考虑短裂纹的应力-能量耦合法预测缺口疲劳极限
PREDICTING NOTCH FATIGUE LIMITS BY COUPLED STRESS-ENERGY CRITERION WITH CONSIDERATION OF THRESHOLD SIF OF SHORT CRACK
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摘要: 基于有限断裂力学理论的应力—能量耦合方法, 有效克服了经典线弹性断裂力学无法求解缺口结构裂纹萌生问题的局限, 因而被广泛应用于缺口疲劳极限的评估. 然而, 该方法通常会高估疲劳极限, 导致预测结果偏于非保守(偏危险), 因此亟需对原方法进行修正. 鉴于缺口萌生裂纹本质上属于疲劳短裂纹, 而传统应力—能量耦合方法采用长裂纹门槛值计算等效应力强度因子, 这与裂纹演化的物理机制不符. 因此, 将材料短裂纹门槛值参量引入能量方程以修正应力—能量耦合方法. 为了确定缺口短裂纹门槛值的大小, 选取Meggiolaro缺口短裂纹门槛值模型, 并基于大量实验结果确定了Meggiolaro模型参数的最优值. 为验证方法的有效性, 对50组V型和中心圆孔缺口试样疲劳极限进行预测, 并与试验结果进行了比较. 结果表明, 修正后的平均预测误差由原方法的9.9%降至3.9%, 预测精度显著提升. 同时, 正向预测误差(即预测值大于试验值)的比例从78%降至60%, 预测结果更趋于中性, 有效改进了原方法预测结果偏危险的缺点. 进一步分析结果表明, 随着缺口半径的减小, 基于缺口疲劳极限和停留裂纹计算得到的应力强度因子值与短裂纹门槛值越接近, 因而预测误差也越小.Abstract: The stress-energy coupled criterion based on the finite fracture mechanics is widely used to evaluate the notch fatigue limit, as it overcomes the disadvantage that the classic linear elastic fracture mechanics is not suitable for solving the crack initiation of notches. However, the prediction results of this method are often larger than the experimental results. Therefore, it is necessary to modify the coupled criterion. It is clear that the cracks initiated at notches are actually fatigue short cracks. And the stress-energy coupling method adopts the long crack threshold to calculate the equivalent stress intensity factor, which is inconsistent with the actual situation. Hence, the material parameter of the short crack threshold is incorporated into the energy equation to modify the coupled criterion. In order to determine the value of the notch short crack threshold, the Meggiolaro notch short crack threshold model was employed, and the optimal value of the parameter in the Meggiolaro model was determined based on a large number of experimental results. To verify the effectiveness of the modified method, the fatigue limits of 50 groups of notched specimens were predicted and compared with the experimental results. The results show that the average prediction error is significantly reduced from 9.9% to 3.9% after modification, indicating a substantial improvement in prediction accuracy. Meanwhile, the proportion of positive prediction errors (i.e., predicted values are higher than experimental values) decreases from 78% to 60%, making the prediction results more neutral and effectively improving the shortcoming of non-conservative predictions in the original method. Further analysis results reveal that as the notch radius decreases, the stress intensity factor calculated based on the notch fatigue limit and the initiating crack becomes closer to the short crack threshold, leading to smaller prediction errors.
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