Ti60钛合金中富钕稀土相颗粒对叶片室温振动疲劳性能的影响

采用振动疲劳实验及SEM断口分析等方法,研究了含稀土元素Nd的Ti60高温钛合金稀土相颗粒对叶片振动疲劳裂纹萌生的影响.结果表明:叶片振动疲劳寿命与稀土相颗粒的尺寸和分布位置具有密切关系.稀土相颗粒尺寸越大,对叶片振动疲劳裂纹的萌生作用也越大,疲劳寿命降低;外露于或镶嵌于叶片表面的稀土相颗粒明显促进了叶片振动疲劳裂纹的萌生.     

曲轴弯曲疲劳试验裂纹研究与失效判定

通过对疲劳裂纹的产生、扩展机理研究和试验,结合疲劳试验数据分析了疲劳裂纹扩展与疲劳寿命的关系。试验结果及分析表明,大量经圆角滚压强化的球墨铸铁曲轴,疲劳试验运行107次在连杆颈与曲柄臂过度圆角产生微裂纹,该裂纹属于非扩展裂纹,继续试验时裂纹不扩展,试样在该载荷下具有无限疲劳寿命。     

伺服机构涡轮泵转子叶片断裂失效分析

某伺服液压源涡轮泵转子在测试过程中转子叶片发生断裂,通过对失效转子的观察、测试与分析认为:转子叶片的断裂性质为疲劳断裂,断裂原因是在叶片根部存在疲劳裂纹,疲劳裂纹在动静载荷作用下失稳扩展而发生疲劳断裂.分析认为转子叶片发生疲劳破坏与其组织在锻造成型工艺过程中存在工艺缺陷,导致材料疲劳寿命下降有关.     

某压气机钛合金叶片掉块失效分析

通过对某高压压气机工作叶片掉块的宏观、微观分析,认为叶片的掉块原因是叶片长度超差使叶尖与机匣产生摩擦,磨屑堆积在叶背产生裂纹,形成疲劳源,最后造成疲劳裂纹扩展.     

超声疲劳试验方法对S06钢疲劳性能及裂纹萌生机制的影响

采用超声疲劳试验方法和试样尺寸相近的高频疲劳试验方法对S06钢进行疲劳性能试验,将试验数据和试样断口形貌进行对比,研究超声疲劳试验方法对S06钢疲劳性能和裂纹萌生机制的影响.结果表明:在相同的应力水平下,超声加载频率下S06钢的疲劳寿命高于高频疲劳试验测得的疲劳寿命;超声疲劳试验中裂纹全部从表面萌生,而高频疲劳试样裂纹有表面萌生和内部萌生两种机制.分析了超声加载频率对S06钢疲劳性能和裂纹萌生机制产生影响的原因:对疲劳性能的影响与金属材料的晶体结构和裂纹尖端的化学反应有关,对裂纹萌生机制的影响与试样表层残余应力松弛有关.     

黄台发电厂7#机末级叶片断口分析

对黄台发电厂７＃机末级叶片断口进行了分析。认为,在该叶片进汽侧靠近头部的硬质合金堆焊层中,由于成分偏析,夹杂等原因产生微小裂纹,并在运行中产生应力腐蚀或腐蚀疲劳裂纹并扩展,最终导致叶片断裂。     

42CrMo钢疲劳裂纹扩展剩余寿命评估

以高频三点弯曲疲劳试验机为平台,进行42CrMo钢疲劳裂纹扩展试验研究,通过建立裂纹扩展剩余寿命评估模型,实现对存在裂纹的工程机械零部件剩余寿命的评估。采用显微成像测试系统实时采集并测量疲劳扩展裂纹,使用声发射系统监测整个疲劳裂纹扩展过程。结果表明:声发射幅值、能量等特征参数可以实时反应疲劳裂纹萌生、稳定扩展和失稳扩展等各个损伤阶段,并在疲劳断裂时产生急剧的突变;裂纹扩展速率的对数值与应力强度因子幅的对数值具有较高的线性相关性,建立了不同应力工况条件下裂纹扩展剩余寿命评估模型,以双排链轮轴为例进行裂纹扩展剩余寿命评估;随着疲劳应力的增加,裂纹扩展剩余寿命减小。     

疲劳破坏概率分析中的细节疲劳额定值研究

本文对小裂纹阶段疲劳寿命与裂纹长度之间的关联进行了探讨，给出了结构细节疲劳额定值（ＤＦＲ）与疲劳裂纹长度之间关系的数学模型，并通过试验进行了分析和验证，提供了基于概率疲劳的较小裂纹的疲劳破坏概率的分析方法。     

考虑结构裂纹扩展的振动疲劳寿命计算方法

工程实践中任何结构都存在不同程度的裂纹损伤,振动激励下动响应与疲劳裂纹扩展之间互相耦合,直接影响结构振动疲劳寿命.为了考虑结构振动疲劳耦合效应对疲劳寿命的影响,提出了一种考虑结构裂纹扩展的振动疲劳寿命计算方法.分析时,通过建立若干个含不同长度裂纹的结构有限元模型模拟结构裂纹扩展,采用Paris方程分段计算结构振动疲劳裂纹扩展寿命,通过试验确定的固有频率降变化规律反推结构裂纹萌生寿命,最后累计得到结构疲劳总寿命.结论表明,仿真计算结果与试验结果比较吻合.     

