SKEDSOFT

Reliability based design

Objectives

•Learn formulation of reliability design problem.  Understand difference between reliability-based design and deterministic design

•Learn how to quickly approximate reliability of component using mean value, first order, second moment method

•Be exposed to three example problems involving analysis and design

Deterministic vs. Reliability Design

Deterministic

•Find design variables

•To minimize cost (or weight)

•so that safety factor >= minimum acceptable value

Reliability

•Find design variables

•To minimize cost (or weight)

•so that reliability >= minimum acceptable value

Approximate calculation of reliability

•Mean value, first order, second moment method (MVFOSM) method

•Assumptions:

–Random variables are Gaussian

–Linear Taylor approximation of performance function, g(x₁,…,x_n), is accurate (performance function; positive implies survival, negative implies failure)

Linear Taylor approximation of gabout mean values of random variables:

Mean value of g

Standard deviation of g

The derivatives are calculated at the mean values of r.v.s

Failure probability = Φ(-β)

Reliability=1-failure probability