When are Allowables Conservative?

Abstract

Aircraft designers are legally required to treat manufacturing variability so as to minimize the probability of structural failure, pursuant to Code of Federal Regulations (CFR) 25.613. The usual design strategy employed for compliance with the regulations is the use of allowable values. However, allowables are not guaranteed to satisfy the regulations as written. In this work, we thoroughly investigate the use of allowables via a formal reliability analysis using quantile evaluation. We find that allowables are provably conservative for simple settings but admit anticonservative structural analysis under nonmonotone structural response, strongly nonnormal distributions, correlated material properties, and high-dimensional material property spaces such as those arising in laminate composites. This analysis develops intuition for when allowables are a conservative approximation of a true reliability analysis. We introduce an inexpensive procedure to detect analysis pathologies and illustrate its use on a set of aerospace-relevant structural analysis problems.