合金材料超高周疲劳行为的基本特征和影响因素

合金材料在超高周疲劳下具有与低周和高周疲劳不同的裂纹萌生和扩展行为以及不同的 S--N曲线特征. 材料的强度、循环加载的频率、所处的环境等都显著影响超高周疲劳的特性. 本文综述了合金材料超高周疲劳行为的基本特征和影响因素的研究进展.     

Influence of cubic boron nitride grinding on the fatigue strengths of carbon steels and a nickel-base superalloy

The influence of cubic boron nitride (CBN) grinding on fatigue strength was investigated on an annealed carbon steel, a quenched and tempered carbon steel at room temperature, and a nickel-base superalloy, Inconel 718, at room temperature and 500 °C. The results were discussed from several viewpoints, including surface roughness, residual stress, and work hardening or softening due to CBN grinding. The fatigue strength increased upon CBN grinding at room temperature, primarily because of the generation of compressive residual stress in the surface region. However, in the case of Inconel 718, this marked increase in the fatigue strength tended to disappear at the elevated temperature due to the release of compressive residual stress and the decrease of crack growth resistance at an elevated temperature.     

Fatigue Behavior of Friction Stir-Welded Joints Repaired by Grinding

Fatigue is undoubtedly the most important design criterion in aeronautic structures. Although friction stir-welded joints are characterized by a high mechanical performance, they can enclose some defects, especially in their root. These defects along with the relatively low residual stresses of the friction stir-welding thermomechanical cycle can turn into primary sources of crack initiation. In this context, this article deals with the fatigue behavior of friction stir-welded joints subjected to surface smoothing by grinding improvement technique. The 4-mm-thick aluminum alloy 2024-T351 was used in this study. The fatigue strength of the base material, joints in the as-welded condition, and the sound and defective friction stir-welded joints improved by grinding were investigated in detail. The tests were carried out with a constant amplitude loading and with a stress ratio of R = 0. The fatigue results show that an improvement in fatigue behavior was obtained in the joints repaired by superficial grinding technique. The weld grinding technique is better especially for lower loads and increases the high cycle fatigue strength. The fatigue strength of the improved welded joints was higher than that of the base material.     

超声冲击改进地铁车辆焊接裂纹修复质量的探究

目的 针对地铁车辆车体、转向架构架疲劳裂纹频发的问题,探索超声冲击工艺增强焊接结构件疲劳强度的机理和可行性.方法 分别利用机械打磨和不同工艺参数的超声冲击处理SMA490BW钢焊接对接接头,借助应力分析仪、金相显微镜、超声疲劳机、扫描电镜等,综合分析试样的应力分布、接头形貌、晶粒组织、疲劳寿命以及断口形貌,以对比不同处理方式的强化效果.结果 超声冲击处理可有效改善对接接头的应力集中,细化表层晶粒,并调整表面应力场的分布,将焊接残余拉应力转为较大的残余压应力.在工艺参数1.5 A/10 min的冲击下,试样可获得最高平均寿命3.35×107周次,是机械打磨试样的3.8倍,是原始试样的10倍,且疲劳断裂失效模式不变.结论 超声冲击通过表面改性,显著提高焊接结构的疲劳寿命,且优于机械打磨方法.超声冲击法高度契合车辆裂纹修复的需求,但相关标准、设备、工艺不够完善,建议对超声冲击方法进一步探索,并最终将其纳入地铁车辆裂纹修复工艺.     

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.     

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.     

