Ultra-low noise front-end electronics for air-coupled ultrasonic non-destructive evaluation

Air-coupled ultrasonic inspection has been demonstrated to be a non-contact method of great interest in non-destructive evaluation (NDE) applications. The absence of direct contact or a liquid couplant provides this technique very attractive benefits in front of the well-known and well-developed liquid-coupled ultrasonic inspection systems. A wide range of defects can be detected by means of ultrasound coupled to and harvested from the specimen in absence of contact when using appropriate transducers designed for their operation in air.This paper presents an easy way to integrate air-coupled piezoelectric transducers in conventional ultrasonic NDE equipment. The design of a specific front-end electronics by using an ultra-low noise amplifier enables existing inspection systems to be used for the dry-coupled ultrasonic test of materials and structures. The amplifier provides the receiver with a signal-to-noise ratio large enough for good quality signal processing and imaging. System dynamic ranges of more than 100 dB are achieved.     

Removing approach for flashes of friction stir spot welds

A friction stir spot welding tool with a threaded probe and a flat shoulder was made to join 6061-T6 aluminum sheets under various indentation depths, rotational speeds, indentation rates and dwelling times. Two types of flashes, ring-shape and petal-shape, were found and their forming processes were identified. A concept of combining friction stir spot welding and end milling was proposed to make welds and remove flashes simultaneously. Pictures and micrographs were examined to study the effects of processing parameters on the flash thickness, height, and type, and to obtain the maximum flash thickness/cutter width for tool design. Based on experimental results, a welding tool with two flash-removing cutters was developed to remove flashes during welding. The mechanical properties of friction stir spot welds with and without flashes are further examined to evaluate the performance of the proposed welding tool.     

基于空气耦合超声的激光焊缝质量评估

随着激光焊接技术在汽车及轨道交通上的广泛应用,对激光焊缝的无损检测技术要求越来越高. 针对3 mm以下的两层金属薄板激光焊缝为对象,探讨了空气耦合超声检测技术对激光焊缝检测的可能性. 利用数值解析和试验分析两种方法,在空气中对铝板激励兰姆(Lamb)波,通过Lamb波在激光焊缝试件中的传播模拟,分析了激光焊缝的宽度以及焊缝的良否对反射率及透射率的影响,解明了Lamb波在激光焊缝试件中的传播规律. 结果表明,可以利用Lamb波A0模式对激光焊缝的质量进行评估.     

薄钢板点焊接头超声信号分析

采用水浸超声聚焦直入射法对镀锌薄钢板点焊接头进行了质量评价。详细分析了点焊接头的A扫描信号、B扫描及C扫描图像特征,探讨了沿焊核直径B扫描评价焊点质量方法的可行性。研究结果表明,采用这种方法不但可以定性评价虚焊接头与完好接头,还可以定量计算出上下表面压痕深度及熔核直径。对比沿焊核直径B扫描图像与横截面的金相图像,两者吻合较好,证实了检测结果的准确性。     

SUS304奥氏体不锈钢板点焊接头的超声成像分析

利用超声波水浸聚焦入射法对1 mm厚的SUS304奥氏体不锈钢板点焊接头进行超声C扫描成像检测.分析了不同焊接工艺参数下的C扫描图像特征,甄别了飞溅、焊穿等典型焊接缺陷,并提取其对应的A扫描信号.基于C扫描图像对焊核直径进行了测量,并与焊核切口端面尺寸进行了比较.结果表明,基于超声波水浸聚焦入射法得到的C扫描图像,能有效观测焊核内部形貌特征.焊接电流超过8 kA,电极力小于2 700 N时,超声波C扫描图像中清晰反映出飞溅、焊穿等缺陷,其对应区域的A扫描信号与正常熔核区波形特征有明显差异;借助超声C扫描图像测得的焊核直径为4.39~5.25 mm.     

Development of spot weld failure parameters for full vehicle crash modelling

Abstract

A cost and time effective process was developed to create spot weld failure parameters for crash models implemented in LS-DYNA. The process includes a design of experiment (DOE) approach for coordinating data collection, welding and testing, finite element modelling, statistical analysis, validation, and implementation. The DOE approach was used to coordinate testing of a reduced set of samples over a large range of material strengths and gauges. This testing included cross-tension, lap shea, and coach peel type evaluations. Computer models for each metal combination and sample geometry were developed to obtain normalised tensile, shear and bending stress at the incidence of weld failure. These normalised loads were then regressed to extend these results to all possible material combinations within the DOE space. These results were then used to estimate the spot weld failure parameters for all the stack-ups of interest. A set of multiweld T-section samples were then welded, physically tested and computer modelled to validate the failure parameters developed from the small single weld tests.     

