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首页> 外文会议>Annual Symposium on Quantitative Nondestructive Evaluation; 19980719-24; Snowbird,UT(US) >ULTRASONIC DETECTION OF FATIGUE CRACKS BY THERMO-OPTICAL MODULATION
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ULTRASONIC DETECTION OF FATIGUE CRACKS BY THERMO-OPTICAL MODULATION

机译：热光调制超声检测疲劳裂纹

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摘要

Positive identification of small fatigue cracks presents a challenging problem during nondestructive testing of fatigue damaged structures. First, it is important to distinguish fatigue cracks from primary geometrical features (e.g., nearby holes, corners, and edges) and secondary irregularities (e.g., uneven machining, mechanical wear, corrosion, etc.). Second, it is important to distinguish small fatigue cracks as early as possible after crack nucleation from intrinsic material inhomogeneities such as coarse grains, anomalous microstructure, second phases, precipitates, porosity, various types of reinforcement, etc. Generally, linear acoustic characteristics (attenuation, velocity, backscattering, etc.) are not sufficiently sensitive to very small fatigue cracks. On the other hand, in a great variety of structural materials even very small fatigue damage can produce very significant excess nonlinearity, which can be orders of magnitude higher than the intrinsic nonlinearity of the intact material. The excess nonlinearity is produced by the strong local nonlinearity of a microcrack whose opening is smaller than the particle displacement. Perhaps the simplest way to observe crack-closure under laboratory conditions is to ultrasonically monitor the opening and closing of fatigue cracks when subjecting the specimen to static or quasi-static external loading. The technical realization of the acousto-elastic method must incorporate two tasks. One is to find an effective way to generate crack-closure in the specimen, i.e., the "elastic" problem. The other is to find a way to monitor the resulting parametric modulation by ultrasonic means, i.e., the "acoustic" problem. The modulation stress may be generated through different ways such as external cyclic loading in a typical fatigue test or exploiting the inherent vibration of the structure itself during operation. The main disadvantage of using external mechanical loading is that usually the whole structure must be loaded, which requires very substantial forces and might cause additional damage in certain parts of the structure. More localized temporary stresses can be produced by simply cooling or warming the specimen to be tested.
机译：在疲劳破坏结构的非破坏性测试过程中，对小疲劳裂纹的正确识别是一个具有挑战性的问题。首先，将疲劳裂纹与主要的几何特征（例如，附近的孔，拐角和边缘）和次要的不规则性（例如，不均匀的加工，机械磨损，腐蚀等）区分开是很重要的。其次，重要的是要在形核后尽快尽早将小的疲劳裂纹与固有材料的不均匀性（例如粗晶粒，异常的微观结构，第二相，析出物，孔隙率，各种类型的增强材料等）区分开。通常，线性声学特征（衰减） ，速度，反向散射等）对很小的疲劳裂纹不够敏感。另一方面，在各种各样的结构材料中，即使很小的疲劳损伤也会产生非常明显的过量非线性，该非线性可能比完整材料的固有非线性高几个数量级。过度的非线性是由微裂纹的强局部非线性产生的，该微裂纹的开口小于粒子位移。在实验室条件下观察裂纹闭合的最简单方法也许是在使样品承受静态或准静态外部载荷时超声监测疲劳裂纹的打开和关闭。声弹方法的技术实现必须包含两个任务。一种方法是找到一种在试样中产生裂纹闭合的有效方法，即“弹性”问题。另一种是找到一种方法来监视通过超声手段产生的参数调制，即“声学”问题。可以通过不同的方式来产生调制应力，例如在典型的疲劳测试中通过外部循环载荷或在操作过程中利用结构本身的固有振动来产生调制应力。使用外部机械负载的主要缺点是通常必须对整个结构进行加载，这需要非常大的力，并且可能在结构的某些部分造成额外的损坏。通过简单地冷却或加热要测试的样品，可以产生更多的局部临时应力。

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