Application of Radio Frequency Noiselet Waveforms for Nondestructive Imaging of Multilayered Structures

Open Access
Kim, Tae Hee
Graduate Program:
Electrical Engineering
Doctor of Philosophy
Document Type:
Date of Defense:
August 30, 2018
Committee Members:
  • Ram Mohan Narayanan, Dissertation Advisor
  • Ram Mohan Narayanan, Committee Chair
  • James Kenneth Breakall, Committee Member
  • Timothy Joseph Kane, Committee Member
  • Namiko Yamamoto, Outside Member
  • Radio Frequency
  • Noiselet Waveform
  • Microwave
  • Synthetic Aperture Radar
  • Radar Imaging
  • Nondestructive Testing
Nondestructive testing (NDT) is an inspection method used to determine the inner structure of a material without inflicting damage upon it. There are several NDT methods which are commonly used, including: ultrasonic, eddy current, liquid penetrant and radiographic (or X-ray) testing. Each method has its advantages and disadvantages. For example, certain methods require additive materials, or an in-contact testing scheme, whereas others may raise safety concerns for humans and the environment—microwave NDT, has the potential to address these issues. Fiber Reinforced Polymer or Plastic (FRP) composites have been rapidly increasing in the aerospace, automotive and marine industry, and civil engineering, because these composites show superior characteristics such as outstanding strength and stiffness, low weight, as well as anti-corrosion and easy production. Generally, the advancement of materials calls for correspondingly advanced methods and technologies for inspection and failure detection during production or maintenance, especially in the area of nondestructive testing (NDT). Among numerous inspection techniques, microwave sensing methods can be effectively used for NDT of FRP composites. FRP composite materials can be produced using various structures and materials, and various defects occur due to environmental conditions encountered during operation. However, reliable, low-cost, and easy-to-operate NDT methods have not been developed and tested. FRP composites are usually produced as multilayered structures consisting of fiber plate, matrix and core. Therefore, typical defects appearing in FRP composites are disbondings, delaminations, object inclusions, and certain kinds of barely visible impact damages. In this dissertation, we propose a microwave NDT method, based on synthetic aperture radar (SAR) imaging algorithms and ultrawideband radio frequency waveforms, for stand-off imaging of internal defects. When a microwave signal is incident on a multilayer dielectric material, the reflected signal provides a good response to interfaces and transverse cracks. An electromagnetic wave model is introduced to delineate interface widths or defect depths from the reflected waves. For the purpose of numerical analysis and simulation, multilayered structure samples with artificial defects are assumed, and their SAR images are obtained and analyzed using a variety of high-resolution ultrawideband (UWB) wavelet and noiselet waveforms.