非比例载荷下缺口件疲劳寿命预测

FATIGUE LIFE PREDICTIONS OF NOTCHED SPECIMENS UNDER NON-PROPORTIONAL LOADINGS

针对Mod.9Cr-1Mo铁素体钢缺口件进行了一系列非比例载荷低周疲劳试验,采用直流电位差法测量裂纹萌生寿命,比较了缺口半径和应变路径对疲劳裂纹萌生寿命的影响。结果表明,缺口件裂纹萌生寿命占总寿命的比例与材料类型、应变路径相关,更与缺口半径尺寸直接相关。同一路径下,随着缺口半径增加,裂纹萌生寿命所占比例增大。采用Neuber律进行缺口局部应力-应变损伤的计算,结合Smith-Watson-Topper(SWT)模型和Kandil-Brown-Miller(KBM)模型进行疲劳寿命预测。结果表明,除单轴路径和比例路径外,SWT模型得到的预测结果偏于不安全;KBM模型除对单轴预测偏于保守外,其他预测值较好,总体预测结果位于2倍分散带内。

A series of low cycle fatigue tests were conducted for notched specimens of Mod.9Cr-1Mo ferrite steel under multi-axial loadings. The crack initiation life was measured by the direct current potential difference method(DC-PDM). The effects of the notched radius and the strain path on crack initiation life were discussed. It is shown that the ratio of the crack initiation life to the entire fatigue life is related to material type, strain path, and especially the notched radius. Under the same strain path, the ratio increases with the notched radius. Local stress and strain were calculated based on the Neuber rule, and the results were employed to predict the fatigue life using the Smith-Watson-Topper(SWT) approach and the Kandil-Brown-Miller(KBM) approach. On the whole, the prediction from the SWT approach was optimistic, except for the uni-axial and proportional strain paths. On the other hand, the KBM approach offered a more reliable prediction, apart from being conservative under circumstances of uni-axial strain paths. The overall prediction results fell within a two-time scatter zone.