Quantitative models on corrosion fatigue crack growth rates in metals: Part I. Overview of quantitative crack growth models

Approaches to predict da/dN-ÀK for environmental situations; including empirical interpolative equations, linear superposition of mechanical fatigue and time-based environmental cracking, and mechanism-based models; are presented. For several material-environment systems, these models were incorporated in fracture mechanics life prediction methods, and successes have been reported in evaluating the corrosion fatigue contribution. Considerable uncertainties are, however, associated with these models. The linear superposition analysis is emphasized; material-environment systems that are severely environment-sensitive should be adequately described by this method. Direct and indirect methods exist to define time-based crack growth rates for use in linear superposition predictions of da/dN. The linear superposition approach is effective, but only for those cases where K_ISCC is high relative to typical flawed component stress intensity levels. Empirical curve-fit models require an extensive environmental crack growth rate data base, which are costly to develop, and are effective for interpolations but not predictions of fatigue crack growth data. Mechanism-based models for broad predictions of cycle-time dependent da/dN versus ÀK, and other variables such as frequency or hold time, are in an infant state.     

Fatigue life estimation of steel using laser speckle sensor

We investigated a method to evaluate fatigue damage of steels without contact using a laser speckle sensor. In the earlier stage of fatigue in steels, slipbands are produced on the surface and the slipband density increases as fatigue damage progresses. When a laser illuminates the surface of the fatigued steel, light intensity distribution of the laser speckle pattern caused by the reflected light changes depending on the change of surface properties caused by slipbands. Width of the speckle pattern broadens correspondingly to spatial frequency distribution of the surface and thus it is presumed that speckle pattern broadens with increase of slipband density. The method presented in this paper is based on this phenomenon. In this study, we observed change of the speckle pattern during fatigue loading. We propose a method to estimate fatigue life by observing change in the speckle pattern depending on number of loading cycles in the earlier stage of fatigue before crack initiation.     

Nondestructive characterization of defects using a novel hybrid ultrasonic array sensor

The concept of a novel hybrid array transducer is introduced in this article. Based on the principles of dynamic beam steering and ultrasonic shadowing, a transducer consisting of an assembly of a linear phased array and a static array is developed. Using a dynamic pitch-catch configuration, the transducer is capable of determining the location, size and orientation of a planar defect without scanning or relocating the transducer. A theoretical shadow analysis carried out in this work allows for developing a scheme to characterize a single defect. To demonstrate the feasibility of the technique, a rail specimen with real defect has been assessed. This scheme proves to be a viable approach for characterizing transverse defects (TD) with good accuracy.     

Nondestructive testing of train wheels using differential-type integrated Hall sensor matrixes embedded in train rails

A nondestructive testing method for inspecting cracks in train wheels is proposed in this study. The train wheels can be inspected without disassembly, as soon as the train enters its shed, using this method. The proposed method uses differential-type integrated Hall sensor matrixes (D-IHaSMs) embedded in the rails. This D-IHaSM can inspect cracks over a large area at high speed with a high spatial resolution. In addition, a custom signal processing circuit is designed to match the requirements of the D-IHaSM. The artificial cracks formed in the train wheels are inspected and the crack volume is estimated using the D-IHaSM system.     

基于超声相控阵的耐候钢接头疲劳裂纹动态监测

借助超声相控阵技术对耐候钢对接接头开展疲劳失效过程动态监测. 基于超声波探头的信号特征,研究其扇形扫描反射过程,建立实时扫查方案, 并对10 mm厚的耐候钢对接接头实施实时监测.结果表明,当疲劳寿命为5 × 104次时,相控阵检测到多个裂纹从对接接头焊趾部位萌生,并沿着板厚扩展,当疲劳寿命超过3.5 × 105次时,裂纹开始快速扩展. 与疲劳试验断口对比发现,基于相控阵检测得到的裂纹尺寸与试验结果基本一致,验证了相控阵裂纹动态检测的准确性. 根据裂纹深度a、裂纹长度2c与循环次数N关系,明确了裂纹动态演化行为,并获得中厚板耐候钢对接接头表面裂纹的扩展演化规律.     

金属磁记忆法检测焊接裂纹的时间空间有效性

预制了焊接裂纹试板,在试板的不同空间区域,以及焊接之后的不同时刻进行了金属磁记忆检测.采用快速傅里叶变换对检测信号进行了特征分析,研究了焊接裂纹周围不同区域在不同时刻的金属磁记忆信号以及其傅里叶变换相位突变特征.结果表明,在焊接裂纹存在的70 mm范围内是金属磁记忆检测的空间有效区域.在焊接完成后,接头温度低于100 ℃以后的任何时间进行金属磁记忆检测,取得的结果都是有效的.     

霍尔电流传感器在电焊机中的应用

介绍了霍尔电流传感器的基本原理及其在电焊机焊接电流测试中的应用,同时提出了在焊机电流测试中提高焊接电流测试精度的措施和需要注意的问题.     

