Propagation of Millimeter Waves in Composite Materials

Composite materials have become indispensable in a wide variety of technical fields, where they are viewed as the key technological component. Due to their structural complexity, these materials are prone to errors while in the manufacturing process, thus non-destructive inspecting becomes a crucial step in the processing chain and further maintenance of said material. However, a tomographic characterization of these components is often only possible to a limited extent-a capability which is essential to exploit the full potential of this technology. Measurement systems based on millimeter waves (mmWs) have the potential to achieve exactly that, by providing valuable information on the internal structure of such components. Yet, these non-destructive and non-contact methods have not found wide usage in practical applications. One reason for this is that the effects composite materials have on the propagation behavior of mmW signals have not been a major subject of study in the past. Therefore, a model for predicting the propagation properties of mmW signals in composite materials is presented here. Based on coupled wave analysis, the propagation of arbitrary electromagnetic signals in composite materials can be simulated. These results can be directly incorporated into the design of respective measurement systems. The introduced model is verified through a comparison of simulation and measurement results, obtained via a focused frequency modulated continuous wave (FMCW) radar, operating in the $H$ -band (220-325 GHz), directed at glass fiber reinforced plastic samples.

I. Introduction

The physical and mechanical properties of composite materials, later in this text referred to as “composites,” can be specifically tailored to meet the requirements of a wide range of applications. Some of the most common composites are glass fiber reinforced plastics (GFRPs) and carbon fiber reinforced plastics (CFRPs), which are made from glass and carbon fibers, respectively, embedded in an epoxy resin matrix. Due to their lightweight, high specific strength, and stiffness, composites make for an ideal choice of construction material for wind turbines [1], ships [2], airplanes, and more [3]. To ensure that all components work as designed, reliable measurement methods are necessary to characterize the composites and classify defects that are not perceivable to optical inspection. Defects in composites that deteriorate the mechanical stability can range from resin nests, delamination, undulations, inclusion of foreign bodies, moisture, and air entrapment. Among the most common non-destructive measurement methods for the characterization of composites are ultrasonics and thermography [4]. However, both these methods have their own limitations, be it the requirement of a coupling medium between sample under test (SUT) and sensor or the fairly limited depth information that can be acquired. It has been shown that a promising extension of the measurement portfolio can be sensor systems based on millimeter waves (mmWs), which enable non-contact and non-destructive tomographic imaging of composites [5]–[10]. In order to exploit the full potential of this technology, theoretical models of mmW signal propagation are required, which enable the optimization and estimation of the performance of systems and applications based on composites. They are necessary to optimize systems for the application and estimate their potential and limitations.