海洋用结构钢在NaCl溶液中疲劳短裂纹的扩展

本文用海洋平台结构钢A537CL1的单边裂纹样品,在空气和3.5％ NaCl水溶液中进行了腐蚀疲劳短裂纹扩展实验,比较不同裂纹长度及环境介质对疲劳短裂纹扩展行为的影响。结果表明,在门槛值附近,对于相同的应力强度因子范围,短裂纹扩展速率比长裂纹扩展快。短裂纹扩展的门槛值随裂纹长度的减小而减小。与空气介质相比较,盐水对疲劳短裂纹扩展加速的程度远大于长裂纹。     

FATIGUE CRACK PROPAGATION IN Al-Li ALLOY 8090

Fatigue crack growth rates of Al-Li alloy 8090 in air were found to be strongly dependent up-on the aging conditions.The naturally aged and underaged specimens showed the highest re-sistance to the fatigue crack propagation.The fatigue crack growth resistance of the overagedspecimen is the lowest and that of the peakaged specimen in between.As compared to air,3.5% NaCl solution does not change the effect of aging conditions on the fatigue crackgrowth,but causes a decrease of the resistance to the fatigue crack growth under the same ag-ing condition.Both short cracks from notch and physically short cracks showed much higherrates of fatigue crack propagation in comparison with long cracks under the same aging condi-tion and stress intensity level.The growth behavior of the short crack depends on its type.Thegrowth rate of short crack from notch decreases first to a minimum and then increases with in-creasing △K.However,the physically short crack grows at a progressively increasing rate.The effects of aging conditions are explained in terms of the slip planarity of dislocations andthe cyclic slip reversibility.The observed short crack behavior is considered to be dependent onthe crack closure and the local plasticity near the crack tip.     

30CrMnSiNi2A高强钢的疲劳小裂纹扩展特性及寿命预测

研究了３０ＣｒＭｎＳｉＮｉ２Ａ高强钢在恒幅载荷下的小裂纹起始特征和裂纹扩展特性。试验结果表明，小裂纹起始于缺口表面初始缺陷（如夹杂或孔洞）处。在应力比Ｒ＝０的恒幅载荷下，未显示小裂纹效应：在Ｒ＝－１的恒幅载荷下，则显示出小裂纹效应的存在。利用裂纹闭合模型和微观结构缺陷作为初始裂纹尺寸的方法预测了小裂纹扩展速率和疲劳全寿命，预测值与实验值符合良好。     

8090Al—Li合金的疲劳裂纹扩展

研究了8090型Al—Li合?疲劳裂纹扩展行为以及组织结构和环境的影响,结果表明,在空气中,自然时效和欠时效?现出最好的疲劳裂纹扩展阻力,其次是峰时效状态,而过时效状态的疲劳裂纹扩展阻力最低,在3.5%NaCl水溶液中,时效状态对疲劳裂纹扩展行为有着同空气中相同的影响规律,但在同样的时效条件下,在3.5%NaCl水溶液中的疲劳裂纹扩展阻力要比空气中的低,在相同的应力强度因子范围△K作用下,缺口短裂纹和物理短裂纹均表现出比长裂纹高的扩展速率,用位错的平面滑移性和循环滑移可逆性解释了时效的影响,用裂纹的闭合效应和裂纹尖端塑性区尺寸说明了长短裂纹扩展行为的差别。     

考虑形状比的焊趾裂纹扩展行为的数值仿真分析

针对焊趾处初始裂纹的不同形状比,基于NASGRO裂纹扩展速率模型对裂纹扩展过程进行了数值仿真. 以具有不同形状比的半椭圆表征焊趾表面裂纹,考虑短裂纹扩展阶段,采用M积分计算整个裂纹前沿的应力强度因子,并将裂纹前沿应力强度因子中值点作为裂纹扩展的主要控制参量,模拟并分析了不同形状比条件下裂纹前沿的形状演变和应力强度因子的分布及变化特点. 结果表明,随着裂纹的扩展,短裂纹阶段的不同形状比变化将趋于一致;短裂纹的形状比对早期裂纹扩展有较大影响,并最终导致整体疲劳寿命的显著差异性;综合初始形状比对裂纹前沿应力强度因子、扩展形状以及疲劳寿命的影响,可将其作用效果的强弱拐点作为界定短裂纹与长裂纹的有效参考. 开展了十字焊接接头疲劳验证试验,试验结果与仿真结果具有较好的一致性.     

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.     