TC4钛合金三点弯曲超高周疲劳性能研究

针对航空发动机压气机叶片复杂载荷环境下的超高周疲劳问题,开展了TC4钛合金三点弯曲超高周疲劳试验,研究其在弯曲加载下的超高周疲劳破坏行为。疲劳试验结果表明：在两种应力比（R=0.3、0.5）下,当循环次数超过107次时,试件仍发生疲劳断裂,S-N曲线均呈现双线性特征;SEM断口分析表明,随着最大应力的降低,裂纹萌生位置由试件表面向次表面转移,疲劳裂纹萌生是表面滑移和内部解理断裂之间相互竞争的结果;基于疲劳寿命建立模型分析了应力比对2种裂纹萌生机制之间竞争行为的影响。采用红外热像仪监测试件表面的温度,高周疲劳试件的温度变化分为四个阶段：稳定升高、稳定降低、快速升高和快速降低阶段,而超高周疲劳试件的温度变化分为三个阶段：稳定升高、快速升高和降低阶段。最后,阐述了疲劳过程中热产生和传热的特点,并分析了温度变化与应力分布的关系。     

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．     

Effect of Residual Stress on Fatigue Failure of Carbonitrided Low-Carbon Steel

The effect of residual stress on fatigue behavior and mechanisms of carbonitrided AISI 1015 steel under uniaxial cyclic loading has been experimentally studied. By progressive removal of thin surface layers using an electropolishing technique and subsequent residual stress measurements using an x-ray diffraction technique, the compressive residual stress at the surface was approximately 900 MPa. The stress decreased toward the center, and became stable tensile residual stress of approximately 20 MPa. The fatigue resistance of carbonitrided AISI 1015 steel was higher than that of AISI 1015 steel due to the presence of compressive residual stress in case layer. The fatigue limit of AISI 1015 steels with and without carbonitriding was 340 and 300 MPa, respectively. Subsurface cracks initiated at the case-core interface, i.e. approximately 400 μm from the surface. With increasing number of stress cycles, the subsurface cracks coalesced and propagated intergranularly through the case layer. After some incubation cycles, the subsurface cracks reached the surface of specimen, and became a main crack. During this stage, the stress increased, and caused the formation of voids in core material. Consequently, the crack propagated through the core material, interacted with voids, and caused complete fracture.     

INFLUENCE OF RESIDUAL STRESS AND SURFACE MORPHOLOGY ON FATIGUE PROPERTIES OF STEEL 60Mn

Influence of residual stress and surface morphology induced by shot-peening on fatigue behav-ior of a medium temperature tempered spring steel 60 Mn has been studied.The compressiveresidual stress induced in the near-surface region may improve fatigue limit from 930 to 1010MPa,and the very high tensile residual stress in the interior may reduce it from 1010 to 940MPa,whereas the severe surface damage may cause a drop-off of it from 1010 down to 800MPa.Fatigue cracks initiated in such position where the equivalent Mises stress,includingresidual stress,exceeded the local strength of the material.The compressive residual stress,induced by shot-peening,may intensify the effect of crack closure,so as to decrease the crackgrowth rate.     

Damage mechanism of a nodular cast iron under the very high cycle fatigue regime

Fatigue behaviour in the very high cycle regime (VHCF) of 10¹⁰ cycles were investigated with a cast iron (GS51) under ultrasonic fatigue test system (20 kHz) in ambient air at room temperature with a stress ratio R = −1.

The influence of frequency was examined by comparing similar data generated on conventional servo hydraulic test systems. An advanced, high-speed, and high-sensitivity infrared imaging system was used to measure the temperature changes during ultrasonic fatigue test at various load levels caused by internal damping due to a very high frequency cycling. The temperature field on the surface specimen was determined by using a non-destructive measurement technique called infrared pyrometer. An infrared camera made up of a matrix of 320 × 240 detectors was used.

The S-N curves obtained show that fatigue failure occurred beyond 10⁹ cycles, fatigue limit does not exist for the cast iron and there is no evidence of frequency effect on the test results. A detailed study on fatigue specimens subjected to ultrasonic frequency shows that the temperature evolution of the cast iron specimen is very evident, the temperature increased just at the beginning of the test, the temperature increased depending on the maximum stress amplitude. Under the current test conditions, the high cycle fatigue (VHCF) behaviour of the cast iron exhibited a typical fatigue crack growth process, that is, fatigue initiation takes place always at the surface graphite or subsurface void; the distinctive stable fatigue crack growth zone can be found around the fatigue crack initiation site, the change of fatigue initiation site from surface to subsurface is associated with the complex effects of applied maximum stress level, surface condition.

