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 |
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... 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.
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| Runouts |
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... 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 |
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... describes the ballooning variance
observed in the lower right hand corner of the HCF s-N curve. |
| Fracture Mechanics |
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... calculations can predict how long a
structure can survive, starting with a crack of a given size. |
| NonDestructive
Evaluation |
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(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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