Safe+Fail



= Definition = Safe-fail (alternately called fail-safe) are systems or devices that in the event of failure, respond in a way that will mitigate harm or damage.

= Issues Addressed = A sampling of safe-fail techniques :

Redundancies
Multiple load paths – if a structural element fails, the load it was carrying will be transferred to other members. Obviously, it is essential that the fracture be detected before multiple members fail.

Intentional “Weak Link”
An inexpensive and easy to replace component may be used to prevent damage to expensive or difficult to repair component. Fuses in electrical circuits are an example of this for electrical systems. Shear pins are used on boat propellers are a mechanical example. These are inexpensive and easy to replace pins that transmit power from the shaft to the propeller. If the propeller strikes an object, the shear pin is designed to fail before the propeller or shaft are damaged.

Physical Law
Designing a system in such a way that failure cannot be catastrophic based on how failure will occur. For example, nature gas pipelines are produced from sufficiently tough material so that it will fail in a ductile manner, rather than brittle. Ductile fractures propagate at about 600 ft/sec. Brittle fractures propagate at about 1500-2500 ft/sec. When a crack forms in a pipe, the gas will immediately begin to decompress. The decompression wave will travel down the pipe at about the speed of sound (1300 ft/sec). If the crack speed is faster than the decompression speed, the crack front will always remain under high pressure and the crack will grow indefinitely. Otherwise, the decompression wave will out run the crack, and the crack will stop growing.

Early Detection
When a structure is designed such that cracks will easily be detected before they reach critical length, it may be considered a fail-safe design. A critical element of this is the detection of the crack before it reaches critical length. It is very important that proper materials (high fracture toughness) be selected that can withstand large cracks before fracturing.

Fracture mechanics must be used:
 * Determine minimum detectable crack length (how small of crack can nondestructive testing detect)
 * Determine critical crack length for the maximum load
 * Create a crack growth curve showing crack length as a function of number of cyclic loads
 * Determine how much time is required from the crack to grow from the minimum detectable length to critical length.

**Leak-before-break** Pressure vessels use this method to prevent explosive failures. Pressure vessels are designed such that a crack will propagate completely through the vessel before it reaches critical length. Generally, the cracks will start at the internal wall and progress outward, radially. Leaks are generally easy to detect, and therefore, should be detected before the crack grows to critical length.



Crack Arresters
To prevent cracks that exceed critical length from fracturing the entire part, crack arresters may be added to the structure. In aircraft these are in the form of riveted straps added to the skin. This will contain the crack to a small area of the structure. Effectively, what is occurring is the crack tip stress intensity decreases as it approaches the arresters. The arresters start to carry more and more load, thus decreasing the load near the crack tip.



**Considerations** These techniques may not be applicable to every type of project, and a qualified engineer must be involved in the assessment and design.

= Applications = Large industrial buildings may be good candidates for some of these techniques due to their dangerous uses and proximity to residential areas in a dense city like NYC.

= Related Reports = Fail-Safe and Safe-Life Designs a nd Factor of Safety

= References =