the cryogen. For the liquid hydrogen composite tank, this hydrogen leakage through the
interface sheet causes the catastrophic failure of the sandwich structure [1].
4 Transverse Crack
--. Interfacial Crack
Figure 1-4. SEM images of micro-crack propagation in composite laminates after thermo-
mechanical cycles.
When a microcrack propagates in a give material, the stress distribution normal to
the crack plane can be described by the equation in Figure 1-5. Normal stresses at a
crack-tip are governed by the stress singularity A and stress intensity factor KI. Zak and
Williams [11] found the normal stresses ahead of a crack-tip are proportional to r~X (0 <
Re [k] <1) where r is the distance from the crack-tip. The stresses normal to the crack
ahead of a crack-tip can be expressed as c =KI r-. The stress singularity of a micro-
crack in laminated composites is governed by anisotropic properties at the vicinity of a
crack-tip.
The stress singularity describes the behavior of crack propagation. The Material 1
and the Material 2 are composite layers with specific fiber orientations. When a
transverse crack reaches a ply-interface of composite layers with identical fiber
orientation, stress singularity is equal to /2 which is conventional singularity for
homogeneous materials. When a transverse crack propagates from the Material 1 which
is stiffer than the Material 2, the transverse crack penetrates the ply-interface and
continues to propagate through the Material 2 until the crack reaches a stiffer layer [12].