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Stitched Composites                         169

































                                             (f)
           Figure 8.4 (Continued) Photographs and illustrations of stitching damage. (e) A region
           of low fibre content due to stitching is shown within the circle, and this develops into a
           resin-rich region when the composite is consolidated.  (f) Schematic of the distortion to
           stitches caused by heavy compaction of the preform (From Mouritz and Cox, 2000).



           8.3 MECHANICAL PROPERTIES OF STITCHED COMPOSITES



           8.3.1 Introduction
          The  application of  stitched composites to  load-bearing structures on aircraft,  such as
          wing  skin  panels  and  fuselage  sections, requires  an  in-depth  understanding  of  their
          mechanical properties and failure mechanisms. The mechanical property data is needed
          to  validate  design  codes  for  stitched  composites  to  be  used  in  high  performance
          structures. In  this  section  the  effect of  stitching on  the  tensile, compressive,  flexure,
          interlaminar  shear,  creep  and  fatigue  properties  of  composite  materials  will  be
          described. It will be shown that there is not a complete understanding of the effect of
          stitching on the mechanical properties of composites.  In addition, models for predicting
          changes to the properties of composites due to stitching are not fully developed.
             Until  a  strong  modeling  capability  combined  with  a  comprehensive  database  of
           mechanical  properties  for  stitched composites is  achieved,  then  the  certification  and
          application of  these materials to  primary aircraft structures will  be difficult.  Despite
           some shortcomings in our knowledge, there is much about the mechanical properties of
           stitched composites that is understood.
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