基于阵列超声成像法的混凝土裂缝深度检测

针对传统超声法检测混凝土裂缝深度时人为影响因素较多,检测效率十分低下的问题,引入了阵列超声成像法,研究了阵列超声成像法检测裂缝深度的原理及其计算方法,并在实验室设计制作了一块预设7条不同深度裂缝的素混凝土试件,进行了跨单裂缝和试件整体成像检测试验。试验结果表明:阵列超声成像法根据缝底反射超声波的传播时间来检测裂缝深度,生成的二维图像可以显示缝底位置,但无法显示缝身;该检测法的检测精度较高,并且随着裂缝深度的加深逐渐提高;检测时应考虑边界效应,否则无法准确检测裂缝深度;检测时存在盲区,当缝底位于盲区内时会出现漏检。     

基于超声横波的混凝土结构无损检测数据成像技术

目前超声波成像主要采用合成孔径聚焦技术(SAFT),成像质量很大程度上依赖于超声波速估计的准确性和噪声压制的效果。超声波和地震波在固体介质中的传播特征具有相似性,故基于成熟的地震数据处理技术,建立了一套适用于商用超声横波反射检测仪的数据成像处理方法。基于炮检互易定理,对仪器的单边观测数据进行重构而转换为双边观测数据,再对双边观测数据采用地震数据处理中的多次叠加和克希霍夫偏移技术进行成像。大型混凝土道床模型应用案例表明,提出的成像技术完全适用于商用超声横波检测仪的数据成像,与仪器配套的国外商业合成孔径聚焦技术成像结果相比,成像分辨率有明显改善,并对混凝土层位界面和内部钢筋结构的反应更为精细。     

Ultrasonic imaging of concrete using scattered elastic wave modes

Two different techniques for ultrasonic imaging of concrete are presented, namely a weighted sum technique and a statistical technique. The techniques utilize both the scattered elastic compressional and the mode converted shear wave fields. Since the conventional imaging techniques utilize either the compressional or the shear wave mode to generate the subsurface image, information contained in the scattered ultrasonic field is partially utilized. The proposed techniques intend to use more information contained in the scattered field leading to an improvement in the final image. Synthetic full matrix data is generated using the finite difference in time domain scheme for simulation of elastic wave propagation in concrete with embedded rebars and aggregates. The proposed imaging techniques are then used to generate the subsurface image of the medium. Experiments were also performed to generate full matrix data on two concrete slabs with different geometry, sizes and distribution of rebars. The applicability of the proposed imaging techniques is demonstrated on the experimentally acquired data. The images when compared to the ones generated by the conventional single mode approach, show improvement in resolution and distinction between the embedded features, e.g. closely spaced rebars. It is also observed that the weighted sum technique performs slightly better in comparison to the statistical technique with respect to suppression of background noise.     

Assessment of porosity of mortar using ultrasonic Rayleigh waves

The relationship between Rayleigh wave velocity at ultrasonic frequencies and porosity is investigated in dry and fully saturated mortar. Porosity is varied by changing the water/cement ratio. Rayleigh wave (RW) pulse velocity is measured accurately using air-coupled ultrasonic transducers as transmitter and receiver. The experimental results are compared with two single-phase models. Due to relatively weak correlation between RW velocity and porosity, measured data and the models are compared using statistical hypothesis tests. Although the parameters of the model are not accurately predicted, these tests conclude on a reasonable agreement between measured data and the models. This result can be useful to develop simplified models of wave propagation in concrete, or to estimate porosity in concrete cover from ultrasonic measurements.     

改进层析算法在混凝土超声成像中的应用

混凝土质量超声CT检测中,为了对混凝土质量进行准确评估,对成像精度有很高要求,由于混凝土自身结构的不均匀性,经典算法很难满足高精度层析计算要求,容易导致检测结果失效。提出了一种基于混沌搜索和自然权函数的改进模拟退火算法,首先采用弯曲射线追踪方法确定最终路径,而后引入了混沌搜索和自然权函数对路径进行反演修正,改善模拟退火算法准确度,最后对结构进行层析计算和成像。数值仿真实验表明:改进算法计算结果更加准确有效,成像分辨力得到了明显改善。     

