船舶柴油机活塞的热疲劳强度分析

运用ＡＤＩＮＡＴ／ＡＤＩＮＡ有限元程序分析了船舶柴油机活塞的三维温度和三维应力应变场,并采用带中央环状缺口的圆柱型试件在单轴疲劳试验机上进行了热疲劳寿命模拟试验,根据高温强度理论归纳了当量应变评价准则并将一准则用于柴油机活塞的寿命预测,并证实了同相热疲劳模式预测结果是合适的。     

应用当量应变法预测柴油机活塞的多维疲劳寿命

运用ADINAT／ADINA有限元程序分析了6110型柴油机活塞的三维温度场和三维应力应变场，并采用5种带缺口试件进行了多维高温低周疲劳寿命模拟试验，根据高温强度理论归纳了当量应变范围评价准则并将这一方法用于柴油机活塞的寿命预测，结果表明，这一方法是适合的，并且对柴油机设计与使用有一定的参考价值。     

启动工况下汽轮机转子疲劳寿命消耗和裂纹扩展量的计算

本文根据汽轮机转子材料30CrMoV钢低周循环疲劳和高温疲劳裂纹扩展试验,用暂态变物性温度热弹塑性有限元对冷态启动工况下的汽轮机转子进行应力场分析结果分别进行无裂纹转子疲劳寿命损耗和转子疲劳裂纹扩展估算,并对估算方法及应力强度因子计算进行了初步分析。     

核压力容器用A508-3锻件和A533B钢板在模拟压水堆冷却水中的腐蚀疲劳裂纹扩展性能

引言由于环境介质和受力状态的交互作用,使环境介质对疲劳裂纹扩展性能的影响十分复杂。腐蚀疲劳裂纹扩展速率数据是进行寿命估算和完整性评定时所必须具备的数据。在进行部件寿命估算时,还必须了解裂纹扩展的机理、部件的受力状态和实际服役工况。     

基于S-N曲线和断裂力学的浮式风力机张力腿疲劳评估

针对风浪流联合作用下张力腿型浮式风力机筋腱的疲劳问题,提出一种基于S-N曲线与断裂力学的疲劳评估方法。通过对浮式风力机进行时域耦合分析获取张力筋腱应力时程,基于裂纹萌生S-N曲线、线性累计损伤理论评估裂纹萌生阶段寿命,并基于断裂力学Forman公式评估未穿透裂纹扩展、穿透裂纹扩展2阶段寿命。以5 MW Seastars式风力机平台为例,验证了该方法的可行性,并研究了顶张力、风浪夹角对疲劳寿命的影响。计算结果表明:穿透裂纹扩展寿命占总寿命比例极小;总寿命随预张力增大而减小,且对风浪流角度的变化较为敏感。     

锅炉汽水联箱疲劳寿命预测和安全评安

本文作者定量分析某型舰用主锅炉汽水联箱的疲劳寿命，并根据汽水联箱的运行情况制定其载荷谱，按联箱的载荷谱计算其应力与应变，然后用局部应力应变法和疲劳循环计数的雨流法，计算汽水联箱的裂纹起始寿命，用断裂力方法计算汽水联箱的裂纹扩展寿命，作者在此基础上还对汽水联箱的安全性进行了评定。     

锅炉汽水联箱疲劳寿命预测和安全评定

本文作者定量分析某型舰用主锅炉汽水联箱的疲劳寿命；并根据汽水联箱的运行情况制定其载荷谱，按联箱的载荷谱计算其应力与应变，然后用局部应力应变法和疲劳循环计数的雨流法，计算汽水联箱的裂纹起始寿命；用断裂力学方法计算汽水联箱的裂纹扩展寿命。作者在此基础上还对汽水联箱的安全性进行了评定。图３表１参４。     

风力机传动链齿轮齿根疲劳裂纹扩展剩余寿命评估

在叶轮气动载荷模型、柔性传动链模型和齿根弯曲应力模型的基础上,建立了随机风速下风力机传动链齿轮齿根疲劳裂纹扩展剩余寿命计算模型,对不同风速下的齿根弯曲应力进行了计算,分析了随机载荷下裂纹的扩展速率.对1.5 MW风力机太阳轮齿根疲劳裂纹的扩展进行了分析.结果表明:风力机齿轮齿根疲劳裂纹扩展寿命主要受风载荷影响,且紧急制动等冲击对裂纹扩展后期的影响不可忽视.     