Under lower stress amplitude and high cycle condition, surface graphite fatigue initiation is predominantly depended on cyclic stress amplitude; subsurface void fatigue initiation is determined by maximum cyclic stress.

In the process of small crack propagation, the temperature in local plastic zone increase very sharply. The temperature field of ultrasonic fatigue specimen can be changed with the cooling condition; internal heating can accelerate surface fatigue crack initiation and propagation.

Fatigue properties in VHCF regime were studied for cast iron (GS51) at 20 kHz frequency and for the first time, crack initiation and propagation stages were analyzed using a high-sensitivity infrared camera. The new Paris's model for fish eye formation in the gigacycle fatigue were also confirmed by this study.     

焊接结构主S-N曲线拟合方法及软件开发

主S-N曲线法是当前焊接结构疲劳计算的新方法,在焊接结构疲劳分析中被广泛采用. 该方法以等效结构应力为核心参量,实现了以一条S-N曲线计算不同载荷模式和焊接形式结构的疲劳寿命. 依据主S-N曲线法,利用不同材料和焊接形式的疲劳试验建立了焊接结构疲劳试验数据库. 考虑厚度、弯曲比及多轴应力等修正参量的影响,采用最小二乘法原理,研究了不同主S-N曲线方程的拟合方法及其标准差. 在此基础上,开发了主S-N曲线拟合方法的专用软件. 基于VC++编程环境实现了名义应力、结构应力、等效结构应力、剪切结构应力、多轴结构应力及初始裂纹修正等主要计算功能. 基于该软件,完成了试验数据的多参量的对比分析,为研究焊接结构疲劳寿命评估及影响因素分析提供了技术基础.     

喷丸对H13钢单边带缺口试样疲劳裂纹扩展行为的影响

喷丸工艺通过改变试件的表面形貌、微观组织及残余应力等表面完整性可以影响裂纹的萌生和扩展;对于存在缺陷的试件,喷丸的作用机制和影响结果有所不同。采用白光干涉仪、SEM、XRD及显微硬度计等对单边带缺口的H13钢薄板试样喷丸前后的表面完整性进行了测定。并借助原位SEM开展了系列裂纹扩展试验,分析了喷丸对试样疲劳寿命、裂纹扩展速率以及疲劳断口特征的影响。研究结果表明,虽然残余压应力诱发裂纹闭合,但由于喷丸后表面粗糙度的大幅提高增强了缺口效应,表面加工硬化使得韧性有所降低,以及残余应力在加载过程中发生松弛等因素,喷丸后单边带缺口试样的裂纹萌生过程缩短,疲劳寿命降低,且裂纹扩展速率的变化较小。喷丸前后疲劳断口形貌均为准解理特征,喷丸后断口近表面处的撕裂棱特征消失。     

残余应力和表面形态对60Mn疲劳性能的影响

喷丸产生的表层残余压应力使中温回火弹簧钢60Mn的疲劳极限从930提高到1010MPa,而心部的高残余拉应力又会使疲劳极限从1010降至940MPa,严重的喷丸表面损伤可以使疲劳极限从1010降至800MPa。裂纹在计入残余应力的Mises等效应力超过材料局部强度的部位萌生。喷丸产生的残余压应力可以增强裂纹闭合效应,从而降低裂纹扩展率。     

不同电火花加工条件对Cr12MoV钢表面残余应力的影响

电火花加工模具所产生的残余应力会使材料的抗疲劳强度下降,造成表面裂纹等缺陷.采用X-350型X射线残余应力分析仪和DJP-2型电解抛光仪逐层去除的方法,在不同的加工条件下,分别测到了冷作模具钢(Cr12MoV)电火花加工表面残余应力的分布,结果表明,表面残余应力的产生和控制取决于放电规准、工作液、电极和工件材料、热处理等因素的选择,为减小残余应力可采用高温回火热处理,但回火温度不得超过电火花加工前的最高回火温度.     

