Acoustoelastic Effects on Borehole Flexural Waves in Anisotropic Formations under Horizontal Terrestrial Stress Field

Applying the Stroh theory and based on the works of Hwu and Ting (1989), the complex function solution of stress and displacement fields around an open borehole in intrinsic anisotropic formation under horizontal terrestrial stress field is obtained. For cross-dipole flexural wave propagation along borehole axis, using the perturbation method, the acoustoelastic equation describing the relation between the alteration in phase velocity and terrestrial stress as well as formation intrinsic anisotropy is derived. At last, the numerical examples are provided for both the cases of fast and slow formation where the symmetry axis of a transversely isotropic (TI) formation makes an angle with the borehole axis. The phase velocity dispersion curves of borehole flexural wave and the corresponding velocity-stress coefficient are investigated. Computational results indicate that different from the stressed intrinsic isotropic formation situation, the variation in the phase velocity of flexural wave in stressed intrinsic anisotropic formation is dominated by two factors, one is the intrinsic formation anisotropy itself and the other is the stress-induced anisotropy. The former factor merely causes the borehole flexural wave split while the latter factor induces the dispersion curves intersection for two flexural waves polarized orthogonally. The combined effect of the two factors could strengthen or weaken the phenomenon of crossover for flexural wave dispersion curves. Thus, the dispersion curves of flexural waves may not intersect even under the unequal horizontal terrestrial stress field, whereas it is still possible to observe the crossover of the flexural wave dispersion curves under the equal horizontal terrestrial stress field. The polarized direction of the low-frequency fast flexural wave is no longer consistent with the direction of the maximum horizontal terrestrial stress all the time. Therefore, the crossover of the borehole flexural wave dispersion curves means that the terrestrial stress must exist. On the other hand, we can't exclude the possibility of the existence of terrestrial stress even if the flexural wave dispersion curves do not intersect. Based on the above researches, the method for terrestrial stress inversion from borehole flexural wave dispersion curves obtained by cross-dipole sonic logging in stressed intrinsic anisotropic formation is simply discussed.     

Diffraction of Internal Waves by a Submerged Circular Cylinder at Forward Speed in a Two-Layer Fluid

Diffraction of internal waves by a submerged body in a uniform current of a two-layer fluid is considered. The layers are infinitely deep, and the flows are two-dimensional. The linearized potential theory is used for the inviscid and incompressible fluid. The solution for the circular cylinder, which is either below or above the interface, is given in the form of rapidly converging series. This is achieved through the use of certain recursive relations. Numerical results are provided for the exciting forces, wave resistance and lift which may be useful in testing numerical methods used for the study of internal wave diffraction by a submerged body of arbitrary form.     

土中爆炸地冲击作用下埋地管线动应力的数值模拟

对于土中爆炸地冲击作用下埋地管线动应力如何计算及其是否安全,是一个值得研究的问题.利用LS-DYNA3D有限元程序,运用非线性动力学基本理论和算法,适当选取材料的本构关系,对半无限土介质中集团药包爆炸地冲击作用下埋地管线的动力响应问题,进行了数值模拟研究,得出了埋地管道的迎爆面上受到很大的轴向拉应力作用,且其为瞬态受力过程的结论.模拟计算结果与实验结果较为吻合,表明采用数值模拟方法研究土中爆炸地冲击对埋地管线的作用是可行的.     

Probabilistic Fracture Mechanics for Analysis of Longitudinal Cracks in Pipes Under Internal Pressure

Journal of Failure Analysis and Prevention - In this paper, we present a probabilistic fracture mechanics methodology to analyze elastic and elastic–plastic fracture of semi-elliptical...     

Solitary Waves of Maxwell's Equations in Nonlinear Anisotropic Media

Abstract

The (1 + 1)-D solitary wave solutions of Maxwell's equations in nonlinearity induced anisotropic media (in liquids such as carbon disulphide, and in crystals, etc.) are investigated. We find that there is no arbitrarily linearly polarized (in the x-y plane perpendicular to the propagation direction z) soliton solution from Maxwell's equations except that with linear polarization either in alignment with or orthogonal to the geometric axis of the light induced refractive index change. This contradicts the prediction of the vector nonlinear Schroedinger equation (an approximation of Maxwell's equations) which yields soliton solutions with an arbitrary linear polarization. However, Maxwell's equations are found to admit stable elliptically polarized solitary wave solutions which reduce to the stable circularly polarized solitary wave solutions of the vector nonlinear Schroedinger equation when the induced refractive index change approaches zero.     

