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通过合理地控制孕育条件和冷却速度，获得了化学成分基本相同，而石墨大小、数量互异的铸态铁素体球铁试样，测定了它们的疲劳门槛值△Ｋ＿（ｔｈ）．结合裂纹附近的金相组织分析发现：在球化良好的情况下，△Ｋ＿（ｔｈ）值有随单位面积石墨球数量Ｎ的减少而增大，或随平均石墨间距和平均石墨球径的增加而增大的趋势，通过简化的力学模型对这种有悖于一般强度变化规律的反常特性进行了分析讨论，并作出了相应的解释。
By reasonably controlling the inoculation conditions and the cooling rate, the as-cast ferritic ductile iron samples with the same chemical composition and different sizes and numbers of graphite were obtained, and their fatigue threshold △ K th was measured. In combination with the analysis of the microstructure of the crack, it is found that the ΔK th value increases with the decrease of the number N of graphite balls per unit area, or increases with the average graphite pitch and the average graphite ball diameter However, with the trend of increasing, the anomalous characteristic which is contrary to the regular change of the general intensity is analyzed and discussed through the simplified mechanical model, and the corresponding explanation is given.