透平转子的可靠性设计

介绍透平转子的可靠性设计方法。该方法以传统的转子强度和振动的设计方法为基础,把透平转子的静应力、材料静强度、应力幅、尺寸系数、表面加工系数、应力集中系数、材料疲劳极限、热疲劳累积损伤程度、材料低周疲劳程度、蠕变寿命、固有频率、激振力频率、不平衡响应等处理为随机变量,使用机械概率设计法确定转子设计的可靠度。文中给出了透平转子静强度、疲劳强度、热疲劳、蠕变寿命、避开弯曲共振、避开扭转共振和限制不平稳响     

THREE-DIMENSIONAL MODELING FOR THIN PLATE-LIKE STRUCTURES INCLUDING SURFACE EFFECTS BY USING STATE SPACE METHOD

<正>A three-dimensional(3-D)approach based on the state space method is proposed to study size-dependent mechanical properties of ultra-thin plate-like elastic structures considering surface effects.The structure is modeled as a laminate composed of a bulk bounded with upper and bottom surface layers,which are allowed to have different material properties from the bulk layer.State equations,including the surface properties of the structure,can be established on the basis of 3-D fundamental elasticity to analyze the size-dependent static characteristics of the thin plate-like structure.Compared with two-dimensional plate theories based size-dependent models for thin film structures in literature,the present 3-D approach is exact,which can provide benchmark results to assess the accuracy of 2-D plate theories and various numerical approaches. To show the feasibility of the proposed approach,a 3-D analytical solution for a simply supported plate-like thin structure including surface layers is derived.An algorithm is proposed for the calculation of the state equations obtained to ensure that the numerical results can reveal the surface effects clearly even for extremely thin surface layers.Numerical examples are carried out to exhibit the surface effects and some discussions are provided based on the results obtained.     

表面机械滚压处理(SMRT)316L不锈钢梯度纳米层在腐蚀介质下的摩擦学行为研究

采用表面机械滚压处理(surface mechanical rolling treatment,SMRT)技术在316L奥氏体不锈钢表面构筑了梯度纳米结构层. 利用透射电子显微镜(TEM)和纳米压痕仪等分析其微观组织、力学性能等基础上,重点探讨了SMRT前后316L在1 mol/L HCl溶液(以纯水环境作为对比组)中的摩擦学行为. 结果表明:经SMRT加工后316L表面梯度纳米晶层厚度达200 μm以上,表面硬化层厚度超过1.5 mm,表面硬度提升至基体近2倍;SMRT大大减缓了材料磨损,与基体试样相比,SMRT试样在腐蚀介质下减摩效果比纯水环境更明显,且在腐蚀环境下表现出优异的耐腐蚀性能,其磨损机制由处理前伴随严重剥落特征的疲劳磨损和磨粒磨损转变为轻微疲劳磨损. 因此,316L不锈钢机械滚压梯度纳米层在腐蚀服役环境下具有较高的潜在工程应用价值.      

SAPH440汽车薄板高低周疲劳特性试验研究

本文通过设计防屈曲装置,对2.3mm厚度汽车薄板SAPH440在室温下进行了单轴拉伸、低周疲劳和高周疲劳等一系列试验．给出该材料静态力学参数,低周疲劳曲线（E-N曲线）、高周疲劳曲线（S-N曲线）以及相关疲劳性能参数．分别得到了在应力比R=0.1和R= -1状态下的疲劳寿命公式．通过考察低周循环应力-应变曲线,发现材料在试验控制的循环应变幅范围内发生了循环软化,具有明显Masing特性．此外,通过对疲劳断口形貌分析,初步给出了疲劳裂纹的扩展机理．     

Swell-induced surface instability of hydrogel layers with material properties varying in thickness direction

Upon swelling in a solvent, a thin hydrogel layer on a rigid substrate may become unstable, developing various surface patterns. Recent experimental studies have explored the possibilities to generate controllable surface patterns by chemically modifying the molecular structures of the hydrogel near the surface. In this paper, we present a theoretical stability analysis for swelling of hydrogel layers with material properties varying in the thickness direction. As a specialization of the general procedure, hydrogel bilayers with different combinations of the material properties are examined in details. For a soft-on-hard bilayer, the onset of surface instability is determined by the short-wave limit, similar to a homogeneous layer. In contrast, for a hard-on-soft bilayer, a long-wave mode with a finite wavelength emerges as the critical mode at the onset of surface instability, similar to wrinkling of an elastic thin film on a compliant substrate, and the critical swelling ratio is much lower than that for a homogeneous hydrogel layer. A smooth transition of the critical mode is predicted as the volume fraction of the top layer changes, linking surface instability of a homogeneous layer to thin film wrinkling as two limiting cases. The results from the present study suggest that both the critical condition and the instability mode depend sensitively on the variation of the material properties in the thickness direction of the hydrogel layer.     

