磁性应力测量条件对应力感度的影响

磁性应力测量具有测量速度高,探测深度大(可达数毫米),无辐射危险,携带方便等优点.可以对使用中的构件进行实时实地且安全的测量.但磁性应力测量法只能用于铁磁材料并且对铁磁材料结构等因素也敏感.因此,在磁性应力测量中影响测量精度的原因很多,其中一个最主要的参数就是应力感度.本文主要研究了两种材料的应力感度与材料测定条件的关系,以及测定环境对应力感度的影响,从而在磁性测量的过程中能排除不利条件,正确地确定磁性应力的感度,提高磁性应力测量技术的准确度.     

应力与渗氮

渗氮试样在２８０℃加热并施以拉力时，促使γ＇相析出的事实表明，应力对扩散起促进作用，且在弹性极限态时作用最大，试验指出，渗氮本身就存在着渗氮应力，由此导致＜＜间断式＞＞渗氮工艺的产生。     

关于残余应力的静载松弛与最佳喷丸残余应力场的研究

本文研究了喷丸残余应力场在疲劳加载初期的静载松弛现象及机理,并对最佳喷丸残余应力场进行了探讨。试验结界表明,残余应力在疲劳过程中的静载松弛是工件表层材料剧烈塑性变形的结果。残余应力的静载松弛会使晶界、相界等障碍物处形成一定数量的微裂纹,给工作表层材料带来损伤,降低疲劳裂纹的形核寿命。为了避免疲劳初期由于残余应力的静载松弛所造成的损伤,在喷丸后采用应力松弛低温回火工艺,预先降低残余应力场中的最大残余压应力值,建立最佳残余应力场。这种通过热激活的方式使残余应力发生的松弛属子非损伤性松弛,因而能够有效地提高材料在S—N曲线上的较高交变应力区的疲劳寿命。     

金属和SiC复合薄膜的应力及应力梯度控制技术

等离子增强化学气相沉积（PECVD）SiC谐振器的制作面临导电和应力及应力梯度控制问题．为了解决导电问题,采用金属W作为导电材料,它耐断腐蚀,可简化工艺．使W薄膜应力最小的溅射条件为：Ar压力2．0Pa,功率300W,反溅射工艺;为了解决应力梯度问题,采用双层金属结构、退火和离子注入3种手段．在离子注入电压120keV、剂量1．6×10^16cm^-2、不退火的条件下,可以得到应力梯度最小、结构完好的PECVD SiC谐振器.     

Analytical and Numerical Solution of the Stress Field and the Dynamic Stress Intensity Factors in a Cracked Plate under Sinusoidal Loading

A cracked plate subjected to a sinusoidal loading perpendicular to its plane is considered, and the analytical solution of the dynamic vibration behavior of a plate, which allowed the determination of the stress field near the crack tip, is developed. A mixed mode of loading near the crack tip has been established and described with dynamic stress intensity factors K _I (z,t) and K _II (z,t) associated with modes I and II crack openings, respectively. To validate the analytical results, a finite element analysis (FEA) of a 1 × 1 m square plate with a thickness of 1 cm, having a middle crack of 10 cm in length, is made. The results have shown significant agreement between analytical and FEA findings.     

支铰钢梁的振动时效与残余应力分析

为消除刘家道口弧形闸门支铰钢梁焊接后的残余应力,通过有限元分析,确定了采用振动时效工艺,并对振动时效的效果进行了分析。结果表明：振动时效方案可有效消除钢梁的残余应力,为同类工程消除残余应力提供了范例和经验。     

超微颗粒与应力释放

颗粒的粒度对变形特性及断裂过程有重要影响．本文从能耗理论入手，分析了超微颗粒的断裂机理，并指出应力释放是实现超微颗粒制备的理想模式．     

硫化物应力腐蚀开裂试验应力环载荷的测量及标定

硫化物应力腐蚀开裂试验的常用载荷测量方法是通过测量应力环的变形量计算载荷。指出了该方法在标定和测量过程中容易产生较大误差的原因。通过测量对比,证明了采用载荷传感器直接测量载荷的方法可减小误差,更适合作为试验过程中的载荷标定方法。     

Non—uniform Stress Field and Stress Concentration Induced by Grain Boundary and Triple Junction of Tricrystal

