Low-cycle fatigue behavior of permanent mold cast and die-cast Al-Si-Cu-Mg alloys

Fatigue failure is one of the main failure forms of Al-Si-Cu-Mg aluminum alloys.To feature their mechanical aspect of fatigue behavior,the low-cycle fatigue behavior of permanent mold cast and die-cast Al-Si-Cu-Mg alloys at room temperature was investigated.The experimental results show that both permanent mold cast and die-cast Al-Si-Cu-Mg alloys mainly exhibit cyclic strain hardening.At the same total strain amplitude,the die-cast Al-Si-Cu-Mg alloy shows higher cyclic deformation resistance and longer fatigue life than does the permanent mold cast Al-Si-Cu-Mg alloy.The relationship between both elastic and plastic strain amplitudes with reversals to failure shows a monotonic linear behavior,and can be described by the Basquin and Coffin-Manson equations,respectively.

Interaction of fatigue and creep of GH33 under multi-axial stress at high temperature

Low-cycle fatigue experiments of tension-compression, torsion andtension-torsion with holding time were performed. The interaction law of creep and fatigue undermultiaxial stress at high temperature was investigated, and the micro-mechanism of equilibriumdiagrams was analyzed. A united equation of fatigue life under multiaxial stress was proposed.

Methodology to Evaluate Fatigue Damage of High-Speed Train Welded Bogie Frames Based on On-Track Dynamic Stress Test Data

The current method of estimating the fatigue life of railway structures is to calculating the equivalent stress amplitude based on the measured stress data. However, the random of the measured data is not considered. In this paper, a new method was established to compute the equivalent stress amplitude to evaluate the fatigue damage based on the measurable randomness, since the equivalent stress is the key parameter for assessment of structure fatigue life and load derivation. The equivalent stress amplitude of a high-speed train welded bogie frame was found to obey normal distribution under uniform operation route that verified by on-track dynamic stress data, and the proposed model is, in effect, an improved version of the mathematical model used to calculate the equivalent stress amplitude. The data of a long-term, on-track dynamic stress test program was analyzed to find that the normal distribution parameters of equivalent stress amplitude values differ across different operation route. Thus, the fatigue damage of the high-speed train welded bogie frame can be evaluated by the proposed method if the running schedule of the train is known a priori. The results also showed that the equivalent stress amplitude of the region connected to the power system is more random than in other regions of the bogie frame.

Experimental study on uniaxial ratchetting-fatigue interaction of extruded AZ31 magnesium alloy with different plastic deformation mechanisms

The uniaxial ratchetting-fatigue interaction of extruded AZ31 magnesium(Mg)alloy is investigated by uniaxial stress-controlled cyclic tests at room temperature and with addressing the roles of different plastic deformation mechanisms.Different stress levels are prescribed to reflect the cyclic plasticity of the alloy controlled by diverse deformation mechanisms(i.e.,dislocation slipping,deformation twinning and detwinning ones),and then the influences of stress level and stress rate on the ratchetting and fatigue life are discussed.The experimental results demonstrate that different evolution characteristics of whole-life ratchetting and fatigue life presented during cyclic tests with various mean stresses,stress amplitudes and stress rates are determined by the dominated plastic deformation mechanisms.It’s worth noting that the ratchetting can occur in the compressive direction even in the cyclic tests with a positive(tensile)mean stress,and the fatigue life increases first and then decreases with the increase of mean stress on account of the interaction between dislocation slipping and twinning/detwinning mechanisms.Comparing the fatigue lives obtained in the asymmetric stress-controlled and symmetrical strain-controlled cycle tests,it is seen that the ratchetting deformation causes an additional damage,and then leads to a shortening of fatigue life.

