十字接头恒幅和变载荷历程疲劳试验结果及预测

对十字接头分别进行恒辐轴向载荷和变载荷历程疲劳试验,接头疲劳裂纹通常开始发生在焊缝根部,但如果接头变形引起的弯曲应力较大,疲劳裂纹也可能发生在焊缝趾部。本研究试图找出影响焊接接头疲劳寿命各参数之间的相互关系,并利用作者引伸的焊接接头疲劳裂纹起始－扩展模型较精确地预测复杂焊件的疲劳寿命。该模型把疲劳裂纹起始寿命看成是疲劳裂纹萌生,早期生成并聚合成主疲劳裂纹的循环次数,利用应变控制疲劳数据和Ｐａｌｍｇ     

Fracture and fatigue crack-growth behavior of single crystal NiAl

The plane strain fracture toughness for pre-cracked, single crystal NiAl + 0.25%Fe was determined to be -6.5 MPa√m for the {110}t Ol crack orientation. The fatigue crack-growth behavior for this orientation was also studied. Fractography indicates the possibility of fatigue crack propagation on planes other than the preferred cleavage plane. Fatigue on these alternate planes also resulted in increased plastic deformation on the fracture surface. Finally, the transient crack-growth response following compressive overloads have been shown to be effective in reinitiating fatigue crack-growth. This effect may be due to a reduction in shielding from roughness induced crack closure.     

Effect of preloading on corrosion fatigue of 70/30 brass

The effects of a single tensile overload and precycling load in air on subsequent corrosion fatigue cracking in 70/30 brass exposed to 1 M sodium nitrite solution were studied. A single tensile overload caused a decrease in subsequent corrosion fatigue crack propagation rate. Precycling load also showed retardation effects, depending on the load conditions for precycling and corrosion fatigue. A transsition from a higher mean stress for precycling to a lower mean stress for corrosion fatigue decreased corrosion fatigue crack growth rate, while an opposite transition did not appreciably change the corrosion fatigue crack propagations. The responses of bulk mechanical properties to a presingle overload and precycling load in air were measured, showing hardening which may probably be responsible for the observed retardation.     

Investigation of Fatigue Crack Propagation in Spot-Welded Joints Based on Fracture Mechanics Approach

In this paper, fatigue crack propagation life of resistance spot welds in tensile-shear specimens is investigated based on the calculation of stress intensity factors and J-integral using three-dimensional finite element method. For comparison, experimental works on 5083-O aluminum alloy spot-welded joints have been carried out to verify the numerical predictions of fatigue crack propagation of welded joints. A lot of analyses have been performed to obtain stress intensity factors and J-integral in tensile-shear specimens of spot-welded joints by using commercial software ANSYS. These gathered data have been formulated by using statistical software SPSS. The results of fatigue propagation life and predicted fatigue crack path revealed very good agreement with the experimental fatigue test data and photograph of cross-section of the fatigued spot-weld specimens.     

Q960E热影响粗晶区疲劳寿命与ΔKth值的关系分析

基于模拟焊接热循环试验及疲劳裂纹扩展试验,对动载结构用高强钢Q960E热影响粗晶区进行了多种应力幅值作用下的疲劳寿命研究.通过得到Paris方程建立了不同焊接热模拟工艺下疲劳裂纹扩展门槛值（ΔK_th）随不同交变载荷下疲劳寿命值的近似线性关系.利用场发射扫描电镜中背散射衍射功能（EBSD）对疲劳裂纹扩展试样中的裂纹尖端进行了晶体学取向分析及扩展机制讨论.结果表明,在应力幅值ΔP固化后,疲劳寿命N随ΔK_th的增大而增加,其延寿微观机理在于组织中的亚结构取向存在差异,所形成的大角度晶界（≥15°）可有效迫使裂纹转向,从而提高材料的疲劳寿命.     

