Terahertz Technique: A Non-Destructive Testing Method for Composite Defect Detection [Measurement Methodology]

Fiber-reinforced composites are currently employed in a multitude of aerospace applications as an alternative to metallic materials due to their high specific strength and high corrosion resistance. However, internal defects such as delamination and cracking can lead to a significant degradation of their mechanical properties over their lifetime service. Delamination is the most common but fatal defect in composites that can occur during manufacture, transport, or utilization, and leads to a drastic reduction in interlaminar strength. It is therefore essential to be able to reliably detect delamination defects to ensure the safety of aerospace equipment in service. Terahertz (THz) techniques utilize electromagnetic waves in the range of 0.1 THz to 10 THz to inspect the internal defects of the non-conductive components, providing high spatial resolution and large penetration into composite materials, rendering them suitable for delamination detection. This article provides a general introduction to the principle and composition of two typical types of THz measurement systems: terahertz time-domain spectroscopy (THz-TDS) and THz continuous wave (CW) measurement systems. Furthermore, this paper discusses how complex THz time-domain signals from multilayer composites can be processed by advanced signal processing techniques.