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A
A single load application to the rotor blade causes it to fracture, this is called metal fatigue.
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B
Cumulative applications of load may cause the blade to fail below the ultimate stress level, this is called metal fatigue.
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C
Multiple applications of load cause the rotor blade to fail at the ultimate stress level.
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D
Major loads and stresses occur at the rotor blade tip.
Refer to figure.
Stresses can be either torsion, tension, compression or shear and can act individually or together.
The stress at which the structure fails is called “Ultimate Stress”. It is the fail point for a single application of a static load.
In flight, the structure is loaded and unloaded many times at levels below the ultimate stress. This causes cumulative damage which in turn allows the structure to fail catastrophically at a stress level well below the ultimate stress.
This cumulative damage and weakening of the structure is called “Metal Fatigue”, from which composite materials also suffer, but react in a different way.
The greatest contributor to fatigue on a helicopter is vibration. Below a certain level little or no fatigue occurs, but any increase above this, will start the process.
The major loads and stresses occur at the blade root and can be reduced with design features, such as washout and dragging-flapping hinges.
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