Three-dimensional local stress analysis on grain boundaries in polycrystalline material

In order to understand the initiation behavior of microstructurally small cracks in a stress corrosion cracking condition, it is important to know the tensile normal stress acting on the grain boundary (normal GB stress). The local stress in a polycrystalline body is enhanced by the inhomogeneity which stems from the shape and orientation of each grain. The stress in a three-dimensional polycrystalline body consisting of 100 grains with random orientation, under a remote uniform tensile stress condition, is evaluated by the finite element method. It was revealed that the local stress on the polycrystalline body is inhomogeneous under uniform applied stress and becomes large at those grain boundaries that are perpendicular to the load axis, though there is large fluctuation. It was also shown that the normal GB stress tends to be large near the triple points due to the deformation constraint caused by adjacent grains. Finally, the maximum stress on the surface of a large component caused by the inhomogeneity was evaluated by using Gumbel statistics.     

Fatigue of polycrystalline copper with different grain sizes and texture

Fatigue experiments of polycrystalline copper with different grain sizes and texture were conducted under tension–compression, torsion, and non-proportional loading. The grain sizes ranged from 10 μm to 2 mm. The stress–strain response was found to be a strong function of the grain size and texture. A plasticity-based critical plane multiaxial fatigue criterion was used to predict the fatigue lives of the polycrystalline copper. It was found that the criterion was able to correlate all the experimental results with one single set of material constants. This indicates that the fatigue failure of the material under consideration is dominated by the fatigue resistance of the grains with an insignificant influence of the grain boundaries on the fatigue of the polycrystalline material. It was found that the fatigue model with the material constants obtained from fatigue experiments can be applied to predict failure under monotonic torsion.     

磨损加疲劳载荷下的协同疲劳行为

自行研制的摩擦磨损装置与轴向疲劳试验机相互配合,实现GDL-1钢试样在疲劳应力(240~280 MPa)及接触载荷(30 N)作用下摩擦磨损疲劳试验.通过对磨损层厚度的分析,研究试样承受摩擦磨损载荷及拉压疲劳载荷双重作用下的疲劳寿命变化,用SEM扫描电镜观察分析次表层内疲劳裂纹扩展的演变过程,并采用Hertz线弹性理论和Smith接触理论计算分析摩擦表面以下切应力值.结果表明:在磨损疲劳载荷作用下,形变层的流变作用将显著影响疲劳小裂纹扩展方向,渐趋于切应力方向,从而提高试样疲劳寿命.在此基础上,建立了在摩擦磨损疲劳载荷下疲劳裂纹扩展模型.此外,计算可知在距表层深度0.03 mm处切应力最大,0.18 mm以内材料产生塑性变形,导致形变层的形成.     

TC21钛合金显微组织对超长寿命疲劳行为的影响

本文主要研究钛合金的网篮尺寸对于超声疲劳行为的影响。研究发现,对于TC21钛合金,高应力下疲劳裂纹萌生于试样表面;低应力下疲劳裂纹萌生于试样亚表面。最大应力-循环次数(S-N)曲线显示,在两段连续下降的曲线之间有一个表面萌生向内部萌生的转变平台。对于网篮尺寸为60μm的钛合金,转变平台对应的应力幅为540MPa,疲劳极限为430MPa。网篮尺寸为40μm的转变平台应力幅为600MPa,疲劳极限为530MPa。分析得知网篮尺寸为60μm的TC21钛合金疲劳性能低于网篮尺寸为40μm的TC21钛合金。     

Sufficient criterion of fracture in the case with a complex stress state and non-proportional deformation of the material in the pre-fracture zone

A general case of proportional loading with a complex stress state of the material in the pre-fracture zone, which is typical for polycrystalline solids with plastic deformation, is considered. A sufficient criterion of fracture is proposed for the case of a complex stress state with non-proportional deformation of the material in the pre-fracture zone. Critical parameters of fracture (pre-fracture zone length and load) for cracks propagating in quasi-brittle materials are obtained with the use of a modified Leonov-Panasyuk-Dugdale model. The pre-fracture zone width is determined by solving the problem of the plasticity theory in the vicinity of the crack tip. The proposed modification of the Leonov-Panasyuk-Dugdale model makes it possible to estimate the critical opening of the crack and the critical displacement of the crack flanks. Inequalities that describe different mechanisms of material fracture under proportional loading (predominantly shear fracture mechanism and fracture mechanism through cleavage) are derived.     

