Abstract-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.

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Carbon fiber reinforced polymer (CFRP) composites are increasingly being used in strengthening structures of civil infrastructures, aerospace and automotive industries. Subsequent to the application of CFRP, detection and evaluation of the structural integrity of the member becomes a critical issue. Microwave non-invasive inspection techniques have been successfully used for inspecting these structures. A novel inspection system with a dual-polarized microwave near-field waveguide probe for detecting defects such as disbond between CFRP laminates and strengthened structures is presented. It is shown that this system provides automatic removal of the influence of undesired standoff distance (or surface roughness) variations. It may simultaneously generate three images of defects: two at orthogonal polarizations and one after the influence of the undesired variations is removed. The novel measurement system provides for detection and evaluation of different types of defects in CFRP reinforced composite structures, reducing the time required for data acquisition. This paper also presents the design of the dual-polarized reflectometer at X-band (8.2-12.4 GHz) and details of the measurement system as well as the results of application of this system for inspecting CFRP reinforced composites cement-based structures including some from an actual bridge. The results clearly illustrate the utility of this system for non-invasive inspection of CFRP strengthened structures.

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