The Fatigue Fracture Criterion Based on the Latent Energy Approach

The extensive literature on the fatigue problem, published for more than one hundred years, is reviewed by the known scientists [1,2]. As it follows from these investigations, the fundamental amount of failures in engineering practice connected with the fatigue fractures of materials and structure elements. The fatigue problem is complicated one and it is not solved yet. So the theoretical and experimental investigations of this problem will be continued. In our paper the energy approach to formulate the fatigue strength criterion is proposed. The criterion is based on the conception of the latent energy [3-7]. This conception was not applied previously to the fatigue problem. The latent energy is consumed to generate the irreversible deformation and to damage and fracture of metallic materials. So the fatigue fracture criterion can be formulated using the results of latent energy measurements in the macro experiments. This is most impotent advantage of the proposed approach. The logistic function is used to describe the dependence of latent energy from the value of irreversible deformation. It is assumed that the cyclic strength of metals is defined by the latent energy, stored in specimen, when it is reached the critical value in accordance with the logistic curve in a saturation zone. This proposal is used to formulate the fatigue strength criterion. The functions and parameters of received criterion are concretized and comparisons with experimental results for axial cyclic tension for sheet aluminum alloy specimens are given.

Simulation of Damage Evolution and Study of Multi-Fatigue Source Fracture of Steel Wire in Bridge Cables under the Action of Pre-Corrosion and Fatigue

A numerical simulation method for the damage evolution of high-strength steel wire in a bridge cable under the action of pre-corrosion and fatigue is presented in this paper.Based on pitting accelerated crack nucleation theory in combination with continuum mechanics,cellular automata technology(CA)and finite element(FE)analysis,the damage evolution process of steel wire under pre-corrosion and fatigue is simulated.This method automatically generates a high-strength steel wire model with initial random pitting defects,and on the basis of this model,the fatigue damage evolution process is simulated;thus,the fatigue life and fatigue performance of the corroded steel wire can be evaluated.A comparison of the numerical simulation results with the experimental results shows that this method has strong reliability and practicability in predicting the fatigue life of corroded steel wire and simulating the damage evolution process.Based on the method proposed in this paper,the fatigue life of steel wires with different degrees of corrosion under the action of different stress levels is predicted.The results show that as the degree of corrosion increases,the fatigue properties of steel wire gradually decrease,and the influence of existing pitting corrosion on fatigue life is far greater than that on mass loss.Stress concentration is the main cause of fatigue life of corroded steel wire in advance attenuation.In addition,the fracture process of steel wire with multi-fatigue sources and the effect of the number and distribution of pits on the fatigue life of steel wire are studied.The results show that,compared with a stepped pitting distribution,a planar pitting distribution has a greater impact on the damage evolution process.The fatigue life of steel wire is positively correlated with the number of pits and the angle and distance between pits.

Fatigue properties analysis of cracked rock based on fracture evolution process

Fracture evolution process (initiation, propagation and coalescence) of cracked rock was observed and the force- displacement curves of cracked rock were measured under uniaxial cyclic loading. The tested specimens made of sandstone-like modeling material contained three pre-existing intermittent cracks with different geometrical distributions. The experimental results indicate that the fatigue deformation limit corresponding to the maximal cyclic load is equal to that of post-peak locus of static complete force?displacement curve; the fatigue deformation process can be divided into three stages: initial deformation, constant deformation rate and accelerative deformation; the time of fracture initiation, propagation and coalescence corresponds to the change of irreversible deformation.

Improvements in the microstructure and fatigue behavior of pure copper using equal channel angular extrusion

In this study, annealed pure copper was extruded using equal channel angular extrusion （ECAE） for a maximum of eight passes. The fatigue resistance of extruded specimens was evaluated for different passes and applied stresses using fatigue tests, fractography, and metallography. The mechanical properties of the extruded material were obtained at a tensile test velocity of 0.5 mm/min. It was found that the maximum increase in strength occurred after the 2nd pass. The total increase in ultimate strength after eight passes was 94%. The results of fatigue tests indicated that a significant improvement in fatigue life occurred after the 2nd pass. In subsequent passes, the fatigue life con-tinued to improve but at a considerably lower rate. The improved fatigue life was dependent on the number of passes and applied stresses. For low stresses （or high-cycle fatigue）, a maximum increase in fatigue resistance of approximately 500%was observed for the extruded material after eight passes, whereas a maximum fatigue resistance of 5000%was obtained for high-applied stresses （or low-cycle fatigue）. Optical microscopic examinations revealed grain refinements in the range of 32 to 4 μm. A maximum increase in impact energy absorption of 100%was achieved after eight passes. Consistent results were obtained from fractography and metallography examinations of the ex-truded material during fatigue tests.

