Fatigue-Nucleating Discontinuities

Compared to the carefully prepared surfaces typical of traditional laboratory fatigue specimens, production aircraft structures have many surface discontinuities. These discontinuities are generally very small, of the order of 0.01 mm deep. When they are sufficiently crack-like and the local cyclic stresses generated by the service loads are sufficiently high, the nucleation and growth of fatigue cracks from the larger discontinuities will commence very early—almost immediately—in an aircraft’s service life or in an FSFT. The importance of the surface finish is stressed. A method of characterizing these discontinuities as equivalent cracks is presented.