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Ultrasonics is the name given to the study and application of ultrasound, which is sound of a pitch too high to be detected by the human ear, i.e. of frequencies greater than about 18 kHz. Ultrasonic waves have a wide variety of applications over an extended range of intensity, including cutting, cleaning and the destruction of tissue at the upper extremity and non-destructive testing (NDT) at the lower end. A non-destructive test is one in which there is no impairment of the properties and performance in future use of the object under examination. With ultrasonic non-destructive testing, which is effectively a mechanical method, periodic mechanical stresses are applied to the object. It is essential that there have been no changes in dimensions and structure of the object when the test is completed. This can only be achieved when the maximum applied stresses do not exceed the elastic limit below which Hookels law is obeyed, so that the resultant strain is proportional to the applied stress. Hence it is necessary that the ultra- sonic intensity is sufficiently low for the elastic limit not to be exceeded. Ultrasonic testing consists effectively of the propagation of low amplitude waves through a material to measure either or both the time of travel and any change of intensity for a given distance. Applications include distance gauging, flaw detection and measuring parameters (such as elastic moduli and grain size) which are related to the material structure. Reasons for using ultrasonic as opposed to audible fre quencies include the following.
The main advantages of ultrasonic testing are:
• testing can be carried out from a single surface;
• a high degree of penetration is possible in many commonly-used materials, which is in contrast with the lower degree of penetration encountered with radiological testing of metals;
• accuracy in locating and measuring defects;
• the ability to detect and size very small defects; and
• compatibility with automatic scanning devices and with micro- processors and computers.
The principal drawbacks are these:
• Operators must be properly trained, highly experienced and possess a high degree of reliability and integrity. However, with 100 % automated testing, these requirements may be somewhat relaxed provided that the functions of the automated system are thoroughly understood.
• With manual operation over a large surface area only a small part of a surface can be scanned at a time, although this can be improved upon, where feasible, with the use of transducer arrays.
• A high degree of coupling between the transducer and the surface to be scanned is required, though much progress is now being made with the development of non-contact transducers.
The conversion of electrical pulses to mechanical vibrations and the conversion of returned mechanical vibrations back into electrical energy is the basis for ultrasonic testing. The active element is the heart of the transducer as it converts the electrical energy to acoustic energy, and vice versa. The active element is basically a piece of polarized material (i.e. some parts of the molecule are positively charged, while other parts of the molecule are negatively charged) with electrodes attached to two of its opposite faces. When an electric field is applied across the material, the polarized molecules will align themselves with the electric field, resulting in induced dipoles within the molecular or crystal structure of the material. This alignment of molecules will cause the material to change dimensions. This phenomenon is known as electrostriction. In addition, a permanently-polarized material such as quartz (SiO2) or barium titanate (BaTiO3) will produce an electric field when the material changes dimensions as a result of an imposed mechanical force. This phenomenon is known as the piezoelectric effect. Additional information on why certain materials produce this effect can be found in the linked presentation material, which was produced by the Valpey Fisher Corporation.