Microwave characterization and microwave non-destructive testing applied to carbon fibre reinforced polymers
Publication Date
December 13, 2022
Creator
Abstract
The instrumentations and the developments presented in this thesis aim to address the challenges of the applicability of microwave characterization and microwave non-destructive testing (MNDT) methods to carbon fibre reinforced polymers (CFRP).
Microwave characterization of carbon fibre composites has become an issue particularly relevant in recent years, as the prevalence of using CFRP based materials has significantly increased in many electromagnetic applications, including radio frequency (RF) absorption or electromagnetic interference (EMI) shielding. In addition, MNDT methods represent an effective solution in detecting defects and flaws within material structures, as they offer the advantage to overcome the problems of traditional NDT techniques such as coupling and danger coming from ionizing radiation.
However, when it comes to the analysis of CFRP, the use of these techniques is limited by the composite relatively high conductivity of ≃10^4 S/m. The electromagnetic signal is unable to penetrate the material, making the electromagnetic characterization difficult and restricting the MNDT applicability to surface defects (e.g., impact damage, indentation).
In regards to these observations, the shielding effectiveness of woven and unidirectional carbon fibre composites is studied at low-band frequencies, to achieve a more detailed definition of the electromagnetic behaviour of woven and unidirectional carbon fibre composites. The percentages of power reflected and transmitted are reported and the electrical behaviour of the samples suggests that a single layer of carbon fibre fabric is able to shield more than 99 % of the electromagnetic radiation.
Consequently, a waveguide-based measurement configuration is proposed, as a modification to the traditional completely-filled waveguide technique, for microwave characterization of relatively thin and high-loss materials. This method is aimed to overcome the large measurement errors caused by gaps and cross-sectional distortions, which are of great concern when measuring high-loss samples, such as CFRP. The influence of critical parameters, namely; dielectric constant, loss factor, material thickness and waveguide misalignment, on measured complex permittivity of several diverse samples, is fully investigated through numerical electromagnetic simulations and measurements at X-band (8.2-12.4 GHz). By the virtue of the electromagnetic properties associated with high-loss dielectric materials, it was found that this technique coupled with the NRW/Backer-Jarvis conversion procedure is a capable method for measuring high-loss dielectric materials.
After having established a reliable method of electromagnetic characterization, the complex permittivities of thin and high-loss unidirectional composites are measured at X-band and Ku-band. The use of unidirectional CFRP coupled with appropriate polarization of the electric field is the key to allow transmission of the signal through the CFRP composite. In addition, a mesoscale model for numerical electromagnetic simulations of unidirectional composites is adopted.
Extensive simulation results and free-space measurements are also presented at Ka-band, to further determine the permittivity of unidirectional CFRP and to assess damage detection in case of delamination of a unidirectional composite laminate.
Finally, synthetic aperture radar (SAR) imaging method is applied at Ka-band frequencies, to detect and characterize manufacturing defects such as gaps, overlays, missing tow, fibre misalignment and fibre waviness in carbon fabric and CFRP laminates. This approach makes particular use of the changes in the scattered signal with the orientation of a polarizing target (i.e., conducting fibres).
Item Type
ethesis
Thesis Type
PhD
Subjects (LC)
Associated Schools / Departments
Department of Electrical and Electronic Engineering (UK)
Department of Mechanical, Materials and Manufacturing Engineering (UK)
eprints ID
71492
UoN Repository URI
Except where otherwise noted, this item's license is described as
File(s)![Thumbnail Image]()
Name
Thesis Munalli.pdf
Type
Full-text
Description
Examined
Size
13.76 MB
Format
Adobe PDF
Checksum (MD5)
c1af14216bc5725bd445162f48969e89