Infrared Thermography for Nondestructive Evaluation of Fiber Reinforced Polymer Composites Bonded to Concrete
M A. Starnes, Nicholas J. Carino
Fiber-reinforced polymer (FRP) composites, in the form of pultruded laminates or built-up woven fabrics, are being used widely to strengthen existing concrete and masonry structures. The success of these materials in performing their intended functions depends, to a large extent, on how well they are bonded to themselves and to the substrate. There is a need for an efficient and reliable method to detect and characterize defects at the substrate interface and within multi-ply systems. Infrared thermography is well suited for this purpose because it is inherently sensitive to the presence of near-surface defects and can interrogate large areas efficiently. Before infrared thermography can be developed into a standard methodology, however, an understanding is needed of the effects of testing parameters and different types of defects. This report focuses on establishing the potential for quantitative infrared thermography, that is, not only detecting but also characterizing subsurface flaws. Numerical and experimental methods are used to investigate the effectiveness of infrared thermography to estimate the width of subsurface flaws in fiber-reinforced polymer laminates bonded to concrete.First, a dimensional analysis of a simplified case of one-dimensional heat diffusion in an infinite half space is performed to establish the parameters that affect the thermal response of the test object. The results from the dimensional analysis identified the factors that had to be investigated in the parametric study.Next, the finite-element method is used to carry out parametric analyses of the thermal response of simulated defects in fiber-reinforced polymer laminates applied to a concrete substrate. In this study, a defect is an air gap between laminates, at the laminate/substrate interface, or in the substrate. The aim is to assess the potential for quantitative infrared thermography in not only detecting a flaw but also being able to describe its physical characteristics. Six parametric studies are presented, namely: 1) relationships between the thermal input, the maximum signal, and the maximum surface temperature; 2) effect of thermal material properties of FRP composites and concrete; 3) effects of flaw depth and the number of FRP layers; 4) effect of flaw thickness; 5) effect of flaw width and estimation of flaw width; and 6) a multi-parameter screening study to determine relevant factors. From these simulations, procedures are established for selecting the thermal input and estimating the flaw depth and width.The third component of the investigation focuses on laboratory studies. Controlled-flaw experiments are performed to evaluate the potential of infrared thermography testing to quantitatively assess subsurface flaw in FRP bonded to concrete. First, a qualitative test is successfully performed to evaluate the potential for detection of each simulated flaw embedded in the test object. The next two experiments involve quantitative thermography testing of an air void embedded at the interface between a pultruded FRP laminate and the concrete substrate. A comparison between the quantitative infrared thermography test and finite-element simulations of the same test is also performed. Good agreement between experimental thermal response parameters and those calculated from finite-element models provides assurance of the validity of parametric studies based on numerical simulations. Controlled-flaw experiments are also performed to verify the procedure for estimating the width of subsurface flaws. Good agreement is found between the estimated and actual flaw dimensions. Data smoothing is shown to be effective in removing noise from measured temperature profiles. An experimental screening experiment is carried out to determine the relevant factors affecting the thermal response of the controlled-flaw specimens. The results indicate that the depth of the flaw is the only relevant factor affecting the time to ma
and Carino, N.
Infrared Thermography for Nondestructive Evaluation of Fiber Reinforced Polymer Composites Bonded to Concrete, NIST Interagency/Internal Report (NISTIR), National Institute of Standards and Technology, Gaithersburg, MD
(Accessed June 7, 2023)