Methods of non-destructive testing are based on measurement of natural oscillation frequency (NOF) and despite the fact, that they are not widely used till now as methods of ultrasonic testing, the scope of their application is considerably expanded during the last few years. In compliance with generally accepted classification of acoustic methods of testing these methods include methods of free and forced oscillations, which may be further split into local and integral methods of control.
The method of free oscillations is one of the most ancient among methods of non-destructive control: tinkling sound, which appeared during tapping of earthenware, enabled to detect articles with cracks and other imperfections. This simplest method survived till nowadays and is used widely by salespersons throughout the whole world.
Along with the development of technology the scope of this method expanded and it became the most applicable method in railway transport. Practically at every railway station where the train stops for 10 minutes and more one can see from the car window that a man with a hammer subjectively, based on indications being clear for him only tries to detect imperfections, which may cause emergency situation. Methods based on NOF measurements are also widely used to determine physical and mechanical properties of different materials and articles. Even 200 years ago Young had used a method of forced oscillations to determine dynamic module of standard elasticity (Young modulus), i.e. one of the main parameters which specify elasticity properties of material.
The basis of acoustic method of non-destructive testing based on NOF measurements is the availability of correlation dependencies between physical and mechanical features such as hardness, porosity, density, strength, etc. and also operating characteristics of articles and manufacturing technologies.