超声端点衍射法测量在役管道母材疲劳裂纹高度

为保证核电站反应堆冷却剂系统管道在役时运行状态的良好,需要有效和准确地检测出该类管道内外壁母材疲劳裂纹高度。文章着力介绍了母材疲劳裂纹高度的检测原理、检测方法、试块设计、超声波探头参数的选择。经过大量的试验和现场检测,证明了根据管道规格,制作相应的对比试块和性能验证试块,采用5 MHz 45°横波斜探头,对于自身高度大于2 mm的裂纹缺陷,利用端点衍射法进行测量可得到准确的裂纹自身高度。     

预应变对工业纯钛TA2疲劳裂纹尖端应变场的影响

钛制承压设备在制造和服役过程中会产生塑性变形,影响其抗疲劳性能。为了研究预应变对工业纯钛TA2疲劳裂纹扩展行为的影响,本文对原始材料、预应变量为10%、20%和30%的材料进行疲劳裂纹扩展试验,结合数字图像相关(Digital image correlation, DIC)技术获取裂纹尖端应变场,研究预应变对裂纹尖端应变场的影响。结果表明：随着预应变量的增加,稳态疲劳裂纹扩展阶段的扩展速率减小,裂纹张开位移减小,裂纹张开载荷增大,裂纹尖端塑性区、塑性变形量以及循环塑性应变累积量均减小。因此预应变抑制了裂纹尖端的塑性变形及循环塑性累积,致使裂纹闭合效应愈加明显,从而抑制了裂纹的扩展。研究结果对钛制承压设备的安全评定具有参考意义。     

FATIGUE CRACK GSOWTH AND CRACK CLOSURE OF SIMULATED OVER-MATCHED WELDED JOINTS

1.~onItiswellknownthatthefatigUefailureisoneofthecornmonfailuresinmetalstructuresincludingweldedstmctures.VariousformulaehavebeensuggestedtopredictfatigUecrackgrowthrates,andthosewidelyusedinpracticeareasfollows:da/dN=cI(AK)acda/dN=c2(ACOD)?/dN=c3(AJ)%FortnulaeaboverelatedfatigUecrackgrowthrate(da/dN)eitherwiththerangeofstressintensityfactor(isK),orwiththerangeofcrackopeningdisplacement(ACOD),orwiththerangeofJ--intendvalue(6J)ofafatigUeloadcycle.However,sincethediscoverybyW.Elb..['…     

Interaction of Rayleigh surface waves with a tightly closed fatigue crack

A miniature interdigital transducer (IDT) tuned to resonate at 3.1 MHz of frequency is designed, fabricated, and used to generate narrow band Rayleigh waves on aircraft grade metallic alloys. Electrodes of the IDT are precisely machined by a laser machining technique and its acoustical wave properties are characterized by using a scanning laser interferometry system with a spatial resolution of less than 10 microns. Unlike the conventional contact surface wave generation methods using wedges or combs, the biggest advantage of an IDT are its miniature physical size and its high efficiency in converting electrical signals to mechanical vibrations. Narrow band surface waves with finite amplitudes generated by an IDT are used to investigate the wave interaction with a tightly closed fatigue crack on a metallic test specimen. High-resolution images for the time-resolved ultrasonic field and time-averaged amplitude displacement are generated to understand the interaction of the wave with a surface-breaking fatigue crack. From the amplitude displacement data, three-dimensional surface contour plots of the wave energy are generated to find out how the elastic energy interacts with the tight fatigue crack interfaces. Results show that a tightly closed fatigue crack can transmit Rayleigh waves through the crack due to interfacial contact within the crack, where transmission of elastic energy was found to be more dominant towards the crack tip as expected. The sum of transmitted and reflected energy at the crack interface suggests that there is an additional energy loss mechanism which is proved experimentally by visualizing part of the incident Rayleigh wave energy propagating along the interfacial surfaces of the crack and continues its propagation along the surface of the opposite side of the test specimen.     