ST12汽车钢板拉剪点焊接头的疲劳强度

针对车身使用的ST12低碳钢板,利用生产线上焊接设备制备了电阻点焊接头拉剪疲劳试件.对点焊接头区域进行了金相组织观测,测量了焊点附近的维氏硬度值分布.在MTS材料疲劳试验机上进行了恒幅疲劳加载,得到了ST12低碳钢板的电阻点焊接头的S-N曲线.基于所得的S-N曲线进行了两级疲劳加载试验,对两级加载下的线性累积疲劳损伤进行了分析.累积损伤结果分析表明,ST12低碳钢板电阻点焊件在两级加载下存在加载次序效应,高-低加载次序下加载次序效应更为明显.     

外加磁场对铝合金电阻点焊质量的影响

分析了外加磁场对铝合金电阻点焊接头质量的影响,包括熔核尺寸、接头力学性能和微观组织. 结果表明,外加磁场增大了熔核直径、提高了点焊接头的剪切力和吸收能. 在不同工艺参数下,熔核直径增长在5%~25%,剪切拉伸力可提高10%~30%. 在一些焊接参数下,外加磁场可以降低焊接电流、减短焊接时间,从而提高焊接效率、降低能耗. 外加磁场可促进点焊熔核内等轴晶的生成和细化晶粒.当焊接电流很小或很大、焊接时间过短或过长时,都会降低外加磁场的效应. 因此当对点焊施加外磁场时,应在合理的工艺参数下进行,以最大限度地发挥外加磁场的作用.     

Segmentation of thermal images for non-destructive evaluation of bridge decks

The objective of the project presented in this paper is to automate the detection of subsurface defects in concrete bridge decks using infrared thermography. The algorithm developed for this purpose is based on the region growing approach, which segments the image and identifies the voids without human interference or prior knowledge of the conditions. The segmentation algorithm starts with the hottest pixels in the image as seed points, and then regions are grown around them based on a neighborhood selection criterion. The algorithm was tested on images collected from concrete bridge deck specimens containing various man-made defects and also on a defect-free control model. The experimental work successfully identified defects in concrete bridge decks up to 3 in. below surface using thermograph imaging.     

Magnetic pulse spot welding of bimetals

One of the main parameters in magnetic pulse welding (MPW) is the requirement of certain distance/spacing called “air gap” between the two sheets or tubes to be welded. This work presents means to alleviate this constraint through stamped humps in one of the sheets. The sheets remain in contact all over except at humps where a local air gap is sufficient to generate impact velocity required to produce spot welds using electromagnetic energy. The developed approach has been validated on the heterogeneous assembly of AA1199 aluminum to EN 355 steel. The microstructural analysis revealed that the weld interface is wavy and the waves are in two opposite directions from the center. Quasi-static tension–shear tests validated the joint efficiency of spot welds.     

Prevention of liquid metal embrittlement cracks in resistance spot welds by adaption of electrode geometry

ABSTRACT

Advanced high strength steels are usually coated by a zinc layer for an increased resistance against corrosion. During the resistance spot welding of zinc coated steel grades, liquid metal embrittlement (LME) may occur. As a result, cracking inside and around the spot weld indentation is observable. The extent of LME cracks is influenced by a variety of different factors. In this study, the impact of the used electrode geometry is investigated over a stepwise varied weld time. A spot welding finite element simulation is used to analyse and explain the observed effects. Results show significant differences especially for highly increased weld times. Based on identical overall dimensions, electrode geometries with a larger working plane allow for longer weld times, while still preventing LME within the investigated material and maintaining accessibility.     

Fatigue behaviour and life estimation of Mg/Al ultrasonic spot weld bonding welds

Mg/Al dissimilar joints with favourable static and dynamic mechanical properties were obtained utilising ultrasonic spot welding and ultrasonic spot weld bonding technology. The fatigue behaviour, fatigue failure mode, and typical fatigue fracture surface morphology of Mg/Al ultrasonic spot welding and ultrasonic spot weld bonding joints were compared and observed to get a better understanding of the fatigue failure mechanism. In general, the Mg/Al ultrasonic spot weld bonding joint exhibited better fatigue performance than the ultrasonic spot welding joint. The two kinds of joints presented different fatigue failure modes with decreasing applied load level. A fatigue life prediction model was established, and the fatigue life prediction results were in good agreement with the experimental fatigue results.     

Mechanical properties and interface morphology of Mg/Al ultrasonic spot weld bonding welds

This paper investigates the mechanical properties and interface morphology of Mg/Al ultrasonic spot welding (USW) joint, adhesive bonding (AB) joint and ultrasonic spot weld bonding (USWB) joint. The peak load and fracture energy of USWB joint increased significantly compared to that of USW joint and AB joint. The USWB joint presented a hybrid fracture mode which was composed of the delamination failure at adhesive/Mg interface, cohesive failure within the adhesive and cleavage failure in the weld zone. The interface morphology suggested that USWB joint exhibited fewer defects in the cured adhesive matrix and elevated connection density of adhesive/metal interface, which improved joint strength and altered the fracture mode.     

