ABSTRACT
In recent years, the terahertz (THz or T-ray) technology (see Ref. [1]) has received a lot of attention because of its unique properties and capabilities that make it very attractive as a nondestructive evaluation (NDE) tool. This chapter covers some NDE applications of THz radiation-focusing on the detection of mechanical and heat-induced anomalies in porous media as well as THz radiation modeling. In many NDE applications, THz radiation is more suitable than ultrasonic energy. For example, ultrasonic energy cannot propagate through ceramic foam tiles used in the space shuttle thermal protection systems (TPS). Therefore, internal defects in these tiles cannot be detected by ultrasonic signals. TPS shows primarily three types of mechanical damage: (1) delaminations at the tile-substrate interface, (2) delaminations between different layers within
15.1 Introduction .......................................................................................................................... 771 15.2 What Is THz Radiation? ....................................................................................................... 772 15.3 Review of THz Radiation Applications ................................................................................ 773 15.4 Introduction to the Theory of Electromagnetic Radiation ................................................... 773
15.4.1 Plane Wave Equations .............................................................................................. 776 15.4.2 Polarized Plane Wave and Interface Interaction ....................................................... 778 15.4.3 Coherent THz Transmission Spectroscopy .............................................................. 781
15.5 Mechanical Damage Detection in Polymer Tiles by THz Radiation ................................... 782 15.5.1 Dielectric Material Property Determination ............................................................ 782 15.5.2 THz Finite Element Analysis ................................................................................... 784 15.5.3 THz-TDS Measurement of Blocks with Mechanical Damage ................................. 787
15.6 Heat-Induced Damage Detection by THz Radiation ............................................................ 792 15.6.1 Specimen Preparation ............................................................................................... 792 15.6.2 THz-TDS Measurement of Heat-Treated Blocks ...................................................... 793
15.7 DPSM for THz Radiation Modeling ..................................................................................... 795 15.7.1 Gaussian Beam Theory ............................................................................................ 797 15.7.2 Green’s Functions and DPSM ................................................................................... 797 15.7.3 Gaussian Beam Modeling in DPSM .........................................................................800 15.7.4 DPSM in Layered Structures ....................................................................................804 15.7.5 Dielectric Scatterer Modeling in DPSM ...................................................................807
