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Use of the stress gradient factor to estimate fatigue stress concentration factors K<inf>f</inf>

Abstract:

Stress concentration effects in unavoidable notches reduce fatigue strengths and lives of most structural components, so they must be properly quantified when designing and analyzing them. The Stress Gradient Method (SGM), based on classic Fracture Mechanics concepts and on short crack ideas, can be used to calculate Stress Gradient Factors (SGF) and to properly estimate fatigue stress concentration factors Kf. The procedures proposed here to calculate them are general and only require the normal stress distribution ahead of the notch tip and material properties. The SGF concept is defined to differentiate it from the geometric stress concentration factor Kt, also present in stress intensity factor equations for cracks. Using short crack, SGM concepts, and SGF, a general equation is proposed to compute Kf, which is discretized to be used with any numerical methods (a short C++ algorithm is presented to solve it). Experimental Kf data are used to validate the proposed method, showing good agreement with its pbkp_redictions, especially for sharp notches.