Fatigue Crack Initiation Potential from Defects in terms of Local Stress Analysis

The competition of surface and subsurface crack initiation induced failure is critical to understand very high cycle fatigue（VHCF） behavior, which necessitates the elucidation of the underlying mechanisms for the transition of crack initiation from surface to interior defects. Crack initiation potential in materials containing defects is investigated numerically by focusing on defect types, size, shape, location, and residual stress influences. Results show that the crack initiation potency is higher in case of serious property mismatching between matrix and defects, and higher strength materials are more sensitive to soft inclusions（elastic modulus lower than the matrix）. The stress localization around inclusions are correlated to interior crack initiation mechanisms in the VHCF regime such as inclusion-matrix debonding at soft inclusions and inclusion-cracking for hard inclusions（elastic modulus higher than the matrix）. It is easier to emanate cracks from the subsurface pores with the depth 0.7 times as large as their diameter. There exists an inclusion size independent region for crack incubation, outside which crack initiation will transfer from the subsurface soft inclusion to the interior larger one. As for elliptical inclusions, reducing the short-axis length can decrease the crack nucleation potential and promote the interior crack formation, whereas the long-axis length controls the site of peak stress concentration. The compressive residual stress at surface is helpful to shift crack initiation from surface to interior inclusions. Some relaxation of residual stress can not change the inherent crack initiation from interior inclusions in the VHCF regime. The work reveals the crack initiation potential and the transition among various defects under the influences of both intrinsic and extrinsic factors in the VHCF regime, and is helpful to understand the failure mechanism of materials containing defects under long-term cyclic loadings.

Fatigue Behavior of High Speed Steel Roll Materials for Hot Rolling by Laser Impacting

The fatigue behavior of high speed steel （HSS） roll materials for hot rolling was researched under watercooling conditions by laser impacting. The microstructure of HSS sample and the morphologies of fatigue samples were observed by scanning electron microscope. The phase structure was detected by XRD. The morphology of situ oxide scale was observed by optical microscope, and the expansion coefficient was measured by TGA. The experiment results indicate that the cracks come into being at the carbide-matrix interface, but there are no cracks in the matrix after many times of laser impacting treatment, for the situ sample taken from the fractured roll surface, big carbides are more sensitive to the fatigue, and peel off prior to small ones. The relevant fatigue mechanisms are also discussed.

High cycle fatigue behavior of the forged Mg-7Gd-5Y-1Nd-0.5Zr alloy

This paper investigated the high cycle fatigue behavior of a forged Mg-7Gd-5Y-1Nd-0.5Zr alloy with different stress concentration factor(Kt),under different stress ratio(R),and along different loading direction.The smooth specimen(Kt=1),under R=0.1 and along longitude direction,shows a high fatigue strength of 162 MPa at 107 cycles.The fatigue behavior of the forged Mg-7Gd-5Y-1Nd-0.5Zr alloy exhibits a high sensitive to the notch.Moreover,change of stress ratio from 0.1 to−1 may also result in a bad fatigue property.The flux inclusions were elongated along longitude direction and/or transverse direction during the forging process of the Mg-7Gd-5Y-1Nd-0.5Zr alloy.The interface between the flux inclusion and the matrix may debond and serve as the crack initiation site during the fatigue loading process,leading to the deterioration of the fatigue property along thickness direction and a high anisotropic fatigue behavior between longitude direction and thickness direction.

High-cycle fatigue behavior of nickel-base single crystal superalloy

High-cycle rotating bending fatigue behavior of SRR99 nickel-base single crystal alloy at 700 and 900℃ was investigated. The fatigue strengths for 107 cycles are 350 and 335MPa at 700 and 900℃, respectively. The total fatigue life becomes shorter when the temperature increases regardless of the loading stress and frequency. With the number of cycles decreasing, the difference in fatigue strength at the two temperatures becomes smaller. Typical fatigue rupture process including crack initiation site, crack propagation region and final rupture region exhibits at 700℃. The fracture surface is basically characterized by cleavage rupture at 900℃.

