Abstract:
The subject of this thesis is the development of optical instrumentation for surface strain
measurement of non-planar objects. The speckle interferometry technique of
shearography is used to perform quantitative measurements of surface strain on nonplanar
objects and to compensate these measurements for the errors that are due to the
shape and slope of the object.
Shearography is an optical technique that is usually used for defect location and for
qualitative strain characterisation. In this thesis a multi-component shearography system
is described that can measure the six components of displacement gradient. From these
measurements the surface strain can be fully characterised. For non-planar objects an
error is introduced into the displacement gradient measurement due to the variation of
the sensitivity vector across the field of view and the variation in the magnitude of
applied shear due to the curvature of the object surface. To correct for these errors
requires a knowledge of the slope and shape of the object.
Shearography may also be used to measure object slope and shape by a source
displacement technique. Therefore slope, shape and surface strain may be measured
using the same optical system.
The thesis describes a method of multiplexing the shear direction using polarisation
switching, a method of measuring the source position using shadow Moire and the
shearography source displacement technique for measuring the surface slope and shape
of objects. The multi-component shearography system is used to perform measurements
of the six components of surface strain, on an industrial component, with a correction
applied for errors due to the shape and slope of the object.
