金属材料疲劳裂纹扩展曲线的拟合方法研究

提出了一种金属材料疲劳裂纹扩展曲线(a~N曲线)的拟合方法,即修正的双曲函数拟合方法。为检验这种曲线拟合方法的适用性,利用2024-T42铝合金CCT试样的疲劳裂纹扩展试验数据,对该铝合金的疲劳裂纹扩展曲线进行了拟合分析,并给出了疲劳裂纹扩展速率和扩展寿命。通过疲劳裂纹扩展寿命计算结果和试验结果的对比分析,证明这种拟合方法具有较高的拟合精度。     

2A12铝合金疲劳裂纹的成核与扩展机理EI北大核心CSCD

进行了不同应力水平下和不同应力比下2A12铝合金试验件的疲劳试验,并对试验件疲劳断口显微结构进行分析和对比,以揭示疲劳裂纹成核与扩展的微观特征。结果表明:2A12铝合金的疲劳裂纹在靠近试验件表面较为粗大的第二相粒子或试验件棱角缺陷处成核;成核位置距试验件表面的距离与应力水平和应力比有关,疲劳裂纹扩展区域也与应力水平和应力比有关。应力水平较小或应力较大时,疲劳裂纹沿晶扩展和穿晶扩展尤为明显。疲劳断口的裂纹成核粒子越小(大),试验件疲劳寿命越短(长),微裂纹成核寿命是疲劳全寿命计算中不可忽视的部分。     

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.     

金属材料疲劳裂纹扩展速率不同模型应用分析

金属材料疲劳裂纹扩展速率是对金属材料及其零构件含有裂纹体后安全服役评价及疲劳寿命评估的一项重要力学性能指标.基于金属材料疲劳裂纹扩展过程的不同阶段、闭合效应、应力比对疲劳裂纹扩展速率的影响等多个方面,对几种常见金属材料疲劳裂纹扩展速率模型的应用特点进行了对比分析,为安全设计及寿命评估时不同模型的选择应用提供参考.     

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。     

不同回火温度下贝氏体热轧钢轨疲劳裂纹的扩展速率

用MTS810试验机进行了贝氏体热轧钢轨疲劳裂纹扩展速率试验,并用Paris公式分析了不同回火温度下贝氏体钢轨疲劳裂纹扩展速率试验数据。结果表明,Paris公式可以描述不同回火温度下贝氏体热轧钢轨疲劳裂纹扩展速率,并可用来预测裂纹扩展情况。     

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。     

金属材料疲劳裂纹扩展速率Paris模型中材料常数的相关性

金属材料疲劳裂纹扩展速率是工程应用中进行损伤容限设计、疲劳寿命评估的一项重要力学性能指标.Paris模型是疲劳裂纹扩展速率最常用的表达形式,该模型认为疲劳裂纹扩展速率随裂纹前缘应力强度因子范围的变化呈幂函数关系,涉及到两个材料常数C和m.本研究借助部分文献中的试验数据,分析合金钢、铜合金、钛合金、铝合金等不同金属材料疲...     

7050铝合金Ⅰ-Ⅱ复合型疲劳裂纹研究

为研究7050铝合金在Ⅰ-Ⅱ型复合加载下疲劳裂纹扩展规律,在Amsler HFP5000高频试验机上利用Richard加载装置,完成紧凑拉剪(CTS)试样疲劳裂纹扩展试验,利用有限元对Ⅰ-Ⅱ复合型裂纹进行数值模拟,采用APDL命令流计算不同裂纹长度的应力强度因子,并引入最大周向应力准则计算裂纹扩展角,用有限元计算等效应...     