不同疲劳载荷下工业纯钛再结晶热处理对疲劳裂纹扩展行为影响

本文开展热处理后工业纯钛TA2不同载荷水平下疲劳裂纹扩展实验,考虑裂尖塑性变形程度,研究疲劳裂纹扩展规律以及热处理状态对疲劳裂纹扩展不同阶段的适应性。结果表明,不同疲劳载荷下热处理对疲劳扩展速率产生不同的影响。A类加载热处理后的疲劳裂纹扩展速率下降是由于近门槛区有效载荷的降低,以及近门槛值的提高。B类加载下热处理对有效载荷以及裂尖塑性变形几乎没有什么影响。C类与D类加载下热处理后裂尖塑性变形受到限制,而导致疲劳裂纹扩展速率下降。     

Fibre bridging during high temperature fatigue crack growth in Ti/SiC composites

Synchrotron X-ray diffraction has been used to map the crack tip stress field, load redistribution and the variation in interfacial shear stress along bridging fibres local to a matrix crack during fatigue crack growth in Ti-6Al-4V/unidirectional SCS-6 SiC monofilament composite at elevated temperature. Quasi-static fatigue cycles were applied in a diffractometer at the same stress amplitudes and temperatures (120 and 300 °C) as those used in prior off-line fatigue testing. The elastic fibre strains were measured ply by ply along the fibres and in the matrix in the region of the crack. In this manner the crack affected zone was mapped, and subsequently the interfacial shear stress levels deduced as a function of distance from the crack at these temperatures. The results are compared with previous studies of load redistribution at room temperature and the fibre sliding stresses compared with those needed to slide pristine fibres to evaluate degradation of the interfacial shear strength caused by interfacial wear during fatigue. The implications for the use of such composites at elevated temperatures, for example in aero engines, are discussed.     

Ultrasonic wave intensity reflected from fretting fatigue cracks at bolt joints of aluminum alloy plates

The in-process ultrasonic measurement was carried out during fatigue testing of the bolted aluminum alloy 2024-T3 plates by using a water bag to obtain the surface acoustic wave (SAW) reflected from a fretting fatigue crack. The echo reflected from the fretting fatigue crack was detected at a position 1.5–2.1 mm ahead of the bolt hole under fastened conditions. In the fatigue process, the intensity of the scattering wave gradually increased in the fretted region ahead of the bolt hole with the number of fatigue cycles, and then a steep increase in the scattering wave intensity was observed. Relationships between the crack length and the average and the peak intensities were also obtained, and the average and peak intensities increased with the crack length. A relationship between the average intensity of the scattering wave and the surface roughness was also obtained. Experimental results suggested that appearance of a fretting fatigue crack in the fretted region caused a steep increase in the scattered wave intensity.     

In-situ monitoring of ultrasonic attenuation during rotating bending fatigue of carbon steel with electromagnetic acoustic resonance

Using electromagnetic acoustic resonance (EMAR), we studied the evolution of the surface-shear-wave attenuation and phase velocity during rotating bending fatigue of a 0.45% C steel. In the EMAR method, we used a magnetostrictively-coupled electromagnetic acoustic transducer (EMAT) for the contactless measurements of the axial shear wave that is a surface-shear wave, propagating in a cylinder-specimen circumference direction with the axial polarization. The attenuation coefficient always showed sharp peaks around 90% of the fatigue life, independent of the fatigue-stress amplitude. In addition to the ultrasonic measurements, we made crack-growth observations using replicas and measured recovery of attenuation and velocity after stopping the cyclic loading just before and after the peak. From these results, we concluded that the evolution of the ultrasonic properties is caused by a drastic change in dislocation mobility being accompanied by the crack growth.     

