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Lifetime

Structural Life prediction is usually based on three technologies: Fatigue, Fracture Mechanics, and NDE.  Fatigue - to estimate how long it takes for a crack to develop, Fracture Mechanics - to calculate how long it takes for the crack to grow to critical size, and NDE - to determine the probability of finding it before that happens.

 

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Fatigue

... is the   process of cumulative material degradation with repeated loading - is often described chronologically in micromechanical terms as "initiation" - creating a crack - followed by crack propagation, modeled mathematically using fracture mechanics.

 

Runouts
... are laboratory tests to determine cycles-to-failure that are stopped before the specimen fails.  These suspended tests do not contain the same information as tests to failure do.   Here's how to analyze them correctly.

 

Random Fatigue Limit
... describes the ballooning variance observed in the lower right hand corner of the HCF s-N curve.

 

Fracture Mechanics
... calculations can predict how long a structure can survive, starting with a crack of a given size.

 

NonDestructive Evaluation
(NDE)  Since the cracksize is unknown, it is often assumed to be a size large enough to grow, but small enough to be missed by inspection.  Thus, the quality of the life prediction depends on measuring the effectiveness on the NDE.


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Last modified: June 08, 2014