Nondestructive Evaluation of the Depth of Cracks in Concrete Plates Using Surface Waves
Nondestructive Evaluation of the Depth of Cracks in Concrete Plates Using Surface Waves
复制标题
利用表面波无损评估混凝土板裂纹深度
DOI：
10.1121/1.381866
复制
发表时间：
2009
影响因子：
2.4
通讯作者：
Yanjun Yang
中科院分区：
文献类型：
--
作者：
Yanjun Yang
混凝土结构通常可以建模为板，例如桥梁、隧道壁和管道。混凝土结构的近表面损伤主要表现为开裂。混凝土表面破裂裂缝影响混凝土性能和结构完整性，因此，裂缝深度的无损评估对于结构监测、加固和修复具有重要意义。另一方面，材料阻尼是材料试件和结构动力分析的基本参数。监测阻尼变化对于评估材料状况和结构劣化是有用的。本研究的主要目的是开发新的方法，表面断裂裂纹的深度评估和阻尼混凝土板的评估。基于波传播的非破坏性技术是有用的，因为它们是非侵入性的，高效的和成本有效的。以前的研究，在混凝土中的表面破裂裂缝的深度评估使用衍射压缩波（P波）。然而，表面波对于近表面缺陷的表征表现出更好的性质，因为（a）表面波主导表面响应，它们携带67%的波传播能量，并且由于传播波前是圆柱形的，因此呈现较低的几何衰减;以及（B）瑞利波（R波）的穿透深度取决于它们的频率。大部分R波能量集中在其波长的三分之一的深度。通过表面破裂裂纹的R波的传输取决于裂纹深度;这种深度敏感性是所谓的傅立叶传输系数（FTC）方法的基础。R波只存在于半空间（一个无摩擦表面）中;而在板（两个无摩擦表面）的情况下，会产生兰姆模式。基本兰姆模式在高频下表现得像R波，因为它们的波长相对于板厚度很小。在标准的FTC方法中不考虑兰姆模式，并且FTC方法还受接收器之间的所选间距的影响。FTC计算需要使用明确的时间窗口来识别表面波的到达，并选择可靠的频率范围。本研究提出了理论，数值和实验结果。讨论了Lamb模的理论问题，导出了Lamb模的理论传递函数，该函数可用于研究Lamb模在时域和频域中随距离的变化。小波变换的最大振幅随距离而变化，因为Lamb模式的色散和较高Lamb模式的参与。通过数值模拟研究了兰姆波在不同深度的表面裂纹中的传播。表面响应被发现是占主导地位的基本兰姆模式。使用iii二维傅里叶变换，提取入射，透射和反射的基本兰姆模式。计算了透射模式和入射模式之间的透射比，该透射比在（d/λ）= 0.1至1/3的范围内对归一化到波长（λ）的裂纹深度（d）敏感。针对小波透射系数法的局限性，提出了一种新的混凝土表面断裂裂缝深度评价方法。WTC方法给出了与裂纹深度相关的全局系数，该系数不需要时间窗和预先选择频率带宽。为了减少由于非等间距配置而存在于FTC方法中的波反射的影响，在WTC方法中使用新的等间距配置。兰姆模色散的影响也被降低。在实验室测试中，使用中心频率为50 kHz的超声波发射器作为源; 50 kHz频率适合于测试的混凝土板（厚度80 mm），因为基本兰姆模式已经收敛到瑞利波模式。汉森管道和预制件公司也在现场使用了这种新方法，剑桥，安大略，加拿大，它显示了实际应用的潜力。通常，材料阻尼的评估比波速的测量更困难;结构振动的动态响应和衰减主要由阻尼控制，并且通常使用模态分析技术来评估阻尼，这需要相当大的努力。现有的基于表面波的方法，使用傅里叶变换来测量材料阻尼，但是谱比方法需要一个明确的时间窗口来提取表面波的到达，此外，谱比的斜率对于不同的频率范围是不同的，因此需要确定可靠的频率范围。这项研究使用小波变换来测量板中的材料阻尼，其中既不需要显式时间窗口，也不需要预先选择频率带宽。测量的材料阻尼表示由源确定的频率范围的平均阻尼。数值和实验结果表明，良好的协议和实际应用的潜力。
Concrete structures can often be modeled as plates, for example, bridges, tunnel walls and pipes. Near-surface damage in concrete structures mostly takes the form of cracking. Surface-breaking cracks affect concrete properties and structural integrity; therefore, the nondestructive evaluation of crack depth is important for structural monitoring, strengthening and rehabilitation. On the other hand, material damping is a fundamental parameter for the dynamic analysis of material specimens and structures. Monitoring damping changes is useful for the assessment of material conditions and structural deterioration. The main objective of this research is to develop new methodologies for depth evaluation of surface-breaking cracks and the evaluation of damping in concrete plates. Nondestructive techniques based on wave propagation are useful because they are non-intrusive, efficient and cost effective. Previous studies for the depth evaluation of surface-breaking cracks in concrete have used diffracted compressional waves (P-waves). However, surface waves exhibit better properties for the characterization of near surface defects, because (a) surface waves dominate the surface response, they carry 67% of the wave propagation energy, and present lower geometrical attenuation because the propagating wave front is cylindrical; and (b) the penetration depth of Rayleigh waves (R-waves) depends on their frequency. Most of the R-wave energy concentrates at a depth of one-third of their wavelengths. The transmission of R-waves through a surfacebreaking crack depends on the crack depth; this depth sensitivity is the basis for the so-called Fourier transmission coefficient (FTC) method. R-waves only exist in a half-space (one traction-free surface); whereas in the case of a plate (two traction-free surfaces), Lamb