Effect of plastic zone on the fatigue crack propagation behavior between two fatigue cracks

In this study, the fatigue crack propagation behavior in the stress interaction field between two different fatigue cracks is studied by experiment and finite element analysis. In the experiment, the offset distance between two cracks and the applied stress are varied to create different stress interaction fields. The size of the plastic zone area is used to examine the crack propagation path and rate. Three types of crack propagation in the interaction field were found by experiment, and the crack propagation behavior of two cracks was significantly changed as different stresses were applied. The size of the plastic zone obtained by finite element analysis can be used to explain crack propagation behavior qualitatively.     

沥青混凝土面板低温疲劳破坏的实验研究

我国北方地区的土石坝和水池的防渗沥青混凝土面板,常出现低温裂缝。因此,对沥青混凝土在低温条件下的温度应力疲劳进行实验研究很有必要.本项研究研制了适用于沥青混凝土低温低周大变形的弯曲疲劳试验机,进行了沥青混凝土在不同低温下的疲劳破坏性能试验。对试验结果用粘弹性理论和推广的应变区分法(SRP)和应变能区分法(SEP)进行了分析,首次得出了沥青混凝土的疲劳寿命与弯曲应力、应变和应变能的关系,并试用于实际工程的予测.     

FATIGUE LIFE PREDICTION OF SUS 630 （H900） STEEL UNDER HIGH CYCLE LOADING

The fatigue life prediction of high strength steel SUS 630 （H900） under high cycle loading is conducted with consideration of a characteristic fatigue length of material. Based on the WShler curve of smooth materials, a modified method for fatigue life prediction is approached. The characteristic fatigue length of material under cyclic loading is associated with the polycrystalline material. Rather than the stress at a point, the average stress within the characteristic fatigue length is implemented for the fatigue life prediction. The method can be applied to both the smooth and the defected material. The fatigue life prediction is also verified experimentally by specimens with various small circular holes. Through the comparison, it is found that the method can be adopted to predict the fatigue lives with different size effects.     

不均匀组织ER8车轮滚动接触疲劳性能研究

针对含不均匀组织ER8车轮服役过程中过早出现滚动接触疲劳(RCF)损伤这一实际问题,通过RCF试验,获得了车轮不均匀组织和正常组织的RCF极限;利用光学显微镜(OM)和透射电镜(TEM)重点表征了RCF裂纹萌生处的微观组织. 借助扫描电镜(SEM)和原位拉伸试验台测试了两种组织的弹、塑性能并原位观察了裂纹的扩展行为. 结果表明:车轮踏面不均匀组织的RCF极限低于轮辋正常组织,不均匀组织中除车轮正常组织应有的珠光体和先共析铁素体组织外,还存在大量的上贝氏体,上贝氏体的存在破坏了车轮正常组织的连续均匀性. RCF裂纹的萌生和扩展主要发生在上贝和正常组织的边界处,上贝氏体的硬度、弹性高于正常组织,但塑性小于正常组织,在相同接触应力作用下,两者弹-塑性变形的不协调是导致其组织边界处产生应力集中,进而诱发并促进疲劳裂纹的萌生和扩展,加速车轮RCF损伤出现的主要原因.      

核电690合金传热管微动疲劳试验及损伤机理分析

核电蒸汽发生器传热管在微幅磨损与交变载荷的作用下形成微动疲劳,导致其表面裂纹萌生和扩展乃至破裂,从而影响反应堆的安全. 为研究径向载荷以及轴向交变应力对690合金管微动疲劳寿命的影响规律,开展690合金管管材的微动疲劳试验,获得690合金管管材的微动疲劳寿命曲线,并与相关研究数据进行对比分析,以便探讨材料在微动疲劳下的寿命模型. 对不同载荷下的690合金管试样的磨痕表面进行三维形貌和扫描电镜观测,分析磨损表面的损伤机理;对不同载荷下的690合金管试样断口的宏观与微观形貌进行表征,分析裂纹萌生、起裂过程及其失效机理. 结果表明690合金管与403不锈钢(SS)抗振条间的磨损机理为剥层及磨粒磨损;690合金管在径向载荷作用下于微动磨损处产生裂纹源,裂纹在轴向交变应力的作用下不断向内部扩展,最终导致断裂;其断裂形式为解理疲劳断裂.      

