Reduction of Sensitivity to Surface Roughness and Slight Standoff Distance Variations in Microwave Testing of Thick Composite Structures


Testing of thick sandwich composite structures is an important and challenging issue. Standard nondestructive testing (NDT) methods are not always capable of testing these structures for the presence of thin disbands and delaminations. However, near field microwave NDT methods have shown great potential for this purpose. Although these methods are very sensitive to the presence of thin disbands and delaminations, they are also very sensitive to changes in the standoff distance. This change can be produced as a result of the scanning procedure, surface roughness or slowly changing undulations associated with a composite specimen. This paper presents the results of an investigation in which complex sandwich composites with authentic anomalous regions were used, as well as the development of a microwave testing system capable of automatically eliminating the adverse influence of standoff distance and surface, roughness changes. Results produced at a frequency of 23.8 GHz showed many of the indications to be easily detected including a very thin disband, delamination in foam core, and impact damage. In addition, the versatility of this system to be successfully applied to very thick and relatively thinner composite structures is also demonstrated. The overall approach and its subsequent results are presented in this paper.


Electrical and Computer Engineering

Keywords and Phrases

Disbond; Microwave Nondestructive Testing; Near Field; Standoff Distance; Surface Height; Thick Composites; Composite Structures; Delamination; Microwaves; Parameter Estimation; Permittivity; Sandwich Structures; Surface Roughness; Nondestructive Examination; Microwave NDT; Near Field; Standoff Distance; Surface Height; Thick Composites

International Standard Serial Number (ISSN)


Document Type

Article - Journal

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© 2002 American Society for Nondestructive Testing, Inc., All rights reserved.

Publication Date

01 Feb 2002

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