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Which of these statements about structural design principles are correct?

1. Fail safe implies the structure will fail, when one component fails.
2. A safe life structure is based on use during a limited time period or number of cycles.

  • A
    1 is correct, 2 is incorrect.
  • B
    Both 1 and 2 are correct.
  • C
    1 is incorrect, 2 is correct.
  • D
    Both 1 and 2 are incorrect.

For this question you need to know the difference between Safe-Life and Fail-Safe design.

Safe-Life: This is de minimum life during which it is known that no catastrophic damage should occur. The life of such a structure can be defined in different ways, cycles, flight hours, pressurization events or calendar time. After this interval has been reached the structure should be replaced, inspected or overhauled per manufacturers recommendations.

Fail-safe: When a structure fails there is alternative load-path provided by an adjacent part which is able to take over the loads for a limited amount of time. This normally should be enough time until the next periodic inspection where the damage should be identified and the damaged part replaced.

Therefore;

Statement 1: Fail safe implies the structure will fail, when one component fails.
Is incorrect, because if a structure fails there will be adjacent parts which can carry the load for a limited amount of time

Statement 2: A safe life structure is based on use during a limited time period or number of cycles.
Is correct, this is the definition of a Safe-life design

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