Is a Costly Re-Design Really Justified if Slightly Negative Safety Margins are Encountered?

by Prof. Dr. Ralf Cuntze

Part 1:

The paper intends to contribute to a general discussion: How safe is a positive Margin of Safety (MS)?

What is the real meaning of a slightly negative MS value because a MS value itself does neither outline the size of risk nor specify the probability of failure? A value of MS = +4% in the one case may incorporate a higher risk than –2% in another case.The author’s intention is to make those engineers, who are responsible for ‘approving and releasing’ structural designs, aware of the real sources of risk. There is no compulsory need to reject a design if a Margin of Safety becomes slightly negative, e.g. –0.5%. No part of the structure will necessarily loose its functionality or will fracture, but an outright rejection could result in high additional costs as well as in a delay of the whole project due to re-designing efforts needed. A numerical example from the aerospace industry is used here to illustrate this topic.

Part 2:

The in KONSTRUKTION 3-2005 previously published partof the article gave some hints why a costly re-design in case of a slightly negative safety margin is not mandatory. Scatte-ring design parameters such as load or strength were addres-sed as well as the terms risk, reliability, factor of safety and margin of safety (MS) were discussed. In this second part the deterministic MS is assessed by a probabilistically computed MS. This assessment is substantiated by a parameter investi-gation. The main conclusion outlines: In case of slightly ne-gative MS values one just should try –instead of stopping the development- to increase as a counteracting means the relia-bility of the input parameters. This conclusion primarily aims at aerospace engineering, but may also be applicable to other engineering disciplines where risk analysis and mar-gins of safety are of importance.