不同腐蚀环境对7475-T7351铝合金疲劳性能及裂纹扩展速率的影响

采用轴向加载疲劳和疲劳裂纹扩展速率性能测试方法,研究了不同腐蚀环境对7475-T7351铝合金厚板疲劳及裂纹扩展性能的影响.结果表明:腐蚀环境对7475铝合金的疲劳性能有较大影响,油箱积水和3.5%(质量分数,下同)NaCl溶液中光滑试样的疲劳强度较室温下降约68%,油箱积水和3.5%NaCl溶液环境对材料疲劳强度的影响程度基本相同;不同环境腐蚀(空气和3.5%NaCl)和不同温度(室温和125℃)对材料的低周疲劳性能影响不大;腐蚀环境对裂纹扩展有较明显的加速作用,油箱积水和3.5%NaCl溶液环境对裂纹扩展的加速规律基本一致.     

喷射成形GH738合金的疲劳裂纹扩展行为

进行了不同温度、频率和应力比条件下喷射成形GH738合金紧凑拉伸(CT)试样的疲劳裂纹扩展试验,分析了相应条件下的疲劳裂纹扩展速率及其对疲劳裂纹扩展行为的影响规律。结果表明:随着温度的升高,裂纹扩展速率略有加快;加载频率降低,疲劳裂纹扩展加速;裂纹扩展速率da/d N随应力比R的增大而增大。疲劳断口呈现多裂纹源特征,裂纹稳定扩展为疲劳条带机制。     

应力比对航空高强LD2CZ铝合金腐蚀疲劳裂纹扩展的影响

利用扫描电镜联合液压伺服试验机,并借助于Walker公式研究了应力比对预腐蚀不同时间航空高强LD2CZ铝合金疲劳裂纹扩展的影响,在应力比分别为0．05,0．5,0．7的条件下对预腐蚀0,15,30d的LD2CZ铝合金单边缺口板状试样进行了疲劳加载试验,得到了其疲劳裂纹扩展速率曲线,并拟合出了Walker公式中的材料常数。结果表明：裂纹扩展速率会随着应力比的增加以及腐蚀损伤的加深而增大,拟舍得到的Walker公式可用来定量化地表征应力比和腐蚀损伤对疲劳裂纹扩展速率的影响。     

沉积颗粒对7N01-T6铝合金疲劳裂纹扩展行为的影响

利用自主设计的实验方法,结合疲劳裂纹扩展速率测试以及断口微观形貌观察研究了R=0.1和R=0.5两种应力比下,石墨和氧化铝沉积颗粒对7N01-T6铝合金疲劳裂纹扩展行为的影响。结果表明:在两种应力比条件下,裂纹扩展Ⅰ、Ⅱ阶段中,合金在石墨颗粒环境下的疲劳裂纹扩展速率均最快。当R=0.5时,在裂纹扩展Ⅰ、Ⅱ阶段,合金在氧化铝颗粒环境下的疲劳裂纹扩展速率最慢。当R=0.1时,在应力强度因子ΔK15 MPa·m~(1/2)阶段,合金在氧化铝颗粒环境下的疲劳裂纹扩展速率最慢,在ΔK=15～30 MPa·m~(1/2)阶段,合金在氧化铝颗粒环境和大气环境下的疲劳裂纹扩展速率相当。石墨颗粒环境下合金疲劳裂纹扩展速率的增加是由于石墨颗粒的润滑作用降低了疲劳卸载过程中的裂纹闭合效应。氧化铝颗粒环境下合金疲劳裂纹扩展速率的降低是由于氧化铝颗粒在断面的沉积增强了疲劳卸载过程中的裂纹闭合效应。     

