Equivalent Level of Safety Approach to Damage-Tolerant Aircraft Structural Design

Abstract

A theoretical approach to damage-tolerant structural design has been proposed based on a probabilistic characterization of relative structural safety. The equations necessary to quantify damage-tolerant structural safety are developed, and their use in the design of a generic composite sandwich panel are demonstrated. Structural safety is identified by the term level of safety, which is defined, for a single inspection event, as the compliment of the probability that a single flaw size larger than the critical flaw size for residual strength of the structure exists and that the flaw will not be detected. The equations derived from this definition incorporate a probabilistic treatment of damage sizes and inspection capabilities. Utilizing damage size data from existing composite aircraft components along with the level of safety, formulas, design charts for residual strength vs safety of a generic composite sandwich panel were constructed. An example design problem is presented that demonstrates the sensitivity of the facesheet thickness sizing parameters to the relative safety of the design. Bayesian statistical techniques are also incorporated to enable the subsequent use of service inspection data to reduce uncertainty in the damage size distributions and to update the structural level of safety value as service experience is acquired.