超声波检测受硫酸盐腐蚀混凝土强度的研究

用超声波对测法对受硫酸盐腐蚀的混凝土强度进行了检测。结果表明,超声波在不同腐蚀程度的混凝土中的传播速度是不同的,传播速度与受腐蚀混凝土的抗压强度具有一定的回归关系。在分析实验数据的基础上,建立了超声声速与受腐蚀混凝土强度之间的函数关系,为用超声波法检测受腐蚀混凝土强度提供了数学依据。     

3D simulations and experiments of guided wave propagation in adhesively bonded multi-layered structures

Understanding guided wave propagation in multi-layered plates and interaction with discontinuities can be difficult, as well as the interpretation of the ultrasonic signals. Propagation of guided waves can be studied analytically solving the equations of motion with the proper boundary conditions; nevertheless analytical models can be difficult to solve for complex multi-layered structures or having inner discontinuities. The problem can be efficiently studied using numerical techniques. Simulation of guided wave propagation in multi-layered structures, for ultrasonic waves in the MHz range, is solved here with the finite element analysis based on an explicit integration rule to solve the equations of motion in a dynamic analysis. Simulation allows a better understanding of propagation and interference phenomena by creating a window of observation in the multi-layered plate. Numerical results determined for a three-layer Al plate, without or with discontinuities, matched very well with experiments, providing an efficient tool to visualize and extract significant information in the transmitted waves and to optimize wave mode and configuration for a rigorous ultrasonic inspection.     

Acoustoelastic effect in concrete material under uni-axial compressive loading

This study deals with the general matter of non-destructive evaluation of pre-stressed structures in civil engineering. Usually such structures are composed of concrete and are steel reinforced. Proposed idea is the evaluation of mechanical stress state of a concrete body (instead of steel cables) via ultrasonic non-destructive evaluation (NDE), by using the link between ultrasonic velocities and mechanical stresses provided by the acoustoelasticity theory. Velocities of the ultrasonic waves (longitudinal and transversal with different polarizations) are observed during propagation through a concrete body submitted to uni-axial loading (compressive testing). Obvious variations in velocity are found depending on the mechanical stress state (e.g. Δc=92 m/s at σ=16 MPa for longitudinal waves). Thus acoustoelastic behavior of concrete is demonstrated. Further analyses provide acoustoelastic coefficients of concrete about ten times higher than the common ones of steel. The feasibility of stress evaluation using ultrasounds in concrete structures is proved under laboratory conditions.     

流体多参数超声波测量仪设计

针对目前流体测量仪测量参数单一的问题,提出了一种用超声波技术测量流体多参数的方法。当传播距离不变时,利用超声波在流体中的传播速度间接得到流体的体积流量、密度、质量流量。由于流体流量、密度等参数均受温度影响,故设计过程中以温度作为参考信号,为所求参数就行实时补偿。同时,测量仪以FPGA为控制核心,实现高速信号采集与处理,以软件插补算法实现超声波传播时间的精密计算,使测量结果更加稳定可靠。     

插入式超声波温度计设计

基于超声波在棒状固体介质中传播速度与温度的关系,运用高速高分辨率的硬件电路和软件细分插补算法,精确地测量超声波的回波时间,从而精确地测量超声波的传播时间并计算出对应的温度值.根据在固体介质中传播时间和温度的关系建立数学模型,通过已测得的传播时间求出对应的温度值.设计的温度计结构简单,能够实时地测量液体温度的变化,并且能在各种高温环境中广泛应用.     

Radar survey of concrete elements: Effect of concrete properties on propagation velocity and time zero

Ground-penetrating radar (GPR) is widely used to detect and locate steel reinforcement in concrete. However, there are many factors which need to be considered, if accurate depth measurements are to be made. Knowledge of the reference time zero and the propagation velocity of radar waves is essential. This paper presents experimental results on the effect of the physical properties of concrete on the time zero and the propagation velocity of both direct and reflected waves radiated by a ground-coupled antenna. Laboratory experiments were conducted on reinforced concrete slabs and involved various porosities, water contents and depths of steel reinforcing bars. The results of this research demonstrate clearly that the direct wave, which is radiated laterally, propagates at the same velocity as the reflected waves. The implication of such a result is that the direct wave time position is not constant and is affected by the concrete porosity and water content. Therefore, in cases where the direct wave is used as the time reference, this paper proposes to improve delay measurements by applying a delay correction corresponding to the travel time of the direct wave and taking account of the transmitter–receiver offset and of the propagation velocity of radar waves in concrete.     