某柴油机高压油管冷镦接头断裂问题分析

针对某型机高压油管使用中冷镦接头发生断裂的原因进行分析。分析表明：断裂方式为疲劳断裂,并在此基础上针对疲劳裂纹萌生及扩展两大影响零件疲劳寿命的主要因素进行了分析。结合疲劳裂纹萌生及扩展的基本理论,提出了改善高压油管疲劳性能的生产工艺改进措施。     

某型柴油机活塞开裂分析

某型国V柴油机进行试验时,曲轴箱压力突然增大,拆开发动机,发现二缸活塞沿活塞销方向的后端有熔洞,前端有裂缝。分别针对活塞加工与图纸要求一致性、活塞材料化学成分、材料金相组织、活塞材料在常温、高温下的抗拉强度等方面分析了活塞产生裂纹的原因。同时对开裂活塞的断口进行了理化分析,结果表明,活塞在交变载荷的作用下,顶部的温差较大,产生较大的热应力,导致热疲劳裂纹的产生;后续在交变燃气爆发高压应力的作用下,裂纹发生了机械载荷疲劳扩展,最终导致活塞顶部开裂。     

ENLARGEMENT OF A THERMALLY DRIVEN PENNY-SHAPED CRACK: STABILITY CONSIDERATIONS

Quasistatic extension of a penny-shaped crack, which occurs prior to its spontaneous propagation, is studied by means of nonlinear fracture mechanics analogous to the methods used to describe plastic deformation in metals. Wnuk's model of final stretch is employed to describe the displacement and stress fields in the vicinity of the crack border. Three kinds of thermal loadings are considered: (a)prescribed heat flux across the surface of the crack, (b) prescribed temperature difference, and (c) given heat extraction rate resulting from the steady-state flow of a cooling fluid circulating in and out of the crack. The latter case is directly applicable to power-generating geothermal systems. All three problems, if treated by linear elastic fracture mechanics methods, lead to singular thermal stresses around the circumference of the crack. Although the first two problems are of a positive K-gradient nature, i.e., inherently unstable, the third one results in a negative K-gradient. Through the approach used in this work the existence of a preliminary phase of stable crack growth, which precedes the catastrophic (cases a and b) or the spontaneous (case c) crack extension, has been demonstrated. The critical parameters involving the thermal load and the terminal dimension of the crack, at which the transition from stable to unstable propagation takes place, are predicted.     

TWO-CRACK PROPAGATION PATHS IN A FUNCTIONALLY GRADED MATERIAL PLATE SUBJECTED TO THERMAL LOADINGS

Two-crack propagation paths in a ceramic/metal functionally graded material plate (FGP) under one-cycle temperature change of heating and cooling are considered. When the FGP is subjected to thermal shock, a single crack or multiple cracks often initiate on the ceramic surface during the cooling process and propagate in the FGP. Crack paths are influenced by the heating temperature conditions, a compositional profile of the FGP, the fracture toughness, interaction among multiple cracks, and so on. Transient thermal stresses are treated as a linear quasi-static thermoelastic problem for a plane-strain state. The crack paths are treated under fracture mechanics using the finite-element method. The effects of heating temperature conditions, a compositional profile of the FGP, the fracture toughness, and a crack space on the crack propagation pattern are discussed and are shown in figures.     