拉压循环载荷下正交异性钢桥面板肋-面板焊缝裂纹扩展分析

采用四步法计算了考虑循环载荷中压应力影响的正交异性钢桥面板的肋-面板焊缝表面裂纹扩展。第一步是基于正交异性钢桥面板的疲劳分析模型,计算肋-面板焊缝处的应力,第二步是通过肋-面板焊缝的三维局部模型,用Schwartz-Neumann交替法计算焊缝表面裂纹的应力强度因子分布,第三步是用二维断裂力学模型和增量塑性损伤模型,计算循环载荷中的压应力对裂纹扩展的影响,第四步是用第二步中的三维裂纹分析结果和第三步中的二维断裂力学模型得到的裂纹扩展公式,计算钢桥面板的肋-面板焊缝表面裂纹扩展。计算结果表明,对应于正交异性钢桥面板肋-面板焊缝处的循环应力,本文所用模型的裂纹尖端反向塑性区导致裂纹扩展率增加50%以上。研究结果为正交异性钢桥面板肋-面板焊缝裂纹的疲劳寿命分析提供了研究基础。     

Fatigue crack initaition and propagation from hole defects

The aim of this study is to investigate the interaction of two hole defects affecting fatigue crack initiation life and propagation behavior. The location of two hole defects was characterized by an angle of alignment and the distance between the centers of two hole defects. The fatigue cracking behavior is studies under bending for AI 5086. When defects are located close to each other, the fatigue crack initiation lives are varied with their relative locations. In the experiments, the area of the local plastic stratin strongly played a role in the fatigue crack initiation lives. Therefore, the authors introduce a parameter which contains the plastic deformation are at stress concentrations and proposed a fatigue crack initiation life prediction curve. In addition, the directions and propagation rates of fatigue cracks initiated at two hole defects were studied experimentally.     

45CrNiMoVA钢的低周疲劳特性和表面疲劳裂纹的在位观测

利用MTS810材料试验机对带中心圆孔的45CrNiMoVA钢低周疲劳特性进行了研究,并借助长焦显微镜和CCD摄像头对试样表面裂纹的演化规律进行了在位停机观测和在位不停机连续观测尝试,试验发现,裂纹均萌生于与拉伸应力垂直的圆孔边缘;疲劳裂纹的萌生与Lueders形变带密切相关,萌生期超过整个疲劳寿命的70%,主裂纹的发展既可以共面扩展的方式向前连续延伸,也可以裂纹连接的方式向前跳跃传播,应力控制的低周疲劳条件下,疲劳寿命与应力面形貌图像,频闪光源照明可以实现表面疲劳裂纹的在位不停机观测。     

Three-dimensional mixed-mode stress intensity factors for cracks in functionally graded materials using enriched finite elements

Three-dimensional enriched finite elements are used to compute mixed-mode stress intensity factors (SIFs) for three-dimensional cracks in elastic functionally graded materials (FGMs) that are subject to general mixed-mode loading and constraint conditions. The method, which advantageously does not require special mesh configuration/modifications and post-processing of finite element results, is an enhancement of previous developments applied so far on isotropic homogeneous and isotropic interface cracks. The spatial variation of FGM material properties is taken into account at the level of element integration points. To validate the developed method, two- and three-dimensional mixed-mode fracture problems are selected from the literature for comparison. Two-dimensional cases include: inclined central crack in a large FGM medium under uniform tensile strain and stress loadings, a slanted crack in a finite-size FGM plate under exponentially varying tensile stress loading and an edge crack in a finite-size plate under shear traction load. The three-dimensional example models a deflected surface crack in a finite-size FGM plate under uniform tensile stress loading. Comparisons between current results and those from analytical and other numerical methods yield good agreement. Thus, it is concluded that the developed three-dimensional enriched finite elements are capable of accurately computing mixed-mode fracture parameters for cracks in FGMs.     

应力作用下NiTi形状记忆合金微结构演化的相场模拟及其本征应变率敏感性

基于Ginzburg-Landau动力学控制方程建立了NiTi形状记忆合金非等温相场模型,实现了对NiTi合金内应力诱导马氏体相变的数值模拟。同时将晶界能密度引入系统局部自由能密度,从而考虑多晶系统中晶界的重要作用。数值计算了单晶和多晶NiTi形状记忆合金在单轴机械载荷作用下微结构的动态演化过程和宏观力学行为,并重点研究了晶粒尺寸为60 nm的NiTi纳米多晶在低应变率下（0.0005～15 s?1）力学行为的本征应变率敏感性。研究结果表明,单晶NiTi合金系统高温拉伸-卸载过程中马氏体相变均匀发生,未形成奥氏体-马氏体界面。而纳米多晶系统在加载阶段出现了马氏体带的形成-扩展现象,在卸载阶段出现了马氏体带的收缩-消失现象。相同外载作用过程中,NiTi单晶系统的宏观应力-应变曲线具有更大的滞回环面积,拥有更优的超弹性变形能力。计算结果显示,在中低应变率下纳米晶NiTi形状记忆合金应力-应变关系表现出较明显的应变率相关性,应变率升高导致材料相变应力提升。这一应变率相关性主要源于相场模型中外加载荷速率与马氏体空间演化速度的相互竞争关系。     

