Original Article Structural Health Monitoring 2018, Vol. 17(2) 135–144 Ó The Author(s) 2017 A photophone-based remote Reprints and permissions: sagepub.co.uk/journalsPermissions.nav nondestructive testing approach to DOI: 10.1177/1475921716686772 interfacial defect detection in journals.sagepub.com/home/shm fiber-reinforced polymer-bonded systems Tin Kei Cheng1 and Denvid Lau1,2 Abstract Externally bonded fiber-reinforced polymer is an increasingly popular material to be used in strengthening and retrofitting aging structures. In such structures, debonding defects may occur at or near the interface between fiber-reinforced poly- mer and concrete. As such debonding in fiber-reinforced polymer-bonded systems is generally brittle in nature, there is a need of a reliable inspection technique that can provide early warning of interfacial defects such that premature failure of fiber-reinforced polymer-strengthened structures can be avoided. A remote nondestructive testing approach based on the working principle of a photophone is presented here as an economical alternative to laser Doppler vibrometry for detect- ing interfacial defects. Concrete specimens retrofitted with fiber-reinforced polymer are excited acoustically by white noise, while the surface of the structure is illuminated by a light source. If an interfacial defect exists beneath the surface, the surface will exhibit a frequency response different from an intact surface. The surface of the fiber-reinforced polymer portrays the role of flexible mirror in a photophone, which encodes information about surface vibration into amplitude- modulated light signal. A light detector then captures the irradiance of the reflected beam, and the amplitude modulation is converted into frequency domain in post-processing. With this technique, defect dimensions and thus damage extent can be inferred from the frequency spectrum obtained. The obtained results correspond well with the theoretical calcula- tion, demonstrating the robustness and the applicability of the proposed technique in civil infrastructure. Keywords Acoustic-laser, amplitude-modulated modal analysis, fiber-reinforced polymer, interfacial defect, nondestructive testing, photophone Introduction good environmental resistance, and excellent resistance against fatigue. Because externally bonded FRP becomes The maintenance of aging structures is a challenging issue 1 more popular in retrofitted structures, the failure modes faced by many nations. In most cases, strengthening and of FRP-bonded systems have thus been studied exten- retrofitting of these deteriorated structures may be more sively to better understand the mechanical behavior of economical than reconstruction, especially in situations such composite structures.2–6 Failures in FRP-bonded involving the preservation of historic sites. The use of reinforced concrete (RC) beams can be classified into fiber-reinforced polymer (FRP) composites as an exter- nally bonded element to retrofit structural elements such 1 as beams, columns, slabs, and bridge decks in order to Department of Architecture and Civil Engineering, City University of Hong Kong, Hong Kong restore the design capacity and structural redundancy of 2Department of Civil and Environmental Engineering, Massachusetts deteriorated structures has emerged as a popular Institute of Technology, Cambridge, MA, USA strengthening and retrofitting technique. FRP, as a thin sheet of composite material, has a lot of desirable proper- Corresponding author: ties when compared with the conventional bulk construc- Denvid Lau, Department of Architecture and Civil Engineering, City University of Hong Kong, Hong Kong. tion materials, such as high strength-to-weight ratio, Email: [email protected] 136 Structural Health Monitoring 17(2) two major mechanisms: (1) flexural failures of critical sec- tions, such as crushing of concrete and FRP rupture and (2) debonding of FRP from the RC beams.2–6 The near- surface debonding defects resulting from the latter case will be referred to as interfacial defects in this article, and they are the focus of this study. Interfacial defects often occur in regions with high stress concentration and/or shear stress discontinuity, which are positivelycorrelated with (1) regions of material discontinuity, (2) poor instal- lation including inadequate surface preparation of the substrate, and (3) the presence of cracks (e.g. propagation of flexural or flexural-shear cracks from the concrete sub- Figure 1. A representative arrangement of the photophone.26 strate to the FRP-concrete interface, resulting in delami- A beam of sunlight is concentrated upon a flexible mirror on the nation). The failures modes due to interfacial defects are transmitter side. After reflection, the beam is again