Experimental Study on Mechanical Properties of Q690 High-Strength Steel after High Cycle Fatigue Damage

Through the static tensile test of Q690 high-strength steel, the relevant mechanical parameters are obtained and the maximum fatigue load is determined. The fatigue life is measured by the fatigue test under the load. According to the fatigue cumulative damage method, the number of fatigue pre-damage vibration is designed in proportion. Then the fatigue pre-damage test is carried out on the high-strength steel, the stress-strain curve and the variation of residual mechanical property reduction coefficient with fatigue damage were drawn. The results show that: compared with the undamaged specimens, the yield strength and tensile strength of Q690 steel are less affected by fatigue damage, but the elongation changes more significantly, and the elastic modulus is not significantly affected. Finally, through the change of mechanical properties of Q690 high-strength steel with different fatigue damage, it provides a scientific basis for the performance evaluation of existing Q690 high-strength steel structure after fatigue damage.

Coupling damage and reliability model of low-cycle fatigue and high energy impact based on the local stress–strain approach

Fatigue induced products generally bear fatigue loads accompanied by impact processes,which reduces their reliable life rapidly. This paper introduces a reliability assessment model based on a local stress–strain approach considering both low-cycle fatigue and high energy impact loads.Two coupling relationships between fatigue and impact are given with effects of an impact process on fatigue damage and effects of fatigue damage on impact performance. The analysis of the former modifies the fatigue parameters and the Manson–Coffin equation for fatigue life based on material theories. On the other hand, the latter proposes the coupling variables and the difference of fracture toughness caused by accumulative fatigue damage. To form an overall reliability model including both fatigue failure and impact failure, a competing risk model is developed. A case study of an actuator cylinder is given to validate this method.

EH36钢焊接接头焊趾处应力集中对高低周复合疲劳的影响

为研究船舶结构用钢EH36钢焊接接头焊趾处应力集中对高低周复合疲劳的影响,对不同焊缝形貌20 mm厚EH36钢双面全熔透对接接头进行高低周复合疲劳试验.研究发现,高低周复合疲劳对接头寿命的降低幅度随着高周应力比的增大而增大;结合数字图像相关法测量结果分析,发现焊趾处应力集中系数越大,接头的高低周复合疲劳寿命越低,其降低幅度随着高周应力比的增大而增大.基于试验数据对模型进行验证,结果表明,应力集中系数高的接头疲劳寿命低且寿命预测结果分散性增大;Palmgren-Miner模型和Zhu模型由于未充分考虑高低周耦合损伤作用,导致寿命预测结果相对危险.修正后Zhu模型在充分考虑低周疲劳对高低周复合疲劳寿命的影响后,预测寿命大于实际寿命的预测点较少,预测结果相对安全,且分散性较小,误差也较小.

深井超深井大尺寸井眼钻具疲劳失效分析与防控

地球深部蕴藏着丰富的油气资源,随着深井超深井钻井数量的递增,钻具失效(断钻具、钻具刺漏)事故呈现逐年递增趋势。为进一步明确深井超深井大尺寸井眼钻具失效原因、减少钻具失效事故,梳理了近年来中国石油西部三大油田钻具失效情况及特征,并以西部油田某口典型超深井?444.50 mm井眼连续5次断钻具事故为对象开展原因分析。首先基于弹性动力学密哈顿原理(Hamilton’s principle)和纽马克方法(Newmark-β)建立了钻具动力学模型,结合雨流法和Miner疲劳损伤累积理论实现对钻具运动、受力及疲劳寿命的定量评价,并验证了模型的准确性,最后提出了大尺寸井眼预防断钻具配套工艺措施。研究结果表明:(1)深井超深井中钻具失效主要发生在上部大尺寸井段,失效位置多集中在下部钻具接头或本体尺寸较小的薄弱位置;(2)因钻具横向振动诱发高频弯曲应力是导致钻具疲劳失效的主要因素,持续剧烈的横向振动在数小时内会导致钻具疲劳失效;(3)提高下部钻具组合刚度、使用“高钻压+低转速”参数钻进有助于降低钻具横振强度;(4)因钻具组合刚度偏小、钻井参数不合理导致下部钻具剧烈横向振动,叠加“中性点”在钻具接头及本体尺寸较小等薄弱位置反复交替,是典型案例井连续断钻具的直接原因。结论认为,建立的模型可以定量评估钻具疲劳失效情况,提出的预防措施可为深井超深井大尺寸井眼预防钻具失效提供技术参考。