在役转轴表面疲劳裂纹的爬波检测

采用常规超声检测方法对在役转轴表面疲劳裂纹检测的效果很差。针对此类在役转轴疲劳裂纹产生的机理,结合超声爬波传播的特性,选用超声爬波对在役转轴产生的表面疲劳裂纹进行检测。对某规格的阶梯转轴进行的检测结果表明,超声爬波检测方法简便可行,能够有效地检测出转轴在役运行状态中出现的疲劳裂纹,因此可用于在役转轴表面疲劳裂纹的日常检测。     

Fatigue crack monitoring in engine-grade titanium alloy by dynamic subtraction of surface acoustic wavemodulation

The initiation and evolution of fatigue cracks in forged titanium alloy samples are monitored ultrasonically during fatigue testing. An in-situ surface wave acoustic method is applied during fatigue with an overlaid small low-frequency periodic loading, resulting in a nonlinear modulation of reflected ultrasonic pulses. The acoustic wave time traces in the sample are collected for a range of applied fatigue and modulation load levels and for a range of spatial propagation positions within each fatigue cycle. These samples are characterised by strong microstructure-induced ultrasonic scattering. To improve the signal-to-noise ratio a post-processing subtraction technique is introduced with the aim of enhancing initiated crack detectability.     

The evolved microstructure ahead of an arrested fatigue crack in Haynes 230

The dislocation microstructure beneath surface slip traces produced by fatigue loading of Haynes 230 was revealed to be a function of distance from a crack tip. The microstructure beneath these traces evolves from planar slip bands with increasing dislocation density and decreasing interband spacing as the crack tip approaches one of refined subgrains and lamellar bands at and in the vicinity of the crack tip. Similarly, beneath fatigue striations the microstructure evolves from nanosized subgrains to a banded structure with increasing distance from the fracture surface. These structures are significantly different to those predicted to develop under fatigue loading of a planar slip material. The evolved structures are considered in terms of the microstructure generated by severe plastic deformation.     

45钢构件电磁热裂纹止裂后残余应力分析

对带有半埋藏裂纹的45钢金属构件进行电磁热止裂实验发现,止裂后,半埋藏裂纹裂尖处熔化、钝化,并有金属喷射现象;采用超声波对比测试法,分析了止裂前后试件内残余应力的变化情况,实验测得止裂后超声波声程减小,表明试件内部残余应力增大;通过ANSYS有限元软件,采用间接耦合方法,对半埋藏裂纹电磁热止裂过程进行数值分析,得到了止裂后构件内残余应力场的具体类型及分布情况。     

Nondestructive sizing and localization of internal microcracks in fatigue samples

A large number of fatigue experiments on standardized samples is required for the development of databases of the fatigue properties of specific material systems. To facilitate such studies, different visual monitoring methods for surface fatigue cracks have been used; however, the problem of monitoring internal fatigue crack initiating during cold dwell fatigue of Ti is much more complicated. This paper describes the development and integration of several nondestructive evaluation methods for monitoring and sizing microcracks in titanium fatigue samples. For in situ monitoring of crack initiation and evolution ultrasonic Lamb wave signals are excited and acquired in the sample continuously during fatigue tests at different levels of fatigue load using a high-speed data acquisition system. Localization of the secondary cracks is done by both the in situ ultrasonic method and an ultrasonic immersion scanning method here referred to as “vertical C-scan” (VC-scan). The VC-scan is developed for imaging small cracks aligned normal to the fatigue sample axis. Microradiography has been performed on fatigue samples to confirm the localization and sizing of the detected cracks with other ultrasonic NDE techniques. The fusion of data from different NDE techniques provides useful information on the initiation, location, shape, size and growth history of fatigue cracks.     

Direct high resolution in situ SEM observations of very small fatigue crack growth in the ultra-fine grain aluminium alloy IN 9052

A study of the early stage of fatigue crack growth for a very small contained fatigue crack (∼2 μm) in an ultra-fine grain size aluminium alloy (IN 9052) has been carried out. Direct in situ SEM observations revealed that for this material very small contained fatigue cracks could propagate along the shear band associated with a blunted crack tip and could close at a positive applied load.     

非线性超声脉冲反转法在铝合金焊缝疲劳寿命预测中的应用

以不同疲劳寿命的铝合金焊缝为例研究了疲劳过程中的超声波非线性效应,同时建立了非线性参数与疲劳寿命的关系曲线(S-N曲线).采用脉冲反转法对非线性超声信号进行处理.结果表明,经过脉冲反转法获得的S-N曲线与经过滤波模块获得的S-N曲线相比谐波幅值提高了一倍,所以脉冲反转法可以提高非线性超声检测法表征疲劳损伤程度能力,而S-N曲线在疲劳过程中呈先缓慢增长后快速增长,最后下降的趋势,此趋势对应了材料疲劳的3个阶段,而金相组织验证了曲线的准确性,所以通过非线性超声脉冲反转法来预测疲劳寿命是可行的.     

