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。     

WE94镁合金长寿命疲劳小裂纹萌生和扩展行为研究

镁合金作为结构材料在汽车、航空、航天等领域有广泛的应用前景,在实际服役过程中存在高频振动荷载的长期作用。为解决镁合金结构件在长寿命服役条件下的安全与可靠性问题,基于旋转弯曲和超声振动加载两种疲劳实验方法,研究WE94镁合金在高周与超高周区间（104～109周次）的疲劳强度与失效机制。结果表明,疲劳裂纹萌生于沿基面的滑移带并在断裂面上形成解理状“小平面”形貌;而在疲劳小裂纹扩展初期,裂纹面存在大量细小平行状条带,该条带于裂纹尖端与孪晶带的交互作用过程中形成。因此,镁合金疲劳小裂纹的萌生与扩展过程对材料局部微结构形态与变形机制表现出密切的相关性。     

基于连续损伤力学的UHTCC疲劳损伤扩展模型研究

为研究超高韧性水泥基复合材料的损伤扩展规律,该文对该材料的预制单边切口试件进行了三点弯曲疲劳试验。基于连续损伤力学,建立两种疲劳损伤扩展模型：一是以J积分作为自变量的双对数线性模型,二是以疲劳应力水平作为自变量的单对数线性模型。结果表明,两种模型计算结果与试验结果吻合良好。但J积分取值很大程度上依赖于试件的几何特征,导致模型1受试件形状影响较大;实际应用中,由于疲劳应力水平的获得相对直观方便,故模型2更加实用。     

腐蚀疲劳裂纹扩展研究近况

综述了腐蚀疲劳裂纹扩展的研究现状，对腐蚀疲劳今后的发展趋势作了展望。     

疲劳裂纹测试的裂纹扩展片技术

提出了一种间接直流电位技术——裂纹扩展片技术,用于测量疲劳裂纹长度。该技术具有测量精度高、操作简单、可实现数据自动采集处理和测试设备成本较低等优点,并可用于绝缘材料裂纹和表面裂纹测量,是一种较有前途的裂纹测试技术。     

正交异性钢桥面板疲劳裂纹扩展机理及数值模拟研究

为追踪正交异性钢桥面板的疲劳裂纹扩展过程及通用的正交异性板钢桥抗疲劳设计与开裂加固提供理论指导,提出基于实桥有限元模型进行正交异性钢桥面板疲劳裂纹扩展模拟方法及流程。建立桥梁整体有限元模型进行恒活载作用下整体分析,结合实桥调查结果确定全桥疲劳关键部位;建立含焊接细节的疲劳关键部位精细化模型进行疲劳应力幅分析,并基于车桥耦合振动分析考虑冲击系数对应力幅影响,确定裂纹扩展方向、路径及寿命,进行疲劳裂纹扩展全过程分析;以既有大跨度斜拉桥正交异性桥面板疲劳裂纹扩展分析为例,验证该方法与计算流程的可行性、准确性,并为该桥运营期疲劳失效维修、加固提供理论依据。     

疲劳损伤和中间退火修复对疲劳裂纹扩展速率的影响

用电火花予制裂纹的40Cr钢试样与经受不同周次塑性应变的试样在相同的弹性应力加载条件下测得的裂纹扩展速率da/dn进了对比.结果表明随着预先疲劳损伤周次的增加,da/dn也增大,中间退火后可以明显降低da/dn.TEM和SEM分析表明改变da/dn大小的主要原因是损伤和修复影响碳化物在晶界区的沉淀、形态和断口形貌.     

轴承钢疲劳裂纹扩展速率的研究

研究了轴承钢碳化物及晶粒细化对轴承钢疲劳寿命的影响。结果表明;细化轴承钢中的碳化物可以使其疲劳裂纹扩展速率下降,而同时细化轴承钢中的碳化物和晶粒,会使其疲劳裂纹扩展速率下降更明显。     

A Continuum Mechanics Approach for Crack Initiation and Crack Growth Predictions

Very often, different approaches are used for crack initiation and crack growth predictions. The current article introduces a recently developed approach that can be used for the predictions of both crack initiation and crack propagation. A basic assumption is that both crack nucleation and crack growth are governed by the same fatigue damage mechanisms and a single fatigue damage criterion can model both stages. A rule is that any material point fails to form a fresh crack if the total accumulated fatigue damage reaches a limit. For crack initiation predictions, the stresses and strains are obtained either directly from experiments or though a numerical analysis. For the prediction of crack growth, the approach consists of two steps. Elastic‐plastic stress analysis is conducted to obtain the detailed stress‐strain responses. A general fatigue criterion is used to predict fatigue crack growth. Compact specimens made of 1070 steel were experimentally tested under constant amplitude loading with different R‐ratios and the overloading influence. The capability of the approach to predict both crack initiation and the crack growth under these loading conditions was demonstrated by comparing the predictions with the experimental observations.     

A Probabilistic Method to Predict Fatigue Crack Initiation

A probabilistic method to predict macrocrack initiation due to fatigue damage is presented in this paper. Acoustic non-linearity is used to quantify pre-macrocrack initiation damage. This data is then used in a probabilistic analysis of fatigue damage. The probabilistic fatigue damage analysis consists of a suitably chosen damage evolution equation to model accumulated damage coupled with a procedure to calculate the probability of macrocrack initiation. The probability of macrocrack initiation is evaluated using the Monte Carlo Method with Importance Sampling. Numerical results for the probabilistic assessment of fatigue damage for a sample problem are presented and compared with experimental results.     

