High resolution interferometric measurements of residual strains in composites
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As composites have been more widely accepted as structural materials, residual stresses in them have become a more serious issue. Various experimental methods have been developed and used to measure residual strains in different kinds and shapes of materials. The anisotropic and heterogeneous nature of composites and the complexity of residual stresses, however, pose limitations on current techniques. Those techniques lack the sensitivity or spatial resolution that is required for the measurement of local deformation of composites on a plyÂ·by-ply basis, or give point-by-point and averaged information. Also they are incapable of resolving the complexity of combined effect of different residual stress components. I n order to measure residual strains more effectively, a new method of measuring then1 is required.
Moire interferometry combined with the cut-and-sectioning method has been developed for effective measurement of residual strains in fiber-reinforced composites. This optical technique provided the capability of studying separately the effect of each component of residual stresses. It also allowed the determination of high-sensitivity full-field deformation information. This approach was applied to thick composite cylinders for measuring residual strains. The results showed a strong influence of curing procedure on residual stresses.
Also, in order to determine residual strains on a within-the-ply basis, a new high-resolution data reduction procedure has been developed. This procedure enhanced the resolution of the existing data reduction technique without losing qualitative information. The combination of both aforementioned techniques provided an effective tool for measuring residual strains of composite materials. The technique is illustrated in an investigation of the effect of stacking sequence on residual strains in flat composite panels.
- Doctoral Dissertations