On the physical differences between tensile testing of type 304 and 316 austenitic stainless steels with internal hydrogen and in external hydrogen

Seventeen metastable austenitic stainless steels (type 304 and 316 alloys) were tested in tension both with internal hydrogen and in external hydrogen. Hydrogen-assisted fracture in both environments is a competition between hydrogen-affected ductile overload and hydrogen-assisted crack propagation. In general, hydrogen localizes the fracture process, which results in crack propagation of particularly susceptible materials at an apparent engineering stress that is less than the tensile strength of the material. Hydrogen-assisted crack propagation in this class of alloys becomes more prevalent at lower nickel content and lower temperature. In addition, for the tests in this study, external hydrogen reduces tensile ductility more than internal hydrogen. External hydrogen promotes crack initiation and propagation at the surface, while with internal hydrogen surface cracking is largely absent, thus preempting hydrogen-assisted crack propagation from the surface. This is not a general result, however, because the reduction of ductility with internal and external hydrogen depends on the specifics of the testing conditions that are compared (e.g., hydrogen gas pressure); in addition, internal hydrogen can promote the formation of internal cracks, which can propagate similar to surface cracks.     

Analysis of Thermoelastic Crack Problems Using Green–Lindsay Theory

A boundary element method using the Laplace transform in time domain is presented for the analysis of fracture mechanics under thermal shock using the Green and Lindsay (GL) theory of thermoelasticity. The dynamic thermoelastic model of Green and Lindsay is selected to show the effect of thermal wave propagation at finite speed on crack tip stress intensity factor evaluation. The singular behavior of the temperature and displacement fields in the vicinity of the crack tip is modeled by the quarter-point elements. Thermal dynamic stress intensity factor for mode I is evaluated from computed nodal values, using the well-known displacement and traction formulas. The accuracy of the method is investigated through comparison of the results with the available data in literature. Condition where the inertia term plays important role is discussed and variations of dynamic stress intensity factor is investigated. Different relaxation times are chosen to briefly show their effects on stress intensity factor in the Green and Lindsay theory.     

TEMPERATURE AT A PROPAGATING CRACK TIP IN A VISCOPLASTIC MATERIAL

Abstract

Rupture processes at a propagating crack tip and the dissipation of mechanical energy in the vicinity of a crack tip can give rise to substantial temperature increases. In this paper we consider the (;transient) time-dependent temperature field. It is shown that under certain conditions. which are discussed in some detail, the adiabatic approximation gives a very acceptable approximation for the near-tip temperature. The adiabatic approximation has been used to calculate the crack tip temperature for a crack that propagates in a viscoplastic material. The mechanical behavior of the material is represented by a model proposed by Bodner and Partom. The Bodner-Partom model accounts for flux of energy into the crack tip as well as for dissipation of mechanical energy in the near-tip region. The temperatures due to both effects have been analyzed. and estimates are presented for the maximum crack tip temperature that may be reached during the crack propagation process.     

Green's function approach for crack propagation problem subjected to high cycle thermal fatigue loading

An efficient numerical approach using Green's function for the analysis of crack propagation under thermal transient load has been presented. The present approach based on multi-Green's functions pre-determined for each stage of the incremental crack growth substantially shortens the calculation time of the stress intensity factor (SIF) ranges. It was shown that the Green's function method (GFM) can be efficiently used to evaluate not only thermal stresses for fatigue analysis but also the SIF for crack propagation analysis. The crack propagation analysis results have been compared with those of the actual observation for the piping structure subjected to thermal striping load in a liquid metal fast breeder reactor. It was shown that the function determined at a fixed temperature can be applied to a relatively wide range of temperatures because of the compensation effect of the material properties, that is, some properties increase while the others decrease as the temperature increases.     

