48




 Case 3 (Mid-layer thickness= 3 0 mm)
 160

 140

 120

 100

 g80

















 8E+07
 60 -

 40 -

 20

 0 0002 0 004 0 006 0008
 Displacement (m)

Figure 3-18. Load-displacement curves from four-point bending tests at cryogenic
 temperature for the Specimen 3.




 8E+07 -

 7E+07 -

 6E+07 -

 5 5E+07

 4E+07

 S 3E+07

 LL 2E+07 --- Case 1
 Case 2
 ---a-- Case 3
 1E+07


 0 1E-05 2E-05 3E-05
 Distance from a crack tip, r (m)



Figure 3-19. Variation of K = -r with the distance from the crack tip at cryogenic
 temperature.


 The load-displacement results are shown in Figures 3-16 through 3-18. Fracture


load decreases 15% with 67% increase of the mid-ply thickness in Table 3-5. Fracture


load decrease may be due to material degradation as micro-crack density increases at


cryogenic temperature. Fracture toughness at cryogenic temperature is predicted using


the finite element analysis. The contour plot of the stress distribution is shown in Figure


3-9. Fracture toughness is calculated following the same procedure as described in the