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.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.