New concept for crack detection

The purpose of this paper is to describe the development, application techniques and the mechanical properties of a new and unique crack-detection coating. The coating consists of a base-layer matrix of epoxy resin and specially formulated, very delicate microcapsules, 250–500 μm in size, which contain black conductive liquid. A second layer of a thin, conductive silver coating then is added on the top of the base layer. Whenever a structure using this coating develops a fatigue crack, the base matrix over the fatigue crack also cracks, rupturing the microcapsules and releasing the conductive liquid into the crack. An electrical circuit is thus completed between the structure and the top conductive silver layer. A change in electrical resistance from 50 K megohms to one-tenth (0.1) of a megohm occurs and the conductive liquid, containing carbon black, will show a visual fine-hairline crack on the upper conductive silver coat (Figs. 1,2). The crack-detection coating is so formulated that the mechanical characteristics and fatigue-crack resistance are greater than the metal structure.     

钢纤维聚合物高强混凝土疲劳性能的实验研究

钢纤维聚合物高强混凝土(SPHC)是本课题组最近研发的新型混凝土桥梁结构材料。为了探明SPHC的疲劳性能,本文采用对比分析的方法,对实桥上应用的SPHC及其静力学性能相近的C60混凝土的疲劳性能展开实验研究,得到了这两种材料的S-N实验曲线。研究结果表明,与C60混凝土相比,SPHC材料由于掺入了钢纤维和聚合物乳胶,增加了韧性及抗裂性能,使其抗疲劳性能得到了大幅度的提高。该新材料的实桥应用,将有效地提高桥梁结构的疲劳寿命。     

Modeling the contact wear fracture of a two-layer basement

We develop a method for calculating the kinetics of fatigue fracture of a two-layer elastic basement by a periodic system of indenters sliding on the surface and modeling the surface microroughness. The method is based on solving the contact problem for a periodic system of indenters and a two-layer elastic basement, determining the internal stresses with friction forces taken into account, and constructing the damage function in the two-layer basement. We calculate the kinetics of the fatigue fracture of the surface layer (the coating) and find the characteristics of the process depending on the strength and mechanical properties of the coating and basement materials, the loading and geometric characteristics of the system, and the friction coefficient.     

The mechanical strength of polysilicon films: Part 1. The influence of fabrication governed surface conditions

In an effort to explain the considerable variations in measured mechanical strength of polysilicon films doped with phosphorous for use in MEMS applications, the influence of the specimen manufacturing processes on the mechanical properties has been examined in connection with varying exposure to 49% hydrofluoric acid (HF). It was found that surface roughness as characterized by groove formation along grain boundaries depends on the HF release time. Surface undulations and crevasses related to grain structure result thus in reduced fracture strength and, in addition, induce errors into the determination of the effective elastic modulus—especially when the latter is determined from flexure configurations. Extensive exposure to HF results in pervasive material degradation, as evidenced by a transition from transgranular to intergranular fracture, and a correspondingly precipitous drop of the film strength with attendant increase in grain boundary material removal. Short times of exposure to HF can result in delamination of a thin surface layer, which is sufficient to initiate an “early” failure. Longer exposure allows HF permeation into the intergranular domains, degrading the body of the material significantly. On the other hand, tests on material from a different source that has undergone different doping and post-processing demonstrated a suppression of this degradation resulting in film strengths that are higher by a factor of two or more. Thus, consideration of additional influences of doping and electro-chemical phenomena during the HF wet release, in association with silicon-metal contacts, is necessary.     

电镀铜薄膜力学性能的实验研究

电子器件中大量使用铜膜作为电信号通道,而且一般采用电镀工艺制成.铜膜的力学性能参数对于其热疲劳可靠性的研究非常重要.目前有关该材料的力学性能研究尚不充分,而且数据极为不统一.本文借助于纳米压痕法、声发射等实验手段对电镀铜薄膜的静态力学性能(包括弹性模量和屈服强度等)及疲劳性能进行了测试.结果发现,与大块铜材料相比,电镀铜薄膜的弹性模量低很多,但屈服强度与大块铜材料相当,甚至高出200％.同时,本文采用弯曲疲劳实验,以电阻变化为失效判据,对镀铜材料的疲劳性能进行了测试,获取了该材料不同失效判据下的疲劳寿命预测模型的系数.     