The stress characteristics in the anisotropic bicrystal and tricrystal specimens were analyzed using the anisotropic elastic model, orthotropic Hill‘s model and rate-dependent crystallographic model. The finite element analysis results show that non-uniform stresses are induced by the grain boundary. For bicrystal specimens in different crystallographic orientations, there exist stress concentrations and high stress gradients nearby the boundaries. The activation and slipping of the slip systems are dependent on the crystallographic orientations of the grains and also on the relative crystallographic orientations of the two adjoining grains. For the tricrystal specimens, there is not always any stress concentrations in the triple junction, and the concentration degree depends on the relative crystallographic orientations of the three grains. Different from the bicrystal specimens, there may be or no stress concentration in the vicinity of grain boundaries for the tricrystal specimens, which depends on the relative crystallographic orientations of the three grains. The stress concentration near to the grain boundaries and triple junction can be high enough for the local plastic deformation, damage and voiding or cracking even when the whole specimen is still under the elastic state.It can be further concluded that homogeneous assumption for polycrystalline materials is not suitable to study the detailed meso- or micro-mechanisms for damaging and fracturing.     

High-Temperature Stress Relaxation Cracking and Stress Rupture Observed in a Coke Gasifier Failure

This article discusses the high-temperature metal degradation mechanisms that occurred in the failure of a nine-story tall coke gasifier, located in a refinery power plant. Cracking of gasifier internals, bulging and stress rupture of the vessel shell, and escape of hot syngas resulted in an external fire. The failure mechanisms include stress relaxation cracking of a large diameter Incoloy Alloy 825 tube, stress rupture of the 4.65-in. thick, 1¼ chromium steel shell wall, and oxidation of 1¼ chromium steel exposed to hot syngas. The gasifier process and operating conditions that contributed to the high-temperature degradation will be discussed.     

Multiaxial Fatigue Criterion for Notched Specimens Including the Effective Stress Range,Relative Stress Gradient,and Hydrostatic Pressure

Multiaxial fatigue criteria for notched specimens must take into account the equivalent stress range, the relative stress gradient, and hydrostatic pressure. The effective shearing stresses take into account the relative gradient via the weight function. As a result of testing notched specimens of mild steel under combined torsion and tension, it was established that the effective shear stresses are linearly decreasing.     

氨基磺酸盐镀镍过程中内应力产生及变化的因素与机理

电沉积镍用途广泛,但常因应力问题而影响其应用.为此,采用薄片阴极弯曲法研究了氨基磺酸盐体系电镀镍层内应力的影响因素并对镀层内应力的形成机理进行了初步探讨.结果表明,升高电解液温度或降低阴极电流密度可以降低电镀镍层的拉应力;当电解液pH值为4.3时,电镀镍层具有最大的压应力,高于或低于此值时镀层的压应力减小,直至出现拉应力;电解液中Cl-浓度的增加会增加电镀镍层的拉应力.X射线衍射分析表明,电镀镍层的应力特性与其微观结构密切相关,镀层处于压应力时具有(200)择优取向;镀层处于拉应力时具有(220)和(111)择优取向.     

A New Theory of Strain Hardening and its Consequences for Yield Stress and Failure Stress

A new theory of strain hardening is developed. Important in its own right, the strain hardening solution is also of decisive use in rigorously defining the historically broad concepts of yield stress and failure stress. Under ideal conditions yield stress is found to represent a 3rd order transition. Failure stress is an explicit "failure of function" criterion rather than just being the primitive notion of breaking into pieces. Computational extensions and opportunities are discussed.     

残余应力测试与校准方法研究现状与展望

简单介绍了残余应力产生的原因及其对材料性能的影响,分析了现阶段残余应力测试方法的基本原理、应用场合和优缺点等,尤其是对目前普遍使用的X射线衍射法、中子衍射法、纳米压痕法、钻孔法等残余应力测试方法进行了详细说明.针对残余应力准确测量需求,分析现阶段基于标样法和仪器性能校准的残余应力校准方法,并说明了校准对残余应力准确测量...     

Stress and failure analysis of the crankshaft of diesel engine

In this work the failure analysis of the crankshaft of diesel engine was performed. Visual examination of the crankshaft fracture showed that beach marks, typical for fatigue failure were observed. Additional observations of the crack initiation zone indicated that crack origin was not covered by material defects or corrosion products. Performed hardness test of the fractured crank pin showed that large HRC values were observed in central part of the pin only. On the corner of cylindrical pin surface where the crack origin was located the hardness of material was much smaller. In order to explain the reason of premature crankshaft damage, the finite element method was utilized. The results of nonlinear static analysis showed that during work of the engine with maximum power the high stress area was located in crack initiation zone. Based on results of performed investigations it was concluded that the main reason of premature fatigue failure was high-cycle fatigue of the material in external zone of the crank pin where the small structural radius was designed. In final part of the work the recommendations for increase of the fatigue life of analyzed crankshaft were formulated.     