Investigation of Stress and Temperature Effect on the Longitudinal Ultrasonic Waves in Polymers

This work aims at establishing the effect of stress and temperature on the velocity of ultrasonic longitudinal waves in typical engineering polymers, and evaluating the potential of ultrasonic stress measurement in the evaluation of residual stresses in polymer parts. In order to estimate the effect of material morphology, two amorphous and two semicrystalline polymers have been considered. A series of tests are implemented, to determine the acoustoelastic constants and temperature constant of materials, by using the designed transducer fixtures for in situ measurement of longitudinal wave velocity. As expected, the velocity changes linearly with stress and temperature, and the temperature effect is as important as the acoustoelastic effect. It shows that this kind of nondestructive method is a valuable quantitative tool to estimate the residual stress in polymer products, but the material temperature influence must be considered during the estimation.     

角联管网瓦斯爆炸冲击波与火焰波的传播特性

为了得出实际管网中瓦斯爆炸冲击波、火焰波的普适性传播特性,搭建了角联管网实验系统。通过平均升压速率和爆炸威力指数表征冲击波传播特性;通过火焰传播速率表征火焰波传播特性。结果表明:冲击波在角联管网中传播时出现了多次衰减与叠加,冲击波的正向传播和冲击波在管网所有互连管道中的反向传播叠加在一起,导致冲击波的传播呈现复杂无序状态。冲击波传播在不同的管道结构时,经过45°分岔管道时爆炸威力最大;在斜角联支管中,压力损失最大,超压爆炸威力下降幅度最大,火焰波传播速率最快;底部直管中的火焰波传播速率最慢。可为瓦斯爆炸灾害发生后应急救援方案的制定提供理论参考。     

Generalized Magneto-thermoelastic Interaction in a Fiber-Reinforced Anisotropic Hollow Cylinder

In this article, an estimation is made to investigate the transient phenomena in the magneto-thermoelastic model in the context of the Lord and Shulman theory in a perfectly conducting medium. A finite element method is proposed to analyze the problem and obtain numerical solutions for the displacement, temperature, and radial and hoop stresses. The boundary conditions for the mechanical and Maxwell’s stresses at the internal and outer surfaces are considered. An application of a hollow cylinder is investigated where the inner surface is traction free and subjected to thermal shock, while the outer surface is traction free and thermally isolated. The displacement, incremental temperature, and the stress components are obtained and then presented graphically. Finally, the effects of the presence and absence of reinforcement on the temperature, stress, and displacement are studied.     

Crack Front Waves in Dynamic Fracture*

A rapidly moving tensile crack is often idealized as a one-dimensional object moving through an ideal two-dimensional material, where the crack tip is a singular point. When a material is translationally invariant in the direction normal to the crack's propagation direction, this idealization is justified. A real tensile crack, however, is a planar object whose leading edge forms a propagating one-dimensional singular front (a `crack front'). We consider the interaction of a crack front with localized material inhomogeneities (asperities), in otherwise ideal brittle amorphous materials. We review experiments in these materials which indicate that this interaction excites a new type of elastic wave, a front wave, which propagates along the crack front. We will show that front waves (FW) are highly localized nonlinear entities that propagate along the front at approximately the Rayleigh wave speed, relative to the material. We will first review some of their characteristics. We then show that by breaking the translational invariance of the material, FW effectively act as a mechanism by which initially `massless' cracks acquire inertia.     

Diffraction of Elastic Waves by a Spherical Inclusion with an Anisotropic Graded Interfacial Layer and Dynamic Stress Concentrations

Scattering of compressional waves in multiphase metal matrix composites containing spherical particles with spherically isotropic graded interfacial layers is investigated using a state-space approach. A continuous transition from the particle to the matrix with the change of volume fraction of one of the constituents is assumed to exist across the thickness of the interphase zone. A simplified multilayer model for the interphase complications including both anisotropy and inhomogeneity is considered. Taylor’s expansion theorem is employed to solve a modal state equation leading to a global transfer matrix that directly links the boundary conditions at the outer surface of the interface layer to those at the inner surface. Numerical calculations reveal the important effects of interphase anisotropy and inhomogeneity on the total scattering cross section and dynamic stress concentrations for a moderately wide range of frequencies and interface layer thicknesses.     

Corrosion Induced Explosion of a High-Pressure Fire-Extinguishing Gas Cylinder

The failure of a high-pressure fire-extinguishing cylinder was investigated. Failure was induced by internal surface corrosion and stress corrosion cracking (SCC) due to condensation of carbonic acid. In internal surface, especially the area near the bottom of the exploded cylinder, severe corrosion was characterized by local pits. SCC initiated from these local corrosion pits was observed by metallurgical analysis. Microstructure of the failure cylinder near the internal surface consisted of multiple-banded structure and the banded structure could accelerate local corrosion initiation and propagation. The corrosion products built up on the fracture surface were primarily ferrous carbonate (FeCO₃). The determination of moisture in fire-extinguishing gas was also examined.     