Quantitative assessment of through-thickness crack size based on Lamb wave scattering in aluminium plates

The interaction of Lamb wave modes at varying frequencies with a through-thickness crack of different lengths in aluminium plates was analysed in terms of finite element method and experimental study. For oblique-wave incidence, both numerical and experimental results showed that the wave scattering from a crack leads to complicated transmission, reflection and diffraction accompanied by possible wave-mode conversion. A dual-PZT actuation scheme was therefore applied to generate the fundamental symmetrical mode (S₀) with enhanced energy to facilitate the identification of crack-scattered wave components. The relationship between crack length and the reflection/transmission coefficient obtained with the aid of the Hilbert transform was established, through which the crack length was quantitatively evaluated. The effects of wavelength of Lamb waves and wave diffraction on the properties of the reflection and transmission coefficients were analysed.     

Localization of defects in magnetic permeable bar by multiple signal classification approach using rotating dipole model

A simplified search algorithm based on multiple signal classification (MUSIC) approach with rotating dipole model has been proposed to identify the defect locations inside a magnetically permeable bar. This new algorithm identifies the defect locations and the moments of the modelled anti-dipoles without the necessity to decompose the single column lead vector by singular value decomposition (SVD). Here, we find the location of the first dipole by using the rotating dipole model of MUSIC approach and the rest of the defect locations are identified using the orthogonal projection method coupled with rotating dipole model of MUSIC. Three deeply buried defects were modelled as anti-dipoles and the leakage fields due to these three deeply buried defects were simulated by direct forward calculation. Using the calculated data as an input, it has been possible to identify the number of defects and their locations by this new approach with significant reduction in computations, even in the presence of reasonable levels of additive noise.     

Study of acoustic signal in the process of resistance spot welding based on array sensor system

This investigation was performed to study acoustic field signal in order to improve RSW quality. Researchers firstly built an acoustic array sensor system, which included 8 MPA-416 acoustic sensors, data acquisition card and LabVIEW. The system obtained the acoustic field information in the process of nugget growing. Due to the nonlinearity field signal, array sensor algorithm was utilized to quantitatively analyze the characteristics of acoustic field and reduced noise. The experiment and calculation results show that array sensor system can acquire acoustic field signal of nugget growing in the RSW process and array processing algorithm based on acoustic field can extract characteristic parameters to evaluate RSW quality. It was concluded that the acoustic array sensor system offers a new methodology for RSW quality inspection.     

Ultrasound monitoring of bubble size and velocity in a fluid model using phased array transducer

Experimental studies on composite materials highlighted the existence of gas bubbles (voids) at different scales (micro and macro). These voids, resulting from the fabrication process, are sources of weakness for the end-user material. Therefore, several studies focus on the evaluation and the minimization of void rate. During injection of resin into the fibrous matrix, bubbles appear. Some are dispersed while others persist and diminish the overall quality of the finished product. This study consists in detecting bubbles during the resin transfer molding (RTM) process using an original method based on ultrasound. However, in practice we have to face with several problems due to the heterogeneity of the environment, such as the differentiation of a bubble from a strand or fiber. Due to the complexity of the problem, an experimental setup was built in order to detect bubbles using an ultrasonic phased array transducer in a flow of viscous fluid only, without the presence of fiber matrix. The study deals with the flow of a model fluid, which simulates the injected resin, with presence of air bubbles trapped in a channel production. Since the ultrasonic characteristics of the experimental setup are well known and characterized, the number, velocity and size of air bubbles could be evaluated.     

Nondestructive testing of express train wheel using the linearly integrated Hall sensors array on a curved surface

A novel nondestructive testing (NDT) system of high speed and high spatial resolution is needed to detect small cracks on the wheels of an express train. This paper proposes a high-speed NDT system, which uses the Hall sensors array and parallel signal processing circuits. Linearly integrated Hall sensors are arrayed on the cross-section line of a wheel, and the Hall voltage are obtained in the rotation direction. High-speed NDT can be carried out using analog-to-digital converters and micro-processors. The ?V_H/?x value, which provides the most important information about a crack, can be obtained by buffering and calculations. The effectiveness of the novel method was verified by applying it in the examination of cracks on a wheel specimen model.     

磁性寡层石墨烯用于高效去除水中重金属

用简单的原位共沉淀法制备了磁性寡层石墨烯(MFLG),将其用于吸附水中的Ni(Ⅱ)和Pb(Ⅱ).在25℃下,以用量为500 mg·L-1的MFLG对初始浓度为200 mg·L-1的两种重金属水溶液进行吸附,5 min达到吸附平衡,对Ni(Ⅱ)的吸附率和吸附量分别为97.91％和391.63 mg·L-1;对Pb(Ⅱ)的...     

