摘要：
Efficient and accurate nondestructive testing of surface or sub-surface flaws is essential for metal components. This study focuses on the high-quality image reconstruction of these flaws. An imaging detection method is introduced which combines the leaky Rayleigh waves testing and the synthetic aperture focusing technique (SAFT) with an immersion pulse-echo scanning. The principal component analysis (PCA) is used to reduce the noise of the leaky Rayleigh waves, and then the imaging is obtained using the SAFT algorithm. The experimental results show that the PCA-SAFT imaging method can reduce structural noise and imaging artifacts comparing with the B-scan imaging method. The lateral resolution of defects is improved and the mean error of defects sizing can be reduced by 48.81%. The signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) of the ultrasonic image are increased by 6.95 dB and 4.20 dB respectively. The proposed method can be a new choice for effective evaluating the surface quality of key components.
摘要：
超声表面波是检测激光熔覆层质量的重要手段, 为提高检测分辨率, 采用可达到声束聚焦效果的相控阵表面波对激光熔覆层进行检测。建立了单探头与相控阵表面波传播的有限元模型, 基于Fermat原理研究超声波传播路径并分析了阵元延时特性, 实现了相控阵表面波的聚焦和偏转, 研究了熔覆层厚度对相控阵表面波聚焦特性的影响。结果表明, 对于基体材料为铝, 熔覆层材料为45#钢时, 熔覆层厚度在2.5 mm内, 聚焦点的能量随厚度增加而减小, 如1 mm厚相对于0.2 mm厚的聚焦点能量减小了58.8%; 当厚度大于2 mm时, 聚焦点处能量变化不明显, 表明超声相控阵表面波对薄的熔覆层具有较好的检测效果。 The ultrasonic surface wave is an important method to detect the quality of laser cladding layer. In order to improve the detection resolution, the phased array surface wave which can achieve the focusing effect of acoustic beam is used to detect laser cladding layer. Firstly the finite element model of surface wave propagation between single probe and phased array is established. Subsequently, the ultrasonic wave propagation path is studied based on the Fermat principle and the delay characteristics of the array elements. The focusing and deflection of the phased array surface wave are realized, and the influence of the thickness of the cladding layer on the focusing characteristics of the phased array surface wave is studied. The results show that when the base material is aluminum and the cladding layer material is 45# steel, the thickness of the cladding layer is within 2.5 mm, and the energy of the focal point decreases with the increase of thickness. Especially, the energy of the focal point decreases by 58.8% with the thickness of 1 mm compared to 0.2 mm. When the thickness is greater than 2 mm, the energy changes little at the focus point, indicating that the ultrasonic phased array surface wave has a good detection effect on the thin cladding layer.
关键词：
microcrack;colinear wave-mixing;nonlinear ultrasonic;finite element simulation
摘要：
The timely detection of microcracks is of great significance for ensuring the safety of key components. In this paper, colinear wave-mixing technology using the ultrasonic surface wave was applied for the detection of microcracks on metal surfaces. The propagation model of colinear wave-mixing surface wave was first established by the finite element method, and the effect of propagation distance was studied to determine an appropriate observation point to receive signals. The effect of the size of the microcracks on the nonlinear coefficient was then studied. The simulated results show that the nonlinear coefficient decreases with the increasing width of the microcracks and increases with increasing length within a certain range. The results provide a useful reference for the quantitative nonlinear ultrasonic wave-mixing testing of microcracks.
关键词：
coating thickness;flaw sizing;ultrasonic measurement model;corrections for transmission coefficient
摘要：
When magnesium alloys are tested using ultrasonic techniques, their oxide coating will affect the transmitted ultrasonic energy and the accuracy of the flaw sizing. In this work, the effects of the coating thickness on the flaw sizing are investigated. An ultrasonic measurement model is employed to predict the flaw response signal, and the reflected and transmitted ultrasonic energy in the coated surface are corrected by modifying the reflection and transmission coefficients related to the effects of the coating thickness. The effectiveness of these coefficients and the ultrasonic measurement model are verified through experiments. With the correction of the coating thickness effects, the flaw-sizing curves predicted using the ultrasonic measurement model are provided. The flaws in magnesium alloy specimens with different coating thicknesses are tested, and the determined flaw sizes from these curves agree well with the actual sizes, which reveals the effectiveness of the proposed work. This work provides an effective tool to improve the flaw sizing performance using ultrasonic techniques in practical applications.