Influence of the depth and morphology of real cracks on diffuse ultrasound in concrete: A simulation study

The aim of the present paper is to simulate the propagation of diffuse ultrasonic energy in concrete in the presence of a real crack. The numerical model is presented and validated by the comparison with experimental data from the literature. Unlike most of the studies which consider a crack as a notch, a realistic crack morphology exhibits partial contacts along its lips. These contacts are modeled in order to study their influence on the diffusion parameters. The feasibility of determining the contact density of the crack is shown, revealing practice implications for non-destructive crack sizing and imaging in concrete.     

Characterization of surface cracks with Rayleigh waves: a numerical model

In the non-destructive testing of concrete structures, the use of Rayleigh waves shows some advantages to characterise surface cracks: easiness of excitation and recording, access to only one surface of the structure required, great spectral sensitivity to the propagation medium…. But the behaviour of Rayleigh waves on surface defects in concrete is difficult to perceive in the field, even if the dependence of the diffraction pattern on the crack's geometrical features is significant.A numerical model is adapted from earth physics in order to better understand the influence of the crack geometry on Rayleigh-wave propagation. This model, based on an indirect boundary element method, calculates the three-dimensional seismic response of two-dimensional structures. Synthetic seismograms are obtained for the propagation of a Rayleigh wave across various crack geometries. The variations of spectral ratios between the transmitted and incident waves are studied as a function of the crack depth. They are used to design an efficient procedure for the determination of crack depths.     

A time-of-flight mapping method for laser ultrasound guided in a pipe and its application to wall thinning visualization

Piping systems in nuclear power plants (NPPs) are susceptible to flow-accelerated corrosion (FAC) that results in local wall thinning of the piping system. FAC can cause severe piping failure such as leakage and rupture. In this study, a reliable ultrasonic time-of-flight (ToF) mapping system in the pipe is proposed with the goal of improving ultrasonic NPP pipe inspection. The system consists of a high-speed laser ultrasonic scanning system (50 mm/s at intervals of 0.5 mm) and two ultrasonic sensors. The ToF mapping algorithm uses mode identification based on wavelet transform, an automatic threshold setting method based on the statistics of the spatial noise map, and two-dimensional (2-D) ToF extraction based on the threshold crossing time method. The Lamb-like waves guided in the wall of a stainless steel (SUS) straight pipe were simultaneously interrogated by the proposed two-channel laser ultrasonic system. The proposed ToF mapping algorithm enabled the generation of a smooth ToF distribution within the beam incidence angle of ±60° in the curved surface of the pipe. Then a ToF mapping method was applied to a wall-thinned pipe, and an ultrasonic wave propagation movie was generated to help understand the complex ultrasonic propagation pattern. Based on an understanding of the ToF change in the wall-thinned region, and on the mode collapse and attenuation that occurred because of the ultrasonic bottleneck phenomenon, the wall-thinned region was evaluated. Since this method allows in-situ and automatic field inspection, it can contribute to NPP pipe health management.     

Characterization of ultrasonic Rayleigh surface waves in asphaltic concrete

This research examines the application of ultrasonic Rayleigh surface waves to the non-destructive evaluation (NDE) of asphaltic concrete in the frequency range 40–100 kHz. Two ultrasonic parameters, the phase velocity and amplitude attenuation, are considered to be sensitive to local compositional and structural variations and are measured. A wedge technique is used to generate Rayleigh surface waves in an asphaltic concrete beam with uniformly distributed aggregate, and this technique is shown to be effective in launching Rayleigh waves in this highly viscoelastic (absorptive) and heterogeneous medium. Three different ultrasonic detection setups using contact and non-contact transducers as receivers are examined and their results are compared. The experimental results show that the wedge generation technique along with an air-coupled receiving transducer with a finite-size aperture is the most reliable for characterizing Rayleigh waves in asphaltic concrete. It is also experimentally demonstrated that the proposed setup may be used to detect aggregate segregation in asphaltic concrete.     

