Optimizations Allowing Partial Matrix Cracking
Plies of angle 61 are the plies allowed for matrix cracking for optimizations
allowing partial matrix cracking. The 82ti Of the 61 plies was increased to 0.0154, while
the rest of the laminate still used 82ti Of 0.01 1. The lower limit of t2 WAS increased to 0.010
inch (total + 62 thickness of 0.04 inch) to prevent hydrogen leakage. Table 4-6 shows the
optimal design allowing partial matrix cracking. Its thickness is the same as that of the
design without partial matrix cracking (Table 4-5), and the ply angle of the cracked plies
increased due to the increased strain limit, 82". However, the failure probability is higher
than the design that does not allow matrix cracking, which indicates that this option does
not help. The active constraint is still the tensile strain limit 82ti Of 0.011 at cryogenic
temperatures for the un-cracked plies.
Table 4-6. Optimal laminate for temperature dependent material properties allowing
partial matrix cracking: 82ti Of 0.01 1 for uncracked plies and 0.0154 for
cracked plies
6, 6,t; tha (inch) Probability of failure
(degree) (degree) (inch) (inch)
36.07 25.24 0.015 0.010 0.100 (0.097) 0.003716 (0.004582)
a Numbers in parentheses indicate unrounded thickness.
Optimizations with Reduced Axial Load Ny
With small ply angles, the critical component of the load is the axial load Ny,
induced by pressure on the caps of the propellant tank. A smaller axial load may be
obtained by using an auxiliary structure to carry part of this load, such as axial stiffeners
or a cable connecting the caps. If the auxiliary structure does not directly connect to the
wall of the hydrogen tank (such as attached to the caps of the tank), it will not be affected
by the mismatch of the thermal expansion coefficients, i.e., the residual thermal strains.
Here the possibility of reducing the axial load by half by carrying 1200 lb./inch of the