## 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.

### 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 statistical 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.