modes are generated. Fundamental Lamb modes behave like R-waves at high frequencies, because their wavelengths are small relative to the plate thickness. Lamb modes are not considered in the standard FTC method, and the FTC method is also affected by the selected spacing between receivers. The FTC calculation requires the use of an explicit time window for the identification of the arrival of surface waves, and the selection of a reliable frequency range. This research presents theoretical, numerical and experimental results. Theoretical aspects of Lamb modes are discussed, and a theoretical transfer function is derived, which can be used to study changes of Lamb modes in the time and frequency domains as a function of distance. The maximum amplitude of the wavelet transform varies with distance because of the dispersion of Lamb modes and the participation of higher Lamb modes in the response. Numerical simulations are conducted to study the wave propagation of Lamb modes through a surface-breaking crack with different depths. The surface response is found to be dominated by the fundamental Lamb mode. Using the iii 2D Fourier transform, the incident, transmitted and reflected fundamental Lamb modes are extracted. A transmission ratio between the transmitted and incident modes is calculated, which is sensitive to crack depths (d) normalized to the wavelength (λ ) in a range (d/λ ) = 0.1 to 1/3. A new wavelet transmission coefficient (WTC) method for the depth evaluation of surface-breaking cracks in concrete is proposed to overcome the main limitations of the FTC method. The WTC method gives a global coefficient that is correlated with the crack depth, which does not require time windowing and the pre-selection of a frequency bandwidth. To reduce the effects of wave reflections, which are present in the FTC method because of the non-equal spacing configuration, a new equal spacing configuration is used in the WTC method. The effects of Lamb mode dispersion are also reduced. In laboratory tests, an ultrasonic transmitter with central frequency at 50kHz is used as a source; the 50kHz frequency is appropriate for the concrete plate tested (thickness 80mm), because the fundamental Lamb modes have converged to the Rayleigh wave mode. The new method has also been used in-situ at Hanson Pipe and Precast Inc., Cambridge, Ontario, Canada, and it shows potential for practical applications. In general, the evaluation of material damping is more difficult than the measurement of wave velocity; the dynamic response and attenuation of structural vibrations are predominantly controlled by damping, and the damping is typically evaluated using the modal analysis technique, which requires considerable efforts. The existing methods based on surface waves, use the Fourier transform to measure material damping; however, an explicit time window is required for the spectral ratio method to extract the arrival of surface wave; in addition, a slope of the spectral ratio varies for different frequency ranges, and thus a reliable frequency range needs to be determined. This research uses the wavelet transform to measure material damping in plates, where neither an explicit time window nor the pre-selection of a frequency bandwidth are required. The measured material damping represents an average damping for a frequency range determined by source. Both numerical and experimental results show good agreement and the potential for practical applications.