THERMOELASTIC PROBLEM OF STEADY-STATE HEAT FLOW DISTURBED BY A CRACK PERPENDICULAR TO THE GRADED INTERFACIAL ZONE IN BONDED MATERIALS

The plane thermoelasticity equations are used to investigate the steady-state nonisothermal crack problem for bonded materials with a graded interfacial zone. The interfacial zone is modeled as a nonhomogeneous interlayer having continuously varying thermoelastic moduli in the exponential form between the dissimilar, homogeneous half-planes. A crack is assumed to exist in one of the half-planes oriented perpendicular to the nominal interface, disturbing a uniform heat flow. Based on the method of Fourier integral transform, formulation of the crack problem is reduced to solving two sets of Cauchy-type singular integral equations for temperature and thermal stress fields. The heat-flux intensity factors and the thermally induced mode II stress intensity factors are defined in order to characterize the singular behavior of temperature gradients and thermal stresses, respectively, in the vicinity of the crack tips. In the numerical results, the values of heat-flux and thermal-stress intensity factors are presented for various combinations of material and geometric parameters of the dissimilar media bonded through a thermoelastically graded interfacial zone. The influence of crack-surface partial conductance on the near-tip temperature and thermal stress fields is also addressed.     

Mechanism of hydrogen-restrained crack propagation and practical application research of thermohydrogen treatment in a TiAl-based alloy

The mechanism of hydrogen-restrained crack propagation and practical application of thermohydrogen treatment in a TiAl-based alloy was investigated in this study. Hydrogenated and unhydrogenated alloys were subjected to high-temperature compression test, with a temperature range 1050–1200 °C and strain rate range 0.001–1 s⁻¹. The results showed that crack propagation was restrained due to hydrogen addition. The main mechanism of hydrogen-restrained crack propagation of such alloy was revealed that hydrogen-promoted lamella bending and hydrogen-decreased Young's modulus induced inter-lamellar cracks transforming into trans-lamellar cracks, decreasing cracks in the hydrogenated alloy. Additionally, hydrogen-induced mechanical twinning in γ-phase lamellae partly restrained inter-lamellar crack propagation. In the two-step forging process, the optimum forging parameters were determined. It was found that hydrogen could effectively restrain crack propagation during the two-step forging process. Hydrogen refined grains of the forged billets, which improved toughness of such billets. The hydrogen content of the forged hydrogenated billets could be decreased to a desired value, and the phase composition and content were basically identical to those of the initial unhydrogenated alloy.     

THERMAL SINGULAR STRESSES NEAR A CIRCUMFERENTIAL EDGE CRACK IN A SPHERICAL CAVITY UNDER UNIFORM AXIAL HEAT FLOW

The linear thermoelastic problem of a spherical cavity with a circumferential edge crack is solved. The thermal stresses are caused by a uniform heat flow disturbed by the presence of the crack and the cavity. The surfaces of the crack and the cavity are assumed to be insulated. Integral transform techniques are used to reduce the problem concerning the temperature and thermoelastic fields to that of solving two singular integral equations of the first kind. The integral equations are solved numerically and the variation of the thermal stress intensity factor with the crack depth and the crack opening displacement are shown graphically.     

含交叉裂隙类岩石试样物理力学特性试验及其RFPA模拟

为探究含交叉裂隙试样在单轴应力下的物理力学特性,制备了与岩石力学性质相似的类岩石材料,利用预制树脂片法模拟了不同倾角交叉裂隙的情况,对其进行了单轴压缩试验,同时利用RFPA软件对不同工况进行了数值模拟。结果表明,含交叉裂隙类岩石试样在单轴应力下主要有单一裂隙扩展模式、主裂纹扩展贯通模式、次生裂纹扩展贯通模式三种破坏模式;单一裂隙扩展模式与主裂纹扩展模式下,次生裂隙尖端产生剪破坏,主裂纹尖端产生拉破坏,而次生裂纹扩展模式下主裂纹尖端产生剪破坏,次生裂纹尖端产生拉破坏,最终产生的翼裂纹与反翼裂纹属于拉剪破坏;单轴应力下的含交叉裂隙试样应力-应变曲线经历弹性变形阶段、非线性变形阶段、残余变形阶段三个阶段;主裂纹倾角对含交叉裂隙试样的峰值强度起到控制性作用,而次生裂隙仅在主裂纹倾角为0°时对试样的峰值强度有较大影响。