Measurement of the Stress Transmission Coefficient of Material Slugs in an Aerated Radial Stress Chamber

In dense-phase pneumatic conveying, particles are transported along a pipeline at relatively low conveying speeds. Due to the relatively gentle handling characteristics of this mode of flow, it is suitable for conveying fragile and brittle bulk materials used in the food and chemical industries. The simulation of the stress field within a slug aims at developing an accurate prediction model for the pressure drop along a pneumatic conveying line. A reliable prediction of the pressure drop strongly depends on an accurate assessment of the particle properties, the pipeline dimensions, and the operating conditions. In past decades, a few models have been developed to serve this purpose, most of them including the mean particle diameter as a crucial parameter. This generally limits the selection of materials to those of nearly spherical particle shape, as it is extremely difficult to obtain a representative diameter for irregularly shaped particles or bulk commodities comprising differently sized and/or shaped particles. Another previously conflicting parameter is the so-called stress transmission coefficient k w , which relates the lateral wall stress within a slug of material to the axial stress. Previously, this parameter could not be measured directly in a test rig and had to be estimated; therefore, inaccuracies within the prediction were unavoidable. Consequently, a new test chamber was developed to measure the lateral and axial stresses within a slug, which leads directly to the stress transmission coefficient. The design of the test apparatus is outlined and the initial tests undertaken are reported. A strong dependence of the radial stress measurements on temperature changes of the test rig induced by the airflow was discovered. Possible solutions to compensate for this influence are addressed and further discussed.     

Stress and Failure Analysis of Brittle Matrix Composites. Part I: Stress Analysis

Damage initiation in brittle matrix composites with ultimate strain of the matrix less than that of the fiber under longitudinal tensile load takes place with the formation of cracks in the matrix. This is followed by debonding at the fiber-matrix interface and fiber fractures depending on the properties of the interface. Study of the various failure mechanisms of the composite has a prerequisite the analysis of the stress field around matrix cracks, fiber-matrix disbonds, fiber failures, etc. In the present work stress analysis for a cylindrical element of matrix with a single fiber and two matrix annular cracks perpendicular to the fiber direction under longitudinal tensile load takes place. The results of the finite element stress analysis are used to predict the overall tensile stress-strain curve of the composite which is compared with the predictions based on a modified shear lag analysis. This revised version was published online in July 2006 with corrections to the Cover Date.     

Stress relaxation and creep of cork

The stress relaxation and creep behaviour of cork under compression were characterized in tests done with the compression axis parallel to each of the three principal directions in the tree (radial, tangential and axial). All stress relaxations lead to a linear variation of stress with the logarithm of time, the slopes being nearly independent of stress and direction of compression. Creep stresses in the range 0.36 to 1.72 MPa were used. The strain rate continuously decreases during creep, from initial values around 10^–4sec^–1 to 10-7 sec^–1 after 8 h, but its dependence on the creep stress and direction of compression is not simple, mainly because different deformation regimes may operate during a single creep test. Compression loading-relaxation-unloading cycles were imposed on specimens, with compression either in the radial or in the tangential direction, with the purpose of simulating the performance of a cork stopper. A work softening is observed, i.e. the resistance decreases in successive compressions, particularly between the first two. This is explained in terms of an increased undulation of the cell walls produced in the first compression.     

Proportional Limit Stress and Residual Thermal Stress of 3D SiC/SiC Composite

Proportional limit stress （PLS） and residual thermal stresses （RTS） of 3D SiC/SiC composite were investigated. PLS was obtained by four different methods from the monotonic stress-strain response curve to get a convincing value. RTS in the SiC matrix was quantified by solving the geometric intersection point of the regression lines of hysteresis loops from the periodical loading-unloading-reloading cycle test curve. Classical ACK model and analytical formulas were used to analytically calculate the PLS and RTS of 3D SiC/ SiC composite. Good agreement between the experimental results and the analytical calculation was observed. And relationship between the PLS and the RTS of 3D SiC/SiC was discussed.