Research On Methods For Fatigue Crack Opening Load Determination In C35Steel

The assessment method based on the crack propagation theory iswidely used in the fatigue design nowadays. The crack closurephenomenon is observed during the fatigue crack growth. The cracks growonly when the load reaches the crack opening load Fop. Thus the influenceof the crack closure should be considered for evaluating accurately thefatigue residual life. During the analysis of the crack closure, thedetermination of Fopconcerns the estimation of the crack closure influence.Therefore, the research of the Fopdetermination methods is very important.The compliance-based Fopdetermination methods were researched inthis paper. The experiment scheme based on compliance was designed. Theback-face strain method was adopted for the measurement of crack openingdisplacement. The compliance curves of C35steel were measured with thisexperimental platform. An experimental equipment was designed for thecamera installation. The stability of this equipment was analyzed by thetheory method and the numerical simulation method. The results showedits stability is good.A new method based on the compliance differential theory was designed for Fopdetermination. Two-times differential conversion methodwas proposed for data processing. And the Fopvalue was determined by thelogarithmic fitting and the offset method. For better applying the newmethod to the practical test, a Fopsoftware with customer interface wasdesigned with the help of MATLAB GUI. The automatic calculation wasrealized.The crack propagation tests of C35steel were conducted with thesoftware designed in this paper. The effective stress intensity factor ΔKeffwas calculated by the method proposed by this paper. The modified Parisparameters of C35steel were obtained. The results of the new method,curve fitting methods, ASTM standard method were compared. The resultsshowed the new method was in accord with other Fopdetermination method,and its result floating is smaller. It shows that the stability and thefeasibility of the Fopdetermination method proposed by this paper are good.