疲劳裂纹起始的超载效应及新的寿命估算模型

Overloading Effect on Fatigue Grack Initiation in LY12-CZ Alloy and a New Model for Life Prediction

本文根据对LY 12-CZ铝合金的疲劳裂纹起始超载效应的实验研究结果,提出了完善的疲劳裂纹起始寿命表达式和新的疲劳裂纹起始寿命估算方法。上述裂纹起始寿命表达式揭示了裂纹起始寿命与切口构件几何、循环加载条件、拉伸性能、裂纹起始门槛值以及超载应力幅之间的关系。考虑了裂纹起始的超载效应,提出的估算变幅载荷下切口构件裂纹起始寿命的新方法,不作任何工程修正,估算的裂纹起始寿命与实验结果符合很好。

Up to now the local strain approach(LSA) is still widely used to predictthe fatigue crack initiation life(FCIL) of components under variable amplitudeloading[17, 18]. After summarizing the research on LSA, Buch gave nine reasonsto explain the poor accuracy of the FCIL prediction by using LSA[17]. Basedon our recent research, it may be thought that the principal reason for thepoor accuracy of FCIL prediction by LSA lies in that the interaction betweenthe higher load and the lower load in the load spectrum could not be taken into account[2,4, 6]. In the present study, a good formula for FCIL is givenand a new model is tentatively proposed to predict FCIL of notched elementsunder variable amplitude loading. which might be thought of as two contribut-ions of this paper. In order to give a good formula for FCIL, fatigue tests were performed byusing the notched elements of LY12-CZ aluminum alloy sheets after overload-ing of given magnitudes and then fatigue-tested at constant amplitude loading.The formulas proposed in Refs. [1,6] were used to analyze the test results ofFCIL. Thus, the quantitative relation, eq. [11] in this paper, between theFCIL after overloading and the equivalent stress amplitude of overloading wasobtained. In order to develop a new model for predicting FCIL under variable amp-litude loading, the effects of some factors on FCIL after overloading shouldbe investigated. Test results show that the cyclic loading pattern and the resttime after overloading have no appreciable effect on that part of FCIL whichwas increased by overloading, and that after overloading the FCI threshold isstill the upper limit of the equivalent stress amplitude, below or equal towhich no fatigue damage occurs in the metal at notch root. Based on the above-mentioned test results and analysis and other considera-tions, a new model and its corresponding method of computation could be dev-eloped to predict FCIL of notched elememts under variable amplitude loading.The FCIL of LY12-CZ alloy notched elements under random load spectrumpredicted by above-mentioned model is in good agreement with test results. asreported in Ref. [16].