无损检测专业英语教学课件ppt作者晏荣明unit6Eddycurrenttesting课件.pptVIP

6.1 Introduction If another electrical conductor (test material) is brought into the close proximity to this changing magnetic field, eddy current that flow in a circular path will be induced in this second conductor due to mutual induction By measuring changes in the resistance and inductive reactance of the coil, information can be gathered about the test material. This information includes the electrical conductivity and magnetic permeability of the material, and whether it contains cracks or other defects. 6.1 概论 如果另一个导体（被检材料）靠近变化的磁场，由于互感就会在该导体中感应出沿圆形流动的涡流。 通过测量线圈中阻抗和感抗的变化，就可以得到被件材料中的信息。这些信息包括被件材料的电导率、磁导率以及是否存在裂纹以及其他的缺陷。 6.2 Theory and Principles6.2.1 Skin effect and Standard Penetration Depth The depth that eddy currents penetrate into a material is affected by the frequency of the excitation current and the electrical conductivity and magnetic permeability of the specimen. The depth at which eddy current density has decreased to 1/e, or about 37% of the surface density, is called the standard depth of penetration 6.2 理论和原理6.2.1 集肤效应和标准透入深度 涡流在材料中的渗透深度受激励电流的频率以及材料的电导率和磁导率的影响。 电流密度下降到表面的1/e 或大约37%的深度称为标准透入深度。 6.2 Theory and Principles6.2.2 Lift-off effect  Liftoff can be used to make measurements of the thickness of nonconductive coating such as paint that hold the probe a certain distance from the surface of the conductive material. 6.2 理论和原理6.2.2 提离效应 提离效应可以用于测量像漆层这样的非导电涂层的厚度，因为涂层使探头与导体表面之间保持一定的距离。 6.2 Theory and Principles6.2.3 Selection of Inspection Frequencies At the lower frequencies, the penetration δ is relatively high, but the sensitivity to the detection of discontinuities is relatively low; the reverse is true at higher frequencies. In the case of a cylindrical shape of radius r, frequency selection depends on the characteristic frequency νc (also known as limit frequency) which is derived by considering the strength of the magnetic field penetrating into the specimen. 6.2 理论和原理6.2.3 检测频率选择 频率较低时，渗透深度比较深，但是检测灵敏度相对要低，反之亦然。 对于半径为r的圆柱体，检