压应力场中爆生裂纹分布与扩展特征实验分析

采用动静组合加载实验装置和数字激光焦散线实验系统,进行了0、3、6、9 MPa等4种压应力场中PMMA试件的爆破致裂实验,分析了沿静态主应力方向扩展的裂纹运动学和力学行为。实验结果表明:首先,静态竖向载荷在预制炮孔周围产生应力集中,在炮孔壁上下端部处出现最大拉应力;随后,在动态爆炸载荷的叠加作用下,裂纹优先在炮孔壁上最大拉应力位置处起裂,并沿最大主应力方向扩展;裂纹扩展过程中,静态竖向载荷越大,裂纹扩展速度越大,且裂纹尖端应力强度因子值越大。     

Analytical solutions of thermal stress distribution in plastic encapsulated integrated circuit packages

IntroductionElectronicpackagesarecomplexcompositestructure.Theyhavemyriadcornersandinterfacesthatcanactaspotentialfailuresitesduringfieldoperation .Atpresenttime ,plasticencapsulatedintegratedcircuits (ICs)arewidelyusedinengineering .AplasticencapsulatedICpackageconsistsofasilicondie,dieattachment,passivation ,wireinterconnects,aleadframe ,andplasticmoldingcompound .Thermalexcursionduringpackagequalificationfromthermalcycling thermalshock ,orpowercyclingduringnormaloperationcansetupthermomec…     

多簇压裂干扰应力变化规律及对裂纹扩展的影响

为研究水平井分段多簇压裂缝间的干扰应力及其对裂纹扩展的影响,在现有二维未考虑地应力的单裂缝干扰应力解析解的基础上,利用双平面复变函数保角变换得到了包含地应力项的三维干扰应力解析解。基于扩展有限元法建立三维多裂缝扩展力学模型,利用Python脚本二次开发平台实现了三维多裂缝水力压裂参数化建模,通过解析解与数值计算对比分析,得到如下结论。裂纹两侧裂纹面法向和走向干扰正应力分别为压应力和拉应力,均呈纺锤形,法向干扰应力影响范围大约为走向干扰应力的5倍;裂纹尖端裂纹面法向和走向干扰正应力分别为拉应力和压应力;裂纹尖端两侧存在干扰剪应力;考虑初始地应力对干扰应力解析解进行修正后的干扰应力值均变小;多簇压裂中裂缝间的干扰应力叠加,簇间距越小,叠加效果越强;多簇压裂的干扰应力使裂缝间裂纹面法向压应力增大,走向压应力减小,导致裂纹扩展注水压力升高,裂缝张开的宽度降低,不利于单裂缝的扩展;干扰应力使裂缝间应力差降低,甚至局部最小地应力方向发生改变,有利于形成复杂缝网。     

Photoelastic simulation of the stress wave field around a tunnel in an anisotropic rock mass subject to shock load

The stress wave field around a tunnel in an anisotropic medium subject to shock load is analyzed using the dynamic photoelastic method. The influence of various factors on the distribution and magnitude of boundary stresses around the tunnel are studied by simulating the deformation process. The time dependence of the dynamic stress concentration factor on the tunnel walls is established __________ Translated from Prikladnaya Mekhanika, Vol. 42, No. 8, pp. 122–125, August 2006.     

Hoek–Brown criterion applied to circular tunnel using elastoplasticity and in situ axial stress

Based on the nonlinear Hoek–Brown failure criterion, elastoplastic analytical solutions are developed for the elastoplastic stresses, strains and plastic zones around a circular tunnel subjected to different value of the axial in situ stress. Effects of the transverse in situ stress, the axial in situ stress and the strength parameters of rock masses on the elastoplastic stresses, strains and plastic zones in the surrounding rock masses are investigated. It is found from the numerical results that the stresses, strains, and plastic zones in the surrounding rock depend not only on the transverse in situ stress but also on the axial in situ stress as well as the mechanical parameters of rock masses.     

应用干涉云纹测量工艺应力分布

本文就冷胀孔周残余应力分布的测量技术和干涉螺接孔周干涉应力分布的测量技术进行了实验研究。沿着径向将试件切割,以释放欲测的应力,同时用干涉云纹技术测量由于释放应力而引起的附加应变场。文中还就残余应力在循环载荷作用下的松弛问题进行了实验研究。提供了典型的实验结果,讨论了引起实验误差的主要原因。     

3D dynamic stress intensity factors around two parallel square cracks in an infinite elastic medium under impact load

