| dc.description.abstract | 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. | en_US |