表层强度及其对疲劳的影响

光滑试件疲劳时因表面处于平面应力状态其屈服强度比整体材料低，首先发生滑移，因而造成疲劳损伤，达到一定疲劳周次后积累的损伤导致断裂，因而表层屈服强度和疲劳极限之间存在对应关系。缺口试件引入残余压应力，由于应力集中效应，缺口根部的残余应力集中系数可能高于理论应力集中系数，因而其疲劳强度将超过最高的光滑试件疲劳强度。考虑残余应力对裂纹扩展的作用时应计及其衰减的作用。     

Study on residual stress in socket weld by numerical simulation and experiment

In this paper, welding residual stress in socket weld of 304L stainless steel pipe was investigated using numerical simulation and validated by X-ray stress measurement. From the simulation results, the maximum tensile residual stresses were located at weld root and weld toe on both sides of the weld along pipe, which led to the fatigue failure. Pre-bevelling and low transformation temperature (LTT) dressing could decrease tensile residual stress both in hoop and axial direction at weld root and weld toe. After LTT dressing, compressive residual stress was generated throughout weld toe. Compressive stress can delay fatigue crack initiation and propagation. Therefore, pre-bevelling and LTT dressing can improve the fatigue life of socket weld.     

旋回破碎机4Cr13钢主轴套激光淬火后的磨削裂纹分析

旋回破碎机4Cr13钢主轴套在激光淬火后磨削阶段发现多处裂纹,且与外圆轴向呈约15°夹角。通过宏观分析、化学成分分析、硬度、有效淬硬层深度检测及显微组织观察等,确认了4Cr13钢主轴套裂纹为磨削裂纹。裂纹的产生主要是由于砂轮磨削过程中存在切向与轴向两个方向的运动,切向转速远大于轴向移动速度,两个方向产生的磨削拉应力合力造成裂纹与轴向呈约15°的夹角。同时,激光熔凝产生的放射状柱状晶降低了晶体间的结合力,加之采用激光淬火余温自回火的方式,使淬硬层应力分布较复杂,使其在磨削过程中易出现裂纹。     

Defect assessment of welded specimen considering weld induced residual stresses using SINTAP procedure and FEA

The defect assessment in butt-welded joint of ASTM A36 steel plates and 7075-T7351 aluminum alloy plates containing transverse through thickness crack was analyzed using SINTAP procedure and FEA incorporating weld induced residual stresses. Weld induced longitudinal residual stress profile can be obtained through SINTAP procedure, FEA or experimental analysis. This residual stress profile can be fitted with the trapezoidal residual stress profile available in SINTAP. For three different cases, crack length and residual stress intensity factor (SIF) are calculated and its comparison with the results obtained through FEA is plotted with respect to crack length. The stress intensity factor for mechanical loading is also plotted in the same graph. Using this graphical plot, the total SIF, including residual stress and mechanical loading, can be calculated for any particular crack size. The total SIF can be compared with the fracture toughness of the material for damage tolerance analysis. Also a failure assessment diagram is drawn for welded 7075-T7351 aluminum alloy plates with different crack sizes for as-welded (only residual stress) and mechanical loading along with the existing weld induced residual stresses to show the safety level for a particular crack size and mechanical loading.     

喷丸强化后30CrMnSiNi2A钢表面完整性对其抗疲劳性能的影响*

采用气动式喷丸机对铣削加工和磨削加工的30CrMnSiNi2A钢疲劳试样进行喷丸强化处理,利用扫描电镜、表面粗糙度仪和X射线应力测试仪分别对试样的表面形貌、粗糙度和残余应力进行测试,并对不同表面状态的试样进行疲劳试验,分析表面完整性对其疲劳寿命的影响。结果表明:该钢铣削加工时易在表面留下撕裂、鳞刺和折皱等加工缺陷,导致严重的应力集中,试样疲劳寿命较低;喷丸强化处理能有效降低铣削加工试样的表面粗糙度,增加残余压应力水平,从而提高了其疲劳寿命。磨削试样喷丸处理后试样表面残余应力增大,但表面质量大幅下降,疲劳寿命略有降低。