Metamaterials have been increasingly used in the past few decades in the design of novel microwave circuits and sensor systems. This paper deals with the design of a split-ring resonator (SRR) sensor for microwave nondestructive evaluation (NDE) applications. Building on prior work in this field, a modified design of the sensor is used for imaging composites. The tip of the sensing ring is extended, and a reference ring is introduced to tailor fit the sensor for imaging and NDE applications. A built-in calibration system, integration of multiple sensors (high and low frequencies), super resolution capability and high Q factor are some of the benefits of the sensor. This paper presents the design and development of the sensor system along with experimental results demonstrating the feasibility of the approach for detection of sub-wavelength defects in composites and additive manufactured metals.

Metamaterials are engineered periodic structures designed to have unique properties not encountered in naturally occurring materials. One such unusual property of metamaterials is the ability to exhibit negative refractive index over a prescribed range of frequencies. A lens made of negative refractive index metamaterials can achieve resolution beyond the diffraction limit. This paper presents the design of a metamaterial lens and its use in far-field microwave imaging for subwavelength defect detection in nondestructive evaluation (NDE). Theoretical formulation and numerical studies of the metamaterial lens design are presented followed by experimental demonstration and characterization of metamaterial behavior. Finally, a microwave homodyne receiver-based system is used in conjunction with the metamaterial lens to develop a far-field microwave NDE sensor system. A subwavelength focal spot of size 0.82λ was obtained. The system is shown to be sensitive to a defect of size 0.17λ × 0.06λ in a Teflon sample. Consecutive positions of the defect with a separation of 0.23λ was resolvable using the proposed system.

The roots are a vital organ for plant growth and health. The opaque surrounding environment of the roots and the complicated growth process means that in situ and non-destructive root phenotyping face great challenges, which thus spur great research interests. The existing methods for root phenotyping are either unable to provide high-precision and high accuracy in situ detection, or they change the surrounding root environment and are destructive to root growth and health. Thus,we propose and develop an ultra-wideband microwave scanning method that uses time reversal to achieve in situ root phenotyping nondestructively. To verify the method’s feasibility, we studied an electromagnetic numerical model that simulates the transmission signal of two ultra-wideband microwave antennas. The simulated signal of roots with different shapes shows the proposed system’s capability to measure the root size in the soil. Experimental validations were conducted considering three sets of measurements with different sizes, numbers and locations, and the experimental results indicate that the developed imaging system was able to differentiate root sizes and numbers with high contrast. The reconstruction from both simulations and experimental measurements provided accurate size estimation of the carrots in the soil, which indicates the system’s potential for root imaging.

Composites are being increasingly used in various industries due to their lower cost and superior mechanical properties over traditional materials. They are nevertheless vulnerable to various defects during manufacturing or usage which can cause failure of critical engineering structures. Hence, there is a growing need for nondestructive evaluation (NDE) of composites to detect such defective structures and avoid significant loss and damages. Microwave NDE has several advantages over other existing NDE techniques for detecting defects or faults in non-conducting composites or dielectrics. One of the primary benefits of microwaves is large probe-standoff distances which allow for rapid scan times. However, the resolution of such far-field microwave sensors is diffraction limited. Metamaterial-based lens, also known as “superlens,” can achieve resolution beyond the diffraction limits due to its unique electromagnetic (EM) properties. This contribution focuses on the physical design of a metamaterial lens. The theory underlying the design of a metamaterial lens is presented followed by simulation and experimental results. This paper also investigates the feasibility of using the metamaterial lens for improving the resolution of microwave imaging in NDE of composites.