Summary  Transient stresses around two parallel cracks in an infinite elastic medium are investigated in the present paper. The shape of the cracks is assumed to be square. Incoming shock stress waves impinge upon the two cracks normal to tzheir surfaces. The mixed boundary value equations with respect to stresses and displacements are reduced to two sets of dual integral equations in the Laplace transform domain using the Fourier transform technique. These equations are solved by expanding the differences in the crack surface displacements in a double series of a function that is equal to zero outside the cracks. Unknown coefficients in the series are calculated using the Schmidt method. Stress intensity factors defined in the Laplace transform domain are inverted numerically to the physical space. Numerical calculations are carried out for transient dynamic stress intensity factors under the assumption that the shape of the upper crack is identical to that of the lower crack. Received 2 February 2000; accepted for publication 10 May 2000     

Determination of the stress distribution in assemblies of photoelastic particles

An optical measuring method and a calculation procedure for determining the distribution of the stress tensor in a plane-strained three-dimensional assembly of random-shaped photoelastic particles are described. The stress tensor at an arbitrary point of the model is determined by an integration procedure, based on the equations of equilibrium of stresses. The distribution of the principal-stress direction and the relative principal-stress difference and at least two normal stresses in a plane have to be known to perform the integration. The distribution of the principal-stress directions and their difference are measured optically by scanning the model with an optical filter system with a single rotating polarizer.     

Laser heating of multilayer assembly and stress levels: elasto-plastic consideration

Laser heating of multilayer assembly results in temperature profiles, which differ in each layer. Depending upon the material and laser pulse properties, high temperature gradients can occur. This, in turn, results in excessive stress levels in the region irradiated by a laser beam. When the magnitude of stress level exceeds the yield stress of the material, plastic deformation is resulted. In this case, elasto-plastic analysis should be applied when modeling the thermal stresses due to a laser heating pulse. In the present study, time exponentially varying pulse laser heating of four and two layer assemblies is considered. The four layer assembly composes of gold, chromium, gold and silicon while two layer assembly is formed from gold and silicon. The resulting temperature field is obtained numerically using a control volume approach. The elasto-plastic analysis employed to compute the stress levels in the substrate material. It is found that stress levels higher than the yield stress of the substrate material occurs in the surface region. This, in turn, results in plastic zone in this region. The size of the plastic region extends towards the solid bulk as the heating progresses.     

Photoelastic analysis of the stress field surrounding a fatigue crack

A photelastic analysis was carried out on plane polyester specimens containing a fatigue crack, in order to study the effect of plastic yielding around the crack tip on the elastic stress distribution in the vicinity of the crack. In general, results were in good agreement with values calculated for the case of a sharp-tipped crack. However, very near the crack tip, principal stresses obtained experimentally were slightly lower than calculated stresses, probably due to the bluntness of the fatigue crack. Also lines of constant stress tended to move behind the crack tip, in contrast with the calculated stresses, which occurred further forward over the field of investigation.     

3D dynamic stress intensity factors at three rectangular cracks in an infinite elastic medium subjected to a time-harmonic stress wave

Summary Dynamic stresses around three coplanar cracks in an infinite elastic medium are determined in the paper. Two of the cracks are equal, rectangular and symmetrically situated on either side of the centrally located rectangular crack. Time-harmonic normal traction acts on each surface of the three cracks. To solve the problem, two kind of solutions are superposed: one is a solution for a rectangular crack in an infinite elastic medium, and the other one is that for two rectangular cracks in an infinite elastic medium. The unknown coefficients in the combined solution are determined by applying the boundary conditions at the surfaces of the cracks. Finally, stress intensity factors are calculated numerically for several crack configurations. Received 14 July 1998; accepted for publication 2 December 1998     

Elastostatic stress fields from pressurized cavities

The elastostatic version of the null field approach is applied to the problem of computing the stress field around one or more pressurized cavities. The translation properties of the spherical solutions to the elastostatic equation of equilibrium are derived, and it is shown how to compute stresses also inside the circumscribing spheres of the cavities.     

Inversion of speckle interferometer fringes for hole-drilling residual stress determinations

Speckle interferometric fringe patterns record stress-relief displacements induced by the drilling of blind-holes into prestressed objects. The quantitative determination of residual stress state from such stress patterns is difficult because of the ambiguity in the order of the observed fringes. The plane stress magnitudes are provided directly from selected fringe positions using a stochastic, iterative least squares minimization approach. The inversion requires prior knowledge of the experimental geometry and an appropriate uniaxial stress-relief displacement basis function derived from three-dimensional finite element calculations. Superpositioning of the rotated and scaled displacement basis functions allows the stress-relief relaxation for any biaxial state of stress to be determined. In this paper, fringe patterns were forward modeled from a large ensemble of calculated biaxial stress-relief displacement fields. Inversion of these noise-free fringe patterns reproduced the biaxial stresses with negligible error. Analysis of more realistic fringe patterns that include speckle noise gave stress magnitude errors that diminished rapidly with the number of selected points to better than 3 percent for 100 points. Sensitivity of the optical method is influenced by a number of factors, but the ensemble of model fringe patterns studied indicates that the stress magnitudes (nomalized with respect to the material's Young's modulus) from 3×10^–4 to 10^–2 can accurately be determined with visible laser radiation. The method is amenable to automation and can easily be extended to study near surface gradients in the residual stresses or applied to other optical recording techniques such as moiré and phase-shifting interferometry.     