某电机齿轮冷挤压裂纹形成机理研究

针对材料为20Cr的某电机齿轮在冷挤压成形时内齿齿顶面上出现的裂纹缺陷,基于DEFORM-3D对其冷挤压过程进行了模拟,研究了其裂纹形成机理。有限元结果表明,内齿的金属流动速度小于中部及下部外齿的流动速度,内齿齿顶面上齿根处的金属流动速度最小,此处流动最不均匀,易产生应力集中;对齿面的最大主应力及损伤分布进行了分析,得出在内齿齿顶面上,中部及距离下端面1 mm左右的位置应力集中最明显,拉应力较大且大于材料抗拉强度,损伤值也较大且大于临界损伤值,会产生裂纹,与实际情况相符合。通过改变凹模入模角来控制成形缺陷情况,当凹模入模角为23°时,齿根处拉应力下降,损伤值减小,投入生产后次品率从原本的50%降为1%。

循环荷载下硬质层理砂岩疲劳损伤机制试验研究

列车动载扰动作用下隧道基底围岩的变形开裂蕴含着复杂的力学损伤问题。为探究循环荷载下硬质层理砂岩的疲劳损伤演化机制,对层理砂岩进行了疲劳力学试验及电镜扫描试验。结果表明:层理砂岩的宏观裂隙与动态应力-应变曲线存在时效对应关系,层理效应越明显,滞回环跃迁越显著。层理砂岩力学参数具有明显的层理劣化效应。随层理倾角增大,峰值强度呈缓慢衰减—快速降低—急剧增大的趋势,疲劳寿命与峰值强度呈正相关。弹性模量呈急剧增大—缓慢增大—趋于平缓或降低的趋势,周期比分界点为28.57%和81.81%。砂岩破裂模式与层理效应密切相关,包括张拉破裂(Ⅰ型和Ⅱ型)、斜剪破裂及复合破裂;张拉破裂Ⅱ型具有压杆效应,破裂面粗糙度略小;斜剪破裂面粗糙度显著降低,压—剪作用导致复合破裂断面出现破碎带和光滑断口。循环加卸载效应导致层理砂岩的临界损伤呈非线性快速增大—近似线性增长—非线性急剧增大演变,Logistic反函数损伤模型可很好描述临界损伤规律,疲劳敏感性顺序依次为斜剪破裂型>复合破裂型>张拉破裂型。

不同灰砂比岩充组合体疲劳损伤与破裂特性试验研究

为了揭示地下采场中岩充组合结构体在频发应力扰动作用下的损伤演化和破裂机制,对4种不同灰砂比的岩充组合体试样开展多级增幅疲劳试验并进行试验后CT扫描。结果表明:(1)充填体的灰砂比对岩充组合体的变形、刚度劣化、损伤扩展和破坏模式均有影响。灰砂比从1:4下降到1:12,体积膨胀增加,割线模量先增加后减小。(2)提出了基于不可逆轴向应变的累积损伤演化模型,该模型与试验数据拟合良好,灰砂比为1:4和1:8的岩充组合体累积损伤呈现2阶段增加模式,即最初稳定增加,随后急剧增加。灰砂比为1:10和1:12的岩充组合体呈现倒“S”形损伤累积模式,即初始、稳定和加速增加3个阶段。(3)随着灰砂比减小,岩充组合体的宏观破坏模式从拉伸−剪切混合破坏过渡为拉伸破坏。试验后的CT图像揭示了岩充组合体细观破裂演化模式,包括充填体中的剪切破坏、沿着岩石和充填体接触面的拉伸破裂以及岩石中的拉伸和剪切破裂。采用“柔性充填体”有利于防止岩石剥落、坍塌等灾害。研究成果可为矿山充填配比优化和深部矿产资源安全开采提供理论依据。

