The load-deflection results of various specimens are shown in Figures 3-6-3-8. The

load increases linearly until the crack-tip reaches the ply-interface of the top layer. When

the crack reaches the interface, the load does not increase further since more strain energy

is required to deflect the crack to the ply-interface. After the interfacial fracture initiates,

and as the crack propagates as a delamination, the stiffness of the specimen reduces as

indicated by the slope of the load-deflection curve. The fracture loads are measured

immediately before interfacial fracture initiates. As results, the fracture load is linearly

proportional to the thickness of the mid-layer in Table 3-5. The fracture loads for the

three different specimens are listed in Table 3-6. One can note that the fracture load

increases with the thickness of the middle 900 layer.

Table 3-4. Dimensions of specimens and various mid-ply thickness.
 Layer Thickness
 [0/90/0] eng Width (mm) Top and bottom layer, 0 Mid layer, 900 (mm)
 (mm) o (mm)
 (mm)
 Specimen 1 145.4 18.6 2.4 1.8
 Specimen 2 146.2 18.7 2.4 2.4
 Specimen 3 145.7 18.8 2.4 3.0

Table 3-5. Fracture load and fracture toughness at room and cryogenic temperatures.
 0/90/0] Room Temperature (T= 300 K) Cryogenic Temperature (T= 77 K)
 Ff (N) Kic (MPa-m29) Ff(N) Kic (MPa-m 29)
 Specimen 1 122 58.1 88.1 55.6
 Specimen 2 127 57.9 81.5 58.1
 Specimen 3 133 58.0 74.7 58.8

 Finite element analyses of the test specimens were performed to determine the

detailed stress field in the vicinity of the crack tip corresponding to the fracture loads.

Due to symmetry one-half of the specimen is modeled. The laminate properties of

graphite/epoxy given in Table 3-1 are used for the FE model. A contour plot of the stress

distribution is shown in Figure 3-9.