Evaluation of Microwave Methods for Thickness Measurements of Liquid Shim Material [chapter]

D. D. Palmer, S. C. Buckner, W. S. Samford
1993 Review of Progress in Quantitative Nondestructive Evaluation  
INTRODUCTION This paper describes the use of a microwave interferometer technique which measures the thickness of liquid shim material applied to composite surfaces. Liquid shim, which is a low dielectric material, is applied to spar cap surfaces in order to maintain wing skin mold-line tolerances while reducing stresses at the location of fastener holes. For this application, the thickness of the shim material must be controlled within specified limits. Microwave reflection techniques provide
more » ... techniques provide an alternative nondestructive approach to liquid shim thickness measurements. Microwave nondestructive testing (NDT) methods have been applied successfully to specific testing problems for more than 40 years [1]. However, microwave techniques can be best classified as complementary or specialized techniques when compared to ultrasonic and eddy current inspection methods. Their general use has been limited and microwave NDT installations, as used by the aerospace industry, have typically been more experimental than full-scale production facilities [2]. Microwave NDT applications have been successfully applied to both non-metallic and metallic materials. A wider range of applications can be found for non-metallic, nonconductive materials as microwaves can freely penetrate these materials. Some applications for non-metallic materials include detection of delaminations and porosity, measurement of anisotropy and thickness, and determination of moisture content. Applications for metallic materials are limited to spatial measurements or surface imperfections as microwaves readily reflect from these materials. Some of these applications include measurement of displacement or detection of surface breaking anomalies [2]. BACKGROUND
doi:10.1007/978-1-4615-2848-7_251 fatcat:g3cozs652rclthiwhw3l5djgoq