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。     

低温环境下桥梁钢Q345qD疲劳裂纹扩展行为研究

为了明确在寒冷地区服役桥梁钢的疲劳裂纹扩展行为,以16 mm厚桥梁钢Q345qD为研究对象,完成了室温和低温下的夏比冲击韧性试验、疲劳裂纹扩展速率试验和疲劳裂纹扩展门槛值试验。结果表明,夏比冲击功和试样断口剪切断面率随温度的降低而减少;在应力比0.1、0.2和0.5条件下,疲劳裂纹扩展速率均随温度降低而变缓,该桥梁钢的疲劳韧-脆转变温度点在-60℃以下;在室温~-60℃,其裂纹扩展速率均对应力比的变化不敏感;应力比0.1条件下的疲劳裂纹扩展门槛值随温度的降低有略微增大的趋势。该批次桥梁钢表现出了良好的抵抗低温疲劳裂纹扩展性能,防止低温脆性破坏成为疲劳设计的重点;试验数据能为钢结构桥梁的进一步抗低温疲劳和防低温冷脆断裂设计提供参考。     

基于遗传规划算法的不同应力比下不同厚度7050铝合金疲劳裂纹扩展寿命预测

针对不同厚度7050铝合金试样进行了不同应力比条件下的一系列疲劳裂纹扩展试验,并运用遗传规划算法对疲劳裂纹扩展寿命进行预测。遗传规划算法是模拟自然界中生物的进化策略,通过交换、突变等遗传操作,搜索目标的最优解。建立7050铝合金疲劳裂纹扩展速率的遗传规划模型,并利用试验数据对模型进行测试,后与其他典型疲劳裂纹扩展模型进行比较。研究结果表明:GP模型预测的7050铝合金疲劳裂纹扩展寿命结果与试验值基本吻合,相对误差小于1.5%,且GP模型预测结果的准确性高于Paris模型和Walker模型。     

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。     

激光增材制造体育器材用TC4钛合金疲劳裂纹扩展行为研究

目的 揭示应力比对增材制造TC4钛合金疲劳裂纹扩展行为的影响规律。方法 采用紧凑型拉伸试样,在恒载荷幅条件下对激光增材制造TC4钛合金进行了应力比为0.1、0.3和0.5的疲劳裂纹扩展实验,定量评价了不同应力比下合金的疲劳裂纹扩展速率和变化规律。基于Paris公式对裂纹扩展速率进行了拟合,分析了应力比对各参数的影响规律。最后通过扫描电镜对断口表面形貌进行了观察,分析了应力比对断裂模型的影响。结果 在相同的?K条件下,疲劳裂纹扩展速率随着应力比的增大而增大。在Paris公式中,参数C随应力比的增大而减小,参数m随应力比的增大而增大,并且m和lg C呈现线性关系。随应力比的增大,断口表面的河流花样增多、疲劳辉纹变浅、二次裂纹数量增加。结论 应力比引起的裂纹尖端闭合效应和平面应力比变化是导致裂纹扩展速率发生改变的主要原因。     

14MnNbq焊接桥梁钢的疲劳裂纹扩展行为研究

该文系统地研究了14MnNbq桥梁钢焊接热影响区的疲劳裂纹扩展行为。首先,由中心穿透裂纹(MT)试样疲劳裂纹扩展试验,获得了不同应力比R下的疲劳裂纹扩展速率和门槛值;然后考察了应力比R的影响,给出了适于不同应力比的疲劳裂纹扩展速率和门槛值的一般表达式;最后提出了一种由疲劳裂纹扩展门槛值▽Kth确定闭合参数U的新方法,将控制疲劳裂纹扩展的有效应力强度因子幅度写为▽Keff=▽K-▽Kth,由此讨论闭合参数U的确定方法。研究结果表明:对于14MnNbq焊接桥梁钢,该文给出的疲劳裂纹扩展速率表达式与试验结果符合得相当好。     

Fatigue damage discrepancies,the intrinsic fatigue damage map locus and the multiple stages of the similitude concept

The work demonstrates how the theory of the fatigue damage map (FDM) can be used in order to define a closed form locus where fatigue crack growth can be sought. The Intrinsic FDM Locus depending only on readily material properties represents a tool able to accurately predict crack growth of polycrystals. If the Locus is expressed in terms of a surface plot after triangulation of the data, it concludes into visualization of the potential for multiple similitude stages. The stages are defined as a function of the maximum far field stress and ΔK. Multiple similitude stages are found to dominate the short and near threshold area (Stage I growth) and represent direct result of the effect of polycrystalline behaviour to flow resistance. The work concludes that interrogation of the points defining the Intrinsic FDM Locus and related to the three thresholds can provide potential characteristics in the quest for an ideal damage tolerance material.     