高速铁路接触网弹性吊索疲劳仿真研究

为研究弹性吊索的服役状态与疲劳寿命,对高铁接触网出现的弹性吊索断线现象进行调研并开展疲劳研究。基于武广高铁的线路设计参数构建了弓网动态仿真模型,对300 km/h速度下的弹性吊索振动状态进行分析。由于受电弓运行通过时对接触悬挂的抬升作用,弹性吊索主要沿竖直方向振动,并在线夹位置不断承受交变弯曲载荷。在UG软件中建立了弹性吊索绞线(1×7)及其线夹的实体模型并将其导入到LS-DYNA软件中进行了应力载荷计算;其应力集中位置位于弹性吊索与其线夹连接处。将应力历程数据导入到ANSYS nCode DesignLife软件中分析弹性吊索在指定载荷下的疲劳寿命。最后,根据Miner累积损伤原则计算了300 km/h运行速度下武广高铁弹性吊索的疲劳寿命为4.58×10^(6)弓架次;当运行速度增加至350 km/h时,疲劳寿命为1.17×10^(6)弓架次;当运行速度增加至400 km/h时,疲劳寿命为4.80×10^(5)弓架次。

旋转弯曲和超声波循环加载高周疲劳性能对比分析

通过旋转弯曲(频率52.5 Hz,10^(3)~10^(8)周次)和超声波(频率20 kHz,5×10^(4)~10^(9)周次)轴向循环载荷疲劳实验,研究不同应力比(R)、加载方式和夹杂物对CL60钢高周、超高周疲劳性能的影响。结果表明:所有S-N曲线均呈现水平渐进趋势,并具有明确的疲劳极限;应力比R=-1情况下,通过超声波实验获得的疲劳强度(260~270MPa)比旋转弯曲获得的疲劳强度(400~410 MPa)低140~150 MPa,并且应力比R=0.3的超声波疲劳极限值在195~205 MPa的范围内;旋转弯曲疲劳实验的疲劳裂纹起源于试件的表面,超声波疲劳实验的表面和内部均有裂纹萌生,且裂纹均在MnS夹杂物附近产生,与应力比大小无关。采用有限元方法研究了MnS夹杂物对裂纹萌生和扩展的影响,椭圆MnS夹杂物的侧面相较于尖端位置应力集中水平更高,导致裂纹从夹杂物侧面萌生。

喷丸对TNM-TiAl合金组织以及450℃下高周疲劳性能的影响

目的针对TNM-TiAl合金(Ti-43.5Al-4Nb-1Mo-0.1B)服役时受高周疲劳影响的问题,揭示喷丸处理技术对TNM-TiAl合金疲劳性能的影响规律。方法通过扫描电镜和透射电镜分析喷丸前后的组织演变,测量喷丸前后合金表面残余应力、表面粗糙度和显微硬度分布规律,采用梯度法测量喷丸前后的高周疲劳极限,并分析断口形貌和断口附近组织。结果喷丸后合金表面粗糙并且存在烧蚀坑,表面附近的片层团发生弯曲,在组织内部引入大量位错、位错缠结以及位错胞等缺陷。喷丸后表面粗糙度明显提高,显微硬度提高了209HV0.05,从表面到内部形成深度为260μm的硬度梯度。喷丸后试样表面的残余压应力为446 MPa,深度约为85μm。喷丸后在450℃下保温30 h后合金表面最大硬度仍比喷丸前的硬度高166HV0.05,硬化深度约为150μm。未喷丸光滑试样、喷丸光滑试样、未喷丸缺口试样和喷丸缺口试样的疲劳极限分别为615、637、385、455 MPa,光滑试样断裂类型为穿晶解理型脆性断裂,而缺口试样为穿晶脆性断裂。结论喷丸能够显著提高最大残余压应力和引入应力的深度,且喷丸引入的组织具有一定的热稳定性,喷丸明显提高了TNM-TiAl合金的疲劳极限。

