Non-destructive testing (NDT) of glass-fibre reinforced polyester (GRP) composite
materials has been becoming increasingly important due to their wide applications in
engineering components and structures. Electronic Speckle Pattern Interferometry
(ESPI) has promising potential in this context because it is a non-contact, whole-field
and real-time measurement system. This potential has never been fully exploited and
there is only limited knowledge and understanding available in this area. This reality
constrains the wide popularity and acceptance of ESPI as a novel NDT technique.
Therefore it is of considerable importance to develop an understanding of the
capability of ESPI with respect to damage evaluation in GRP composite materials.
The research described in this thesis is concerned with an investigation into the
applicability of ESPI in the NDT of GRP composite materials. Firstly, a study was
carried out to determine excitation techniques in terms of practicality and
effectiveness in the ESPI system. Three categories of defects were artificially
introduced in GRP composite materials, namely holes, cracks and delaminations each
with different geometrical features. ESPI was then employed to evaluate the three
kinds of defects individually. It has been found that cracks and holes on back surfaces
can be defined when the technique is used in conjunction with thermal excitation.
Internal Temperature Differential (ITD) induced fringe patterns were more efficient
than External Thermal Source (ETS) induced fringe patterns with regard to detecting
the presence of holes and cracks. In the case of delamination, ESPI was found to be
capable of detecting the damage when used in combination with mechanical
excitation originating from a force transducer hammer. The geometrical features and
magnitudes of delaminations were also established as being quantifiable.
The validation of ESPI as an NDT technique was carried out in an attempt to establish
a better understanding of its suitability and have more confidence in its applications.
Four damaged specimens were Subjected to ESPI examination in conjunction with
visual inspection, ultrasonic C-scan and sectioning techniques. The geometrical
features and magnitudes of damage evaluated using ESPI showed a good correlation
with those evaluated by conventional techniques.
Poor visibility and readability is an inherent problem associated with ESP! due to an
overlapping between the noise and signal frequencies. An improvement of image
quality is expected in an attempt to achieve a wide acceptance of ESPI as a novel
NDT technique. It has also been demonstrated that this problem can be tackled using
optical phase stepping techniques in which optical phase data can be extracted from
the intensity fringes. A three-frame optical phase stepping technique was employed
to produce the "wrapped" and "unwrapped" phase maps which are capable of
indicating internal damage with high visibility and clarity.
Finally ESPI was practically employed to evaluate damage in GRP composites
introduced by quasi-static and dynamic mechanical loading. It was found that ESP!
was capable of monitoring the progressive damage development of specimens
subjected to incremental flexural loading. The initial elastic response, damage
initiation, propagation and ultimate failure of specimens were clearly characterised by
the abnormal fringe pattern variations. In a similar manner, ESPI was employed to
evaluate the low velocity falling weight impact induced damage. A correlation was
established between the magnitude of damage and the impact event parameters as
well as the residual flexural properties.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.