用变频扫描励磁方法定量检测裂纹

目的 实现对疵病的定量检测和对材料的定性检测。方法 通过改变磁场交变频率从而引致导体中的涡流发生变化，来观测工件材料的疵病。结果 实验证明文该方法对于提高检测精度是有效的。结论 用变频扫描励磁方法进行裂纹定量检测可提高检测的可靠性。     

基于兰姆波的管件直焊缝缺陷检测系统设计

板波是在板厚与波长相当的薄板中传播的波,就实用意义而言板波往往仅指兰姆波.本文主要设计利用施耐德PLC自动控制系统实现对一定长度薄壁压力管件的夹持、探头直焊缝轨迹行走,以及利用探头发射的兰姆波对管件直焊缝进行缺陷的检测与判别.利用C#语言编写上位机实现对缺陷信号的显示与处理.整体设计可靠性高,实用性强,满足了对薄壁压力管件直焊缝缺陷进行工业自动化检测的需求.     

超声检测信号的特征分析

目的 在无损检测中提高疵病实时检测和识别的正确性和可靠性。方法 提出了利用检测信号的波形特征分析进行疵病实时检测和识别的方法。结果 通过一应用实例,表明该方法对于提高检测可靠性是行之有效的。结论 用波形特征分析的方法可实现疵病的实时检测和提高检测精度。     

金属薄板中导波的模态识别和波速测定

按照Lamb波理论,板内导波由多阶对称波和反对称波组成。当波长远大于板厚时,利用经典平板理论对Lamb方程进行简化,此时板内主要有零阶对称波S0和反对称波A0两种模态。认识薄板内声波模态,不仅有助于分析声源性质,提取有效信号,而且可以提高声发射检测中时差定位的准确性。利用PAC公司的Fieldcal标准信号发生器和Nielsen Hsu断铅法作为信号源在0.7 mm薄钢板上进行声波传播实验。通过比较实测波速和理论波速甄别板内的声波模态。实验发现对称波S0实测波速和理论波速比较吻合,而反对称波A0的波速由于频散而较难精确测量。综合两种模态波的波速情况,声发射检测中利用对称波速度进行声源定位计算更加接近薄壁结构声波的传播状况。     

薄板腐蚀缺陷兰姆波成像检测的有限元模拟

基于A0模式兰姆波对薄板腐蚀缺陷进行成像检测,在频率一定时,由于腐蚀造成板厚的改变将进一步影响检测兰姆波速度的变化,通过检测兰姆波波速的变化可以对板材腐蚀类缺陷进行监测.根据A0模式兰姆波的频率特征,测量发射探头与接收探头之间的兰姆波走时,结合波的实际传播路径,计算出实际检测兰姆波的速度,采用联合迭代重建技术(SIRT)对走时数据进行群速度图像重建.基于有限元数值方法,采用A0模式兰姆波对薄铝板中的腐蚀类缺陷进行模拟检测,给出了成像检测结果.     

超声导波用于液层负载板损伤检测的数值模拟

液层负载薄板结构的损伤检测是结构无损检测和健康监测中的一个重要问题。为寻求合适的液层负载薄板结构损伤检测信号及分析损伤缺陷对检测信号的响应,求解了双侧液层负载薄板的频散方程,计算得到4 mm双侧水域负载铝板的频散曲线,并利用有限元模拟方法研究了缺陷位置、角度及大小对铝板中检测信号的影响。研究结果表明:中心频率小于100 kHz的S0模式漏Lamb波衰减因子趋近于0,适合长距离损伤检测。此外,当缺陷的存在造成板结构的不对称性时,声信号在缺陷处发生明显的模式转换,且转换出的A0模式信号的透射系数随缺陷位置到板厚中心距离的增加而减小,随缺陷与铝板中间面的角度和缺陷长度的增加先增大后减小,并随缺陷宽度的增加而增大。     

50 SiMnVB 钢椭圆凹坑疵病原因分析

基于某产品壳体用50SiMnVB钢在冲拔过程中毛坯内膛出现椭圆形凹坑疵病的现象,采用理化检测和工艺试验等手段对该疵病产生的原因机理进行了系统分析与验证,准确地找到了该疵病产生的原因,使生产顺利进行,有效保证了产品质量.     

