46


















Figure 3-14. Contour plot of stresses normal to ply direction near the free-edge in a
 graphite/epoxy laminate at cryogenic temperature.



 5E+07
 4 5E+07
 4E+07 -
 3 5E+07
 3E+07
 2 5E+07
 2E+07
 1 5E+07
 1E+07
 5E+06

 0 0002 0004 0 006 0008
 Distance from a center to an edge (m)


Figure 3-15. Load-displacement curves from four-point bending tests at cryogenic
 temperature. The mid-ply thickness and hence the specimen thickness varied
 from specimen to specimen.

 The stresses in 2-direction stay constant in 80% of the width and increases sharply


near the edge as shown in Figure 3-15. Since graphite/epoxy composite has negative


longitudinal CTE in 3-direction (see Table 3-1), the mid-layer expands and the top and


bottom layer shrinks at cryogenic temperature. Therefore, Mode I fracture behavior can


be expected at the edge. The result predicts that crack length of the edge delamination is


not too deep to affect the experiment results.