2A12铝合金的多轴加载疲劳行为

采用SDN100/1000电液伺服拉扭复合疲劳试验机对2A12铝合金进行多关键参数的多轴疲劳性能研究,通过对断口的微观分析探究疲劳失效机理。结果表明:等效应力加载条件下,随拉扭相位差的增加疲劳寿命降低,0°相位差下断面裂纹源区能观察到轮胎状、鱼骨状以及钟乳石状的特殊形貌,裂纹扩展区存在二次裂纹和模糊的疲劳条带;分别改变拉、扭平均应力,多轴疲劳寿命均降低,裂纹源区能看到白色絮状的氧化物,瞬断区存在二次裂纹和剪切型韧窝;不同加载波形条件下,正弦波对应最长的多轴疲劳寿命,三角波次之,方波时最短且体现出最大的结构耗能。低-高两级加载条件下,材料产生"锻炼效应"。     

铝合金冲击凹坑残余应力显微压痕测定方法

金属结构的表面缺陷(如冲击、刮蹭等造成的损伤)可能会严重影响到其疲劳寿命,缺陷处的局部残余应力测定对缺陷处起裂寿命评估具有重要参考意义。采用有限元方法模拟了2A12铝合金冲击凹坑缺陷的产生和凹坑处的显微压痕试验,以此为依据评估了采用Lee模型计算缺陷处残余应力的精度。结果表明:凹坑底部残余应力(接近等轴情况)估算值与有限元模拟的标准值相比误差在6%以内;而凹坑边缘的残余应力(非等轴情况)估算值与有限元模拟的标准值之间的误差接近25%,通过对非等轴Lee模型进行修正,可将凹坑边缘残余应力的估算误差缩小到8%之内。     

高速齿轮轴齿表面凹坑产生原因分析

高速齿轮轴在经过渗碳和淬、回火热处理后的喷丸过程中,其齿部表面出现多处凹坑。利用光学显微镜、扫描电子显微镜、直读光谱仪和显微硬度计等对齿部表面的凹坑进行了失效分析。结果表明：齿部表面的凹坑区域存在沿晶扩展的微裂纹,另还存在沿晶界分布的碳化物;表面出现凹坑是由于在渗碳后的空冷过程中表面产生了“⊥”型裂纹,在随后的停放期间和等温淬火时“⊥”型裂纹沿轴向或径向进一步扩展形成剥离裂纹,继而于喷丸过程中表层断裂脱落,产生凹坑。     

钢桥面板纵肋与横隔板焊接细节表面缺陷及疲劳效应研究

初始焊接缺陷是影响结构件疲劳性能的关键因素之一。在断裂力学评估框架下引入特征化初始焊接缺陷,结合相互作用积分法与复合断裂准则解决由表面缺陷所导致的复合型疲劳裂纹扩展问题,在此基础上编写裂纹扩展模拟程序,建立表面焊接缺陷效应评价方法,通过分析揭示了不同形态和尺度的初始焊接缺陷对于钢桥面板纵肋与横隔板构造细节裂纹扩展关键性度量指标和疲劳寿命预测的影响。结果表明:所建立的方法可有效用于评估焊趾部位表面焊接缺陷对于疲劳性能的效应;面状缺陷对于裂纹扩展度量指标和疲劳寿命预测结果的影响更为显著,其初始缺陷深度和形态均是影响疲劳性能的关键因素,体积型缺陷对于疲劳寿命的影响主要由深度方向的缺陷尺寸决定;焊接缺陷的形态和几何参数取值应根据工程实际和规范建议值共同确定,直接简化为面状缺陷会低估结构件的疲劳寿命;考虑焊接缺陷不确定性的可靠度评估方法尚需进一步研究。     

TC4钛合金复合加载下超高周疲劳性能研究EI北大核心CSCD

针对航空发动机压气机叶片在实际工况下的超高周疲劳断裂问题,研究了三种锻造温度下TC4钛合金三点弯曲–轴向拉伸复合加载的疲劳破坏行为。试验结果表明,S-N曲线呈直线下降型和双平台型,采用985℃近锻造时疲劳性能最好。随着应力幅值降低,裂纹由表面萌生向次表面萌生转变,断口形貌呈现准解理断裂特征。表面裂纹萌生于α晶界或α-β相界,由位错滑移堆积导致;而次表面裂纹萌生于刻面,由初生α相解理导致。疲劳寿命由裂纹萌生阶段主导,且所占比例随总寿命的增加而变大。双态组织中初生α含量和尺寸均小于等轴组织,且β转变组织含量更高,从而具备更好的疲劳性能。轴向拉伸改变了试件的轴向应力分布,有利于提高裂纹萌生于次表面的概率,使裂纹起源点向内部迁移。     