某产品823钢壳体内膛疵病原因分析

目的研究某产品关键零件用壳体823钢在冲拔过程中,毛坯内膛出现侧壁拉裂和底部缩孔疵病的原因。方法通过理论分析毛坯在冲拔过程中的变形机理和金属流动规律,结合理化检测和工艺试验等手段,对该疵病产生的原因进行了系统分析与验证。结果准确找到了该疵病产生的原因。结论 823钢原材料存在缩孔残余及二次缩孔,是造成弹体毛坯底部缩孔疵病和侧壁拉裂疵病的根本原因;通过靶场射击动态考核试验和增加超声波探伤检验工序的产品,即使存在缩孔残余及二次缩孔,也不影响产品使用性能。     

有对称液层负载时各向异性板中Lamb波的传播

1　引　言　　 L amb波是固体薄板中传播的一种弹性波 ,最初有关 Lamb波的微传感的理论研究大都只局限在各向同性板材中 ,由于在各向异性压电板上可以高效激发 L amb波 ,灵敏度高 ,体积小 ,性价比高等特点 ,因而各向异性材料有更大的实用性 ,实验上已有各向异性压电板传感器的尝试 [1] ,特别是在生物传感领域 ,由于能检测到病毒等 ,因而特别具有吸引力。在理论上 S.G.Joshi[2 ] ,Chimenti[3 ] ,Vinay Day-al[4 ] 都对各向异性板中的 L amb波进行了理论推导和数值计算 ,为了对各向异性材料微传感性的机理有进一步深入了解 ,本文研究若干…     

某炮弹弹体内膛裂纹原因分析

目的某弹体作为某炮弾一重要零件,材料采用9260钢。研究其在冲压过程中,毛坯内膛出现侧壁拉裂和底部裂纹疵病的原因。方法利用理论分析毛坯在冲压过程中的变形机理和金属流动规律,结合理化检测等手段,对弹体疵病产生的原因进行了系统分析。结果通过对毛坯成形原理分析和理化检测分析,对裂纹产生的原因进行逐一排查,最终准确找到了该弹体疵病产生的原因。结论弹体用9260钢原材料存在缩孔残余及沿晶低熔点物质,是造成弹体毛坯侧壁拉裂和裂纹的根本原因。     

板材厚度变化对Lamb波透射系数的影响

对Lamb波与厚度变化的板材之间的相互作用进行了分析。针对S0模式Lamb波入射情况,利用混合边界元模型计算了台阶状散射体的透射系数与厚度变化的关系,并进行了实验验证。结果表明S0模式的透射系数能够很好的反映板材的厚度变化,从而为该类材料的定量无损检测提供一定的理论支撑,在实际工业中具有重要的应用意义。     

用于多色光的波片的设计

在偏振光路中有时需要交替使用不同波长的光束,为使光路保持最佳状态,常需要根据不同的工作波长选用相应波长的波片。为此提出了多波长波片的概念,给出了多波长波片的设计方法,具体设计了有实用价值的双波长四边之一波片,并成功地应用于一个实际的光学系统中,从而验证了多皮长波片设计思想的正确性。虽然多波长波片不一定能完全等效于多个严格意义上的单波长波片,但在一定的误差范围内所设计的多波长波片是能满足实际需要的,从而避免了频繁更换不同波长波片的麻烦。     

Free-surface wave interaction with a thick flexible dock or very large floating platform

In this paper the recently developed semi-analytic method to solve the free-surface wave interaction with a thin elastic plate is extended to the case of a plate of finite thickness. The method used is based on the reformulation of the differential–integral equation for this problem. The thickness of the plate is chosen such that the elastic behavior of the plate can be described by means of thin-plate theory, while the water pressure at the plate is applied at finite depth. The water depth is finite.     

Analytical solutions for sagittal plane waves in three-layer composites

The analytic dispersion equations for the symmetric and antisymmetric saggital plane plate modes of a three-layer composite system are presented. The composite consists of a solid isotropic plate sandwiched between two acoustically thin (<0.02 lambda) isotopic solid layers, where lambda is the acoustic wavelength. The thin layers are considered either as the mass loading or the chemical selective coating layers for the plate wave sensors. Explicit formulas which identify the contributions of the elasticity and inertia effects for the phase velocity and mass loading sensitivity of the lowest symmetric (S(0)) and antisymmetric (A(0)) mode for the case where the thickness of the composite plate is much less than lambda are obtained. The amounts by which the elasticity of the thin layer and the inertia decrease the mass loading sensitivity is found for both sensors. It is also found that the sensitivity of the A(0) mode significantly depends on the operating frequency but that of the S(0) mode does not. Specific examples are given for the case of a fused silica plate sandwiched by two thin lucite layers.     

