possibility. However matrix strength measured at cryogenic temperatures should be used

for accurate prediction of formation of micro-cracks.

Fracture Toughness for a Transverse Crack in Laminated Composites at Cryogenic
 Temperatures

 At cryogenic temperature, the transverse cracks propagate mostly at transverse

plies of composite laminates. The transverse crack and delamination provide the leakage

path of the liquid hydrogen permeation which causes the catastrophic failure of the

composite tank. The stress singularity describes the fracture propagation of composite

laminates. The singularities for glass/epoxy and graphite/epoxy composites with various

stacking orientations are estimated using analytical and finite element methods.

Singularities calculated by both methods are in good agreement. The results provide the

confident of capturing the singularity using the finite element model. When the transverse

crack propagates in the unidirectional laminates, the singularity becomes 0.5 as a general

homogenous material. When the transverse crack propagates from the 0-degree to the 90-

degree layer, the singularity becomes lager than 0.5 and vice versa.

 The finite element analysis was performed to estimate fracture toughness of a

transverse crack under the fracture load measured from the fracture experiment. The

results of fracture toughness are compared at room and cryogenic conditions. Four-point

bending experiments are performed at room and cryogenic temperatures to obtain

fracture loads of graphite/epoxy specimens. The specimens had different mid-ply

thicknesses, i.e, the crack lengths were different in different specimens. The results of

fracture load and fracture toughness are listed in Table 3-6. At room temperature, fracture

load increases 8% when the mid-ply thickness increases 67%. However, the variation of

fracture toughness is less than 0.5%. Fracture toughness is only dependent to local