双向应力状态下三维球形空穴应力集中的实验研究

用光弹三维应力冻结切片法研究了双向应力状态下球穴附近应力分布和集中问题，介绍了含球穴的三维光弹性模型的尺寸确定、浇注工艺和实现双向应力状态及大面积均匀拉伸加载的方法，分析了在特定平面上的正应力σ，剪应力τ沿球穴边缘的分布及衰减规律。文中对实验数据进行了统计分析处理，证明了实验与理论结果符合一致     

Residual stress distribution on surface-treated Ti-6Al-4V by X-ray diffraction

The x-ray diffraction technique has been used to measure surface residual stress in Ti-6Al-4V samples subjected to shot peening (SP), laser shock peening (LSP) and low plasticity burnishing (LPB). The magnitude, spatial and directional dependence and uniformity of the surface residual stresses have been investigated. The results show that residual stresses due to SP are uniform and independent of direction. LSP has been observed to produce non-uniform residual stress varying from one region to another, and also within a single laser shock. In the case of LPB, residual stresses have uniform spatial distribution but have been observed to be direction-dependent. Various components of the residual stress tensor in the LPB sample have been determined following the Dolle-Hauk method. The results of the residual stress due to three surface treatments are compared, and possible reasons for spatial and directional dependence are discussed.     

Hybrid stress analysis of perforated composites using strain gages

A strain gage hybrid method is described for determining individual stresses on the boundary and in the neighborhood of cutouts in orthotropic composites. Results agree with independent measurements and finite element analysis. Few measured strain data are needed, and the measured strains originate away from the hole. Ability to determine the stresses on the edge of a cutout from nonboundary measurements recognizes the difficulties in obtaining reliable measurements very near an edge while circumventing the challenge of attempting to bond gages to the transverse curved surface of a small hole or notch. The method also alleviates the problem of not knowing a priori where the most serious stress will occur on the geometric boundary and, hence, where to locate strain gages.     

The general stress strain relation of soils involving the rotation of principal stress axes

1.IntroductionThestudyofsoils'constitutiverelationismostlylimitedinsuchStressstatethattherotationofprincipalstressaxesisabletobeignored.Soonlythevalueofprincipalstressneedtobeconsideredandthedirectionofprincipal'stressisregardedtobeunchangeable.Onthiscondition,theprincipalaxesof'stressincrement,strainincrementjstressandstrainarethesame.Therelevantexperimentsandengineeringsshowthattherotationofprincipalstressaxeswillgeneratesignificantplasticdeformationandthenoncoaxalityofstressandstrain.First…     

含一个无外力圆柱面带转动的三维杂交应力元简

根据一种修正的余能原理,建立了一类具有一个无外力圆柱表面及结点含转动自由度的8 结点新型三维杂交应力元. 单元边界位移场选择二次位移插值函数,且与相邻元协调;单元内假定应力场满足以柱坐标表示的平衡方程及圆柱面上无外力边界条件. 数值算例表明,这种特殊杂交应力元在相当粗的网格下即能十分准确地分析圆弧形槽口附近及曲梁的三维（及二维）的孔边应力分布.     

Determining stress intensity factors in orthotropic composites from far-field measured temperatures

We demonstrate the ability to determine stress intensity factors in orthotropic materials directly from measured temperatures away from the crack and using far-field expressions for the stresses. This is advantageous, recognizing that recorded thermoelastic data can be very unreliable near the tip of a crack. In addition to singular terms that govern in the immediate vicinity of the crack tip, the present series expressions for the stresses contain higher-order finite terms. Little measured input information is needed and data acquisition positions can be selected largely at the user's discretion.     

Image enhancement and stress separation of thermoelastically measured data under random loading

Traditional thermoelastic stress analysis (TSA) presupposes that the structure being analyzed is cyclically loaded at a constant amplitude and frequency. This approach typically has been used to satisfy the adiabatic reversible assumptions. The authors employ an alternative signal analysis technique that enables one to evaluate the magnitude of the individual components of stress in a component subjected to a loading that is random in both frequency and magnitude. However, the nature of the measured information does not change; i.e., data are inherently noisy, and edge information is unreliable. The latter two aspects have caused many thermoelastic stress analyses to be more qualitative than quantitative. The present paper emphasizes developing the TSA technique into a practical, noncontacting quantitative method for stress analyzing actual engineering structures that are randomly loaded. In particular, ability to determine the individual stresses thermoelastically under random loading is demonstrated.