Active Microwave Thermography Research Articles

Nondestructive evaluation of carbon fiber reinforced polymer (CFRP)-timber interfacial debonding using active microwave thermography (AMT)

Carbon fiber reinforced polymer (CFRP) is increasingly used to strengthen the flexural, shear, axial, and seismic performance of existing timber structures, but CFRP-timber interfacial debonding can substantially weaken the composite action and trigger catastrophic structural failure. Therefore, efficient nondestructive evaluation method is needed to inspect the interfacial debonding, flaws, and imperfections after the installation and periodically during the service of CFRP strengthened timber structures. This study is the first work to investigate nondestructive evaluation of CFRP-timber interfacial condition using active microwave thermography (AMT). AMT utilized microwave as an excitation to generate heat in CFRP and to indirectly detect the interfacial defect from the temperature anomaly on the surface of CFRP. Ten CFRP-timber specimens with different shapes and sizes of interfacial defects were tested under AMT. The experimental results showed that AMT can intuitively detect the CFRP-timber interfacial debonding at the earliest within 8 s to 11 s, which has advantages in speed and visualization. It should be noted that the blur effect can decrease the precision of detection of the shape, especially sharp corners, of the defect. The comparison between specimens with different parameters showed that larger areas of debonding can be detected easily while small ones cannot. In this study, defects with a diameter ≥ 7 mm can be detected.

Modified Thermographic Signal-to-Noise Ratio for Active Microwave Thermography

Modified Thermographic Signal-to-Noise Ratio for Active Microwave Thermography

Debonding detection of defected CFRP-concrete interface using active microwave thermography

This study presents an investigation of non-destructive evaluation (NDE) of carbon fiber reinforced polymer (CFRP)-concrete interfacial debonding using a novel technology referred to as active microwave thermography (AMT). AMT utilizes microwave energy as a heating excitation to the structure and captures the anomalies of temperature contrast (TC) at the structure surface using a thermal camera. In this study, AMT was used first as an inspection method of CFRP-concrete joints. Comparative tests on CFRP-concrete joints with and without an initial man-made defect showed AMT can detect initial defects at the CFRP-concrete interface in around 15 s. Then, AMT was used to monitor the time-history progressive interfacial debonding process of the CFRP-concrete joints tested in direct shear under monotonically increasing loading. The test results showed that: (i) the initial defect at the CFRP-concrete interface can be detected by AMT quickly; (ii) the presence of an initial defect at the CFRP-concrete interface can substantially decrease the load-carrying capacity of the CFRP-concrete joint; (iii) the initial defect can alter the interfacial debonding process and global load response; and (iv) significant load drops, indicative of interfacial damage formation, could be detected by a change of slope of the local CFRP surface TC determined by AMT.

Health Monitoring of RAM-Coated Structures by Active Microwave Thermography

In this article, the application of active microwave thermography (AMT) is investigated as an efficient nondestructive technique (NDT) for health monitoring of structures coated with radio frequency absorbing materials (RAMs). The uniqueness of AMT for this particular application is the use of microwave excitation, which acts as a highly efficient thermal source due to the inherent absorptive electromagnetic properties of RAM-coated structures. In addition, the microwave-induced heating takes place subsurface within the absorbing material of an RAM-coated structure, as opposed to the sole surface heating of conventional thermography. Specifically, this work focuses on detection of delamination, a common defect in carbon fiber reinforced polymer (CFRP) structures, via AMT. To this end, a formulation and analysis method based on electromagnetic/thermal modeling is presented, which enables estimation of the output of an AMT inspection of an RAM-coated CFRP structure suffering from delamination. The effect of the structure’s constitutive properties, the microwave excitation parameters, and the delamination size and depth on its detectability is investigated. In addition, several simulated and experimental results are provided, which verifies the accuracy of the proposed approach and shows the high efficiency of AMT for detection of delamination in such structures.

Active Microwave Thermography to Detect and Locate Water Ingress

This article reports on the application of a new and efficient nondestructive testing and evaluation (NDT&E) technique referred to as active microwave thermography (AMT) used to detect and locate water ingress within a structure. First, a mathematical formulation and analysis is established that relates the water extension and location to the temperature distribution over the inspection surface and enables evaluation of water from the measured temperature data. Following the analysis, the AMT measurement results of a small volume of water, placed in a rubber sample at a predetermined location, are reported. The results verify the analysis and demonstrate the potential of AMT for water detection. In addition, analysis of the measurement data indicates the potential of AMT for not only detection of water ingress but also accurate depth estimation of small amounts of water ingress.

Detection of CFRP-concrete interfacial debonding using active microwave thermography

Active microwave thermography (AMT), a thermographic technique that utilizes a microwave-based thermal excitation, is proposed as an efficient technique for non-contact, real-time inspection of embedded flaws. This study examines the use of AMT to detect interfacial debonding of the carbon fiber reinforced polymer (CFRP)-concrete interface. CFRP-concrete interfacial damage is caused by external mechanical loading and monitored under AMT. CFRP strips bonded to concrete prisms are tested in direct shear to initiate debonding. AMT is used to monitor the CFRP surface thermal profile in an attempt to detect and monitor interfacial damage. Correlations between the mechanical response (namely, applied load vs. composite displacement relative to the substrate, i.e., slip) and AMT results (namely, temporal CFRP thermal response) are found. High thermal contrast, TC, defined as the CFRP surface thermal response caused by loading, is observed where interfacial debonding initiates and propagates along the bonded length. An increase in the TC slope occurs when a load drop occurs, which indicates local debonding occurrence or mobilization. At different load levels, the interfacial slip and surface temperature distributions along the bonded length show similar trends. These correlations indicate that initiation and propagation of CFRP-concrete interfacial damage can be monitored by the AMT technique.