非对称循环加载下燃气轮机叶片材料疲劳寿命预测

燃气轮机叶片在实际工作过程中,易受到非对称循环载荷作用而发生疲劳失效。为便于准确地预测非对称循环加载下叶片材料的裂纹萌生及扩展寿命,首先考虑平均应力效应和高应力区的塑性变形影响,对Chaboche模型进行改进,然后将改进的Chaboche模型和Walker裂纹扩展公式相结合,建立了一种非对称循环载荷作用下裂纹萌生及扩展综合寿命模型,并用试验数据进行验证分析。结果表明,该模型预测得到的裂纹萌生寿命、裂纹扩展寿命与试验数据基本吻合,验证了新模型的适用性和准确性,为燃气轮机叶片损伤分析和寿命预测奠定理论基础。     

Evaluation of corrosion fatigue crack propagation life at low-pressure steam turbine rotor groove

The corrosion fatigue crack propagation life of Christmas-tree type rotor groove with three hooks is studied. Each corner of the hook can be a candidate for crack initiation site therefore the condition where cracks initiate and propagate simultaneously at several hook corners must be considered. When a blade is inserted in the rotor groove, narrow gap is introduced unavoidably between the rotor groove and the blade root. The effect of this narrow gap on the crack behavior must also be considered. A procedure was presented to assess the crack initiation and propagation behavior under such a condition. Using the procedure, crack initiation and propagation behavior was evaluated for several gap conditions. It was revealed that the gap condition had little effect on the relation between crack depth at the third hook corner and life consumption ratio (ratio of loading cycle to final failure life). A corrosion fatigue test was performed using a rotor groove model specimen, and the results were compared with the evaluation results.     

Multiaxial small fatigue crack growth in metals

Observations related to the formation and growth of small cracks ranging from subgrain dimension up to the order of 1 mm are summarized for amplitudes ranging from low cycle fatigue (LCF) to high cycle fatigue (HCF) conditions for polycrystalline metals. Further efforts to improve the accuracy of life estimation which address LCF, HCF and LCF–HCF interactions must consider various factors that are not presently addressed by conventional elastic–plastic fracture mechanics (EPFM) or linear elastic fracture mechanics (LEFM) approaches based on long, self-similar cracks in homogeneous, isotropic materials, nor by conventional HCF design tools such as the ε–N curve, the S–N curve, modified Goodman diagram and fatigue limit.Development of microstructure-sensitive fatigue crack propagation relations relies on deeper understanding of small crack behavior, including (a) interactions with microstructure and lack of constraint for microstructurally small cracks, (b) heterogeneity and anisotropy of cyclic slip processes associated with the orientation distribution of grains, and (c) local mode mixity effects on small crack growth. The basic technology is not yet sufficiently advanced in these areas to implement robust damage tolerant design for HCF. This paper introduces an engineering model which approximates the results of slip transfer calculations related to crack blockage by microstructure barriers; the model is consistent with critical plane concepts for Stage I growth of small cracks, standard cyclic stress–strain and strain–life equations above threshold, and the Kitagawa diagram for HCF threshold behaviors. It is able to correlate the most relevant trends of small crack growth behavior, including crack arrest at the fatigue limit, load sequence effects, and stress state effects.     

Influence of surface recrystallization on the low cycle fatigue behaviour of a single crystal superalloy

This paper investigated the effect of surface recrystallization (RX) on the low cycle fatigue (LCF) behaviour of a single crystal (SX) superalloy. LCF tests on both raw and recrystallized samples showed that fatigue life was significantly reduced by surface RX. Fractography indicated that fatigue cracks initiated from the casting defects in RX layer and multiple crack initiations were commonly observed. Moreover, RX grains exhibited predominantly transgranular cracking, in contrast to the intergranular fracture reported in literature. The fatigue crack propagation behaviour was discussed in light of fracture mechanics and crack growth life model. The fatigue cycles required to penetrate RX layer were estimated to be about one magnitude lower than that in forming an equivalent crack in SX specimens. It is suggested that the earlier crack initiation and promoted crack propagation in RX layer, as well as the trend of multiple initiations, are responsible for the fatigue degradation by RX.     

