Fatigue cracks generated by repeated loads cause structural failures. Such cracks grow continuously and at an increasing speed owing to the concentration of stresses near the crack tips. Therefore, the early detection of fatigue cracks is imperative in the field of structural-health monitoring for the safety of structures exposed to dynamic loading. In particular, the detection of those cracks subjected to compression is known as a challenging problem in the nondestructive inspection area. The nonlinear ultrasonic modulation technique is effective for the detection of microcracks smaller than the size of a wavelength because this technique uses the deformation of waves passing through the crack surfaces. However, the technique has not been thoroughly verified for detecting cracks subjected to external forces. In this study, nonlinear ultrasonic modulation tests are performed on two types of crack specimens under compressive forces. The results show that in fatigue-cracked specimens, the cracks can be detected using modulated waves even under strong compressions. With artificial cracks, buckling occurs at a relatively low compression, and the amounts of modulated waves rapidly increase due to the bending of the specimen before buckling failure takes place. In this study, the crack detection methodology under compression is proposed and experimentally verified. The proposed method might be beneficial to find cracks under compression in various structural components.

中文翻译：

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裂纹闭合对调制波幅值的影响

重复载荷产生的疲劳裂纹会导致结构失效。由于裂纹尖端附近的应力集中，这种裂纹会以不断增加的速度连续增长。因此，在结构健康监测领域，为了确保承受动态载荷的结构的安全，早期检测疲劳裂纹势在必行。特别是，在无损检测领域中，检测受压裂纹是一个具有挑战性的问题。非线性超声调制技术对于检测小于波长尺寸的微裂纹是有效的，因为该技术利用穿过裂纹表面的波的变形。然而，该技术尚未彻底验证用于检测受到外力的裂缝。在这项研究中，在压缩力作用下对两种类型的裂纹试样进行了非线性超声调制试验。结果表明，在疲劳裂纹试样中，即使在强压缩下，也可以使用调制波检测裂纹。对于人工裂缝，屈曲发生在相对较低的压缩下，并且在屈曲破坏发生之前，由于试样的弯曲，调制波的数量迅速增加。在这项研究中，提出了压缩裂纹检测方法并进行了实验验证。所提出的方法可能有助于在各种结构部件中发现受压裂纹。即使在强烈的压缩下，也可以使用调制波检测裂缝。对于人工裂缝，屈曲发生在相对较低的压缩下，并且在屈曲破坏发生之前，由于试样的弯曲，调制波的数量迅速增加。在这项研究中，提出了压缩裂纹检测方法并进行了实验验证。所提出的方法可能有助于在各种结构部件中发现受压裂纹。即使在强烈的压缩下，也可以使用调制波检测裂缝。对于人工裂缝，屈曲发生在相对较低的压缩下，并且在屈曲破坏发生之前，由于试样的弯曲，调制波的数量迅速增加。在这项研究中，提出了压缩裂纹检测方法并进行了实验验证。所提出的方法可能有助于在各种结构部件中发现受压裂纹。