Double-Sided Terahertz Imaging of Multilayered Glass Fiber-Reinforced Polymer

Przemyslaw Lopato
2017 Applied Sciences  
Polymer matrix composites (PMC) play important roles in modern industry. Increasing the number of such structures in aerospace, construction, and automotive applications enforces continuous monitoring of their condition. Nondestructive inspection of layered composite materials is much more complicated process than evaluation of homogenous, (mostly metallic) structures. Several nondestructive methods are utilized in this case (ultrasonics, shearography, tap testing, acoustic emission, digital
more » ... iography, infrared imaging) but none of them gives full description of evaluated structures. Thus, further development of NDT techniques should be studied. A pulsed terahertz method seems to be a good candidate for layered PMC inspection. It is based on picosecond electromagnetic pulses interacting with the evaluated structure. Differences of dielectric parameters enables detection of a particular layer in a layered material. In the case of multilayered structures, only layers close to surface can be detected. The response of deeper ones is averaged because of multiple reflections. In this paper a novel inspection procedure with a data processing algorithm is introduced. It is based on a double-sided measurement, acquired signal deconvolution, and data combining. In order to verify the application of the algorithm stress-subjected glass fiber-reinforced polymer (GFRP) was evaluated. The obtained results enabled detection and detailed analysis of delaminations introduced by stress treatment and proved the applicability of the proposed algorithm. (TDS) are utilized in this frequency range [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] . The first group of applications refers to spectroscopic evaluation of materials or potential inclusions compositions, i.e., polymer evaluation [16], drugs identification [19, 20] , explosives detection [19] , biological tissue identification in the case of biotechnological and medical applications [21, 22] , hydration/moisture level determination [23, 24] , polymerization state monitoring [25] , chemical mixtures evaluation [26] , and determination of tea geographical origin [27] . The second group of applications refers to observations of the inner structure of an object under test (OUT), similar to ultrasonic testing or the pulsed ground penetrating radar (GPR) technique. In this case homogenous or layered dielectric materials are examined in order to detect irregularities or defects [14, 15, [28] [29] [30] . Non-conductive coatings of dielectric or conductive materials, as well, as layers thicknesses can be also evaluated [28] . In the case of integrated circuits the inner structure is monitored [31] . TDS is also utilized in order to evaluate the integrity of layered tablet structures. Another group of applications refers to terahertz tomographic imaging [32] [33] [34] [35] [36] . Tomography refers to the cross-sectional imaging of an object by measuring transmitted or/and reflected waves (ray). There are various terahertz imaging options: time of flight reflection tomography (THz ToFRT), diffraction tomography (THz DT), computed tomography (THz CT), tomography with a binary lens, and digital holography. Continuous wave terahertz imaging [37-39] is a generally faster and less complicated technique compared to TDS. In this case the source (typically horn antenna, photoconductive antenna or far infrared laser) illuminates object under test and detector (photoconductive, pyroelectric, or bolometric) or a matrix of detectors receive the electromagnetic wave. In most cases this technique provides power or intensity information and phase data is not available. Higher power of sources and utilization of detector matrices enables inspection of thicker and bigger structures in comparison to TDS. Main advantages associated with terahertz imaging technique are:
doi:10.3390/app7070661 fatcat:hfiunzagcnaybdui2ssh2srkpq