Nb微合金化对Cr-Co-Ni-Mo系超高强度不锈钢腐蚀疲劳性能的影响

为探讨超高强度不锈钢的应力腐蚀开裂行为,采用Cr-Co-Ni-Mo系超高强度不锈钢为研究对象,利用OM、XRD、TEM等测试手段,结合腐蚀疲劳试验,研究了Nb微合金化对Cr-Co-Ni-Mo系超高强度不锈钢腐蚀疲劳性能的影响。结果表明,试验钢在3.5%NaCl溶液中具有一定的应力腐蚀敏感性,其应力腐蚀开裂机理为氢致开裂和阳极溶解的混合机制。Nb微合金化提高了钢的腐蚀疲劳性能,钢中添加0.11%的Nb后,钢的腐蚀疲劳强度由440 MPa提高至495 MPa,其主要原因是,Nb微合金化可以细化钢的晶粒尺寸,促进钢中不可逆氢陷阱NbC的析出,增加了钢中原奥氏体晶界总量、小角晶界所占比例、Σ3晶界数量、奥氏体体积分数等。

Q960D高性能钢疲劳裂纹扩展影响因素分析

为研究Q960D国产高性能钢疲劳裂纹扩展行为,采用CT紧凑拉伸试样数值分析了若干关键参数对疲劳裂纹扩展的影响规律。基于数值模拟的疲劳裂纹扩展长度、循环次数、应力强度因子与试验数据进行对比,发现所建立有限元模型有较好的吻合度。结合断裂力学理论及围线积分,得到了循环次数与裂纹扩展长度之间的关系,并求出了对应的应力强度因子,揭示了载荷、应力比、试样厚度、裂纹深度比、初始裂纹角度和腐蚀坑对该高性能钢裂纹扩展过程中应力强度因子的影响。结果表明:载荷越大,裂纹扩展速率越大。应力比、试样厚度越大,裂纹扩展速率越小,但是疲劳寿命增加。裂纹深度比越大越容易发生断裂,工程实际中应多关注有效裂纹长度。初始裂纹角度46°为应力强度因子突变的临界角,初始裂纹角度为0°时裂尖的应力最大,最容易发生断裂。在有腐蚀坑的情况下,蚀坑周围的K值沿厚度方向逐渐减小,腐蚀坑周围的裂纹尖端将先达到屈服状态,加速诱发试件疲劳断裂,最终导致寿命降低。

渗碳工艺对18CrNiMo7-6合金钢缺口件疲劳性能的影响

为分析渗碳工艺对18CrNiMo7-6缺口构件疲劳性能的影响,对应力集中系数Kt=1.86的缺口试样进行渗碳热处理并测定其旋转弯曲疲劳性能,同时对试样表层组织、硬度以及应力场进行了表征。结果表明,渗碳热处理可以明显提升试样表面硬度并引入残余压应力,并且可以显著提升缺口试样的疲劳性能,相较于未渗碳试样疲劳极限提升超过100%;随着有效硬化层深度的增加,缺口试样的疲劳极限均呈现先增后减的趋势。未渗碳试样和渗碳试样的疲劳源均在缺口根部表面处,且均为多源断裂。疲劳过程中渗碳试样表面残余奥氏体在循环应力作用下诱导马氏体相变,其转变量存在一个临界值。

焊接残余应力对高频机械冲击处理焊接结构疲劳行为的影响

利用高频机械冲击处理装置对Q355钢纵向角接接头进行处理,研究了残余应力对焊接接头疲劳性能的影响。结果发现:高频机械冲击处理可显著提高接头的疲劳寿命,而消应力+高频机械冲击复合处理则会大幅度改善焊接接头的疲劳性能。对比焊态接头特征疲劳强度(FAT),在含焊接残余应力条件下直接进行冲击处理,接头的FAT提高了143.9%,而复合处理的FAT提高了210.3%。焊态和高频机械冲击态接头的疲劳裂纹源位于焊趾,而经过焊后消应力+高频机械冲击复合处理的接头,其断裂位置转移至母材。