The use of a weak element that will fail in order to protect other elements in the system from damage.
It is said that a chain is only as strong as its weakest link. This suggests that the weakest link in a chain is also the least valuable and most expendable link—a liability to the system that should be reinforced, replaced, or removed. However, the weakest element in a system can be used to protect other more important elements, essentially making the weakest link one of the most important elements in the system. For example, electrical circuits are protected by fuses, which are designed to fail so that a power surge doesn’t damage the circuit. The fuse is the weakest link in the system. As such, the fuse is also the most valuable link in the system.
The weakest link in a system can function in one of two ways: it can fail and passively minimize damage, or it can fail and activate additional systems that actively minimize damage. An example of a passive design is the use of fuses in electrical circuits as described above. An example of an active design is the use of automatic sprinklers in a building. Sprinkler systems are typically activated by components that fail (e.g., liquid in a glass cell that expands to break the glass when heated), which then activate the release of the water.
Applying the weakest link principle involves several steps: identify a failure condition; identify or define the weakest link in the system for that failure condition; further weaken the weakest link and strengthen the other links as necessary to address the failure condition; and ensure that the weakest link will only fail under the appropriate, predefined failure conditions. The weakest link principle is limited in application to systems in which a particular failure condition affects multiple elements in the system. Systems with decentralized and disconnected elements cannot benefit from the principle since the links in the chain are not connected.
The weakest link in a system exists by design or by default—either way, it is always present. Therefore, consider the weakest link principle when designing systems in which failures affect multiple elements. Use the weakest link to shut down the system or activate other protective systems. Perform adequate testing to ensure that only specified failure conditions cause the weakest link to fail. Further weaken the weakest element and harden other elements as needed to ensure the proper failure response.
See also Factor of Safety, Modularity, and Structural Forms.
262 Universal Principles of Design
UPOD p198-272_.indd 262 1st Proof Title: Provision_Universal Principles Of Design Text 9/17/099/17/09 10:31:16 9:49:50 PM Job No:D909-63/4270 Black Crumple Zone Passenger Shell Crumple Zone
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Weakest Link 263
UPOD p198-272_.indd 263 1st Proof Title: Provision_Universal Principles Of Design Text 9/17/099/17/09 10:31:16 9:49:50 PM Job No:D909-63/4270 Black