根据裂纹的特点，可以把它近似为一个没有厚度的面缺陷，其作用由分布在裂纹面上的等效电流偶极子层代替。
Considering that the crack thickness is usually very small, it can be taken as a'surface defect' with zero thickness.
若忽略磁偶极子对的边缘效应，并将其等效成束缚电流元，可用安培定律来确定其受力大小和方向。
To lose sight of paired magnetic dipole brim and to equate bound current element, making certain direction and magnitude of a force upon it by Ampere Law.
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