Fatigue threshold and crack propagation in γ-TiAl sheets

The fatigue crack propagation behaviour of two different microstructures — a coarse-grained designed fully lamellar (DFL), and a fine-grained near γ (FG) — of a Ti–46.5 at.% Al–4 at.% (Cr, Nb, Ta, B) alloy was studied. Both the threshold of stress intensity range and standard long crack growth behavior were determined. A special technique was applied to separate the different mechanisms — intrinsic and extrinsic effects — and their changes with crack length. The fatigue crack propagation rate of long cracks is much smaller in the DFL microstructure than in the FG microstructure at the same stress intensity range. The effective threshold of stress intensity range of both microstructures is about 1.7 MPa√m. The threshold of stress intensity range shows a strong R-curve behavior. In other words the propagation–non-propagation conditions of cracks are significantly influenced by the crack extension. The long crack thresholds of stress intensity range at the stress ratio 0.1 are relatively large; they are about 4.5 and 8 MPa√m in the DFL and the FG microstructure, respectively. The differences in the crack growth behavior between the two microstructures are mainly induced by extrinsic resistance mechanisms.     

THRESHOLD OF SHORT FATIGUE CRACK FOR Al-Li ALLOY 8090

Threshold of fatigue for short through cracks for Al-Li alloy 8090 was evaluated to be lowerthan that of long cracks,and to decrease with the shortening of the crack length.The correla-tion among the threshold,crack wake,and crack tip shielding of the short fatigue cracks wasdiscussed.The super demarcation length and lowest theoretical threshold of short through fa-tigue cracks were presented.     

Short fatigue crack growth behaviour of a ferritic steel weld metal

Abstract

The present study deals with the short fatigue crack growth behaviour of a ferritic steel (nuclear grade SA 333 Grade 6 steel) weld metal at two load ratios R of 0.1 and –1. Single edge notched weld joint specimens were tested at different stress levels. The acceleration and deceleration in short crack growth with increasing stress intensity factor range was explained in terms of microstructure of weld metal. Acicular ferrite and grain boundary ferrite acted as barriers to crack growth. Non-propagating cracks were observed due to multiple cracks and blocking by ferrite grain boundaries. Transition crack length, the limiting crack length above which the crack exhibited a typical long crack behaviour, was determined to be 1 mm at R=0.1 and 1.5 mm at R=2 1.     

BEHAVIOUR OF INITIATION AND GROWTH OF SHORT FATIGUE CRACKS

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45钢疲劳短裂纹的形成和扩展

本文对４５钢光滑试样疲劳短裂的纹的形成和扩展进行了试验，利用复型技术和光学显微镜观测了裂纹尺寸演化全过程，发现疲劳短裂纹形成寿命小于失效寿命的１０％，根据裂纹的扩展速度将短裂纹形成分成微观短裂纹和物理短裂纹两个阶段，给出了区分两种短裂纹的裂纹特征尺寸值和两种短裂纹的扩展速度公式。     

组合加载下碳素工具钢中疲劳小裂纹的门槛区行为特征

通过考察碳素工具钢的表面滑移、疲劳破坏及表面裂纹的扩展条件，以统一的局部应力讨论了光滑试样和裂纹试样的疲劳特征。分析和实验结果表明，在裂纹试样的名义应力小于光滑试样表面滑移门槛应力的范围内，线弹性断裂力学是有效的，对应裂纹扩展门槛值的裂尖性区为一常数。     

Discrete dislocation plasticity modeling of short cracks in single crystals

The mode-I crack growth behavior of geometrically similar edge-cracked single crystal specimens of varying size subject to both monotonic and cyclic axial loading is analyzed using discrete dislocation dynamics. Plastic deformation is modeled through the motion of edge dislocations in an elastic solid with the lattice resistance to dislocation motion, dislocation nucleation, dislocation interaction with obstacles and dislocation annihilation incorporated through a set of constitutive rules. The fracture properties are specified through an irreversible cohesive relation. Under monotonic loading conditions, with the applied stress below the yield strength of the uncracked specimen, the initiation of crack growth is found to be governed by the mode-I stress intensity factor, calculated from the applied stress, with the value of K_init decreasing slightly with crack size due to the reduction in shielding associated with dislocations near a free surface. Under cyclic loading, the fatigue threshold is ΔK-governed for sufficiently long cracks. Below a critical crack size the value of ΔK_I at the fatigue threshold is found to decrease substantially with crack size and progressive cyclic crack growth occurs even when K_max is less than that required for the initiation of crack crack growth in an elastic solid. The reduction in the fatigue threshold with crack size is associated with a progressive increase in internal stress under cyclic loading. However, for sufficiently small cracks, the dislocation structure generated is sparse and the internal stresses and plastic dissipation associated with this structure alone are not sufficient to drive fatigue crack growth.     