X56海底管道在腐蚀环境下疲劳裂纹扩展过程预测

海底管道浸没在海洋环境中,腐蚀疲劳破坏十分常见,因此为这些管道的剩余疲劳寿命提供一种准确的预测方法至关重要。本工作研究了海浪循环荷载下含裂纹管道的疲劳裂纹扩展行为,提出了一种新的管道寿命预测方法。使用新提出的形状因子处理试验数据获得X56钢的Paris常数,并分析了其在腐蚀环境下的疲劳性能。运用有限元软件模拟管道疲劳裂纹的扩展过程并计算其应力强度因子(Δ~K),随后基于Paris公式对管道的剩余寿命进行预测。结果表明,对于给定的Δ~K,海水中管道的疲劳裂纹扩展速率(da/d N)为空气中的1. 6倍,且随着疲劳裂纹扩展速率的增加,海水对管道腐蚀疲劳的影响逐渐降低。运用新提出的方法预测管道剩余寿命并与足尺寸X56管道疲劳试验结果进行对比,结果表明该方法能够有效地预测管道的疲劳裂纹扩展状态和剩余疲劳寿命。     

高能喷丸表层纳米化对纯钛旋转弯曲疲劳性能的影响

采用高能喷丸方法使工业纯钛疲劳试样的表层实现纳米化,并进行了疲劳试验。结果表明,高能喷丸后其表层组织发生严重塑性变形并实现组织纳米化,是提高弯曲疲劳寿命的主要原因。扫描电子显微镜分析发现,疲劳裂纹源的位置主要呈现为表层和次表层两种情况。在相同应力水平下,疲劳裂纹源在次表层时疲劳寿命较长,而疲劳裂纹源在表面时疲劳寿命很短,相应的疲劳试样的表面损伤也比较严重。     

轧制组织对镁合金AM60疲劳性能的影响

研究了AM60轧制后挤压镁合金的组织对其机械性能和疲劳裂纹扩展性能的影响。实验表明:轧制使晶粒细化,强度显著提高。沿纵轴轧制方向出现大量等轴李晶组织,而在横向原来的孪晶组织消失。对于存在大量孪晶组织的方向,其抗拉强度明显低于其它方向。轧制AM60的横向疲劳裂纹扩展速度(FCPR)明显地高于纵向。当疲劳裂纹尖端塑性区的尺寸与组织的晶粒度接近时,挤压AM60组织中晶粒大小的不均匀引起裂纹分叉,裂纹分叉和粗糙度诱发的裂纹闭合对疲劳裂纹扩展产生严重的阻滞作用在挤压镁合金AM60的疲劳裂纹扩展速度(da/dN)与应力强度因子范围(△K)的关系曲线上出现拐点(△K=64～7.5 MPa·m~(1/2))。疲劳裂纹扩展为沿晶和穿晶混合方式。     

550kV断路器操作机构轴销的断裂原因分析

在断路器1 000~2 000次机械操作试验中,30CrNi3合金钢渗氮轴销发生断裂。通过化学成分分析、断口分析、低倍检验、金相检验、力学性能测试的方法,对轴销的断裂原因进行了分析。结果表明:该轴销的断裂性质为双向弯曲疲劳断裂;轴销材料渗氮层存在脉状氮化物,内部存在疏松、大尺寸夹杂物和气泡缺陷,降低了材料的力学性能;在周期性旋转弯曲力作用下,轴销表面缺陷处出现裂纹并不断扩展,最终导致轴销疲劳断裂。     

Ti60钛合金中富钕稀土相颗粒对叶片室温振动疲劳性能的影响

采用振动疲劳实验及SEM断口分析等方法,研究了含稀土元素Nd的Ti60高温钛合金稀土相颗粒对叶片振动疲劳裂纹萌生的影响.结果表明:叶片振动疲劳寿命与稀土相颗粒的尺寸和分布位置具有密切关系.稀土相颗粒尺寸越大,对叶片振动疲劳裂纹的萌生作用也越大,疲劳寿命降低;外露于或镶嵌于叶片表面的稀土相颗粒明显促进了叶片振动疲劳裂纹的萌生.     

