A modulation crack-detection technique: I. Instrumental method of implementation

The results of the development of a nonlinear modulation technique for diagnosing materials and structures containing flaws, which are characterized by linear (cavities) and nonlinear (cracks) acoustic properties, are presented. This technique is based on the modulation of the acoustic characteristics of flaws with low-frequency bending vibrations of an object that are accompanied by simultaneous coherent pulsed ultrasonic location of flaws. The pulsed location mode is used to determine the coordinates of a flaw, while the presence or absence of modulation of the received ultrasonic pulses allows the following problems to be solved: identification of the type of a flaw (a cavity or crack), detection of closed isolated cracks, and identification of the presence of cracks (microcracks) at early stages of their development. The literature on this measurement technique is reviewed. Specific features of measurements and two possible (hardware and software) methods for implementing this technique are discussed. The results of detecting cracks in model (metal plates) and real (pipeline fragments) objects via an instrumental technique are presented.