On the design and test of equivalent configurations for notch and fretting fatigue

This work investigates the possibility of designing fretting and notch fatigue experiments that are nominally equivalent in terms of damage evaluated by a multiaxial fatigue model. The methodology adopted to carry out this search considered a cylinder‐on‐flat contact geometry and a V‐notched plate. The loading conditions and geometries of the experimental configurations were adjusted to obtain the same decay of the multiaxial fatigue index from the hot spot up to a critical distance. Aluminium alloy 7050‐T7451 was used in the experimental evaluation of the methodology. Considering the well‐known scatter of fatigue data and the limited number of specimens available, the obtained results suggest that the use of the notch analogy in fretting fatigue is appropriate.     

On the life-controlling microstructural fatigue mechanisms in ductile metals and alloys in the gigacycle regime

The high-cycle fatigue (HCF) behaviour of ductile metals and alloys, and the life-controlling microstructural fatigue mechanisms known from HCF are reviewed critically with respect to their possible role in the gigacycle or ultra-high-cycle fatigue (UHCF) regime. Arguments are presented to support the hypothesis that, at the very low amplitudes of the UHCF regime, fatigue crack initiation, resulting from cyclic strain localization, and slow early Stage I fatigue crack propagation are the life-controlling mechanisms and that these processes can essentially be described in terms of the microstructurally irreversible portion of the cumulative cyclic plastic strain. Emphasis is placed on the important role of the so-called slip irreversibility which decreases as the amplitude becomes lower and lower. Finally, the Manson–Coffin law is reformulated for very low amplitudes in terms of microstructurally relevant parameters, and a fatigue life diagram is developed, based on these preceding microstructural considerations. Important features of this diagram are: (i) the plastic strain fatigue limit in the HCF regime which is related to the threshold for cyclic strain localization in persistent slip bands; and (ii) the transition from this plastic strain fatigue limit to a threshold of negligible slip irreversibility at still lower amplitudes in the UHCF regime.     

On the estimation of the material fatigue properties required to perform the multiaxial fatigue assessment

To accurately perform the fatigue assessment of engineering components subjected to in‐service multiaxial fatigue loading, the adopted design criterion must properly be calibrated, the used information usually being the fatigue strength under both pure uniaxial and pure torsional fatigue loading. Because of the complex fatigue response of metallic materials to multiaxial loading paths, the only reliable way to generate the necessary pieces of calibration information is by running appropriate experiments. Unfortunately, because of a lack of both time and resources, very often, structural engineers are requested to perform the multiaxial fatigue assessment by guessing the necessary fatigue properties. In this complex scenario, initially, the available empirical rules suitable for estimating fatigue strength under both pure axial and pure torsional fatigue loading are reviewed in detail. Subsequently, several experimental results taken from the literature and generated by testing metallic materials under a variety of proportional and non‐proportional multiaxial loading paths are used to investigate the way such empirical rules affect the accuracy in estimating fatigue strength, the damage extent being evaluated according to the modified Wöhler curve method. Such a systematic validation exercise allowed us to prove that under proportional loading (with both zero and non‐zero mean stresses), an adequate margin of safety can be reached even when the necessary calibration information is directly estimated from the material ultimate tensile strength. On the contrary, in the presence of non‐proportional loading, the use of the empirical rules reviewed in the present paper can result, under particular circumstances, in a non‐conservative fatigue design.     