摘要：
The ultrasonic phased array total focusing method (TFM) has the advantages of full-range dynamic focusing and high imaging resolution, but the problem of long imaging time limits its practically industrial applications. To reduce the imaging calculation demand of TFM, the locations of active array elements in the sparse array are optimized by combining almost different sets with the genetic algorithm (ADSGA), and corrected based on the consistency of the effective aperture with the equivalent point diffusion function. At the same time, to further increase the imaging efficiency, a sparse-TFM image with lower resolution is obtained by reducing the number of focus points and then interpolated by the new edge-directed interpolation algorithm (NEDI) to obtain a high quality sparse-TFM image. Compared with TFM, the experimental results show that the quantitative accuracy of the proposed method is only decreased by 1.09% when the number of sparse transmitting elements reaches 8 for a 32-element transducer, and the imaging speed is improved by about 16 times with the same final pixel resolution.
作者机构：
[杜剑; 胡宏伟] School of Automotive and Mechanical Engineering, Changsha University of Science and Technology, Changsha, 410114, China;[李洋; 周正干; 胡宏伟] School of Mechanical Engineering and Automation, Beihang University, Beijing, 100191, China
通讯机构：
School of Automotive and Mechanical Engineering, Changsha University of Science and Technology, Changsha, China
摘要：
In the production of polypropylene, powder conglutination in pipelines easily leads to blockage, which can seriously affect the operation safety of the pipeline, so it is very important to detect and quantitatively evaluate the powder conglutination. This paper proposed an acoustic-ultrasonic (AU) quantitative evaluation method for powder conglutination detection in polypropylene production pipelines. A simulation model was developed to investigate the wave propagation characteristics of conglutinated layers with different areas and thicknesses using stress wave factors (SWF). Experiments were then conducted to develop a quantitative evaluation method for polypropylene powder conglutination. The results show that the relative attenuation coefficients of peak amplitude, peak-to-peak amplitude and the energy and the peak of the power spectrum all follow an approximate linear relation with the areas and thicknesses of the conglutinated layers. For either area or thickness evaluation, the energy or the peak of the power spectrum of AU signals has higher sensitivity than peak amplitude or peak-to-peak amplitude. Moreover, compared with conglutinated area evaluation, all the SWF models for thickness evaluation were more reliable, where the errors were all less than 7%. As a result, the AU technique is an effective means to detect powder conglutination in polypropylene production pipelines, and high sensitive and accurate quantitative evaluation is feasible with some of the stress wave factors of AU signals. (C) 2017 Elsevier B.V. All rights reserved.
摘要：
The threshold for binarisation of an ultrasonic C-scan image has a significant effect on flaw sizing. The 6 dB drop method shows poor performance when extracting flaws from an ultrasonic image. A novel method is presented to improve the accuracy of flaw sizing, whereby the flaws are separated from the background using digital image processing techniques including the Otsu method, bilateral filtering and dilation. The optimised thresholds for each flaw are calculated using the enumeration method to binarise the original image and then the flaw size is predicted using the 6 dB drop method. The dynamic threshold model for flaw sizing is developed on the basis of the generalised regression neural network theory, where the flaw peak value, the depth of flaw echo and the pre-judgement size of the flaw are all taken into consideration. Finally, ultrasonic C-scan experiments are conducted on an AZ80 magnesium alloy shell with natural flaws. Compared with the traditional 6 dB drop method, the experimental results show that C-scan imaging using the dynamic threshold method can reduce the average relative error of flaw sizing from 40.6% to 7.4%.
摘要：
Surface roughness of a sample has a great effect on the calculated grain size when measurements are based on ultrasonic attenuation. Combining modified transmission and reflection coefficients at the rough interface with a Multi-Gaussian beam model of the transducer, a comprehensive correction scheme for the attenuation coefficient is developed. An approximate inverse model of the calculated attenuation, based on Weaver's diffuse scattering theory, is established to evaluate grain size in polycrystals. The experimental results showed that for samples with varying surface roughness and matching microstructures, the fluctuation of evaluated average grain size was +/- 1.17 mu m. For polished samples with different microstructures, the relative errors to optical microscopy were no more than +/- 3.61%. The presented method provides an effective nondestructive tool for evaluating the grain size in metals with rough surfaces. (C) 2017 Elsevier B.V. All rights reserved.
作者机构：
[王向红; 王泽湘; 胡宏伟] Hunan Province Key Laboratory of Safety Design and Reliability Technology for Engineering Vehicle, Changsha University of Science &, Technology, Changsha, 410114, China;[徐娜] AVIC Beijing Institute of Aeronautical Materials, Beijing, 100191, China;[胡宏伟] Hunan Province Engineering Laboratory of Bridge Structure, Changsha, 410114, China
通讯机构：
Hunan Province Key Laboratory of Safety Design and Reliability Technology for Engineering Vehicle, Changsha University of Science & Technology, Changsha, China