预腐蚀条件下不同孔数冷挤压强化7075-T6铝合金板件疲劳寿命退化规律

目的研究腐蚀环境下单孔和多孔冷挤压强化铝合金板件的疲劳寿命退化规律和损伤机理。方法针对2种不同孔距的三孔冷挤压强化7075-T6铝合金板件,采用模拟热带海洋大气环境的实验室加速腐蚀谱,开展不同腐蚀时间下的预腐蚀疲劳试验。结果获得了3倍孔距的三孔挤压强化铝板以及单孔挤压强化铝板的C(t)曲线、疲劳S-N曲线和损伤形貌特征,分析了腐蚀后挤压强化试件的腐蚀机理及挤压强化作用。同时,将不同孔数试件的试验结果进行比较,探明了孔数对挤压强化铝板腐蚀和疲劳性能的影响。结论腐蚀后挤压强化铝合金板件的疲劳性能发生显著退化,腐蚀损伤主要为剥蚀,疲劳裂纹萌生位置由孔边转移至表面。挤压孔数量对挤压强化铝合金板件的疲劳和腐蚀疲劳性能的影响较小。

拉伸载荷下碳纤维/环氧树脂织物层合板疲劳断裂特性

为探究纤维增强树脂基复合材料疲劳寿命预测和损伤演化问题,通过复合材料层合板拉伸和疲劳实验,获得了4种不同循环应力下碳纤维/环氧基平纹织物层合板的疲劳数据,并以此建立了一种织物层合板的疲劳模型。采用有限元方法分析了层合板的失效过程和性能退化,模拟了织物层合板的疲劳寿命和损伤扩展过程。利用超声波C扫、超景深显微镜和扫描电子显微镜观察疲劳断口形貌及内部损伤,并与仿真结果对比,研究了纤维增强复合材料疲劳损伤演化和断裂机理。结果表明,复合材料层合板疲劳损伤演化主要分3个阶段。在疲劳初期,层合板两侧边表面最先出现层内分层和基体裂纹;中期损伤扩展加快,损伤形式主要以层间损伤为主;末期层合板内严重分层,裂纹趋于饱和,大量纤维断裂,性能迅速退化;最后,损伤贯穿整个横面,发生疲劳断裂。

低碳钢Q345多尺度疲劳断裂行为研究

为了解决钢结构疲劳断裂问题,该文基于应力等效原理引入了材料内部微、细观缺陷的影响,利用有限元软件ABAQUS模拟了含损伤的低碳钢Q345多尺度疲劳裂纹扩展过程,研究了裂纹尖端场分布及其疲劳断裂行为。研究表明,材料内部微、细观缺陷引起的损伤对宏观裂纹尖端塑性区分布、尖端应力强度因子和J积分均具有较大影响,裂纹达到一定长度后,影响更为显著,且会较为明显地加速宏观裂纹扩展。

基于损伤等效的压裂泵柱塞疲劳载荷谱研究

在页岩气作业中,压裂泵柱塞受到周期性载荷的作业易出现疲劳破坏,而压裂泵柱塞产品开发初期没有疲劳计算所需的实测疲劳载荷谱。针对此问题,根据6000型机械压裂泵的百万冲次试验测试柱塞组件的应变载荷谱,采用一定方法对柱塞的疲劳载荷谱进行等效编谱,评估柱塞疲劳寿命并对柱塞的结构进行优化设计,最后锁定柱塞设计数据。通过分析证明了通过测试与仿真相结合可评估柱塞疲劳耐久性,提升产品质量。为了减少重型件产品设计的验证时间,基于采集的柱塞载荷谱以一定方法获得柱塞疲劳载荷谱。基于等效损伤原理,采用block cycle方法进行柱塞台架试验载荷谱的提取,为后续搭建柱塞台架试验台提供设计依据。

一起新能源发电机组CC20型汽轮机叶片断裂故障原因分析

针对一起新能源发电汽轮机复速级叶片失效断裂故障进行强度校核和理化分析,结果表明失效的12Cr13叶片中Mn元素含量高于标准要求,导致叶片在高温工作时形成较大内应力从而引起叶片变形与开裂倾向,并且在失效叶片短弧侧下端根部台阶处尖角位置存在应力集中,两者共同作用使叶片在运行中产生早期疲劳微裂纹,在交变应力作用下加速微裂纹扩展,最终导致叶片疲劳断裂,最后提出预防和质量控制措施。

Tensile and fracture properties of Ti-62A alloy plate with different microstructures

