I. Introduction
Acarbon fiber-reinforced polymer (CFRP) is widely used in aerospace, automotive industry, and civil engineering, due to its extraordinarily high specific strength and specific modulus. However, in the long-term applications, CFRP inevitably suffers from visible degradation as well as barely visible internal damage before catastrophic failure of the structure, such as delamination, fiber breakage, and matrix crack [1]. To prevent costly sudden fail, it is essential to regularly detect and assess the structural damage of CFRP [2], [3], [4].