Dual-polarized near-field microwave reflectometer for noninvasive inspection of carbon fiber reinforced polymer-strengthened structures

Carbon fiber reinforced polymer (CFRP) composites are increasingly being used to strengthen various components of civil, aerospace, and automotive infrastructures. Subsequent to the application of CFRP, evaluation of the structural integrity of the strengthened members becomes a critical issue. Microwave noninvasive inspection techniques have been successfully used for this purpose. To this end, a novel near-field microwave inspection system that employs a dual-polarized reflectometer for detecting defects, such as disbonds between CFRP laminates and strengthened structures, was developed. It is shown that this system is capable of automatic removal of the influence of undesired standoff distance (or surface roughness) variations. It can simultaneously generate three images of a defect: two at orthogonal polarizations and one after the influence of standoff distance variations is removed using the information provided by the two images. This paper discusses the unique features of a unidirectional CFRP laminate that enabled the design of this unique system, as well as the design of the dual-polarized reflectometer at the X-band (8.2-12.4 GHz). The detail design of the reflectometer, as well as the results of several measurements conducted on an actual bridge, whose members were strengthened with CFRP laminates, are also presented, illustrating the practical utility of this system.