Kristen M. Donnell

Application of Active Microwave Thermography to delamination detection

Health monitoring of infrastructure is very important in the transportation and infrastructure industries. Many nondestructive testing (NDT) techniques have been applied for structural health monitoring including microwave NDT, ultrasound, thermography, etc. Due to the complex materials (composites, concrete, etc.) commonly used, it may be difficult to thoroughly inspect a structure using one method alone. Thus, hybrid NDT methods have also been developed. Recently, the integration of microwave NDT and thermography, herein referred to as Active Microwave Thermography (AMT), has also been considered as a potential structural health monitoring tool. This hybrid method uses microwave energy to heat a structure of interest, and then the thermal surface profile is measured using a thermal camera. This paper investigates the potential of AMT to inspect rehabilitated cement-based structures. Preliminary simulations and measurements provided herein indicate that AMT has the potential to detect delaminations under carbon fiber patches bonded to concrete.

Characterization of Corroded Reinforced Steel Bars by Active Microwave Thermography

Detection and characterization of corrosion on steel is important in the transportation and infrastructure industries. Many nondestructive testing (NDT) methods have been applied to this inspection need. To overcome the existing limitations of traditional NDT methods, integrated NDT techniques have also been developed. To this end, the integration of microwave and thermography, referred to as active microwave thermography (AMT), is proposed as a potential NDT tool for detection and characterization of corrosion on steel bars. This method utilizes microwave energy to provide heat excitation. Subsequently, a thermal camera is used to monitor the thermal surface profile. This paper presents preliminary simulations and measurements of AMT as a potential method for corrosion detection and characterization.

Dual-Loaded Modulated Dipole Scatterer as an Embedded Sensor

The modulated scatterer technique (MST) has shown great promise as an embedded sensor for material characterization and flaw detection in nondestructive testing and evaluation (NDT&E) applications. In particular, embedded MST probes facilitate real-time measurements of the dielectric properties of a host structure in the vicinity of the probe, i.e., localized measurements. MST is based on illuminating a loaded scatterer/probe, which is usually a dipole antenna loaded with a p-i-n diode, with an electromagnetic wave, and measuring the backscattered signal while the diode is modulated. Modulating the p-i-n diode by turning it “off” and “on” changes the dipole input impedance, which, in turn, results in two different backscattered signals. These signals are functions of the dielectric properties of interest, i.e., dielectric constant, and other measurement parameters, e.g., scatterer-to-antenna distance, incident power level, and incidence angle. Using the complex ratio of these signals renders measurement independence with respect to many measurement parameters and results in a measurement that is sensitive primarily to the parameter of interest (i.e., the dielectric constant of the material surrounding the probe). However, the modulated backscattered signal must be separated from static or unmodulated signals present at the receiving antenna before the ratio can be computed. This can be accomplished in a number of ways; each has its own advantages and drawbacks with respect to system complexity, signal processing requirements, and accuracy. To improve on the previous designs, in this paper, we introduce a new probe that simultaneously utilizes two p-i-n diodes. Using the four possible combinations of the measured backscattered signals from such a probe, in conjunction with a differential measurement scheme and subsequent ratio calculation, simultaneously results in the desired independence from several measurement parameters and the removal of the static terms. The operation of the proposed dual-loaded probe is described and analyzed in this paper. Experimental verification of the probe performance is also reported herein.

Comparison of Alkali–Silica Reaction Gel Behavior in Mortar at Microwave Frequencies

Alkali-silica reaction (ASR) is one common cause of concrete deterioration and has been a growing concern for decades. Water, in the presence of reactive aggregates used to make concrete, plays a major role in the formation, sustainment, and promotion of this reaction. In this process, free water becomes bound within ASR gel, resulting in expansion and deterioration of concrete. Devising a test approach that is sensitive to the state of water (free or bound) has the potential to become a method-of-choice for ASR detection and evaluation, since such measures can be used to detect ASR and potentially quantify reaction progression. Microwave signals are sensitive to the presence of water, since the water relaxation frequency occurs in this frequency range. Recently, microwave nondestructive evaluation techniques have shown great potential to evaluate and distinguish between ASR-affected mortar samples and those without ASR gel. Given the complex chemistry of ASR products, their behavior is expected to differ at different microwave frequency bands. To evaluate the sensitivity of different frequencies to the presence of ASR, dielectric constant measurements were conducted at R-band (1.7-2.6 GHz), S-band (2.6-3.95 GHz), and X-band (8.2-12.4 GHz). This paper presents the measured results for mortar samples made with reactive and nonreactive aggregates. The measurement results and subsequent analyses aid in a better understanding of the microwave signals interaction with ASR-affected cement-based materials. Moreover, the results indicate that S-band appears to be the most appropriate frequency band for ASR evaluation in the microwave regime.

