assumed that specimens with exceptional properties are scarcer than those with poor
properties. The normal distribution will be truncated on the high side at 30 (excluding 14
out of 10,000 specimens) and on the low side at different values corresponding to
different levels of QC. The tradeoff between QC, failure probability and laminate
thickness (weight) will be explored.
Effects of Quality Control on Probability of Failure
Since the primary failure mode of the laminate is micro-cracking, the tensile strain
limit 82u is the first quantity to be improved by quality control. The normal distribution of
82u is truncated at 30 to exclude unrealistically strong specimens, and on the low side QC
at -40, -30, and -20 was checked, which corresponds to rejecting 3 specimens out of
100,000, 14 specimens out of 10,000, and 23 specimens out of 1,000, respectively. Table
5-10 shows the change in the failure probability for selected reliability-based designs.
Quality control on E2u is a very effective way to reduce the probability of failure. A
relatively low cost QC of EZu at 30 will reduce the failure probability by more than two
orders of magnitude.
Table 5-10. Effects of quality control of EZu On probability of failure for 0. 12 inch-thick
(+6)s laminates
Probability of failure from MCS 10,000,000 samples
B Un-truncated Truncate at -40 Truncate at -30 Truncate at -20
Normal (3/100,000) (14/10,000) (23/1,000)
24.00 60.5e-6 30.5e-6 0.0 0.0
25.00 56.5e-6 29.9e-6 0.1e-6 0.0
26.00 60.7e-6 31.0e-6 0.5e-6 0.0
Table 5-11 shows that truncating other strain limits even at -20 will not change the
laminate failure probability substantially. This reveals the fact that the primary failure
mode of the laminate is micro-cracking. Therefore, E2u is the critical parameter to study