Design and assembly of surface analysis set up for holographic Interferometry
Njoroge, Stephen Maina
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Surface analysis plays a crucial role in determining a material character. These have led to increased use of non-destructive tests (NDT) due to their non-invasive nature. However, majority of the NDT rely on data acquisition through point to point scanning process. The techniques are also costly, less sensitive and some require material preparation prior to inspection. To overcome the highlighted challenges, a novel design for surface analysis set up based on Real Time Holographic Interferometry (RTHI) was developed. This was accomplished by aligning the Laser beam, analysing the Laser beam and determination of optimum conditions for recording of phase holograms. The alignment of the Laser beam was performed by adjusting Laser position until the beam struck the same point on a white screen in the near and far field and also ensuring back reflections along the optical axis. The purpose of aligning the Laser beam was to minimize geometrical aberrations in the optical set-up. The aligned Laser beam was analyzed using spectra 320 analyzer and power meter. The wavelength of the He-Ne Laser was 632.8nm and had a beam waist of 1.099rnm. The maximum power was 21.656~W which decreased to 21.197~W on introduction of an objective lens. To determine the optimum conditions for recording and processing of holograms, a transmission hologram recording set up was designed and assembled. Fundamental parameters such as exposure time, object profile, processing chemical compositions and the angle of polarization between the object and reference beams were investigated and analyzed. Optimizing these parameters resulted to high quality holograms as depicted by the diffraction efficiency. The maximum diffraction efficiency of the recorded object holograms was 16% at an exposure time of 20 seconds. Further study using plane wave transmission holograms resulted to hologram images of 21.1% diffraction efficiency at the same exposure time. To test the design concept, three piston rings from a motorcycle piston were used as objects in the holographic set-up. The object holograms of piston rings were recorded and after processing the holographic plate, it was placed back on the holder. It was at this time, the piston ring was subjected to thermal stress and at the same time illuminated by reference and object beams. The generated interferograms were captured by a CCD camera for analysis. The results demonstrated interferograms abnormalities as a result of thermal stress. This was exhibited by interferograms bending and compressions. Using Atmosfringe version 3.3 and origin version 8.0 softwares an analysis of the interferograms was performed. The peak to valley aberrations ranged from O.0128A to 1.2989A for the whole range of measurements. Using the Fast Fourier Transform analysis of the interference fringes, phase changes were determined for each of the rings. From this result, it was evident that the three rings on the piston have different structural characteristics due to their function. The compression ring had less phase change in comparison with the oil control ring.