Dual-Polarized Microwave Near-Field Reflectometer for Non-Invasive Inspection of Carbon Fiber Reinforced Polymer (CFRP) Strengthened Structures

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