PBnJ

2 "Crack like to blunt" notch S/N curve model Introduction In this Chapter the infinite life design philosophy of notched specimens is analyzed, then the same concepts are specialized to finite life design concepts in the context of the “Theory of the Critical Distances” to analyze the transition from “crack-like to blunt” notch. It is shown that a notched specimen behaves very similarly to a crack up to a certain number of fatigue cycles, then its fatigue behavior can be approximated with a plain specimen reduced by the effective stress concentration factor. Accordingly, some fast assessment methods have been suggested: (i) a crack like notch could be replaced by a crack for which there is a wide amount of solutions in Literature and (ii) a blunt notch could be treated through infinite life design concepts only. To verify the analytical method, the SAE Keyhole test program constant amplitude fatigue test data have been used and predictions

1 Overview Introduction This Chapter provides an overview of the notions used in this Thesis, starting with the derivation of stress vs. number of cycles (S/N) curve models for short cracks, since the work of these years is relying on the stress-life approach. Then, an overview of crack propagation models is given, in fact power law type crack propagation equations are integrated to recharacterize the concept of S/N curve for long cracks. Subsequently, the effect of notches in the stress-life formulation is treated mostly by means of the theory of the critical distances. Besides providing a variety of fatigue S/N curve models, a brief overview of damage accumulation rules for fatigue prediction deriving from variable amplitude loading is given. The Chapter concludes with a brief overview of the regulatory aspects of fatigue tolerance evaluation for rotorcrafts. 1.1 The SN curve The advent