Longitudinal waves for evaluation of large concrete blocks after repair

The present paper discusses the repair characterization of large concrete blocks based on the propagation of low-frequency elastic waves. A common repair method applied in damaged structures is cement injection. As a result, cracks and voids are eliminated; however, due to the poor initial mechanical properties of fresh cement, pulse velocity does not recover immediately, complicating the assessment. Results of P-wave velocity measured through the thickness of a large structure show that only after a suitable hydration period wave parameters increase. Laboratory tests were also conducted in order to extract information under more controlled conditions. It is concluded that velocity, frequency and amplitude of the ultrasonic waves are sensitive to the repair condition and the hardening of the injection material, while slight dispersion trends are eliminated after proper repair.     

A simple device for detecting impact time in impact-echo testing of concrete

The objective of this paper is to describe a simple device for detecting the time of an impact and to demonstrate its efficiency in measuring the depth of surface-opening cracks in concrete. Surface-opening cracks in concrete are relatively deep so that a mechanical impact instead of an ultrasonic transmitter is used to generate stress waves with enough energy for successful evaluation. A technique called the time-of-flight diffraction technique with impact-generated stress waves is an efficient method for detection of surface-opening cracks in concrete. Because the initial time of the impact is unavailable in a mechanical impact device, two receivers are employed in the test. One of the receivers is used to determine the time of the impact indirectly. The present study developed a simple device to record the time of impact initiation. The device comprises a conductive impact device, a sensing film, and an auxiliary circuit. With the device, stress waves are generated and the time of impact is detected so that the evaluation work needs only a single receiver. Laboratory studies were carried out on concrete specimens containing cracks with depths of 0.158 and 0.90 m. Field studies were performed on a dam damaged by an earthquake. Experimental results show that the new device can be used to accurately determine the crack depth in concrete. Using the device, the cost and effort for evaluation and signal analysis are reduced, which largely enhances the evaluation efficiency.     

奥氏体不锈钢焊缝超声TOFD检测中声波传播特性分析及其应用

由焊缝组织引起的声波衰减及散射严重影响奥氏体不锈钢焊缝的超声TOFD检测效果,这在无损检测方面是亟待解决的难题.文中分析焊缝中各角度检测声束形成的缺陷衍射波幅度及检测信号信噪比特征,揭示声波传播特性的影响因素.结果表明,不锈钢焊缝中的超声波传播具有明显的各向异性特征,柱状晶生长方向与声束入射方向的夹角对检测波衰减及散射具有显著影响;文中结合金相结构图建立焊缝组织模型进一步解释了声波传播各向异性的形成机理;并以此为基础提出一次波检测方法和二次波检测方法,获得了较好的检测效果.     

钢管混凝土拱桥超声检测技术研究

阐述了超声检测钢管混凝土的基本原理。针对目前钢管混凝土普遍存在脱空问题,为保证巫山长江大桥混凝土拱桥满足工程要求,应用超声波技术,对钢管混凝土进行了全面的检测。发现部分测试区域内声时变长,频率损失增大,波形畸变严重。深入分析,钢管混凝土存在一定程度的脱空,在各抽样钻孔点,声学参数变化反映的脱空值与实际值相吻合,证明超声技术用于钢管混凝土拱桥缺陷检测是可行的。     

Ultrasonic measurements of undamaged concrete layer thickness in a deteriorated concrete structure

Ultrasonic wave propagation in deteriorated concrete structures was studied numerically and experimentally. Ultrasonic single-side access immersion pulse-echo and diffuse field measurements were performed in deteriorated concrete structures at 0.5 MHz center frequency. Numerically and experimentally it is shown that the undamaged layer thickness in a deteriorated concrete structure is measurable using pulse-echo measurements when the deterioration depth is larger than the wavelength. The signal overlapping, which occurs in the thin deteriorated layers, can be overcome using diffuse field measurements or a pattern matching technique. The ultrasonic experimental data were shown to be in good agreement with the widely used phenolphthalein test for concrete degradation.