Failure Analysis of a Locomotive Engine Oil Pump Shaft

In this study, the fracture of a locomotive engine oil pump shaft is investigated. According to the reports, all oil pump shafts of this type fail in short periods of time due to the fracture at a location on the shaft which is in contact with a pinion. Stress analysis showed that the shaft bears torsion and cyclic bending stresses in the fracture site. The fracture surface observations revealed that the cracks are generated from the surface, and fatigue fracture occurs due to the cyclic bending stresses. According to the laboratorial analysis, the shaft is a surface-hardened low alloy steel. The lack of suitable pre-heat treatment and surface hardening and the roughness created by the slippage between the shaft and the pinion are concluded to be responsible for the failure of the oil pump shaft.     

Torsion fatigue failure of bus drive shafts

The mystery surrounding high failure rates in the drive shafts of a large municipal transit agency's fleet of 40 newly acquired articulated buses is investigated. The drive shafts were fabricated from a low-carbon (0.45%) steel such as AISI 5046. An examination of the drive shafts on all 40 buses is conducted, and 6 different drive shaft designs are identified among the fleet, but all of the failures, 14 in all, are limited to just one of the identified designs. Microscopic examination of the fracture surface of one of the failed drive shafts under a scanning electron microscope is conducted to determine the failure mode. Evidence of high-cycle fatigue is found, and a finite-element analysis is conducted to compare the maximum stress of the design exhibiting failures with the most common of the other designs that exhibits no failures. A fatigue life prediction is performed to determine just how much longer the expected fatigue life of the surviving design is compared to the design that suffered the early failures.     

The effect of steady torsion on fatigue crack growth in shafts

Most of catastrophic mechanical failures in power rotor shafts occur under cyclic bending combined with steady torsion: Mode I (ΔK_I) combined with Mode III (K_III). An analysis of the influence of steady torsion loading on fatigue crack growth rates in shafts is presented for short as well as long cracks. Long cracks growth tests have been carried out on cylindrical specimens in DIN Ck45k steel for two types of testing: rotary or alternating bending combined with steady torsion in order to simulate real conditions on power rotor shafts. The growth and shape evolution of semi-elliptical surface cracks, starting from the cylindrical specimen surface, has been measured for several loading conditions and both testing types. Short crack growth tests have been carried out on specimens of the same material DIN Ck45k, under alternating bending combined with steady torsion. The short crack growth rates obtained are compared with long crack growth rates. Results have shown a significant reduction of the crack growth rates when a steady torsion Mode III is superimposed to cyclic Mode I. A 3D Finite Element analysis has also shown that Stress Intensity Factor values at the corner crack surface depend on the steady torsion value and the direction of the applied torque.     

Effect of steady torsion on fatigue crack initiation and propagation under rotating bending: Multiaxial fatigue and mixed-mode cracking

Axles and shafts are of prime importance concerning safety in transportation industry and railway in particular. Knowledge of fatigue crack growth under typical loading conditions of axles and shafts with rotating bending and steady torsion is therefore essential for design and maintenance purposes. The effect of a steady torsion on both small and long crack growth under rotating bending is focused in this paper. For small crack growth, a modified effective strain-based intensity factor range is proposed as the parameter that correlates small fatigue crack growth data under proportional or non-proportional multiaxial loading conditions. Results show that this parameter is appropriate for determining fatigue crack growth of small cracks on rotating bending with an imposed steady torsion.     

Failure of a swing pinion shaft of a dragline

The causes of failure of a swing pinion shaft of a coal handling dragline have been investigated in the present work. Along with the failed shaft, a small piece of material from the tooth of another shaft that rendered design life in service has also been analyzed. The chemical composition and general heat treatment of the shafts were as per recommendation. The microstructure of both the shafts was tempered bainite, which is usual for such pinion shafts. The mechanical properties met the requirement for such applications. The investigation included metallography, microscopy (SEM), hardness and fractography. Analysis revealed fatigue cracks initiated at a number of places along the length of the tooth, and grew to a considerable depth. Improper surface hardening of the pinion teeth is believed to have rendered the teeth susceptible to bending fatigue failure. Other factors like lubrication and impact loads in the tooth roots might have aggravated the failure.     

