The final stages of fatigue in a machine part under a fluctuating load are manifested by an onset, at a specific point in the part, of a crack. The initiation of a crack begins long before that with crystal imperfections and microscopic dislocations which are followed by an outright split, propagating into a visible crack that leads to an ultimate fracture. Chapters 3 and 4 discussed the stress and strain methods to estimate life expectancy. In this chapter is introduced the method of fracture mechanics which complements the strain method for more accurate results of an entire destruction process. Strain method concerns itself with the initial stage, while fracture mechanics analyzes the crack propagation in its entirety. The physical changes in the material follow from one stage to the other with many factors influencing the duration of one stage or the other. Theoretically we need to divide the timing of the process—from the start to a final fracture—into three discrete periods: (a) initial damage in a submicroscopic scale; (b) visible damage, cracks initiation and growth; (c) final instantaneous fracture which is the ultimate failure. It is apparent that the accuracy of specimen’s life expectancy depends to a large degree upon prediction, as accurate as possible, of the indicated destruction period. Both the specimen’s properties and the outer loading are factors that influence the analysis.