Scanning laser doppler vibrometry for strain measurement and damage detection.

Abstract

Numerous strain measurement and damage detection techniques have been developed over the last century. These techniques include strain gauges, digital image correlation, radiography and ultrasonic inspections. All have various advantages, as well as disadvantages, which make each suited to specific applications. With the development of laser Doppler vibrometry, a number of techniques have been established for non-destructive evaluation, such as the measurement of bending strain, as well as damage detection using kinematic parameters, including displacement and curvature. With recent advancements in laser Doppler vibrometry technology (such as 3D scanning laser Doppler vibrometry for three-dimensional displacement measurements, improved velocity decoders and increased spatial resolution) the door has been opened to develop techniques for measuring surface strain from in-plane displacements, as well as the development of new damage detection techniques based on the fundamental principle of deformation:- the governing differential equation of displacement. The extensive literature review contained in this thesis identified a number of gaps in the field, including the evaluation of the accuracy of quasi-static bending strain measurements using current 1D SLDV technology, the precision of full-field surface strain measurement techniques utilising 3D SLDV, and new detection techniques based on the violation of the governing differential equations of displacement. Thus, the research contained in this thesis focussed on these areas. The first part of this thesis presents an investigation into the use of 1D and 3D scanning laser Doppler vibrometry for non-contact measurement of quasi-static bending strain in beams and surface strain in plates, respectively. The second part presents a new damage detection technique based on the governing differential equations of displacement in beam and plate structures. Two algorithms are developed to determine a violation in the governing differential equations created by either a delamination in a composite beam with out-of-plane displacements, or by a crack in a plate with in-plane displacements.