Trading Risk Versus Cost of a Composite-Material Structure

A probabilistic method has been developed for use in designing a composite-material structure to achieve a balance between maximum reliability and minimum cost. This method accounts for all naturally occurring uncertainties in properties of constituent materials, fabrication variables, geometry, and loading conditions. Heretofore, it has been common practice to use safety factors (also called "knockdown factors") to reduce design loads on composite structures in the face of uncertainties. Safety factors often dictate designs of structures substantially heavier than they would otherwise be, but provide no quantifiable measures of reliability. The present method involves a quantitative approach to reliability; the equations of the method are formulated to yield a design that is optimum in the sense that it minimizes a reliability-based cost.