In-situ monitoring of fatigue crack growth using high frequency guided waves

The development of fatigue cracks at fastener holes represents a common maintenance problem for aircraft. High frequency guided ultrasonic waves allow for the monitoring of critical areas without direct access to the defect location. During cyclic loading of tensile, aluminum specimens fatigue crack growth at the side of a fastener hole was monitored. The changes in the energy ratio of the baseline subtracted reflected guided wave signal due to the fatigue damage were monitored from a stand-off distance using standard ultrasonic pulse–echo measurement equipment. Good sensitivity for the detection and monitoring of fatigue crack growth was found.     

核电站控制棒驱动机构驱动杆厚壁管材水浸超声检测

驱动机构控制杆是核电站反应堆的关键部件之一。其质量要求高,原材料中如存在表面开裂等危险性缺陷,可能对反应堆的安全运行造成影响。驱动杆为管状结构,但壁厚／外径比达到了0．35,周向超声检测困难。笔者采用水浸超声检测方法,利用一次折射纵波和反射变型横波实现厚壁管材的全体积检测,提高了超声检测的灵敏度和信噪比。     

MORPHOLOGY OF SUB-CRITICAL GROWTH ON STRESS CORROSION CRACKING AND PLASTIC ZONE AT CRACK TIP

用抛光的恒载荷试样对40CrNiMoA和AISI4330M两种高强度钢在水介质中应力腐蚀裂纹的产生和扩展进行了金相跟踪观察,研究了裂纹亚临界扩展的形貌。结果表明:表面裂纹前端产生阴影区(塑性变形区),它的形貌随钢种不同而不同。对于40CrNiMoA,阴影区是扩展、闭合,再扩展、再闭合。而对于4330M,阴影区不闭合,近乎平行扩展。只有在刚开始加载、阴影区呈耳朵状时,它才反映了裂纹尖端的塑性区。在以后绝大部分的扩展过程中,阴影区并不代表裂纹尖端的塑性,而只反映产生剪切唇所引起的变形,且与裂纹的“突进”(Pop-in)密切有关,必须和小范围屈服断裂力学中裂纹尖端的塑性区相区别。     

In situ characterization of fatigue damage evolution in a cast Al alloy via nonlinear ultrasonic measurements

A new methodology is described for in situ characterization of fatigue damage accumulation using nonlinear ultrasonic measurements via analysis of the feedback signal of a closed-loop ultrasonic fatigue system. In the very high-cycle fatigue regime, ultrasonic nonlinearity increases with initiation and growth of a dominant, life-limiting fatigue crack. Based on the increase in the ultrasonic nonlinearity with fatigue cycling, crack initiation, small fatigue crack growth and fast crack growth regimes have been distinguished during cycling in specimens with different pore sizes tested at various stress amplitudes. The fraction of fatigue life spent in initiation of a life-limiting fatigue crack decreases with increasing stress amplitude. For a constant stress amplitude, the initiation life also decreases with increasing pore size. The present study also demonstrates the applicability of the methodology for fatigue crack growth studies from natural defects located internally or at the surface in smooth specimens.     

腐蚀疲劳裂尖材料损伤研究

金属环境断裂过程依赖于材料,应力与介质参数间的复杂交互作用,这些交互作用的澄清是实现材料断裂控制的重要基础,而裂纹尖端单元损伤过程的研究则是联接微观机制与宏观断裂规律的桥梁。通过对钝化和活化典型腐蚀体系下恒速长大腐蚀疲劳裂纹受力学与电化学参数扰动后的扩展动力学响应规律的系统研究,并结合多种其它分析手段,考察了不同腐蚀因素对裂尖材料的损伤作用机制。在实验基础上,对腐蚀导致裂纹闭合变化,裂纹扩展过载延