Determination of System Structural Reliability Considering Uncertainty in Characterization of Component Strength Distributions

Abstract

Theoretical presentations of structural reliability calculation methodologies in the literature typically presume the existence of well-characterized probability distributions for stress and/or strength. This is certainly valid when considering the strength distributions of commonly-used metallic materials. When using structural reliability methods in the design of composite and/or bonded assemblies in aerospace structures, however, strength distributions are usually derived from testing of a limited number of coupons that are representative of the flight hardware. The test-based strength distributions are then only an estimate of the entire population that represents the actual strength distribution of the flight hardware. When allocating the overall system structural reliability budget down to the component level, the inherent uncertainty in the estimated strength distribution relative to the actual population must be considered. This requires the specification of a confidence value requirement along with a maximum probability of failure requirement. This paper presents a methodology for developing parameters for a coupon test program that will demonstrate compliance with allocated probability of failure and confidence level requirements for a composite structure with a large number of bonded joints.