made brittle in nature and will result in premature failures of parallel by means of another lens. The beam is then received at FRP-strengthened elements if not promptly resolved.2–6 a distant location upon a parabolic reflector, the focus of which is placed on a selenium photoresistor, connected in a local Current nondestructive testing (NDT) approaches circuit with a battery and telephone. developed for civil engineering application include the use of ultrasound, X-ray and neutron radiography, near-field and far-field radar, and infrared thermogra- required to assemble the NDT device put forward in phy.7–19 However, these techniques may not always be this article can readily be purchased from consumer optimal for the remote detection of interfacial defects.1 electronics retailers, and the total cost of a functioning Passive thermography typically has insufficient spatial setup is around a hundredth of the original work. 18 Originally conceived by Alexander Graham Bell who resolution to resolve small interfacial defects. 25 Radiography involves the use of hazardous ionizing invented the telephone in 1880, a typical photophone 10,11 concentrates sunlight onto a flexible mirror which radiation. The radar approach may have issues if 26,27 carbon fiber–reinforced polymer (CFRP) sheets being reflects the sunlight to a remote receiver (Figure 1). penetrated by radar are not arranged in a parallel Excited by the speaker’s voice, the flexible mirror mod- orientation as CFRP is a highly conductive material at ulates the reflected sunlight’s radiant intensity by flex- microwave frequencies.14 Finally, while near-field radar ing convex and concave according to the changing and ultrasound are promising approaches, they require sound pressure, dispersing, and converging the beam of a close proximity to the subject.14,17,19 Recently, high- sunlight. This amplitude-modulated radiant intensity is speed video in conjunction with motion magnification converted back to an audio signal when detected by the has been applied to remotely evaluate modal behavior. receiver. The photophone eventually achieved the first Despite being promising in detecting larger scale dam- wireless telephone communication, predating the first ages, as of the moment of writing, the maximum fre- spoken radio transmission by almost two decades and quency measurement demonstrated by the high-speed laying the groundwork for fiber-optic communication a video technique is 2500 Hz, which is below the typical century later. Although considered by Bell to be one of fundamental frequencies of interfacial defects as his greatest achievements, even more superior than the explored in this article.20 To complement existing NDT telephone, the photophone never went into mainstream 25,28,29 approaches, an acoustic-laser technique has been usage and it is found mostly in military use. devised specifically to detect interfacial defects from a Nowadays, the photophone is commonly known as the distance based on their vibrational behavior.21–24 It has ‘‘spy microphone’’ in hobbyist projects, with few real- been shown that interfacial defects in FRP-bonded sys- life applications. In this article, the interest in photo- tems can be modeled as a combination of a clamped phone is renewed by considering an NDT application plate (representing the FRP surface) and an acoustic in the detection of interfacial defects in FRP-bonded cavity (representing the air gap at the interface), resem- systems. The working principle of the modified photo- bling the physics of a drum. In such defects, the funda- phone will first be introduced with the relation to previ- mental mode of vibration lies typically at the order of ous acoustic-laser studies, followed by experiments with kHz.21 However, the dedicated setup used in previous artificially induced delamination-like interfacial defects acoustic-laser studies may cost a million US dollars on FRP-bonded concrete specimens. Finally, benefits and might prove an obstacle to the commercialization and limitations of this photophone-based technique will of the acoustic-laser technique. This study presents an be addressed based on experimental results, and future economical augmentation to the previous works based research direction including an extension of this tech- on the design of a photophone. Electronic parts nique to general modal analysis will be discussed. Cheng and Lau 137 Figure 2. An illustration of the photophone-based experiment setup. (a) An FRP-bonded concrete slab with artificially induced interfacial defects is excited acoustically by white noise, while a point on the surface of the structure is illuminated by a laser. As specular reflection is strong in our specimens, a lens system is not installed in the light detector, though a collimator