Impact toughness of a gradient hardened layer of Cr5Mo1V steel treated by laser shock peening

Laser shock peening (LSP) is a widely used surface treatment technique that can effectively improve the fatigue life and impact toughness of metal parts. Cr5Mo1V steel exhibits a gradient hardened layer after a LSP process. A new method is proposed to estimate the impact toughness that considers the changing mechanical properties in the gradient hardened layer. Assuming a linearly gradient distribution of impact toughness, the parameters controlling the impact toughness of the gradient hardened layer were given. The influences of laser power densities and the number of laser shots on the impact toughness were investigated. The impact toughness of the laser peened layer improves compared with an untreated specimen, and the impact toughness increases with the laser power densities and decreases with the number of laser shots. Through the fracture morphology analysis by a scanning electron microscope, we established that the Cr5Mo1V steel was fractured by the cleavage fracture mechanism combined with a few dimples. The increase in the impact toughness of the material after LSP is observed because of the decreased dimension and increased fraction of the cleavage fracture in the gradient hardened layer.     

Optimum material design for functionally gradient material plate

Summary Steady thermal stresses in a plate made of a functionally gradient material (FGM) are analyzed theoretically and calculated numerically. An FGM plate composed of PSZ and Ti-6Al-4V is examined, and the temperature dependence of the material properties is considered. A local safety factor is used for evaluation of the FGM's strength. It is assumed that top and bottom surfaces of the plate are heated and kept at constant thermal boundary conditions. The pairs of the surface temperatures, for which the minimum local safety factor can be of more than one, are obtained as available temperature regions. The temperature dependence of the material properties diminishes, available temperature region as compared with that for an FGM plate without it. The available temperature region of the FGM plate is wider than that of the two-layered plate, especially for the surface temperatures which are high at the ceramic surface and low at the metal side. The influence of different mechanical boundary conditions is examined, and available temperature regions are found to be different, depending on the mechanical boundary conditions. The influence of the intermediate composition on the thermal stress reduction is also investigated in detail for the surface temperatures which are kept at 1300 K at the ceramic surface and 300K at the metal side. Appropriate intermediate composition of the FGM plate can yield the local safety factor of one or more for the four mechanical boundary conditions at once. For the two-layered plate there does not exist, however, any appropriate pair of metal and ceramic thicknesses which would yield the local safety factor of one or more for the four mechanical boundary conditions at once. The influence of the intermediate composition on the maximization of the minimum stress ratio depends on the mechanical boundary conditions. Finally, the optimal FGM plates are determined.     

A film-based wall shear stress sensor for wall-bounded turbulent flows

In wall-bounded turbulent flows, determination of wall shear stress is an important task. The main objective of the present work is to develop a sensor which is capable of measuring surface shear stress over an extended region applicable to wall-bounded turbulent flows. This sensor, as a direct method for measuring wall shear stress, consists of mounting a thin flexible film on the solid surface. The sensor is made of a homogeneous, isotropic, and incompressible material. The geometry and mechanical properties of the film are measured, and particles with the nominal size of 11 μm in diameter are embedded on the film’s surface to act as markers. An optical technique is used to measure the film deformation caused by the flow. The film has typically deflection of less than 2% of the material thickness under maximum loading. The sensor sensitivity can be adjusted by changing the thickness of the layer or the shear modulus of the film’s material. The paper reports the sensor fabrication, static and dynamic calibration procedure, and its application to a fully developed turbulent channel flow at Reynolds numbers in the range of 90,000–130,000 based on the bulk velocity and channel full height. The results are compared to alternative wall shear stress measurement methods.     

增材制造微结构演化及疲劳分散性计算

为了预测增材制造中工艺参数?微结构?力学性能之间的关联规律, 提出了集成离散元、相场模拟、晶体塑性有限元和极值概率理论的计算方法, 揭示了激光扫描速度对微结构演化、屈服应力和疲劳分散性的影响. 首先, 采用离散元实现了重力作用下粉床在已凝固层表面上的逐层铺设; 其次, 通过热?熔体?微结构耦合的非等温相场模拟, 获得了熔体、气孔、晶界、晶粒取向等的时空演化以及最终形成的多晶微结构; 然后, 应用晶体塑性有限元计算了增材制造多晶微结构的宏观力学响应, 并得到表征疲劳裂纹萌生驱动力的疲劳指示参数(FIP); 最后, 采用极值概率理论分析了增材制造多晶微结构的FIP极值分布规律及疲劳分散性. 以316L不锈钢选区激光熔化增材制造为例的计算结果表明: 增材制造微结构的宏观屈服强度随激光扫描速度的增加而降低, 且呈各向异性; FIP极值符合Gumbel极值分布规律, 激光扫描速度增加可降低增材制造微结构疲劳分散性, 但会导致FIP极值升高, 使得疲劳裂纹萌生驱动力增加, 疲劳寿命降低.      

