Ultrasonic transducer calibration
When a material is placed under stress, small changes within the specimen release ultrasonic energy in the form of stress waves. The change may, for example, be a dislocation movement or the advancement of a crack tip. These ultrasonic pulses are termed Acoustic Emission and may be detected at the material surface by ultrasonic transducers. The detected pulse shape is related to the generating source, to the material geometry through which the pulse propagates and to the response of the ultrasonic transducer used to detect the waves. Work has been carried out to measure both the effect of wave propagation and to calibrate the response of ultrasonic transducers. Three types of ultrasonic wave may exist in a material with a non-zero shear modulus; these are longitudinal waves, shear waves and surface or Rayleigh waves. In a large number of specimen geometries, the surface wave has the largest amplitude. The response of a transducer to this wave is therefore very important. Most transducers respond to the out of plane motion of a material surface carrying ultrasonic waves. Therefore, to successfully calibrate a transducer, some absolute measurement of the out of plane motion due to surface waves must be made. An interferometer has been designed and constructed for this purpose. The calibration of ultrasonic transducers has enabled some development work to be carried oLt on high-fidelity piezoelectric transducers and on piezomagnetic transducers. It is not always possible to measure an ultrasonic pulse directly with a calibrated interferometric detector and therefore to enable a wider range of propagation problems to be investigated, various methods of ultrasonic pulse generation have been studied. These artificial sources of acoustic emission have included brittle fracture, laser impact and stimulation by piezoelectric transducers. This work has enabled theoretical calculations on pulse propagation to be verified.