Fatigue crack initiation and multiplication of unnotched titanium matrix composites

Fatigue crack initiation and multiplication of the unnotched SCS-6 silicon carbide fiber-reinforced titanium matrix composites with different matrix and interfacial properties have been investigated experimentally and analytically. Ti–15V–3Al, Ti–6Al–4V, and Ti–22Al–23Nb were chosen as matrix materials. The initiation and propagation of each individual matrix crack as a function of fatigue cycles and applied stress levels were carefully monitored. The statistical distribution of crack growth rates in each composite has been constructed and analyzed. The evolution of normalized matrix crack density and stiffness reduction of these composites under fatigue loading also has been characterized. A modified shear-lag model, coupled with the strain-life equation and a fiber bridging model were used to predict the fatigue crack initiation life, matrix crack growth rate, normalized matrix crack density, and residual stiffness of the composites. The predicted fatigue properties correlated well with experimental results.     

Berechnung des Rißfortschrittsverhaltens geschweißter Proben aus Offshore-Stählen in Meerwasser zur Abschätzung der Lebensdauer von Offshore-Komponenten

Calculation of crack growth in welded specimens under seawater conditions in order to predict fatigue life of offshore components Welded components in the offshore field generally are designed that no cracks should occur. Under cyclic loading cracks must be taken in account, yet. In the research work described a suitable calculation method had to be proved to predict the crack growth behaviour of offshore components in seawater and under service-like conditions. Using Paris' law the crack growth behaviour of specimens which are similar to the component (welded V-shape specimens) may be predicted quite well in air, free corrosion, under cathodic protection (?860 mV Ag/AgCl) and under a variable load sequence. The use of Wheeler's model including crack retardation yielded in a significant higher crack propagation life and therefore unconservative predictions of fatigue life.     

不同类型过载下Ⅰ型疲劳裂纹的扩展行为

研究了Ⅱ、Ⅲ型过载对Ⅰ型疲劳裂纹扩展行为的影响并与Ⅰ型过载进行比较．结果表明：单周Ⅱ、Ⅲ型过载也延迟了其后的Ⅰ型疲劳裂纹扩展，但作用程度小于Ⅰ型过载；过载延迟效应随过载时基础载荷的降低和过载比的加大而增强，在近门槛区过载表现为疲劳裂纹扩展停止、疲劳门槛提高，在高基础载荷过载时疲劳裂纹则延缓、减速扩展．过载延迟行为主要由裂尖尾迹塑性诱导裂纹闭合和裂尖附近残余压应力状态引起。     

Influence of the Peak Tensile Overload Cycles on the Fatigue Crack Growth of Aluminum Alloy Under Spectrum Loading

Many structures such as aircrafts, risers, and offshore pipelines that are in contact with fluids, become subjected to complex variable amplitude loading (VAL) stress-time histories during their service lives. As a result, the structural life assessment and damage-tolerant analyses of such structures are considered as two important design criteria. In this paper, a VAL stress-time history is used to study the fatigue life of 6061-T651 aluminum alloy, with focus on the retardation effect resulting from the applied peak tensile overload cycles (TOLCs). Various so-called “clipping” levels are tested, and the results are compared with those obtained through an analytical method, using the Willenborg retardation approach, in conjunction with the Walker fatigue crack growth model. The results would demonstrate the significant influence of the TOLC present within VAL scenarios on retarding the fatigue crack growth rate of the material. The study also investigates the influence of various clipping levels on the fatigue response of the material, also highlighting the limitations of the analytical approach in estimating the resulting crack growth rate. It is observed that the analytical method predicts a higher fatigue life for the material subjected to VAL, which is non-conservative for design purposes. Some suggestions are provided for fatigue life estimation of the material when subjected to VAL scenarios.     

Fatigue lives of friction stir spot welds in aluminum 6061-T6 sheets

The fatigue lives of friction stir spot welds in aluminum 6061-T6 lap-shear specimens under cyclic loading conditions are investigated in this paper. The paths of fatigue cracks near friction stir spot welds in lap-shear specimens are first examined. The experimental observations suggest that under cyclic loading conditions, the fatigue crack is initiated near the possible original notch tip in the stir zone and propagates along the circumference of the nugget, then through the sheet thickness and finally grows in the width direction to cause final fracture. A fatigue crack growth model based on the Paris law for crack propagation and the local stress intensity factors for kinked cracks is then adopted to predict the fatigue lives of friction stir spot welds. The global and local stress intensity factors are used to estimate the local stress intensity factors of kinked cracks with experimentally determined kink angles. The results indicate that the fatigue life predictions based on the Paris law and the local stress intensity factors as functions of the kink length agree well with the experimental results.     

