Page 220 - Mechanics Analysis Composite Materials
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Chapter 4.  Mechanics of a composite layer      205

           shown  in  Fig. 4.75.  Stresses  e,,  z.~~,and  z,,   were  calculated  with  the  aid  of
            Eqs. (4.161), while stresses el,c2,and   in the principal material directions of the
           plies were found using Eqs. (4.69) for the corresponding strains and Hooke's law for
           the plies. As can be seen in Fig. 4.75, there exists a significant concentration of stress
            e'  that  causes cracks in  the matrix.  Moreover, interlaminar shear stress z,~  that
           appears  in  the  vicinity  of  the  specimen  edge  can  induce  delamination  of  the
           specimen. The maximum value of stress e? is






            Using the modified strength condition, i.e., cy = 8;  to evaluate the strength of
           f60" specimen we arrive at the result shown with a triangular in Fig. 4.69. As can
           be seen, the allowance for the stress concentration results in a fair agreement with
           experimental strength (dark circle).
             Thus, the strength of angle-ply specimensis reduced by the free-edge effects which
           causes  the  dependence of  the  observed  material  strength  on  the  width  of  the
           specimen. Such dependence is shown in Fig. 4.76 for 105 mm diameter and 2.5  mm
           thick fiberglass rings made by winding at f35" angles with respect to the axis and
           loaded with internal pressure by two half-discs as in Fig. 3.46 (Fukui et al., 1966).
             It should be emphasized that the free-edge effect occurs in specimens only and
           does not show itself in composite structures which, being properly designed, should
           not have free edges of such a type.


           4.6.  Fabric layers

             Textile preforming plays an important role in composite technology providing
           glass,  aramid,  carbon (see  Fig. 4.77), and  hybrid fabrics that  are widely used  as


                                                      \'





                               0.6

                               0.4
                               0.2


                                0
                                 0    0.2   0.4   0.6   0.8   1
            Fig. 4.75. Distribution of normalized stresses over the width of f45"angle-ply carbon-epoxy specimen.
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