This paper deals with the basic mechanisms of fatigue damage in materials exposed to service and a series of nondestructive techniques for the early detection of this damage. For all materials with reasonably high fracture toughness it is the organized motion of dislocation that either forms extrusions/intrusions (surface roughness) or the piling up of dislocations at grain boundaries or interface boundaries whose unzipping forms small crack.s Some of these small cracks grow, coalesce with other small cracks and eventually form the large crack which will terminate the life of the structure, if it grows to size that is large enough (at a given stress level) to reach the fracture toughness of the material. The time scale of these events is roughtly: First small cracks (a approx approx o.1 #mu# m) initialte at approx approx 10
展开▼