Pitting corrosion of triggering initial fractures of palm oil screw press machine shafts

Investigation was carried out following the frequent occurrences of shaft failures in a number of palm oil screw press machines in a Malaysia palm oil mill company. Visual inspection, chemical analysis on the shaft material, fatigue analyses by utilizing traditional method and microscopic examinations were performed to support the investigation. Overview on pitting corrosion problems is presented. Common locations of the failure are at around bottom edges of keyway and shoulder fillets of the shafts. The findings indicate that pitting corrosion is identified as the main root causes of the premature shaft failures. In turn, poor surface condition would significantly reduce the fatigue endurance limit of the shafts under torsional loading.     

Failure Analysis of Gearbox and Clutch Shaft from a Marine Engine

This article presents metallurgical failure analysis of a gearbox shaft and a clutch shaft from a marine engine. The gearbox shaft was made of low alloy steel, and the clutch shafts were components made of carbon steel. Fracture surface examination revealed circumferential ratchet marks with the presence of inward progressive beach marks suggesting rotary-bending fatigue failure in the case of gearbox shaft. The star-shaped pattern on the clutch shaft fracture surface suggested that the failure was due to torsional overloading which might have initiated at corrosion pits visible around the fracture surface. The gearbox shaft experienced rotational bending stresses which induced fatigue failure because the fatigue strength of the alloy was too low. The fatigue failure of the gearbox shaft led to the torsional failure of the corroded clutch shaft. The sudden, high level failure load on the clutch shaft occurred when the gear box shaft failed.     

Einfluß von last- und verformungsgesteuerten mehrachsigen Beanspruchungen auf die Anrißlebensdauer

Influence of load- and deformation-controlled multiaxial tests on fatigue life to crack initiation Generally, areas of components with notches or geometrical transitions are critical because of the resulting stress/strain concentrations. In these areas due to the stress-gradients and constraint local deformations are displacement controlled even if the material's yield stress is exceeded, as long as the deformations are below the structural yield point. Therefore, load controlled tests in the elasto-plastic region with unnotched specimens from ductile materials under combined axial loading and torsion are not suitable for the interpretation of component's behaviour because of uncontrolled local deformations. Thus, the influence of multiaxial stress/strain states on the fatigue behaviour of a component under elasto-plastic deformations can be determined reliably with unnotched specimens only by deformation controlled tests, if cyclic creep is not expected in critical areas.     

Research of rail traction shafts and axles fractures towards impact of service conditions and fatigue damage accumulation

Traction shafts and axles of railway vehicles are designed to be safe and reliable in normal service conditions. Accidental and unpredictable conditions including bad exploitation (rail tracks, maintenance and extreme long service life) make the probability of shaft and axle fracture realistic. This article contains the experience and analysis of the traction shafts and axles fractures and fracture processes. Research also shows service conditions with results presented in the form of service stress spectrums. Besides normal service conditions, the results also include unpredictable phenomenon such as natural torsion vibrations of the shaft and wheels caused by stick–slip processes at the moment of the set in motion of a full-loaded train or at the moment of braking. These vibrations create an extremely high level of torque which is the main cause of cracks initialization. The next part of the article contains the results of the traction shafts and axles endurance research. The results of testing are transformed using the results of calculation by the FE method and by statistical estimation of the failure probability distribution. The main part refers to the interaction (impact) of service stress probability and failure (endurance) probability, which leads to the traction shaft (axle) reliability definition. Using the established model, locomotive traction shaft reliability is calculated and analyzed. The final part of the article contains suggestions for the traction shafts (axles) failure prevention.     

Load distribution in the axial direction in a spline coupling

A finite element model has been made of a spline coupling between a shaft and a sleeve. The stress concentration factor is calculated and compared to analytical calculations. An analytical model for the load distribution in the axial direction and the torque transfer between shaft and sleeve is presented and compared to finite element results. Methods to uniform the load distribution and thereby decrease the stress concentration factor are tested with the analytical model. The induction hardened shafts with splines are fatigue tested and the lives are presented. The residual stress on the shaft surface is measured. The analytical model shows that the best way uniform the load is to vary the thickness of the spline tooth in the axial direction. Different shapes and sizes of the tooth thickness variation are also investigated. It is probably possible to extend the fatigue life at least two times for this spline, by uniforming the load distribution.