Ti-62A alloy plates with three different types of microstructure, fully equiaxed, bimodal, and Widmanstatten, were obtained by various heat treatments to investigate the effects of microstructure on the tensile and fracture properties at room temperature. The results reveal that Widmanstatten microstructure exhibits good damage tolerance behavior considering strength, fracture toughness, and fatigue crack growth behavior, while the bimodal microstructure shows good comprehensive properties considering the plasticity synthetically. Optical microscopy (OM) and scanning electron microscopy (SEM) microstructure analyses on fracture and fatigue crack path demonstrate that the dependence of mechanical properties and fatigue crack growth behavior on microstructural feature are attributed to the α lamellae width and the α colony size. © The Nonferrous Metals Society of China and Springer-Verlag Berlin Heidelberg 2012.

Experimental study on durability fracture behavior and vibration modal sweep for vehicle rear axle

In order to study the fatigue fracture behavior of the rear axle of certain China-made car, the strain loading spectrum near the rear axle fracture location is collected, the modified Neuber rule and the cyclic stress-strain hysteresis loop curve equation are used to convert the nominal strain his- tory into the local stress-strain response. The impact of mean stress on fatigue damage is corrected according to the Manson-Coffin model, and programming calculation of the fatigue damage of the fracture crack is conducted in the INFIELD software, the electromagnetic vibrators are used to sweep the vibration modal frequencies of the rear axle and car body. The enhancement test and fre- quency sweep results show that the rear axle fatigue damage mainly concentrates on the washboard road, and when the forced vibration excitation frequency is 24. 07 Hz, the vibration modal frequency of the rear axle is close to the excitation frequency of the washboard road, leading to resonance and making the rear axle subjected to large strain and fatigue damage, and then vibration fatigue fracture due to high stress concentration.

基于有限元模拟方法的导管开裂分析

不锈钢导管在固定卡箍位置发生开裂。对断裂导管的安装位置、结构特点及装配情况进行检查,结合宏观观察、断口分析、化学分析、金相检查以及有限元模拟等方法,对其开裂原因进行分析。结果表明:导管的开裂性质为高周疲劳断裂;疲劳裂纹起始于导管外侧的机械损伤部位,导管受弯曲振动载荷作用,在导管应力集中处萌生疲劳裂纹,进而疲劳扩展。建议在对导管零件进行操作时,要严格按照有关要求进行,避免非正常受力带来的机械损伤。

液氮循环压裂裂缝起裂与形态特征可视化研究

针对干热岩储层改造存在的起裂压力高、裂缝单一和易诱发地震的瓶颈难题,本文结合循环水力压裂和液氮压裂技术优势,探索了一种液氮循环压裂开发干热岩的新思路,即通过“注入—停顿”的方式周期性注入低温液氮,使岩石在交变热应力—流体压力耦合作用下发生疲劳损伤,促进裂缝起裂、转向、分叉进而形成复杂缝网,提高储层改造体积。目前,液氮压裂技术的研究多集中在液氮单次或循环冷却致裂岩石力学机制和液氮压裂造缝机理方面,考虑地应力条件下的液氮循环压裂造缝机理方面的研究未见报道。为了验证液氮循环压裂开发干热岩的可行性,基于自主研发的真三轴液氮循环压裂实验装置,采用可视化材料PMMA(Polymethyl Methacrylate),研究了水平应力差异系数和循环次数的影响规律,揭示了液氮循环压裂裂缝起裂与形态特征,并与清水循环压裂进行了对比。结果表明:在较低的循环次数和循环压力下,液氮循环压裂相对于清水循环压裂可显著降低起裂压力(下降47.1%~71.7%),在交变热应力—流体压力耦合作用下液氮循环压裂易形成以“热应力裂缝+主裂缝”为特征的复杂缝网;液氮循环压裂不易受水平应力差异系数控制,在较大水平应力差异系数下仍能取得较好的造缝效果;液氮循环冷却预处理是液氮循环压裂的关键,增大液氮循环冷却次数可显著降低裂缝起裂压力并形成复杂缝网,当直接采用高压液氮压裂时,起裂压力甚至会超过清水压裂;总体来看,液氮循环压裂相对于清水循环压裂能以较低的循环次数和循环注入压力实现较好的造缝效果,有望为干热岩绿色经济高效开发提供新途径。研究结果可望为液氮循环压裂开发干热岩提供理论依据和实验基础。

