Size effect in finite-life fatigue of metals

Many experimental observations show that the finite-life fatigue strength of metallic materials in the high-cycle regime decreases with increasing the specimen size. Such a decrease can be explained by considering the fractal nature of the reacting cross-sections of structures. Accordingly, the so-called fractal fatigue strength is represented by a force amplitude acting on a surface with a fractal dimension lower than 2, where such a dimensional decrement depends on the presence of damage (cracks, voids) and heterogeneity in the material ligament. A monofractal scaling law for the finite-life fatigue strength of metals is herein proposed, and some experimental results are examined to show how to apply such a theoretical approach.