The SEM Investigation of Inconel 718 Fatigue Process at Various Loading Conditions

Abstract

Fracture surfaces of specimens broken by cyclic loading provide valuable information about individual stages of fatigue process. Changes in the loading conditions (character of fatigue loading, stress amplitude level, influence of cycle asymmetry ratio R, testing temperature, environment of test, etc.) and in the structure of tested material as well cause changes in the fatigue process, whose have effect on fracture surface resulting in change of fatigue properties. In this paper, authors describe the changes of fatigue process of nickel base superalloy Inconel 718 as a result of loading conditions change using SEM (Scanning Electron Microscopy) microfractography analysis of fractured surfaces. The various fatigue loading were, at the first, regular push-pull loading with asymmetry ratio R = -1 and frequency f = approx. 20 000 kHz (High Frequency and High Cycles fatigue Loading - HFL) and the second was three-point flexure loading with asymmetry ratio R = 0.116 / 0.507 and frequency f = approx. 150 Hz (Low Frequency and High Cycles fatigue Loading - LFL). All fatigue tests were done at room temperature.