Near-field microwave nondestructive evaluation (NDE) techniques have shown great potential for disbond detection in multilayer dielectric composite structures. The high detection capability associated with these techniques stems from the fact that near-field microwave signals are sensitive to minute variations in the dielectric properties and geometry of the medium in which they propagate. In the past, the sensitivity of the near-field microwave NDE techniques to the presence and properties of disbonds in multilayer dielectric composites has been investigated extensively. However, a quantitative disbond thickness estimation method has yet to be introduced. In this paper, we propose a maximum-likelihood (ML) disbond thickness evaluation method utilizing multiple independent measurements obtained at different frequencies. We also introduce a statistical lower limit on the thickness resolution based on the mean-squared error in thickness estimation and a given confidence interval. The effectiveness of the proposed ML method is also verified by comparing simulation results with actual measurements.


Electrical and Computer Engineering


Air Force Research Laboratory (Wright-Patterson Air Force Base, Ohio)

Keywords and Phrases

Maximum Likelihood (ML); Multiple Frequency Measurements; Nondestructive Evaluation (NDE); Thickness Estimation; Composite Testing And Evaluation

International Standard Serial Number (ISSN)


Document Type

Article - Journal

Document Version

Final Version

File Type





© 2007 Institute of Electrical and Electronics Engineers (IEEE), All rights reserved.

Publication Date

01 Aug 2007