High Cycle Fatigue of Structural Components Using Critical Distance Methods

Abstract

This study explores the high cycle fatigue strength of crack-like discontinuities in metallic structures as well as those made from powder metals using approaches that are based on theory of critical distances (TCD). The methods used in this study consist of (a) the point method, (b) the line method, and (c) the imaginary crack method. The effective parameter for the methods (a), (b), and (c) is the distance 'd' from the material surface, which is a material property and the reference parameter is the fatigue limit. The imaginary crack method involves introduction of a sharp crack at the root of a notch and the length of the crack, 'l o ' assumed a material constant. In the imaginary crack method, the effective crack length is taken as the sum of the actual crack and the material parameter 'l o '. It is concluded that the high cycle fatigue has a volumetric character and the proposed methods introduce the volume effect in the determination of stress and strain fields as well as the fatigue life. Using the material parameters from the various approaches, the number of cycles to initiate a fatigue has been determined for a number of materials and compared with experimental results. NOMENCLATURE a: crack length, m a th : threshold crack size, m d: distance parameter due to Roche [6], m D: grain diameter, m E: modulus of elasticity, GPa K: stress intensity factor, MPa √m ΔK th : threshold stress intensity factor range, MPa √m ΔK th l : long crack threshold stress intensity factor range, MPa √m l 0 : distance parameter due to El Haddad et al [5], m N: number of cycles to initiate a crack ΔS : stress range, MPa (remote from crack) Δσ : stress range, MPa Δσ e : endurance limit stress range, MPa

Assessment of the Effect of Pitting Corrosion on Fatigue Crack Initiation in Hydro Turbine Shaft

“…This analysis was based on aircraft corrosion analysis and on non-ferrous metals. However, Hoeppner's work, and analysis by Chattopadhyay [23] points out that TCD based analysis will be very convenient for the present case study. The Kitagawa -Takahashi diagram for 20GSL steel castings is presented in Fig.…”