The Prediction of Fatigue Crack Initiation Life in Spot Welds

Abstract: In this paper, strain‐based fatigue life prediction method has been used to estimate the fatigue crack initiation life of spot‐welded joints of Mild Steel JSC270D and Ultra‐High Strength Steel JSC980Y. To do so, the joints were simulated using three‐dimensional finite‐element (FE) models, and then nonlinear FE analysis was performed to obtain the local stress and strain ranges and finally, the Morrow equation was applied to estimate the crack initiation lives. The results have been compared with those obtained from experimental crack growth morphology. In addition, the difference between fatigue limits for smooth specimens and spot‐welded joints for mentioned materials has been briefly discussed. It has been shown that mean stress values in the Ultra‐High Strength Steel can significantly decrease the fatigue limit of spot‐welded joint because even at very low load level the stresses exceed the yield point at the root of nugget of spot‐welded joint, while the amount of mean stress in the Mild Steel for the same load level is much less than that of Ultra‐High Strength Steel. The comparison between numerical results of fatigue crack initiation lives and experimental data provided good agreement between numerical predictions and crack growth morphology observations. The results also shows that in some cases, depending on the joint type, the life spent in the nucleation phase can be an important part of the final failure lifetime.     

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。     

单向帘线/橡胶复合材料的疲劳损伤模型与疲劳寿命预报

将不可逆热力学和损伤力学应用于橡胶复合材料的疲劳研究，在大量试验工作的基础上，选择适当的热力学势函数，建立了橡胶复合材料的损伤演变方程和橡胶复合材料的疲劳寿命预报方程。应用本模型对单向橡胶复合材料的疲劳寿命进行了预报，发现理论预测值与试验结果比较符合。     

基于小裂纹理论的铸造钛合金ZTC4疲劳寿命预测

铸造钛合金ZTC4在飞机和航空发动机上应用日益广泛.深入研究ZTC4疲劳全寿命预测方法,旨在为航空构件的损伤容限设计和寿命预测探索新的途径.本文以宏观和微观结合的手段,采用板材试样的高周疲劳试验、中心裂纹试样的长裂纹扩展试验和扫描电子显微镜(SEM)的断口分析等三种试验,研究了ZTC4在室温恒幅载荷条件下的疲劳断口特征和裂纹扩展行为;对引起疲劳失效的主要原因-材料初始缺陷(夹杂或气孔)进行了定量表征;基于Newman裂纹闭合模型建立了ZTC4长裂纹的(da/dN)-△Keff基线数据;通过对平板内埋椭圆裂纹的断裂力学分析,从基于微观结构和断口分析统计确定的初始缺陷尺寸出发,对ZTC4在恒幅载荷条件下两种应力比的疲劳全寿命进行了预测和实验验证,得到了具有较好学术意义和工程应用价值的研究结果.     

疲劳累积损伤对发动机缸盖再制造的影响

通过对新、旧发动机缸盖取样进行疲劳试验,对比了其SN曲线,结果发现：应力幅较大时,新、旧缸盖材料的疲劳寿命相差明显;应力幅较小时,新、旧缸盖材料的疲劳寿命相差不大,表明旧发动机缸盖材料的损伤程度随着应力幅的增加而增大。因此当发动机经过再制造后,有可能因为累积损伤的原因存在安全隐患。     

高压容器用钢疲劳裂纹扩展速率研究

研究了高压容器用Cr Ni Mo钢在空气中与3.5%Na Cl溶液介质中的疲劳裂纹扩展,结果表明,3.5%Na Cl溶液介质的存在加速了Cr Ni Mo钢的裂纹扩展,随着裂纹的进一步扩展,其加速作用越来越不明显。     

2024-T3和2524-T34铝合金疲劳裂纹的萌生机制

通过2024-T3和新型2524-T34铝合金的疲劳实验和对试样表面及疲劳断口的观测,研究了材料的微观结构和疲劳裂纹萌生机制.实验在室温下完成,应力比为0.1、加载频率为15 Hz.结果表明:实验材料呈现了再结晶的层状晶粒结构,晶粒沿着轧制方向被拉长,并较为平坦.2024铝合金中二相粒子的分布更为密集无序,且粗大、不规...     

颗粒和微观结构对Cu/WCp复合材料疲劳裂纹萌生和扩展行为的影响

通过原位扫描电子显微镜(SEM)研究了粉末冶金制备的Cu/WCp复合材料的疲劳裂纹萌生和扩展行为,分析了颗粒和微观结构对Cu/WCp复合材料疲劳裂纹萌生和早期扩展行为的影响。结果表明:疲劳微裂纹萌生于WCp颗粒和基体Cu的界面;微裂纹之间相互连接并形成主裂纹,当主裂纹和颗粒相遇时裂纹沿着颗粒界面扩展。在低应力强度因子幅ΔK区域疲劳小裂纹具有明显的"异常现象",并占据了全寿命的71%左右。疲劳小裂纹的早期扩展阶段易受局部微观结构和颗粒WCp的影响,扩展速率波动性较大,随机性较强;当小裂纹长度超过150μm时,裂纹扩展加快直至试样快速断裂。裂纹偏折、分叉和塑性尾迹降低了疲劳裂纹扩展速率,而颗粒界面脱粘则提高了复合材料的疲劳裂纹扩展速率。通过数值模拟也可以发现颗粒脱粘增大了材料的疲劳扩展驱动力,从而提高了疲劳裂纹扩展速率。