Determination of residual stress in spherical balls by resonant ultrasound spectroscopy

In this study, resonant ultrasound spectroscopy (RUS) for determining residual stress in small size spherical balls was examined. Natural frequencies of spherical balls with residual stress were analysed by finite element method. Resonant frequencies of spherical balls were experimentally measured by a RUS system. Both natural frequencies in the analysis and the resonant frequencies measured in the experiment decreased as the compressive circumferential stress at the ball surface increased. It was concluded, on the basis of both analytical and experimental results, that the measurement of the resonant frequency by the RUS system along with the analysis of natural frequency are effective for determining the values and distributions of unknown residual stress in spheres.     

Behaviour of welds containing cracks in railway bridge trusses

Throughout the years, vision, human beings' favourite sense, along with our great capacity to obtain, process and interpret a great amount of visual data, has been a great inspiration for development of techniques and technological devices that reproduce it into a computational system. In welding processes, vision can supply information on inspection and quality of the welded joint, monitoring of parameters, trajectory correction and even study of the phenomena involved in the process. However, the light radiation emitted from the weld arc is a barrier to studies based on the viewing process. One form currently used to visualize the process, without the interference of the arc's light, consists of illuminating the process with the near-infrared (NIR) light and using interference (band pass) filters, around this same wavelength, during acquisition of the images. One solution for the NIR light that is being increasingly used involves the use of high-power laser diodes, of low-cost and less complex installation than conventional lasers. The proposal of this study is the design, construction and assessment of a low-cost, highly flexible vision system for welding processes. It is based on characterization of the spectrum of the weld arc, definition of a drive topology for the high-power laser diode in NIR within its limitations of use, and maximizing the light power emitted, creation of control circuits, selection of optical equipment and components and, finally, design and apply a prototype for visualization of welding processes. As a whole, the limitations of the viewing system were encountered through their application with tungsten inert gas and metal inert gas/metal active gas welding. In these trials, although it had not been capable of overcoming the arc radiation, the system developed provided homogenous lighting synchronized with the camera, whereby the main limitation was long exposure time of the camera. In conclusion, the use of employing the system to assist with joint tracking is suggested.     

对接焊缝残余应力的有限元分析

采用有限元方法数值模拟了对接焊缝残余应力大小和分布，计算结果与试验结果基本吻合，证明有限元方法是一种经济而有效地预测焊接接头残余应力的方法，可以为制定正确的焊接工艺，改善接头性能提供理论依据。     

Comparative study of fatigue behaviour in dissimilar Al alloy/steel and Mg alloy/steel friction stir spot welds fabricated by scroll grooved tool without probe

Abstract

Aluminium alloy A6061-T6 or magnesium alloy AZ31 sheet was welded to steel sheet by a friction stir spot welding technique using a scroll grooved tool without a probe. The material flow in the nugget of the Mg/steel weld was less than that in the Al/steel one. The Al/steel weld exhibited higher static tensile–shear strength than the Al/Al weld, while the strengths of Mg/steel and Mg/Mg welds were comparable. Tensile–shear fatigue tests were performed using lap shear specimens of both dissimilar and similar welds. The dissimilar welds exhibited nearly the same fatigue strengths as the similar ones. The effective nugget size in the dissimilar welds was defined as the area where Al or Mg alloy remained on the steel side after static fracture. When the fatigue strengths of dissimilar welds were evaluated based on the effective nugget size, the normalised fatigue strengths of Al/steel and Mg/steel welds were comparable.     

Wavelet analysis based deconvolution to improve the resolution of scanning acoustic microscope images for the inspection of thin die layer in semiconductor

Scanning acoustic microscopy (SAM) is used as an important non-destructive test tool in semiconductor reliability evaluation and failure analysis. However, inspection of thin die layer has proven difficult as the reflected signals from the die top and bottom are superimposed. Conventional deconvolution techniques have been used for the improvement of time resolution in A-scan signal, however, they are not effective for SAM signal because the waveform of the reflected echo is quite different from the incident waveform due to the frequency dependent attenuation and the focal effect. In this paper, in order to overcome this difficulty, a new signal processing method, wavelet analysis based deconvolution technique is proposed. Its validity is approved by computer simulations and practical performances are demonstrated by experiments for the fabricated semiconductor sample.     

非承载角焊缝十字接头疲劳性能

文中基于结构应力方法对非承载角焊缝接头的疲劳性能进行了数值分析与试验研究。首先根据不同焊脚尺寸的十字接头焊趾截面结构应力,发现焊脚尺寸越大,接头疲劳性能反而越低,疲劳试验也证明了这一现象。同时,试验数据与BS 7608标准中F级S-N曲线的对比说明了疲劳评估时进行应力修正的必要性。然后,分别利用含装配间隙和熔深的有限元模型计算接头结构应力,证明两者对接头的疲劳性能影响很小。文中还通过与承载角焊缝接头的对比,证明细节特征对接头疲劳性能的影响程度与接头受力状态直接相关。在定义焊缝缺欠质量等级时,应考虑接头在结构中的受力状态,制定合理的控制参数,从而降低结构的制造成本。