Quantitative Strain Analysis With Image Shearing Speckle Pattern Interferometry

Abstract

This thesis reports on the development of a analysis. The method is based on interferometry, which is also known as Shearography Shearography is an quantitative method for strain image-shearing speckle pattern shearography. optical technique to measure the deformation of object surfaces. Its principal field of application is qualitative non¬ destructive testing, e.g. to find defects in structures made from composite materials. Shearography is less susceptible to environmental vibrations than electronic speckle pattern interferometry (ESPI) or holographic interferometry. Furthermore, shearography allows for the adjustment of the measurement sensitivity without changing the optical set-up or the laser wavelength. The goal of this work is to develop shearography into a method for quantitative strain analysis. This development implies a theoretical analysis of shearography, a concept for the extraction of the strains from the measured images, the implementation of image processing routines, and the design of specific measurement equipment. Theoretical analysis Image processing For the theoretical analysis, the relation between measured phase change and mechanical deformation is derived for a general shearography set-up and the simplifications made in the derivation are identified and analysed. In order to extract the strain components from the measured images, the following image processing techniques are needed: Calculation of the phase from the intensity measurements, filtering of the phase fringe patterns, phase unwrapping, removal of the image doubling, isolation of the directional components, and the quantitative evaluation of the strain fields. A new filter technique cosine average filter - the multiply repeated anisotropic sine/ - is introduced and compared with other advanced filter techniques, clearly showing the superiority of the new filter. Another new image processing technique presented herein calculates the displacement field from the original phase map that represents an approximation to the displacement derivative. This procedure takes into account the finite character of the shear distance and removes its image- doubling effect. Shearography measurements usually contain a combination of directional components of the deformation according to the directions of illumination and observation. It is shown here that from three measurements - each with a different illumination direction arranged in a - it is possible to isolate the directional components of deformation. With the light sources specific symmetry and plane wave illumination, this isolation is particularly simple. Specific In order to have a equipment special shearography system has been designed. It consists of a practical system to perform the strain measurement, a compact shearography head and three illumination modules. The system is remotely controlled from the computer. Experimental verification The developed measurement technique, the equipment, and the image processing are verified by means of an exemplary measurement. The strains on the surface of a flat tensile specimen with symmetric notches is determined and the results are compared with the strain distributions calculated with the finite element method. Results The image processing methods developed in this work are verified individually and show a very good performance. In the final experiment for strain measurement, where all processing steps are combined, the result for the largest strain component is acceptable. The peak value is measured with an accuracy of 6 % and in the rest of the sample the strain lies within the expected range. Still, disturbances, which result from the evaluation procedure, do not allow for the accurate determination of the local strain distributions. For the other strain components, the disturbances cover the wanted strain information. Although the accuracy and reliability of the developed strain measurement method is not yet satisfactory, this work shows the feasibility of quantitative strain analysis with shearography and provides all important tools to perform and evaluate the measurements.