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.
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