SiC各向异性的热弹性应力指数

本文讨论了 SiC 立方和六方变体的热膨胀系数α和杨氏模量 E 的各向异性。热膨胀系数高度依赖于温度,仅在很小程度上与晶体结构方向有关,而杨氏模量则恰恰相反,它高度依赖于晶体结构的方向,受温度的影响则很小。热膨胀系数与杨氏模量的乘积αE 称之为“热弹性应力指数”。本文对该指数与晶体结构方向和温度的关系亦进行了讨论。当考虑复合材料中残余应力时,αE 指数或许可以用于确定复合材料增强相、单晶晶须的最佳轴向方向。同时αE 指数也可以显示出温度、热疲劳时温度差对复合材料性质的影响程度。     

Anisotropic Fracture Behavior of Sintered Rare-Earth Permanent Magnets

Sintered rare-earth permanent magnets (for example, Sm$_2$Co$_17$, SmCo$_5$, and Nd$_2$Fe$_14$B magnets) are quite brittle and easily crack in the course of fabrication, machining, and application. Here, we report on an investigation of fracture behavior and mechanical characteristics of sintered rare-earth permanent magnets and discuss the origin of rare-earth magnet brittleness. We studied three groups of bending specimens with different orientations, cut from the same block of magnet. The bending strength was measured. The fracture surfaces were carefully observed by scanning electron microscopy and showed that the fracture behavior and bending strength of sintered rare-earth permanent magnets obviously exhibit anisotropy. Sintered Sm–Co magnets tend to cleavage fracture in the close-packed ($0001$) plane or in the ($10bar11$) plane. The fracture mechanism of sintered Nd$_2$Fe$_14$B magnet appears to be mainly intergranular fracture. Our analysis indicates that the anisotropy of fracture behavior and mechanical strength of sintered rare-earth magnets is caused by the strong crystal-structure anisotropy and grain texture created by magnetic field alignment during the fabrication process.     

The Scattering of Inhomogeneous Electromagnetic Waves by a Cylinder

Abstract

Using vector harmonics the exact solution for diffraction of inhomogeneous waves by a cylinder has been obtained. Numerical calculations are given and discussed along with some potential applications.     

Failure Investigation of Cold-Rolled ZnTiCu Alloy Fracture During Bending Under Ambient Temperature Conditions

Bending ductility is an important quality parameter of the ZnTiCu rolled sheet for the fabrication of specially formed members in construction industry. Fabrication of such components is usually realized under severe loading conditions (i.e., high strain rate forming using manual tools or press brake machinery) and even sometimes under low temperature environment in the construction field, which is very often close to or less than 0 °C. Bending orientation is also an important parameter, since texture-sensitive Zn alloy cold-worked strip affects ductility performance in direction transverse to the rolling direction. A failure analysis process was implemented on a ZnTiCu bent sheet possessing cracks on outer bent areas. Light optical metallography, and scanning electron microscopy coupled with energy dispersive spectrometry, along with mechanical testing were the principal investigation techniques employed to evaluate the failure. The presence of pronounced elongated grain structure combined with coarse and continuous Ti-rich intermetallic phase, likely reduces the fracture resistance against the transverse stress field imposed by bending, stimulating, thus crack initiation and propagation.     

Influence of Metal Layer Thickness on the Stress-Strain State and Strength of Metal Composite Cylinders Under Internal Explosion

Strength of Materials - The effect of metal layer thickness on the stress-strain state and strength of metal composite cylinders of constant total thickness under the action of internal dynamic...     

Investigation on Fracture Behaviour of Turbine Blades Under Self‐Exciting Modes

Abstract: This paper investigates the failure mechanisms of turbine blades occurring during similar conditions of turbine operation. Optical techniques are utilised to identify critical frequencies that may be the cause of blade fractures. For that purpose, time average, stroboscopic holographic interferometry and holographic moiré are utilised to record vibration patterns. Strain maps are computed from stroboscopic and holographic moiré patterns. Stress distributions are computed under the assumption of linear elasticity. It is found that stress trajectories deduced from holographic measurements match well with the initial trajectories of crack observed for the in‐service broken specimens. This fact allows the excitation modes causing blade fractures to be identified. Finite element analyses carried out on models corrected in view of the experimentally observed vibration modes provide the values of stress causing fracture initiation. The paper discusses the failure mechanisms of the blades on the basis the experimental evidence and finite elements results. The observed failure modes seem to be a form of the principal normal stress fracture criterion. An argument based on damage accumulation explains the observed behaviour. Further evidence on failure mechanisms is gathered from scanning electron microscope images of the fractured specimens.