MECHANICAL ANALYSIS OF SIMPLY SUPPORTED BEAM BASED ON CLASSICAL LAMINATED PLATE THEORY1)

与传统的金属材料相比,复合材料具有比强度高、比模量大,耐疲劳性、耐腐蚀性好,且热性能和电性能良好等优点。本文选择复合材料对简支梁进行铺层设计,首先从简支梁的受力分析入手,根据复合材料力学的经典层合板理论和弹性力学的基本方程,建立简支梁的数学模型;然后对简支梁进行结构设计,确定简支梁的铺层角度与铺层数目,构建复合材料结构,对复合材料简支梁进行静力学分析;最后利用蔡吴张量准则进行强度校核。     

定向有机玻璃疲劳断裂性能的频闪法研究

疲劳裂纹扩展前应力集中区域变形场的测试对材料疲劳性能的研究起到至关重要的作用。但由于疲劳试验为动态的过程,在动态下测量比在静态下要复杂得多,这样就给变形场的测量带来了困难。通过调节频闪光源的频率使其与疲劳试验机的频率相同,可把此动态过程转换为准静态过程,采集不同循环周期内的相同时刻的灰度图像,并运用数字图像频域相关分析法进行测试。本文运用此方法对定向有机玻璃缺口应力集中区域的变形场进行了测试,并分析得出其层间疲劳断裂力学性能薄弱的结论。     

Spherical indentation analysis of stress relaxation for thin film viscoelastic materials

The mechanical testing of thin layers of soft materials is an important but difficult task. Spherical indentation provides a convenient method to ascertain material properties whilst minimising damage to the material by allowing testing to take place in situ. However, measurement of the viscoelastic properties of these soft materials is hindered by the absence of a convenient yet accurate model which takes into account the thickness of the material and the effects of the underlying substrate. To this end, the spherical indentation of a thin layer of viscoelastic solid material is analysed. It is assumed that the transient mechanical properties of the material can be described by the generalised standard linear solid model. This model is incorporated into the theory and then solved for the special case of a stress relaxation experiment taking into account the finite ramp time experienced in real experiments. An expression for the force as a function of the viscoelastic properties, layer thickness and indentation depth is given. The theory is then fitted to experimental data for the spherical indentation of poly(dimethyl)siloxane mixed with its curing agent to the ratios of 5:1, 10:1 and 20:1 in order to ascertain its transient shear moduli and relaxation time constants. It is shown that the theory correctly accounts for the effect of the underlying substrate and allows for the accurate measurement of the viscoelastic properties of thin layers of soft materials.     

列车车轮滚动接触疲劳裂纹评价研究

列车车轮踏面表层金属滚动接触疲劳是影响列车运行安全性和舒适性的核心科学问题. 借助金相显微镜、扫描电镜、透射电镜和显微硬度计,通过开展列车车轮材料的标准滚动接触疲劳试验,将标准接触疲劳样品的损伤行为与实际服役车轮的损伤行为对比分析,研究了列车车轮的滚动接触疲劳裂纹评价方法. 结果表明:车轮表层金属接触疲劳开裂是表层金属累积塑性变形损伤的结果;标准滚动接触疲劳样品剥离坑的深度恰好等于硬化层的深度,实际服役车轮剥离坑的深度小于硬化层的深度;将车轮表面的滚动接触疲劳裂纹命名为“三角形指向性裂纹”;初步建立了车轮表面滚动接触疲劳损伤程度的定量评价方法.      

薄钢板低周疲劳测试技术研究

汽车钢板材料由于厚度较薄,在低周应变疲劳性能测试中,试样容易发生屈曲,并且受限于试验段的标距,无法使用传统机械式引伸计进行应变的测量,在工程中,其低周疲劳性能的测试是一项技术难点。本文基于数字图像相关技术,提出了一种非接触式的光学引伸计,结合光学显微镜头和防屈曲装置,实现材料表面应变的实时监测。该动态应变测量系统使用普通的CCD相机,采用了优化的计算方法,应变测量的频率可以高达60Hz。该动态应变测试技术可以广泛地应用于各个工程领域的动态变形测量。     

珍珠母的微结构与力学行为研究进展

珍珠母(nacre或mother-of-pearl)是软体动物贝壳内层的主要结构材料,具有优异的力学性能,已成为当前先进工程材料仿生设计和制备的研究热点.该生物材料具有完善的分级微结构,使其强度与韧性远高于组成这种材料的纯文石相.本文从珍珠母的微结构、实验研究以及力学模型3个方面对珍珠母的研究现状进行了概述,并指出目前研究中一些仍待解决的关键问题.