A review on microwave non-destructive testing (NDT) of composites

Dielectric composite materials are ubiquitously used across a wide variety of industries. Inherent features encompassing immunity against corrosion, light weight and reasonable strength have allowed composites to be seamlessly integrated within critical applications in the oil and gas, aerospace, energy, transportation and defense sectors. While composites are advantageous to their metal counterparts in certain applications, they are not free from vulnerabilities to defects which could potentially compromise their structural integrity. Being prone to numerous flaws and anomalies during operation and manufacturing, routine inspections and periodic health monitoring of composites is of paramount importance. Various non-destructive testing (NDT) techniques have successfully been applied for composite inspection. However, most modalities have had limited success in reliably detecting and localizing flaws/defects in thick dielectric composites. Among the few successful techniques, microwave NDT has repeatedly shown potential for detection of surface/sub-surface defects in such composites. While other NDT techniques have been thoroughly reviewed in the literature, microwave NDT has not been substantially covered. In this paper, a review of various microwave NDT approaches towards the inspection of various composites is presented. The utility of microwave based probes to reliably detect different types of flaws in composites is highlighted. A succinct overview of effectiveness of microwave techniques used to detect a variety of defects in composites is represented in tabular form for convenience of readers to visualize the progress of microwave based NDT methods in the past decade.