper attempts to investigate the problem for the interaction between a uniformly subsonic moving screw dislocation and interface cracks in two dissimilar anisotropic materials. Using Riemann–Schwarz’s symmetry principle integrated with the analysis singularity of complex functions, we present the general elastic solutions of this problem and the closed form solutions for interface containing one and two cracks. The expressions of stress intensity factors at the crack tips and image force acting on moving dislocation are derived explicitly. The results show that the stress intensity factors at the crack tips decrease with increasing velocity of dislocation, and larger dislocation velocity leads to the equilibrium position of dislocation leaving from crack tips. The presented solutions contain previously known results as the special cases.     

Measurements of velocity fields in finite cylinder arrays with and without tip clearance

The velocity field in a finite cylinder array was investigated experimentally in a water towing tank and an acoustic Doppler velocimeter (ADV). The experimental system consisted of a staggered cylinder array having 14 rows to permit streamwise evolution of the flow. The boundaries were manipulated to create several global flow configurations. Three basic configurations were studied: a globally unidirectional flow, a flow with partial lateral blockage at the inlet and outlet planes, and a flow with the top boundary separated from the cylinders creating a tip clearance. The three components of the velocity vector were measured at various points within the model. Time-averaged results are presented for the different flow configurations. The results provide insight into the development of the flow field in cases of a finite array with complex geometry and boundary effects.     

球形装药动态爆炸冲击波超压场计算模型

为获得球形装药动态爆炸冲击波超压场计算模型,对静态爆炸冲击波超压Baker计算公式加入修正因子进行修正,并建立了构造包含装药运动速度、对比距离和方位角的修正因子函数的方法。为获得修正因子的函数表达式,采用高精度显式欧拉流体动力学软件SPEED针对具有典型运动速度的球形装药空中爆炸过程进行了数值模拟,得到了沿装药不同对比距离和方位角处的动态爆炸冲击波超压峰值。在对数值模拟结果处理的基础上,经过数据拟合获得了动态爆炸冲击波超压场计算模型。校验结果表明,该模型能较准确描述动态爆炸冲击波超压分布,具有普适性。     

Heat generation at the tip of a moving crack

The high energy concentration at the tip of a moving crack causes irreversible deformations and produces heat as a consequence. The resulting temperatures were calculated by consideration of the crack tip as a moving heat-source of rectangular shape. In brittle materials with very small plastic zones and high crack velocities, these temperatures are predicted to be higher than 1000 K. For the experimental verification of these calculations, a very sensitive radiation thermometer was developed. It registers the intensity of the radiation at four wavelengths. By comparison of these intensities with that of black body radiation, the temperature was determined as 3200 K for glass and 4700 K for quartz.     

Effect of T-stress on branch angle of moving cracks

The effects of the T-stress on Yoffe crack propagation are analyzed. Using a maximum k_I fracture criterion near the kink of a moving crack tip, a branch angle is determined via asymptotic crack-tip field containing two fracture parameters related to singular and constant terms. Results indicate that crack speeds decrease the T-stress. The crack-tip field and the branch angle depend on the T-stress, especially for higher crack velocities. The critical speed for crack bifurcation is independent of remote transverse loading if neglecting the T-stress. Otherwise, the crack branch speed is reduced or raised, depending on positive or negative transverse loading, respectively.     

Interaction between cracks and effect of microcrack zone on main crack tip

Mechanism interaction between cracks with different orientation angles is analyzed based on the principle of superposition and a flattening method. It is found that the maximum interaction effect does not occur when the microcrack is along the direction parallel or perpendicular to the principal tensile stress, which is different from the conclusion drawn by Ortiz （1987）. The mechanism of microcrack generation and the effect of the microcrack zone on the main crack tip are studied. It is concluded that the microcrack zone has effect on the main crack tip, which increases with the increase of microcrack density and length.     

Magnetoelectroelastic media containing a row of moving shear cracks

The elastic, electric and magnetic fields created within transversely isotropic magnetoelectroelastic media around an infinite row of uniformly-moving, collinear, antiplane shear cracks are studied, using an extension of the powerful method of dislocation layers. This analysis additionally provides the solutions for a finite magnetoelectroelastic plate containing a single crack and a plate with an edge crack.     

Analysis of a model for void growth and coalescence ahead of a moving crack tip

One mechanism for the slow, steady motion of a crack tip in plane strain is void growth and coalescence. A model of this process is formulated by isolating a geometrical cell containing a single void and extending the cell in uniaxial strain. A rigid-plastic material is assumed and the velocity field is analysed by means of a finite element procedure. From the results of the analysis, the dissipation of energy in the creation of crack surfaces is estimated as a function of the original void-size and the volume fraction of voids. Further, the shallow shape of dimples found experimentally is verified by the results of the analysis. A check on the accuracy is obtained by application of the numerical method to cases where analytical solutions are known.     

Effects of tip perturbation and wing locations on rolling oscillation induced by forebody vortices

The wing rock motion is frequently suffered by a wing-body configuration with low swept wing at high angle of attack. It is found from our experimental study that the tip perturbation and wing longitudinal locations affect significantly the wing rock motion of a wing-body. The natural tip perturbation would make the wing rock motion of a nondeterministic nature and an artificial mini-tip perturbation would make the wing rock motion deterministic. The artificial tip perturbation would, as its circumferential location is varied, generate three different types of motion patterns： （1） limit cycle oscillation, （2） irregular oscillation, （3） equilibrium state with tiny oscillation. The amplitude of rolling oscillation corresponding to the limit cycle oscillatory motion pattern is decreased with the wing location shifting downstream along the body axis.