Measurement of the molecular dipole moment using active microwave thermography (AMT)

A method for implementing Active Microwave Thermography (AMT) for use in determining molecular dipole moments is reported. Specifically, the experimental setup along with a mathematical model for determining the dipole moment for deionized water is reported herein. The thermal and spatial resolution of the camera is shown to be of utmost importance in providing statistical and regional relevance, respectfully, of the properties of the material being studied. Deionized water in particular was studied in order to provide foundational knowledge for the veracity of using AMT in determining dipole moment values and representative values are reported herein. In order to extend the technique to more localized regions and composite systems, a more complex model and upgraded hardware are required.

Active Microwave Thermography for Nondestructive Evaluation of Surface Cracks in Metal Structures

Detection of covered surface cracks in metal structures is an important issue in numerous industries. Various nondestructive testing (NDT) and evaluation techniques have been applied for this goal with varying levels of success. Recently, a technique based on the integration of microwave and thermographic NDT, herein referred to as active microwave thermography (AMT), has been considered for various applications. In AMT, electromagnetic energy is utilized for the thermal excitation, and the subsequent surface thermal profile of the structure/material under test is measured with a thermal camera. Utilizing electromagnetic energy allows the inspection to be tailored to the application through choice of frequency, polarization, and power level. It is shown that for metal with a dielectric-filled crack irradiated with an electric field (E-field) polarized perpendicular to the crack length, a propagating mode (TE10) is generated inside the crack, which causes dielectric heating to occur in the (filled) crack. In the particular case of study in this paper, based on the excitation power and the thermal camera sensitivity, the crack can be detected via an AMT inspection as long as the angle between the crack length and incident E-field is between 0° (perpendicular polarization) and ~65°. In addition, from the measured thermal contrast and signal-to-noise ratio, the optimum heating time is ~5–30 s for successful detection.

Detection of Flat-Bottom Holes in Conductive Composites Using Active Microwave Thermography

Active microwave thermography (AMT) is an integrated nondestructive testing (NDT) technique that utilizes a microwave-based thermal excitation and subsequent thermal measurement. AMT has shown potential for applications in the transportation, infrastructure, and aerospace industries. This paper investigates the potential of AMT for detection of defects referred to as flat-bottom holes (FBHs) in composites with high electrical conductivity such as carbon fiber-based composites. Specifically, FBHs of different dimensions machined in a carbon fiber reinforced polymer (CFRP) composite sheet are considered. Simulation and measurement results illustrate the potential for AMT as a NDT tool for inspection of CFRP structures. In addition, a dimensional analysis of detectable defects is provided including a radius-to-depth ratio threshold for successful detection.

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 $\sim 60$ s, even for small defects.

Active microwave thermography - a new twist on microwave NDT [In Microwave Women Research]

The field of microwave nondestructive testing (NDT) includes numerous test and measurement techniques with a wide range of applications. It is a field that I have been a part of since my undergraduate days, and one that I now contribute to as an academician, along with my own undergraduate and graduate students. I feel extremely fortunate to have the job that I have and the research team with whom I work, and as such, would like to share some of what we do on a regular basis. When I give students and visitors tours of my lab, they are often surprised to see the sorts of items sitting on our lab benches. Along with the expected items (antennas, power supplies, etc.), one may also see a microwave oven, a thermal camera, corroded steel rebar, and small concrete samples.

Evaluation of steel fiber distribution in cement-based mortars using active microwave thermography

The non-uniform distribution of steel fibers in fiber-reinforced cement-based mortars (FRCMs) can lead to heterogeneous properties of hardened material with direct impact on mechanical properties. Among various nondestructive testing techniques, the active microwave thermography (AMT) has shown good potential for inspection of cement-based materials. AMT utilizes combination of microwave energy to generate controlled and localized heating and uses commercially-available infrared cameras to capture surface thermal images in real-time. Utilizing AMT, four FRCM samples made with different steel fiber volumes ranging from 0 to 3 % were investigated to evaluate the feasibility of this method for detecting and quantifying fiber distribution. Full-wave coupled electromagnetic-thermal numerical modeling was also conducted to evaluate the effect of dielectric properties, fiber depth, and fiber clumping on surface thermal profile. The results of simulations indicate that increase in fiber depth results in lower surface temperature, due to lower heating associated with induced surface current. Based on AMT measurement results, samples with higher fiber contents were shown to undergo greater increase in surface temperature, as observed for samples with 1 and 2 % fibers. However, this tendency was reversed with further fiber addition from 2 to 3 %, due to the potential of fiber clumping. Furthermore, the fiber clumping and lower level of fiber homogeneity for FRCM with 3 % fiber resulted in 55 % lower flexural toughness compared to the FRCM containing 2 % fiber. The results of fiber homogeneity measured from freshly cast prism were found to correlate well with those obtained from AMT technique determined on hardened samples.

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.

Dynamic Active Microwave Thermography Applied to Hyperthermia Monitoring

The paper presents a numerical simulation of the active tomographic imaging of temperature changes induced in human pelvis and thorax during hyperthermia treatments of cancer. Thermal images would allow monitoring and optimization of treatment, increasing its efficacy and avoiding the heating of healthy tissues. The fields scattered by a numerical reference phantom and the same model locally heated have been calculated at 434 MHz. From this data, differential images have been reconstructed with a first order algorithm showing approximately the heated region