A Low Cost Microwave Imaging System Using a 6-Port Reflectometer for NDE of Composites

Abstract

The proliferation of composites in aerospace, automotive, and other industries has continued to increase largely due to the high strength to weight ratio. Improving the effective use of these materials is a continuing effort. If the material is compromised due to delamination or disbond, during manufacturing or use, the results can be catastrophic [1]. There is an urgent need for accurately detecting such defects in a rapid and reliable manner using cost effective technologies. Microwave NDE is well suited for analysis of low loss, or lossless fiber reinforced polymer (GFRP) composite structures or composites adhesively bonded to metal structures. In low loss dielectric materials, microwaves can penetrate and scatter off discontinuities. The scattered waves can provide information about the structural integrity of the material. In contrast to other NDE techniques, microwave NDE offers advantages such as non-contact detection, rapid far field inspection, and requires no coupling medium. However, microwave instruments such as a vector network analyzer (VNA) are expensive and difficult to transport to conduct experiments in the field. Such a system can be potentially replaced by a low-cost, single frequency device utilizing a 6-port measurement technique. In this paper, the feasibility of using a single frequency microstrip line 6-port reflectometer for NDE of composite samples has been studied. The first part of the paper deals with experiments that have been conducted using a VNA and X-band horn antenna to demonstrate the application of microwaves for defect detection in adhesively bonded metal-composite joints. The second part of the paper focuses on the design of the 6-port reflectometer to be used as a low cost imaging device for NDE of composite samples. Preliminary simulation results have been presented in this paper, followed by a fabricated prototype. Future work involves a thorough evaluation of device performance by conducting a variety of parametric studies and real-time experiments for detecting defects in composites.