基于微分方法的焊接裂纹金属磁记忆特征研究

针对金属磁记忆检测技术的瓶颈问题,以X70钢为研究对象,通过对预制焊接裂纹一阶微分处理后的金属磁记忆信号进行分析,可区别焊接裂纹引起的应力集中与其它缺口效应引起的应力,并可确定焊接裂纹存在的定位信号特征.现场检测表明,利用一阶微分处理后的磁记忆信号定位特征,可准确地对焊接裂纹进行检测.     

On the suitability of induction coils for crack detection and sizing in metals by the surface magnetic field measurement technique

The surface magnetic field measurement (SMFM) technique has proved to be an accurate means for crack detection and sizing cracks in ferrous metals. The technique involves the use of two U-shaped current-carrying wires of sufficiently high frequency while measuring the discontinuity in the resultant magnetic field at the crack edge with an appropriate magnetic field sensor. In this work, we describe a mathematical algorithm to obtain the crack signal from the output of an induction coil used in a SMFM probe. We also discuss the measurement errors due to the coil size and shape. To reduce the measurement errors, we present an algorithm in which the crack signal is recovered by appropriate deconvolution of the coil output signal and its spatial transfer function. The algorithm is then used to recover crack signals for various coil shapes and sizes. The study of the results demonstrates the effectiveness of the algorithm in the case of large coils.     

Quantitative models on corrosion fatigue crack growth rates in metals: Part II. application of fatigue crack growth rate modeling for nickel-based superalloys at elevated temperatures

Approaches to predict da/dN-àK for environmental situations; including empirical interpolative equations, linear superposition of mechanical fatigue and time-based environmental cracking, and mechanism-based models; are presented. For several material-environment systems, these models were incorporated in fracture mechanics life prediction methods, and successes have been reported in evaluating the corrosion fatigue contribution. Considerable uncertainties are, however, associated with these models. The linear superposition analysis is emphasized; material-environment systems that are severely environment-sensitive should be adequately described by this method. Direct and indirect methods exist to define time-based crack growth rates for use in linear superposition predictions of da/dN. The linear superposition approach is effective, but only for those cases where K_ISCC is high relative to typical flawed component stress intensity levels. Empirical curve-fit models require an extensive environmental crack growth rate data base, which are costly to develop, and are effective for interpolations but not predictions of fatigue crack growth data. Mechanism-based models for broad predictions of cycle-time dependent da/dN versus àK, and other variables such as frequency or hold time, are in an infant state.     

基于ACFM技术的焊缝跟踪传感器空间磁场建模

针对窄间隙I形坡口对接焊缝因其结构特殊性而缺乏直接有效的跟踪方法这一难题,设计了一种基于交流电磁场检测（ACFM）技术的新型焊缝跟踪传感器．建立了传感器焊缝表面感应电流的三维空间简化磁场模型,推导出了空间磁场磁感应强度计算解析式,利用MATLAB软件分析了三维方向的磁感应强度分布规律,重点分析工件焊缝引起的磁场畸变信号与焊缝位置之间的内在联系,采用近似二次多项式曲线拟合表征垂直焊缝方向磁感应强度Bz与焊缝左右偏离距离两者的数学关系,提出了一种新的焊缝偏差检测方法,为I形对接坡口焊缝自动跟踪系统的设计提供理论依据．     

The determination of austenite grain size in ferrous metals

A simple uncomplicated method is introduced to render the austenite grain boundaries visible at room temperature. The suggested method is inexpensive and time saving and may be used universally. This is demonstrated on eleven ferrous materials and a good reproducibility is shown. The austenite grain sizes estimated by more complex methods are very similar to those determined by this new method.     

NDE of a 3-D surface crack using magnetic field induced by DC current flow

A method is proposed for nondestructive evaluation of a 3-D surface crack by means of measurement of a magnetic field induced in the air by DC current flow in a material. A semielliptical crack is considered. The crack length is assumed to be known in advance and the crack depth is to be found. The current is applied between two close points on both sides of the crack and the change in magnetic: flux density in the air due to the crack is measured in non-contact with the surface of the material. An approximate equation relating the magnetic flux density to the depth and length of the crack is derived numerically and some experimental results show that the evaluated crack depth is in good agreement with the actual one.     

音频大地电磁磁场传感器的研制

感应式磁场传感器一直是我国电磁法仪器发展的瓶颈,国外大地电磁法中测量大地磁场主要采用感应式磁场传感器。为此,根据音频大地电磁场源的特点,从感应式磁场传感器的基本原理出发,设计传感器的磁芯和线圈绕组结构以及线圈的谐振频率;另外,采用了磁通负反馈技术和频率补偿的方法,扩展了传感器的频带,得到了在1 Hz～9.8 kHz频带范围内平坦度为±1.5 dB的灵敏度曲线,满足音频大地电磁探测的需要。