Microwave detection of carbonation in mortar using dielectric property characterization

Microwave materials characterization techniques based upon dielectric property measurements are well-suited for detection and evaluation of physical and chemical changes in cement-based materials. In this investigation, microwave dielectric properties of several mortar samples were measured at S-band (2.6-3.95 GHz) and X-band (8.2-12.4 GHz) at two different times nearly one year apart. It was found that during this period, while the samples remained in ambient environment conditions, their masses remained essentially constant over time. However their dielectric properties underwent a relatively substantial change. To investigate the reason(s) behind this phenomenon, both pH indicator test and thermogravimetric analysis were conducted and the results confirmed carbonation in the samples. In this paper, the results of these investigations are presented. Additionally, a first-order dielectric mixing model capable of carbonation depth estimation is described.

Application of active microwave thermography to inspection of carbon fiber reinforced composites

Inspection of carbon fiber reinforced polymer (CFRP) composites is very important in the aeronautical and transportation industries. Many nondestructive testing (NDT) techniques have been applied for health monitoring including ultrasound, thermography, etc. Although thermography has been widely used for this purpose, it often requires powerful heat lamps which may increase the risk of (thermal) damage to the structure under test. Thus, hybrid NDT methods have also been developed. Recently, the integration of microwave heating and thermography, herein referred to as Active Microwave Thermography (AMT), has also been considered as a potential health monitoring tool for infrastructure. This hybrid method uses microwave energy to heat a structure of interest, and subsequently the thermal surface profile is measured using a thermal camera. This paper investigates the potential of AMT for inspection of CFRP-rehabilitated airframes. Preliminary simulations and measurements indicate that AMT has the potential to detect disbonds under carbon fiber patches bonded to aluminum.

Detection of Corrosion in Reinforcing Steel Bars Using Microwave Dual-Loaded Differential Modulated Scatterer Technique

Detection and evaluation of corrosion in reinforcing steel bars (rebars) commonly used in cement-based structures is an important ongoing concern. Embedded sensor technologies hold promise for this purpose. In recent years, modulated scatterer technique (MST) has shown great potential as an embedded sensor technology. The first step in evaluating the potential of embedded MST for rebar corrosion detection is to evaluate its potential for detecting the presence of subtle corrosion in a rebar. This short paper investigates the measurement potential for a dual-loaded differential MST approach for detecting relatively subtle corrosion in a steel rebar.

Active Microwave Thermography for Defect Detection of CFRP-Strengthened Cement-Based Materials

Nondestructive testing (NDT) of rehabilitated cement-based materials (RCMs) with carbon-fiber-reinforced polymer (CFRP) composites is quite important in the transportation and infrastructure industries. Among various NDT methods, active microwave thermography (AMT) has shown good potential. This method uses microwave energy to heat a structure of interest, and subsequently the surface thermal profile is measured using a thermal camera. In this paper, the application of AMT for defect detection (unbond, delamination, and crack) in CFRP composites used in RCMs is presented. More specifically, the effect of defect size and depth and polarization on the resultant surface thermal profile with defects is first studied through simulation. The effect of polarization on detection of defects with regard to the orientation of CFRP fibers is also experimentally investigated. Finally, a quantitative analysis of the measured results based on the thermal contrast and signal-to-noise ratio (SNR) for all the three aforementioned defect types is presented. The results show that the SNR is improved when utilizing perpendicular (compared with parallel) polarization, and that the maximum effective heating time is ~60 s, even for small defects.

Application of frequency selective surfaces for inspection of layered structures

Structural health monitoring is important to many industries today. In the case of layered structures (such as composite structures found in the infrastructure and aerospace industries), one failure mechanism that is important to detect is that of delaminations. Numerous nondestructive testing (NDT) methods have been developed for this purpose including acoustic, thermography, microwave, etc. This paper presents a new application of frequency selective surfaces (FSSs) as a potential inspection method for detection of delaminations in layered structures. Simulations and measurements are presented that support the potential of this method to serve as an additional NDT technique for layered structures. The results indicate that FSSs are sensitive to delaminations occurring in the local vicinity of the FSS, but are less sensitive to delaminations occurring at other layer interfaces.

On the Crack Characteristic Signal From an Open-Ended Coaxial Probe

Detection of surface-breaking cracks in metals is an important issue in many industries (e.g., transportation, aerospace, nuclear). Commonly, eddy current and ultrasonic techniques are used for this purpose. In recent years, a significant amount of work has also been conducted using microwave methods. Consequently, to better understand the interaction between a microwave probe (i.e., open-ended rectangular waveguide or coax) and a crack, a number of electromagnetic models have been developed. For an open-ended coaxial probe, when a crack coincides with the center conductor region of the probe, all previously developed models significantly underestimate the results obtained from measurements. This paper examines the primary reason for this discrepancy, which turns out to be due to a geometrical perturbation in the probe center conductor geometry and its subsequent interaction with a crack.