压力容器用钢低周疲劳表面裂纹扩展速率试验研究

压力容器的表面裂纹常常会导致低周疲劳破坏.因此,研究压力容器用钢的低周疲劳表面裂纹扩展规律,具有重要意义.本文在带有上下对称圆弧板状试样的上圆弧几何中心,加工预制了半椭圆表面裂纹,以模拟压力容器具有表面裂纹时的特点.同时,在上圆弧一侧设计加工了特定形状和尺寸的刀口,装卡COD位移规,用来控制表面裂纹前缘的应变.以10C...     

MB8镁合金高周疲劳实验研究

采用升降法对MB8镁合金室温高周疲劳行为进行实验研究。结果表明:利用升降法计算出MB8镁合金在应力比R=0.1,循环基数为107条件下的疲劳强度为90.2MPa,相当于其抗拉强度的34%左右;合金的疲劳裂纹萌生于试样表面,裂纹扩展区由小的平面状断面组成,没有明显的疲劳辉纹存在,合金疲劳断口呈现韧性断裂特征。     

Fretting fatigue crack analysis in Ti–6Al–4V

A study was conducted to verify the efficacy of a fracture mechanics methodology to model the crack growth behavior of fretting fatigue-nucleated cracks obtained under test conditions similar to those found in turbine engine blade attachments. Experiments were performed to produce cracked samples, and fretting fatigue crack propagation lives were calculated for each sample. Cracks were generated at 10⁶ cycles (10%-of-life) under applied stress conditions previously identified as the fretting fatigue limit conditions for a 10⁷ cycle fatigue life. Resulting cracks, ranging in size from 30 to 1200 μm, were identified and measured using scanning electron microscopy. Uniaxial fatigue limit stresses were determined experimentally for the fretting fatigue-cracked samples, using a step loading technique, for R=0.5 at 300 Hz. Fracture surfaces were inspected to characterize the fretting fatigue crack front indicated by heat tinting. The shape and size of the crack front were then used in calculating ΔKth values for each crack. The resulting uniaxial fatigue limit and ΔKth values compared favorably with the baseline fatigue strength (660 MPa) for this material and the ΔKth value (2.9 MPa√m) for naturally initiated cracks tested at R=0.5 on a Kitagawa diagram.Crack propagation lives were calculated using stress results of FEM analysis of the contact conditions and a weight function method for determination of ΔK. Resulting lives were compared with the nine million-cycle propagation life that would have been expected in the experiments, if the contact conditions had not been removed. Scatter in the experimental results for fatigue limit stresses and fatigue lives had to be considered as part of an explanation why the fatigue life calculations were unable to match the experiments that were modeled. Analytical life prediction results for the case where propagation life is observed to be very short experimentally were most accurate when using a coefficient of friction, μ=1.0, rather than for the calculations using μ=0.3     

MSP试验法评价PZT陶瓷的循环疲劳寿命

采用小样品力学性能试验方法(Modified Small Punch Tests,简称MSP)对Pb(Zr,Ti)O3陶瓷(PZT)实施了不同大小应力下的循环疲劳实验,循环应力越大,样品的残余强度和压电常数衰减越快,这是由应力循环过程中大量微裂纹的产生和扩展所致.通过最大强度值与疲劳寿命的对应关系求得100 Hz循环疲劳下该样品的裂纹扩展指数n为395,由此推测了PZT陶瓷样品的使用寿命,在循环应力的最大值不超过79.1 MPa的条件下,该样品的连续使用寿命可达5年以上.     

Prediction of short fatigue crack growth of Ti‐6Al‐4V

It is observed that the short fatigue cracks grow faster than long fatigue cracks at the same nominal driving force and even grow at stress intensity factor range below the threshold value for long cracks in titanium alloy materials. The anomalous behaviours of short cracks have a great influence on the accurate fatigue life prediction of submersible pressure hulls. Based on the unified fatigue life prediction method developed in the authors' group, a modified model for short crack propagation is proposed in this paper. The elastic–plastic behaviour of short cracks in the vicinity of crack tips is considered in the modified model. The model shows that the rate of crack propagation for very short cracks is determined by the range of cyclic stress rather than the range of the stress intensity factor controlling the long crack propagation and the threshold stress intensity factor range of short fatigue cracks is a function of crack length. The proposed model is used to calculate short crack propagation rate of different titanium alloys. The short crack propagation rates of Ti‐6Al‐4V and its corresponding fatigue lives are predicted under different stress ratios and different stress levels. The model is validated by comparing model prediction results with the experimental data.     

High temperature fatigue of nickel-base superalloys – A review with special emphasis on deformation modes and oxidation

Low cycle fatigue, high cycle fatigue, fatigue crack propagation and thermo-mechanical fatigue in Ni-base superalloys are reviewed in terms of fundamental deformation mechanisms, environmental effects, and interactions between environment and deformation mode. These factors are related to the chemical composition and underlying microstructure for all currently-used product forms (i.e. powder metallurgy, wrought, conventionally cast and single crystal). The basic principles that are developed are used to show how both intrinsic and extrinsic variables can be manipulated to control fatigue behaviour and as a guide for formulation of engineering life prediction models.