金属材料低温疲劳性能的预测方法

本文总结了作者近来研究金属低温疲劳取得的进展,主要内容包括:疲劳极限的热激活模型、应变疲劳公式、疲劳始裂寿命和裂纹扩展速率的定量预测方法、疲劳裂纹扩展机制脆转模型。当室温下的疲劳极限和门槛植ΔK_(th)确定后,应用本文的方法可根据拉伸性能定量预测材料在低温下的疲劳极限、应变疲劳寿命、疲劳始裂寿命和裂纹扩展速率,并且不需要低温疲劳试验和经验修正。     

Characterization of the durability of adhesive bonds

The deterioration of adhesive bonds as a result of exposure to aggressive service environments is a problem for both metallic and composite bonded joints in aircraft structures. In this context airworthiness issues arising from the in service growth of small naturally occurring defects in adhesively bonded repairs has led to the need to develop a fracture mechanics based approach for setting the inspection intervals associated with the growth of such naturally occurring defects. This involves issues associated with both durability and damage tolerance that are associated with such small defects because it is well known that, for metals, the use of long crack data can lead to erroneous non‐conservative estimates. As such a means for assessing the issues related to such small naturally occurring defects is urgently needed. In this context the Boeing Wedge test, and its related variants, plays a central role in not only qualitatively assessing environmental performance but also in ranking surface treatments. However, this method has the problem that the initial defect introduced during wedge testing may not be representative of the small initial defects that can arise in service. Thus the relative ranking of various surface treatments obtained using wedge tests may be misleading. To this end, the paper presents a simple fracture mechanics based method that has the potential to account for the discrepancy in the initial defect size. This approach resembles crack growth laws developed to account for the growth of both long and short cracks in metals, environmentally enhanced crack growth in metals and delamination growth in composites.     

裂纹闭合与疲劳裂纹扩展力的测试方法

介绍了一种在近十年里发展的裂纹扩展力测量方法，这种方法将裂纹扩展的门槛值行为与直流电位法结合起来，可以测量某一载荷下裂纹开始扩展的应力强度因子Ｋｐｒ，并由此计算有效应力强度因子范围ΔＫｅｆｆ＝Ｋｍａｘ－Ｋｐｒ。     

Examination of the fatigue crack growth equations

Most of the crack growth equations proposed so far correlate the crack growth rate (da/dN or da/dt) with crack tip parameters such as the stress intensity factor (SIF) or energy release rate (ERR). In our previous works, an experimental setup was designed to examine the applicability and the boundary of the functional relationship between da/dN and the crack tip parameters, particularly, ERR. In the present paper, the variation of the ERR along the experimentally observed curvilinear crack trajectories is obtained by means of the finite element method. The analysis shows that the Paris-Erdogan type of laws are applicable until the crack tip is located outside the strong crack-defect interaction region (SI region). A functional relationship between da/dN and ERR breaks down within this region. This suggests the existence of additional crack tip parameters that are not accounted for within conventional fracture mechanics. An approach to modeling the observed phenomenon is discussed following the concept of the Crack Layer theory.     

Fatigue and creep crack growth of Alloy 800 and Alloy 617 at high temperatures

A common evaluation is given for creep crack growth and fatigue crack growth experiments which have been performed at the companies ABB, Siemens-KWU and KFA. The materials under investigation were X10NiCrAlTi32 20 (Alloy 800) and NiCr22Col2Mo (Alloy 617). Several production lots and semi-finished materials as well as welded materials have been tested. Testing techniques differed at the different labs. In order to eliminate the influence of individual testing techniques, material from some production lots was investigated at different labs. The given data cover fatigue crack growth (the materials were tested between room temperature and 1050°C; the influence of temperature, R?ratio, and frequency was investigated) and creep crack growth (Alloy 800 was tested between 550°C and 900°C, Inconel 617 between 800°C and 1000°C; the evaluation was done on the basis of the fracture mechanics parameters K₁ and C*).     

