satisfying the reliability constraints, which is expressed as the probability of failure of the
panel P less than P"=10-4
z
Figure 8-2. Isogrid-stiffened cylindrical shell with internal isogrid and isogrid pattern
oriented along circumferential direction for increased bending stiffness in
hoop direction; the zero degree direction for the composite laminates in
isogrid and skin panel are shown
Thermal loading from stress free temperature (3520F ) to service temperature
( -4230F ) are considered together with the following two load cases
1. internal proof pressure of 3 5 psi critical for strength;
2. axial compressive load Nx = 1000 lb/in, with an internal (stabilizing) pressure of 5
psi critical for buckling.
The amount of geometric imperfections is the same as those shown in Table 8-1.
The material used in the study is AS4, which has a density of 0.057 lb/in3. For composite
panel, the residual stress induced from cooling for cure temperature to service
temperature must be considered. The residual stress calculation needs the coefficients of
thermal expansion (CTE) along and transverse to fiber direction, a~ and at, respectively.
It is shown in chapter three that a~ is two orders of magnitude smaller than at, a~ is
ignored in the analysis to reduce the number of variables in response surface