Fringe Projection Technique for Deformation Measurements under Impact Loading
Rai, Mani Ratnam
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High-resolution three-dimensional (3D) shape reconstruction of objects has huge potential for applications in the field of design, security, entertainment, biomedicine, industrial quality control etc. Of the available techniques, optical methods have the distinctive advantage of facilitating non-contact and non-intrusive measurements. Of late, integration of optical measurement system with the computer based data processing has improved the quality of the results. Of the available techniques, structured-light illumination (Fringe Projection) is the most effective, owing its simplistic experimental architecture and analysis. Traditional Fringe Projection techniques function with the use of fringes generated using interferometric methods. With the advent of digital light projectors, digitally generated fringes have taken the place of interferometry based fringes. Despite the technological advances that this field has witnessed over last couple of decades, digital fringe projection technique still suffers from various shortcomings. This thesis presents a strategic solution to the challenges faced by the technique in its application to out-of-plane deformation measurement of objects under impact loading. First part of the thesis reports the developmental work on building an LED-Grating based optical projection system for implementation of linear-fringe projection profilometry. Successful use of the developed system in measuring out-of-plane deformation experienced by multiple targets under impact loading with a time sapling of 20,000 frames per second is re-ported. However, for performing ballistic impact measurements using the liner-fringe projection method, an order of magnitude higher time-sampling is needed. This is due to the disadvantages associated with linear fringe projection technique: (1) results in wrapped phase map (2p ambiguity) estimation, and (2) the deformation/shift of the recorded fringe pattern in the modulation direction sets a limit on unambiguously measurable whole-plane displacement. Typically, fringe pitch dictates the limit of maximum detectable displacement, and thus to be able to capture larger deformation from the earlier state, coarser fringe pitch is required to be projected; while this adversely affects the resolution of measurement system. Hence, there is a need to develop a fringe projection system which has capability for whole-plane displacement without affecting the resolution and/or necessitating higher temporal-sampling. Circular Fringe Projection (CFP) technique is proposed in the second part of the thesis as a novel solution to address the above issues. CFP technique offers additional advantage of relaxing the temporal resolution requirements of the imaging system by decoupling the maximum measurable deformation rate and the frame rate of camera. A new image analysis method is also developed to extract the underlying phase distribution of the recorded circular-fringe patterns, as the conventionally used single-frame linear-fringe analysis methods are incompetent at demodulating the circular fringes. Experimental results obtained in 3D shape measurement and whole-field out-of-plane displacement measurements of a deforming object reported in this thesis, not only confirms the ability of the proposed CFP technique in overcoming the shortcomings of the widely used linear-fringe projection technique, but also its suitability for deployment in ballistic-impact measurements.
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