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Queen Mary University of LondonQueen Mary University of London
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School of Engineering and Materials Science
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PhD Thesis: Mechanical characterisation of bone with laser speckle photography

Author: MO, Ning

Year: 1996

Supervisor(s): Julia Shelton

The mechanical characterisation of bone under physiological conditions is required for the design of skeletal implants and prostheses which are mechanically compatible with bone and hence produce a stable implant-tissue interface. This thesis is concerned with the development of a non-contacting optical measurement technique, which can be used in a physiological environment.
Laser speckle photography, in conjunction with an automated image analysis system was developed to measure in-plane deformations. Recordings were made using a 3 J Ruby pulsed laser to form specklegrams. Automated analysis was performed on an image analyser, and a Fast Fourier Transform algorithm was used to determine the fringe spacing and orientation. These data were subsequently used to determine surface displacements. The accuracy and precision of the automated system was verified. It was established that the technique could be used at magnifications of up to 60 X, providing an ultimate resolution of approximately 1 m.

The technique was used to measure the displacements of both tensile specimens and compact tension specimens of bovine cortical bone, which were 3 mm in thickness. The longitudinal elastic modulus and Poisson's ratio were determined to be 20.5 GPa and 0.30, respectively, which equate to values determined by other methods. The determination of mode I critical stress intensity factor gave values of 6.84 MNm-3/2 and 4.86 MNm-3/2 for the transverse and longitudinal specimens, respectively, which correlate well with values from alternative methods. However, derived values for critical strain energy release rate gave conflicting data to conventional tests. At a recording magnification of two, a displacement disorder zone was detected in front of the crack tip on compact tension specimens, which was interrupted as an area of microcracking in the bone.