LF2铝合金搅拌摩擦焊接头的疲劳特性

对LF2铝合金搅拌摩擦焊接头的疲劳特性进行了研究，建立了焊接头的S-N曲线。疲劳试验结果表明，搅拌摩擦焊接头表现出良好的疲劳性能，疲劳寿命N=10^6次的疲劳强度值约为59～65MPa。搅拌摩擦焊接头具有细小的等轴晶粒和狭窄的热影响区，阻碍了滑移带的形成和裂纹的扩展，从而提高了接头的疲劳性能。疲劳断口分析显示，随着驻留滑移带的扩展逐渐萌生裂纹，在交变应力作用下最终导致失效。     

变幅载荷作用下疲劳裂纹扩展研究综述

机械结构主要承受疲劳载荷,工程上尤其承受变幅载荷。本文对目前国内外比较常用的变幅载荷作用下疲劳裂纹扩展的模型理论进行了详细地综合分析,大致包括了塑性诱导闭合理论、裂纹尖端钝化理论和残余压应力理论。目前常用的模型单峰或多峰超载疲劳裂纹扩展寿命的预测结果比较理想,但在高载比以及考虑构件厚度效应时误差往往比较大。所有模型均有其依据的理论、成立的前提条件和适用范围,在实际工程应用中应根据不同的条件选择相应的模型。高载比,裂尖大范围屈服时模型寿命预测的准确度问题是今后重点。本文对读者了解模型的使用范围和简化模型以提高其寿命预测的适用性、方便性和准确度,可以提供借鉴。     

Role of structural parameters of ultra-fine grained Cu for its fatigue and crack growth behaviour

High cycle fatigue (HCF) life time curves and fatigue crack growth rates of bulk ultra-fine grained (UFG) copper deformed by high pressure torsion (HPT) were determined. Cu of two different purities as well as a bimodally structured HPT Cu were investigated. The results show increased HCF properties of the UFG materials compared to coarse grained (CG) Cu. Especially HPT Cu with lower purity shows enhanced fatigue resistance due to higher microstructural stability. Contrary, crack growth rates in HPT Cu are increased. In case of the high purity Cu, cyclic deformation induced coarsening of the UFG microstructure nearby the crack is found at threshold crack growth rates leading to a retardation of the fatigue crack propagation. Within these coarse grains typical fatigue surface slip marks as observed in CG Cu are found.     

结合S-N曲线和断裂力学的焊接结构疲劳寿命分析

针对焊接结构的疲劳裂纹演化过程,将焊接结构的疲劳寿命定义为裂纹萌生寿命N_i和裂纹扩展寿命N_p之和,提出一种结合S-N曲线和断裂力学理论的疲劳寿命分析方法.采用等效结构应力法和99%下限主S-N曲线计算焊接结构的裂纹萌生寿命,并将这一阶段结束时的裂纹看作为半椭圆表面裂纹.采用Paris裂纹扩展模型和半椭圆表面裂纹应力强度因子ΔK计算裂纹扩展寿命N_p.参照某起重机走行梁的疲劳试验结果进行对比和验证研究.结果表明,等效结构应力可以较好地表征复杂焊接结构的裂纹萌生特性,结合S-N曲线和断裂力学的疲劳寿命计算结果与试验结果具有较好的一致性.     

EFFECT OF NON-PROPORTIONAL OVERLOADING ON FATIGUE CRACK GROWTH

The effect of proportional and non-proportional overloading on mode l fatigue crack growthhave been studied,and the influences of crack tip plastic zone,crack tip blunting as well ascrack closure were discussed.Proportional(model I)overloading may cause more seriouscrack growth retardation than non-proportional(mixed mode)overloading.Therefore,forestimating the fatigue life of engineering structures to simplify a real overload which may of-ten be non-proportional as a proportional one is not always safe.     