Inversion of Functional Graded Materials Elastic Properties from Ultrasonic Lamb Wave Phase Velocity Data Using Genetic Algorithm

An inverse method based on genetic algorithm (GA) is presented to determine the elastic properties of functional graded materials (FGMs) plate from lamb wave phase velocity data. The Legendre polynomial expansion method is used as a forward solver to calculate the phase velocity dispersion curves of lamb wave in FGMs plate, which has a distribution function to express the inhomogeneity of materials’ elastic properties. The properties of FGMs plate can be characterized by minimizing the standard deviations between the actual and calculated phase velocities of lamb waves. By using GA, the elastic parameters of FGMs plates with three different distribution functions are inversed, and the convergence and deviations of the inversion are also discussed. The investigation shows that this inverse method is efficient and accurate for determining the materials parameters of FGMs and the deviation can be controlled below 5 %.     

A boundary element model for a partially piston-type porous wave energy converter in gravity waves

The present paper applies the multi-domain boundary element method (BEM) to investigate the performance of a partially piston-type porous wave energy converter (WEC), which consists of a solid wall, a vertical porous plate, a transmission bar, a rigid block constrained by rollers, a spring, and a damper. This WEC is subjected to a dynamic loading external source from a wave attack. For this wave-body interaction problem, a single-degree-of-freedom (SDOF) system is developed to describe the response of the present WEC. Linear wave theory governs the entire fluid domain, which is divided into three regions by a pseudo boundary and the vertical porous plate. Darcy's law applies to the porous plate. Examples are shown to illustrate the wave reflection from the WEC, the response to wave loading, and the instantaneous mechanical power from the wave.     

Laser ultrasonic study of Lamb waves: determination of the thickness and velocities of a thin plate

Transient Lamb waves in a thin copper plate were generated and measured using a laser ultrasonic setup. A two-dimensional Fourier transform method has been employed to analyse the dispersion curves. This allowed a clear identification of multi-mode Lamb waves. Fitting of the dispersion curve allowed a direct determination of the thickness and bulk velocities, as well as the elastic constants. The obtained experimental results of Lamb waves on a copper plate show excellent agreement with the theoretical model.     

Finite element modelling of lamb waves propagation in 2D plates and thin sheets for damage detection

Structural health monitoring is an evolving technology applied to identify, locate and quantify severity of damages in structures before failure. Lamb waves have become a keen interest for inspection since they can be used to monitor a large area from one single location. The objective of this research is to simulate lamb wave response using finite element method and its application to crack detection and identification in thin metallic engineering structures. Two types of specimen i. e. two‐dimensional aluminium plate and thin aluminium sheets are simulated using commercially available finite element package ABAQUS. Initially phase velocity and group velocity dispersion curve are plotted for aluminium material. Thereafter simulation of individual specimens with cracks and without cracks is performed. Simulation results were compared and validated with actual results and were found to be in reasonably good agreement. This is certainly done by employing group velocity and time of flight for the distance travelled between the monitoring point and crack position. Assessment of lamb waves sensitivity to various sizes and shapes of cracks like rectangular and circular are also investigated and its effect on the structure is discussed in detail.     

Air-Coupled Nondestructive Evaluation Dissected

This work develops a two-dimensional theoretical model to simulate the behavior of a fully non-contact air-coupled nondestructive evaluation system for a thin isotropic plate. The model is divided into transmission, guided wave propagation and reception phase. The validation of the complete model was carried out by modeling the same system by means of finite element method using a Multiphysics software. In addition, the dependency of the generated Lamb waves on different transmitter’s parameters and incidence angle is thoroughly investigated. The results of the acoustic pressure excited by the transducer, the out-of-plane velocity amplitudes for the generated first antisymmetric Lamb wave mode, and the radiated pressure from the plate caused by the leaky Lamb wavefield were all compared between the two models and a reasonable degree of similarity was found.     

Noncontact Air-Coupled Guided Wave Ultrasonics for Detection of Thinning Defects in Aluminum Plates

Ultrasonic guided waves are gaining increasing attention for the inspection of platelike and rodlike structures. At the same time, inspection methods that do not require contact with the test piece are being developed for advanced applications. This paper capitalizes on recent advances in the areas of guided wave ultrasonics and noncontact ultrasonics to demonstrate a superior method for the nondestructive detection of thinning defects simulating hidden corrosion in thin aluminum plates. The proposed approach uses micromachined gas (air)-coupled capacitive transducers for the noncontact generation and detection of guided plate waves. Interesting features in the dispersive behavior of selected guided modes are used for the detection of plate thinning. It is shown that mode cutoff measurements provide a qualitative detection of thinning defects, while frequency shift measurements allow quantification of thinning depth. Measurement of the mode group velocity can be also used to quantify thinning depth. Similarly, thinning length can be determined by mode time-of-flight measurements.