A New Shear-Compression Test for Determining the Pressure Influence on the Shear Response of Elastomers

A new shear-compression experiment for investigating the influence of hydrostatic pressure (mean stress) on the large deformation shear response of elastomers is presented. In this new design, a nearly uniform torsional shear strain is superposed on a uniform volumetric compression strain generated by axially deforming specimens confined by a stack of thin steel disks. The new design is effective in applying uniform shear and multiaxial compressive stress on specimens while preventing buckling and barreling during large deformation under high loads. By controlling the applied pressure and shear strain independently of each other, the proposed setup allows for measuring the shear and bulk response of elastomers at arbitrary states within the shear-pressure stress space. Thorough evaluation of the new design is conducted via laboratory measurements and finite element simulations. Practical issues and the need for care in specimen preparation and data reduction are explained and discussed. The main motivation behind developing this setup is to aid in characterizing the influence of pressure or negative dilatation on the constitutive shear response of elastomeric coating materials in general and polyurea in particular. Experimental results obtained with the new design illustrate the significant increase in the shear stiffness of polyurea under moderate to high hydrostatic pressures.     

STUDY ON THE SURFACE CRACK GROWTH BEHAVIOR IN 14MnNbq BRIDGE STEEL

Three-dimensional crack closure correction methods are investigated in this paper.The fatigue crack growth tests of surface cracks in 14MnNbq steel for bridge plate subjected to tensile and bending loadings are systematically conducted.The experimentally measured fatigue crack growth rates of surface cracks are compared with those of through-thickness cracks in detail.It is found that the crack growth rates of surface cracks are lower than those of through-thickness cracks.In order to correct their differences in fatigue crack growth rates, a dimensionless crack closure correction model is proposed.Although this correction model is determined only by the experimental data of surface cracks under tensile loading with a constant ratio R=0.05, it can correlate the surface crack growth rates with reasonable accuracy under tensile and bending loadings with various stress ratios ranging from 0 to 0.5.Furthermore, predictions of fatigue life and crack aspect ratio for surface cracks are discussed, and the predicted results are also compared with those obtained from other prediction approaches.Comparison results show that the proposed crack closure correction model gives better prediction of fatigue life than other models.     

Dynamic stress intensity factors of multiple cracks in a functionally graded orthotropic half-plane

In the present paper dynamic stress intensity factor and strain energy density factor of multiple cracks in the functionally graded orthotropic half-plane under time-harmonic loading are investigated. By utilizing the Fourier transformation technique the stress fields are obtained for a functionally graded orthotropic half-plane containing a Volterra screw dislocation. The variations of the material properties are assumed to be exponential forms which the equilibrium has an analytical solution. The dislocation solution is utilized to formulate integral equation for the half-plane weakened by multiple smooth cracks under anti-plane deformation. The integral equations are of Cauchy singular type at the location of dislocation which are solved numerically to obtain the dislocation density on the faces of the cracks. The dislocation densities are employed to determined stress intensity factor and strain energy density factors (SEDFs) for multiple smooth cracks under anti-plane deformation. Numerical examples are provided to show the effects of material properties and the crack configuration on the dynamic stress intensity factors and SEDFs of the functionally graded orthotropic half-plane with multiple curved cracks.     

Study on Deformation of Polycrystalline Aluminum Alloy Using Moiré Interferometry

Polycrystalline aluminum alloy is manufactured by annealing, compared to the width of the specimen, the size of the grains can not be omitted, which makes the specimen anisotropic. Under uniaxial tension, the deformation field is inhomogeneous. In this study, moiré interferometry is successfully applied to measure the deformation of the polycrystalline specimen. The experimental results presented the stress versus strain responses of the marked grains in different orientations and different shapes. By using fringe centering method, the strain distributions under certain load in the centers and on the boundaries of the grains are analyzed.     

航空薄壁弧形结构件喷丸强化变形预测

喷丸强化是一种有效提高工件抗疲劳性能的表面处理工艺，经喷丸强化处理后的航空薄壁弧形结构件发生变形是航空制造领域的共性难题．本文基于变截面曲梁平面弯曲理论，建立了弧形件喷丸诱导应力与变形方程，将得到的构件最大变形与有限元方法计算结果进行对比，所建立理论模型能有效预测弧形件喷丸强化后的变形．讨论了喷丸压力和喷丸区域对喷丸变形的影响，结果表明：喷丸强化工艺仅作用于弧形件腹板表面时，弧形件变形以径向收缩为主；喷丸强化工艺仅作用于弧形件筋板表面时，弧形件变形以径向扩张为主；与局部喷丸相比，对弧形件所有表面喷丸强化时，弧形件弯曲变形减小，轴向伸长增大．