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High residual stresses develop in SCS-6/Ti-15-3 composites during cooldown from the fabrication temperature; these residual stresses can affect the behavior of the composite. Discrete fibermatrix finite element models were used to study the residual stresses caused by the temperature change during the fabrication process, including the effects of uneven fiber spacing, the free surface, and increased fiber volume fractions.
Results showed that the calculated stress distributions in a surface ply were significantly different from those calculated for an interior ply. However, the stresses were identical for all subsurface plies. In an interior ply, for uneven fiber spacing less than 0.042 mm, the maximum hoop stress was calculated to occur between fibers within a ply and increased as the fiber spacing decreased. For interior plies, the trend in the calculated maximum hoop stress between fibers within a ply agreed with the observed radial cracking between fibers. For the case of touching fibers, the analysis predicted tensile radial stresses at the fiber-matrix interface, which could lead to fiber-matrix debonding during cooldown. Identical trends were predicted for uneven fiber spacing in surface plies with slightly greater values of peak stresses. The calculations for increasing fiber volume fractions were similar to the calculated results for decreasing the fiber spacing between two adjacent fibers within a ply. The results indicate that matrix cracking and fiber-matrix debonding are more likely to occur during processing for laminates with higher fiber volume fractions.
Research engineer, NASA Langley Research Center, Hampton, VA
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