Advanced Microwave Inspection: Evolution of the Method

Introduction

In a world composed of many kinds of materials and structures, nondestructive testing (NDT) has been important in ensuring the quality of those materials and structures. Ever since the first use of the oldest such technique, visual inspection, there has been a need to develop better and cheaper ways of inspecting the many materials of construction that are employed around the world. While visual inspection allows one to verify the external characteristics of the material under test (MUT), it has always been important to be able to view and measure the internal characteristics of the MUT without taking the extreme step of cutting it apart or damaging it. Thus, the notion of nondestructive volumetric inspection techniques was developed.

Over the years, multiple volumetric NDT techniques have appeared, and many are used on a daily basis across the world to inspect and ensure the quality of the MUT. The volumetric methods all involve the introduction of energy of some sort into the material and measurement of the material’s response. The most common energies used for this purpose can be grouped into two primary categories: acoustic and electromagnetic.

Acoustic techniques include ultrasonic methods, while electromagnetic techniques include radiography and microwave techniques. Radiography and ultrasound are used to perform most volumetric inspections on both metallic and non-metallic materials, even though they may not be best suited for them in some cases. This is mainly because the basic equipment is readily available and there is quite a bit of experience in these methods. Microwave inspection, by comparison, is a relatively new method. Although its potential as an NDT technique was described in several research papers published in the 1950s and 1960s, it has not yet gained widespread use. This is partly due to the fact that it is only practical for the volumetric inspection of non-metallic materials and, until recently, the structures and materials of most interest were predominantly metallic.

Microwave Inspection

Microwave NDT is defined as the nondestructive inspection of a material using electromagnetic radiation (EMR) in the microwave frequency range as the interrogating beam. The microwave frequency range is often defined as lying between radio waves and far infrared, or in the 300 MHz to 300 GHz frequencies. These frequencies roughly correspond to wavelengths of between 1m and 1 mm in a vacuum, respectively.

The effectiveness of a microwave inspection relies in large part on the amount of EMR that is absorbed and reflected from defects or flaws within the material under test. These reflected signals are acquired in an XY matrix, and then that data field is used to create an image that can be interpreted. The nature of the flaws that are detected can be properly categorized by size, shape, and magnitude.