循环荷载对单裂隙岩体疲劳损伤的影响

【目的】揭示单裂隙岩体在循环荷载作用下的疲劳损伤演化规律,研究更便于实际工程应用的损伤变量计算方法。【方法】通过开展基于声波测试的模型岩体疲劳试验研究,分析了不同循环次数比、不同单裂隙角度下试样的波速降低率、初始损伤及损伤变量的变化规律;提出不同角度单裂隙岩体的非线性疲劳累积损伤模型。【结果】随着裂隙角度的增加,模型试样的初始波速降低率及初始损伤呈非线性“S”形增大趋势;在循环荷载作用下单裂隙岩体波速降低率具有明显的三阶段衰减特征,初始衰减阶段、稳定衰减阶段及加速衰减阶段的疲劳寿命比例约为2∶7∶1;在循环次数比相同的情况下,岩体的疲劳累积损伤演化随裂隙角度的增大呈加速趋势,但在加速衰减阶段却呈减速趋势;单裂隙岩体疲劳寿命与裂隙角度的关系曲线呈“V”字形,在循环荷载作用下45°单裂隙岩体损伤变量增长最快,且在加速衰减阶段疲劳寿命占比最小,最易发生拉伸-压剪裂纹混合的“X”形裂纹破坏。【结论】在裂隙角度为30°~90°时,若在岩体疲劳破坏前用名义损伤替代实际总损伤,名义损伤偏大;若在疲劳破坏阶段用名义损伤替代实际总损伤,名义损伤偏小;当失稳比例因子一定时,失稳因子及收敛因子均随裂隙角度的增加呈单调递增趋势。

Enhance liquid nitrogen fracturing performance on hot dry rock by cyclic injection

Producing complex fracture networks in a safe way plays a critical role in the hot dry rock (HDR) geothermal energy exploitation. However, conventional hydraulic fracturing (HF) generally produces high breakdown pressure and results only in single main fracture morphology. Furthermore, HF has also other problems such as the increased risk of seismic events and consuption of large amount of water. In this work, a new stimulation method based on cyclic soft stimulation (CSS) and liquid nitrogen (LN2) fracturing, known as cyclic LN2 fracturing is explored, which we believe has the potential to solve the above issues related to HF. The fracturing performances including breakdown pressure and fracture morphology on granites under true-triaxial stresses are investigated and compared with cyclic water fracturing. Cryo-scanning electron microscopy (Cryo-SEM) tests and X-ray computed tomography (CT) scanning tests were used for quantitative characterization of fracture parameters and to evaluate the cyclic LN2 fracturing performances. The results demonstrate that the cyclic LN2 fracturing results in reduced breakdown pressure, with between 21% and 67% lower pressure compared with using cyclic water fracturing. Cyclic LN2 fracturing tends to produce more complex and branched fractures, whereas cyclic water fracturing usually produces a single main fracture under a low number of cycles and pressure levels. Thermally-induced fractures mostly occur around the interfaces of different particles. This study shows the potential benefits of cyclic LN2 fracturing on HDR. It is expected to provide theoretical guidance for the cyclic LN2 fracturing application in HDR reservoirs.

航空快卸卡箍连接结构开裂原因

某航空卡箍连接结构中的燃油分配器主壳体及卡箍发生开裂。采用宏观观察、扫描电镜分析、金相检验及硬度测试等方法对卡箍连接结构开裂的原因进行分析。结果表明:燃油分配器主裂纹起源于安装边R角,呈多源、线源特征,为起始应力较大的双向弯曲疲劳开裂;卡箍发生了起始应力较大的疲劳开裂,裂纹起源于连接环内侧表面,呈多源、线源特征;燃油分配器与卡箍接触面受力不均是其发生早期疲劳开裂的主要原因,结合面挤压摩擦损伤也促进了疲劳裂纹的萌生。

离合器减振弹簧断裂原因

某型号离合器从动盘总成在扭转耐久性试验过程中,其减振弹簧发生断裂。采用宏观观察、化学成分分析、硬度测试、金相检验、扫描电镜及能谱分析等方法研究了该弹簧断裂的原因。结果表明:减振弹簧内侧存在原始裂纹和摩擦损伤,使弹簧在低应力作用下发生弯曲疲劳断裂。