残余压应力场中裂纹扩展的闭合模型

分析了残余压应力对裂纹闭合的影响机理,在此基础上建立残余应力场中裂纹扩展闭合力的预测模型,讨论了裂纹闭合的尺寸效应,将模型延伸至短裂纹和表面裂纹的范围．对形变强化组织影响闭合进行了讨论     

带斜裂纹模拟圆筒容器静压爆破和疲劳试验

本文提出了斜裂纹鼓胀效应修正系数公式.用该公式计算了带斜裂纹圆筒爆破载荷和开裂载荷,结果与实测大致相符,其平均误差在8%左右,可用于工程计算.本文还研究了斜裂纹疲劳扩展规律.采用直线法和投影法计算了疲劳扩展后的斜裂纹尖端应力强度因子。按照Paris 裂纹扩展方程整理了数据,然后获得了该钢种扩展速率方程,并论及了斜裂纹的扩展角问题.     

深层滚压加工对高碳铬轴承钢超长寿命疲劳行为的影响

深层滚压加工对高碳铬轴承钢超长寿命疲劳行为的研究表明：与深层滚压加工前相同,在高应力幅短寿命区,疲劳裂纹萌生于试样表面;由于表面压缩残余应力的影响,疲劳强度有所提高．在低应力幅长寿命区,疲劳裂纹萌生位置从未经深层滚压加工试样的次表面向深层滚压加工试样内部没有压缩残余应力和硬化层的位置转移;因弯曲应力梯度的影响,疲劳强度有了较大的提高．深层滚压加工可以提高高碳铬轴承钢在超长寿命区的旋转弯曲疲劳强度．     

高速缓冲液压缸内表面裂纹扩展特性研究

由于制造缺陷、疲劳及腐蚀等原因,缓冲液压缸在服役过程中内壁会产生轴向裂纹,严重影响液压缸的结构强度、缓冲性能和使用寿命。通过AMESim液压仿真平台建立缓冲系统模型来模拟实际工况,将缓冲油压作为有限元分析的初始载荷,建立缓冲缸有限元模型和裂纹扩展模型,研究不同初始裂纹形状、裂纹位置对疲劳寿命的影响,并分析裂纹前缘动态应力强度因子和裂纹形貌的变化。结果表明：无缺陷的油缸完全能承受缓冲压力;裂纹初始长度一定时,随着深长比增大,裂纹前缘应力强度因子会整体增大,裂纹扩展速度加快;不同深长比的初始裂纹经过多次循环载荷后都更倾向于在长度方向快速扩展;缸体内不同位置的裂纹疲劳寿命存在较大差异,在壁厚较小的边角位置,疲劳裂纹扩展寿命最短。     

Fatigue behaviour of bolted joints

This paper analyzes the tensile fatigue behaviour of bolted joints constituted by commercial steel bolts. They were tested under both monotonic and fatigue tensile loading, with different R-ratio. Results show that under increasing monotonic tensile loading the bolted joint is not the failure zone of the bolt, whereas such a bolted joint is the failure region under cyclic loading. The fatigue life decreases with the increase of the stress range and with the maximum stress, and pre-loading enlarges the fatigue life. Fatigue fracture surface shows a geometry of crescent moon in the case of short cracks and such a shape evolves towards a quasi-straight crack front in the case of long cracks. Fatigue fracture usually happens at the root of the first notch inside the bolted joint, although fracture initiation may happen in several consecutive notch roots, increasing the initiation angle of the fatigue crack as the applied stress diminishes.     

SHORT FATIGUE CRACK BEHAVIOUR IN ISO-STRESS SPECIMENS

Fatigue tests were made on a vibration machine using specially designed specimens to investi-gate the initiation and propagation features of short fatigue cracks in a weld metal.The spec-imens were triangular in shape so that when loaded as a cantilever beam the surface tensilestress is constant.Test results show that short fatigue cracks mainly originate from slip bandswithin ferrite grains.The development of short fatigue cracks is marked by a gradual increasein crack density with fatigue cycles.Coalescence of short cracks leads to crack propagationand the crack path is predominantly transgranular.     

Integrated fracture mechanics approach to analyse fatigue behaviour of welded joints

Abstract

Current fracture mechanics methods for fatigue assessment, including those that consider thresholds for crack propagation, are based on long crack behaviour. The present work is concerned with an attempt to predict the fatigue strength of welded joints using a fracture mechanics approach that takes into account the fatigue behaviour of short cracks. The methodology estimates the fatigue crack propagation rate as a function of the difference between the applied driving force and the material threshold for crack propagation, which is a function of crack length. The fatigue strength of butt welded specimens stressed transversely was analysed. Experimental results from the literature were used for comparison. Estimations are obtained by using only the fatigue limit and the fatigue propagation threshold for long cracks, and the applied stress distribution along the crack path obtained from simple finite element models. The influence of plate thickness, initial crack length, and reinforcement angle on fatigue strength of butt welded joints was analysed. Results show good agreement with experimental trends.