Fatigue Damage and Its Nondestructive Evaluation: an Overview

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 dislocations that either forms extrusions/intrusions (surface roughness) or the piling up of dislocations at grain boundaries or interface boundaries whose unzipping forms small cracks. 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 a 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 roughly: First small cracks (a '" O.l~m) initiate at '" 10% of the total life; large cracks appear (a'" 0.5mm) at '" 90% of the total life. Details depend on the microstructure of the material, the applied stress and other environmental factors. Over the past thirty years, researchers have tried to follow this sequence of events with a variety of experimental techniques that are adjusted to the specific defect type to be detected (dislocations, small cracks, and large cracks). Some of these techniques are briefly reviewed.