含氢NZ2锆合金疲劳裂纹扩展行为

利用板材标准M(T)试样研究了4种不同氢含量(0,200,450,730 (g/g) NZ2锆合金的疲劳裂纹扩展行为.发现不论含氢量的高低,裂纹稳态扩展区的裂纹扩展行为均符合Paris幂律关系,即 da/dN=C((K)n,但随着氢含量的增加,n值不断减小;含氢量增加导致NZ2合金塑性变形能力降低,疲劳裂纹扩展速率增加,尤其是氢含量较高时表现更为明显.     

Three-dimensional fatigue crack growth behavior in an aluminum alloy investigated with in situ high-resolution synchrotron X-ray microtomography

The fatigue crack propagation process in an Al–Mg–Si alloy was investigated using in situ high-resolution synchrotron radiation X-ray microtomography. Tomography datasets were obtained at periodic intervals throughout the 120,000 fatigue cycles. Three-dimensional rendering of the through-thickness crack shape indicates that in a number of regions the adjacent sides of two branched cracks tend to overlap with fatigue cycling and form a crack overlapping region. Measured crack growth rates in each tomographic slice show that crack growth retardation generally occurs in these crack overlapping regions. The through-thickness variation in crack tip opening displacement was also measured and was used to account for the observed crack propagation behavior. Crack morphologies were observed at different load levels in a fatigue cycle. The crack closure level varied for two selected regions comprising different overlapping cracks. The correlation of the crack growth rate with both crack opening and closure levels was discussed and interpreted.     

Behavior of a submicrocrystalline aluminum alloy 1570 under conditions of cyclic loading

A study of the resistance to fatigue-crack growth in a submicrocrystalline alloy Al-6% Mg-0.3% Sc-0.4% Mn in combination with a precision analysis of the fracture surface of the samples has been performed. A comparison of crack resistance between coarse-grained and submicrocrystalline states of this alloy showed that only at the stage of near-threshold crack growth the velocity of fatigue-crack propagation in the submicrocrystalline state proves to be higher than that in the coarse-grained state. At the stage of linear crack growth, the fatigue-crack propagation becomes insensitive to the grain size. Upon transition to the stage of accelerated crack growth, the velocity of crack propagation in the submicrocrystalline alloy is retarded. A fractographic analysis of the fracture surface of the samples indicates that the retardation of the fatigue-crack growth in the submicrocrystalline alloy is connected with a gradual transition from the intercrystalline to the transcrystalline mechanism of fatigue fracture of the material.     

Corrosion fatigue crack growth behaviour of naval steels

Fatigue crack growth behaviour of two varieties of HSLA steels used in naval structural applications have been evaluated in air and 3.5% NaCl solution. In air both the HSLA steels showed similar resistance to fatigue crack growth. However, in 3.5% NaCl, the fatigue crack growth resistance of HSLA-80 steel was superior to that of HSLA-100. The apparent inferiority of HSLA-100 to corrosion fatigue crack growth resistance is attributed to rapid film formation and rupture, and occurrence of planar modes of failure. Effect of R-ratio on air fatigue and corrosion fatigue crack growth behaviour is rationalised by the concept of crack closure. Effect of cyclic frequency on corrosion fatigue behaviour is examined. It is noted that the mechanism of corrosion fatigue crack growth for the two HSLA steels changes with attendant change in the Paris slope. This leads to increase or decrease of crack growth rates, depending up on the ΔK range of interest.     

激光-电弧复合焊接7075-T6铝合金裂纹扩展分析

研究了7075-T6铝合金薄板激光复合焊焊接接头疲劳裂纹扩展寿命.同时通过试验建立了不同热输入与弹性模量之间的函数关系并预测了给定热输入下的疲劳寿命.基于改进的Forman公式,提出了一种考虑7075-T6复合焊接头性能差异性、应力强度因子门槛值、裂纹扩展驱动力以及残余应力等多因素耦合的接头寿命预测模型.通过有限元模拟仿真并结合裂纹扩展和接头拉伸试验,对提出的模型进行了验证.结果表明,该模型预报的接头寿命与试验结果吻合较好.