A numerical study on the effect of symmetric crack flank holes on fatigue life extension of a SENT specimen

The crack growth retardation is studied theoretically by drilling two symmetric holes at appropriate locations along the crack flanks. A fatigue crack growth code for two‐dimensional elastic problems is developed to approve the effectiveness of this method. The crack growth retardation was examined using a parametric study both on the arrangements of the flank holes and the diameters of holes. The numerical results reveal that for some cases, the presence of flank holes significantly decreases the stress concentration around the crack tip and its stress intensity factor. The best positions for crack flank holes to provide the highest crack growth retardation are shown to be where the line connecting the two centres of holes passes the crack tip. A larger hole diameter and a closer distance between the flank holes and the crack tip result in lower crack growth rate and higher fatigue life.     

Mixed mode fatigue crack growth: A literature survey

The applications of fracture mechanics have traditionally concentrated on crack growth problems under an opening or mode I mechanism. However, many service failures occur from growth of cracks subjected to mixed mode loadings. This paper reviews the various criteria and parameters proposed in the literature for predictions of mixed mode crack growth directions and rates. The physical basis and limitations for each criterion are briefly reviewed, and the corresponding experimental supports are discussed. Results from experimental studies using different specimen geometries and loading conditions are presented and discussed. The loading conditions discussed consist of crack growth under mode II, mode III, mixed mode I and II, and mixed mode I and III loads. The effects of important variables such as load magnitudes, material strength, initial crack tip condition, mean stress, load non-proportionality, overloads and crack closure on mixed mode crack growth directions and/or rates are also discussed.     

Effects of annealing and quenching on fatigue behaviour in type 444 ferritic stainless steel

In order to understand the effects of annealing and quenching on fatigue behaviour in type 444 stainless steel, fully reversed axial fatigue tests have been performed using smooth specimens of heat‐treated materials in laboratory air and 3%NaCl aqueous solution. Three materials subjected to different heat treatments, annealing at 960 and 1000 °C, and water‐cooling at 960 °C, were prepared. In laboratory air, the fatigue limit of the annealed specimens was lower than that of the as‐received specimen and decreased with increasing annealing temperature. The subsequent grain coarsening from the heat treatments was primarily responsible for the lower fatigue strength in the annealed specimens. The fatigue strength of the water‐cooled specimen was lower than that of the corresponding annealed specimen. In the annealed specimens, cracks were generated within ferritic grains, while in the water‐cooled specimen, at or near grain boundaries. In 3%NaCl solution, the fatigue strengths of all specimens decreased compared with those in laboratory air. Only in the water‐cooled specimens, crack initiation at grain boundary and intergranular crack growth were observed, indicating the most sensitive to corrosion environment.     

Using fatigue crack propagation mechanism maps to predict changes in propagation mechanisms

The conditions determining the fatigue fracture mechanism in quenched and tempered steel are discussed with reference to fatigue crack propagation mechanism (FCPM) maps. Criteria for the change from one fatigue mechanism to another are presented.     

Micromechanisms of fatigue crack growth in aluminium alloys

The fatigue crack growth characteristics of high-strength aluminium alloys are discussed in terms of behaviour during mechanical testing and fracture surface appearance. For a wide range of crack growth rates, the crack extends both by the formation of ductile striations and by the coalescence of micro-voids. Dimples are observed at stress intensities very much less than the plane strain fracture toughness, and this is explained in terms of the probability of inclusions lying close to the crack tip. The striation formation process is described as a combination of environmentally-enhanced cleavage processes and plastic blunting of the crack tip.     

Mixed-mode crack opening displacement measurements for small fatigue cracks growing in Astroloy at 20 °C

Crack opening displacements were measured for small fatigue cracks in Astroloy being grown with uniaxial stress application under high-cycle fatigue conditions. Four cracks were investigated including one that grew from 27 to 74 μm in three increments. Most of the cracks grew at an angle to the loading axis and all opened bimodally. Crack opening scaled with distance from the crack tip similar to an elastic crack, which allowed the calculation of a local stress intensity factor for both mode I and mode II. The proportion of mode II stress intensity factor was relatively large, varying as 0.06 < Δ K _II /Δ K _I < 0.42, with an average of ~0.3. Thus, uniaxial loading remote to the cracks resulted in a bimodal opening response on the scale of the cracks.