Abstract The metal magnetic memory (MMM) method is a nondestructive testing method to inspect the early damage of steel structures. A four-point bending fatigue beam was inspected by this method during the whole fatigue life. The self-magnetic-leakage field (SMLF) values were measured during the test of the Q345B steel beam. Frequency of 3 Hz and stress ratio of 0.2 contributed to the fatigue life of 100,180 cycles. Test results indicate that both the normal and tangential component of the SMLF values can identify the stress concentration and the crack zones. The gradient can reflect the degree of damage. Furthermore, in order to study the change law of the SMLF values in the crack zone, an improved magnetic dipole model considering the crack dimensions and shape is proposed. The model predictions are in very good agreement with the corresponding test results. Lastly, three characteristic parameters, i.e. average ΔHp(y) field values, resultant magnetic field value H and magnetic parameter m, were extracted from the test data. These parameters can judge the fatigue stage and the damage degree of the specimen. Therefore, the research lays the experimental and theoretical foundation for using the MMM method to inspect steel structures under fatigue bending loads.

中文翻译：

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Q345B钢梁四点弯曲疲劳试验金属磁记忆检测

摘要 金属磁记忆法是一种检测钢结构早期损伤的无损检测方法。在整个疲劳寿命期间，用这种方法检查了四点弯曲疲劳梁。在 Q345B 钢梁的测试过程中测量了自漏磁场 (SMLF) 值。3 Hz 的频率和 0.2 的应力比促成了 100,180 次循环的疲劳寿命。测试结果表明，SMLF 值的法向分量和切向分量都可以识别应力集中和裂纹区域。梯度可以反映损坏程度。此外，为了研究裂纹区SMLF值的变化规律，提出了一种考虑裂纹尺寸和形状的改进磁偶极子模型。模型预测与相应的测试结果非常吻合。最后，从测试数据中提取三个特征参数，即平均ΔHp(y)场值、合成磁场值H和磁参数m。这些参数可以判断试件的疲劳阶段和损伤程度。因此，该研究为使用MMM方法检测疲劳弯曲载荷下的钢结构奠定了实验和理论基础。