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A Measurement System for the Non-Contacting Measurement of Local Strain

Dipl.-Ing. Dr. Sebastian Claus Schneider

Dipl.-Ing. Dr. Sebastian Claus Schneider

Supervisory committee:

Univ.-Prof. Dipl.-Ing. Dr. Bernhard Zagar
Univ.-Prof. Dr. Brigitte Weiss

Final exam:

November 22, 2005

The objective of this thesis is the development of a laser-optical strain and displacement measurement system, which calculates mechanical or thermal strain and the superposed rigid-body-motion from the motion of a laser-speckle-pattern originating from a small illuminated surface spot. The system makes use of objective speckle patterns and therefore does not depend on imaging optics.

Figure 1: Principle measurement set-up for the correlation method using objective laser-speckles. In the figure the rigid body displacements, rotations, and strains are illustrated on a surface element. The speckle displacements which depend on these factors can be observed in the camera plane.

The work comprises amongst others an overview of literature relevant for this area of research, the important physical principles, a detailed description of the measurement principle and also an error analysis of the entire system, which cannot be found in the currently available literature.

Additionally one chapter is dedicated to signal-processing, which is necessary for the calculation of the speckle-pattern motion.

Figure 2: This figure shows a measurement set-up for measuring the elastic modulus of thin fibres and foils. The shown probe is a
122 µm thin copper wire.

Furthermore the applicability of the speckle-strain-sensor is shown by means of various experiments having been accomplished with the developed measurement system and which are of interest to the area of material physics. Besides experiments for the measurement of the coefficient of thermal expansion, tensile-tests with thin foils and fibres down to the micrometer range were performed.

Figure 3: This figure shows the measured stres-strain-relation of a 122 µm thin copper wire. The slope of the linear regression corresponds to the elastic modules of 97,3 GPa.