Answer:
Fracture mechanics is the study of how cracks form and propogate through variuos materials. Often times during design an engineer will assume a crack in the material to or use x-ray to determine the size of cracks in the material. Fracture mechanics approaches require that an initial crack size be known or assumed. For components with imperfections or defects (such as welding porosities, inclusions and casting defects, etc.) an initial crack size may be known.