Chapter 17 - Mechanical Systems Safety

Abstract

The use of safety factors to provide insurance that a structure can perform its intended function, where the failure of that structure would result in negative consequences, is common in all industries. In its most simple form, a structural safety factor is the ratio of the strength of the structure to the maximum expected load. Safety factors as used in the aerospace industry and most other industries are simply design factors imposed as requirements to assure that risk of failure for safety or mission success is reduced to an acceptable level. Safety factors might be better described as ignorance or fudge factors. If the design engineer could determine the exact or, at least with great accuracy, the maximum stress of a structure and knew with similar accuracy the strength of the structural component, the part would need to be only slightly stronger than required, and failure would not be possible. Many factors make it virtually impossible to determine with the required accuracy either the maximum stress or the load capability of the structure. The use of deterministic safety factors provides insight into neither the actual risk of structural failure nor a situation that can result in a catastrophic structural failure; or for the opposite case, a structure that is considerably heavier than would otherwise be required to meet the safety and mission success goals of the project.