中文摘要：

多层金属结构广泛应用于石油、化工、航天和能源等工业领域的大型、复杂装备中。由于服役工况一般较为恶劣，该结构亚表面层材质性能会出现劣化，直接威胁到大型、复杂装备的安全运转。通过脉冲涡流检测理论仿真，发现了一种基于脉冲涡流差分信号的新型提离交汇点。通过对该交汇点的特性研究，提出了一种能够对多层结构亚表面层材质劣化进行检测和评估的方法。由于当前的脉冲涡流检测技术主要针对金属结构表面层材质劣化进行检测，所以该方法的提出拓展了脉冲涡流检测技术的应用领域。通过数值仿真，所提方法的正确性及实用性得到了验证。

英文摘要：

Stratified metallic structures are widely used in various industries such as petroleum, chemical, aerospace and energy fields. Subsurface Material Degradation（SMI）） occurs within the structures due to rigorous working environment, which influences the operation of crucial components in the Iarge-scale complex apparatus. In this paper, a new Lift-off Intersection Point（LOI） has been discovered via theoretical study of Pulsed Eddy Current technique（PEC）. Based on the investigation of the characteristics of the new LOI, a method for evaluation of SMD in stratified metallic structures has been proposed. The method extends the PEC application currently only for evaluation of surface material degradation. Through numerical simulations, the validity and practicability of the proposed method have been investigated.