载荷频率对金属及其合金高周疲劳特性的影响

采用超声高频加载方法进行高周疲劳试验时,应考察载荷频率对材料疲劳特性的影响。本文从微观组织结构和外部环境两方面论述了疲劳特性的频率效应,从裂纹扩展率、断口分析等方面回顾和介绍了频率对合金钢、钛合金、铝合金三类常用金属疲劳特性的影响,比较了三者异同。最后展望了高周和超高周疲劳研究的方向和方法及其在工程中的应用。     

高校“双肩挑”辅导员疲劳现状调研及风险管理

目的调研高校内"双肩挑"辅导员的疲劳现状并提出疲劳管理方案,以帮助辅导员在完成科研任务的同时完成学生培养工作。方法以某高校"双肩挑"辅导员作为研究对象,采用半结构化访谈及疲劳量表结合的方式,调研疲劳状况。结合疲劳理论,提出辅导员的疲劳风险管理措施。结果高校辅导员面临着疲劳问题,认为自己"比以往更容易疲劳",了解自身的疲劳会影响生活、工作状态。工作的压力来源包括对新事物迅速学习的要求以及重复工作较多、工作节奏快、工作内容多、时间不足等。但辅导员周围的导师和同学能够理解辅导员的疲劳状况。结论 "双肩挑"辅导员存在一定的疲劳问题,其疲劳管理可从辅导员自身、管理组织者和导师及同学三方面开展。     

On the Ekberg, Kabo and Andersson calculation of the Dang Van high cycle fatigue limit for rolling contact fatigue

Recently, various methods have been proposed to assess the risk of rolling contact fatigue failure by Ekberg, Kabo and Andersson, and in particular, the Dang Van multiaxial fatigue criterion has been suggested in a simple approximate formulation. In this note, it is found that the approximation implied can be very significant; the calculation is improved and corrected, and focused on the study of plane problems but for a complete range of possible friction coefficients. It is found that predicted fatigue limit could be much higher than that under standard uniaxial tension/compression for ‘hard materials’ than for ‘ductile materials.’ This is in qualitative agreement, for example, with gears' design standards, but in quantitative terms, particularly for frictionless condition, the predicted limit seems possibly too high, indicating the need for careful comparison with experimental results. Some comments are devoted to the interplay of shakedown and fatigue.     

疲劳载荷下工业纯铁表面与内部裂纹长度的研究初探

疲劳裂纹扩展试验中,采用表面裂纹长度作为疲劳裂纹长度进行疲劳裂纹扩展计算和疲劳裂纹扩展模型建立的过程中,会导致计算的最终结果和模型产生一定的误差。对工业纯铁板材试样进行疲劳裂纹扩展试验,分析疲劳裂纹扩展过程中裂纹长度与裂纹扩展速率的关系,以及对疲劳裂纹扩展断口形貌特征进行观察。结果表明,上述2种方法均可来确定表面裂纹长度和内部裂纹长度之间的关系。     

Determining the life cycle of bolts using a local approach and the Dang Van criterion

The fatigue behaviour of bolts under axial load has always been considered from the component point of view for which fatigue limit is usually taken equal to 50 MPa, and few results are available to designers for limited lifetimes. Here, we take up this problem from a material point of view using a local approach. For each case of fatigue testing, using finite‐element (FE) model of the bolt, we determine the stabilized local stress at the root of the first thread in contact with the nut. To characterize bolt behaviour with these numerical results, we use Dang Van multiaxial fatigue criterion for which we extend application to the medium fatigue life. These results can be correlated with the experimental numbers of cycles to failure to determine material parameters of the generalized criterion. Using statistical Gauss method, we can make lifetime predictions for any level of risk of failure. In addition, we propose an analytical model to rapidly determine the local stress condition from nominal loading data (mean stress and alternating stress). This model dispenses us from a new modelling if the bolt is stressed in the same manner as the bolts used for behaviour characterization. Using this model and the generalized criterion, it is extremely easy to make lifetime predictions whatever the risk considered.