A few examples of fiber-optic sensor applications are medical ultrasound (Webb, 1996), ship hull monitoring (Wang, 2001), and embedded monitoring of reinforced concrete (Maalej et al. 2004). Fiber-optic sensors have also been applied to bridge monitoring (Chan et al. 2006) and embedded composite material monitoring (Kuang et al. 2001; Takeda 2002; Takeda, 2005; de Oliveira et al. 2008; Park, Peters, and Zikry 2010).
There has been a large body of research in fiber-optic sensors for acousto-ultrasonic guided wave monitoring (Tsuda, 2006; Betz et al., 2007; Davis et al. 2014), and acoustic emission monitoring (Perez, Cui, and Udd 2001; Chen et al. 2004; Kageyama et al. 2005; de Oliveira et al. 2008; Tsuda, 2009). This chapter covers pertinent research on active and passive SHM using fiber optics for guided wave sensing. It is organized into the following sections
• Section 6.1 covers research on applications in fiber-optic guided waves and acoustic emission sensing
• Section 6.2 covers the topic of mechanical attachment. This section is of particular note, as it discusses mechanical systems that have been used to modify and enhance an FBG response
AE sensing and mechanical fiber-optic sensors are emphasized, as they are core to this dissertation. There is naturally some overlap between sections 6.1 and 6.2, as novel methods of optical fiber mechanical attachment used active SHM and AE sensing for proof-of-concept. The choice to include material in Section 6.2 rather than Chapter 5 depended on whether mechanical attachment was novel or a critical feature of the article. 6.1.1 Fiber-Optic Sensing of Guided Waves
The development of high frequency dynamic sensing capabilities in fiber-optic sensing has largely been motivated by guided wave sensing. The requisite sensing frequencies are high, ranging from tens of kHz to several MHz. Strain resolutions are also low, in the low microstrain range, and even smaller for AE signals. A single sensor is not needed for each type of damage detection method, e.g. impact, AE, pitch-catch, etc. Rather, a sensor must have a suitable size and strain sensitivity; it must be able to detect the motion that propagates in the structure. This can be expressed in terms of in-plane and out-of-plane motion; it can also be expressed in terms of responsivity to guided wave modes (e.g. Lamb wave modes S0, A0, S1, A1, etc.) which can be resolved into the in-plane and out-of-plane components of their motion.
It should be noted that in plates with traction free surfaces, waves propagate as Lamb waves regardless of the source (e.g. pitch-catch, impact, or AE). This is of particular note in AE literature, where recent trends have moved away from using axial and flexural waves towards Lamb wave formulations, to greater predictive and descriptive success. In practice, practical considerations such as amplitude, wave modes, frequency range, etc. are still related to the source.
Betz (Betz et al. 2003) demonstrated FBG intensity modulation acousto-ultrasonic sensing in a pitch-catch configuration across a Perspex plate. A PZT actuator was used as a transmitter, and both FBG and PZT used as receivers. Both symmetric and antisymmetric Lamb wave modes were sensed, with a minimum detectable strain of 16 nanostrain for the optical system; however, this research took advantage of a feature of pitch-catch SHM, in that it allows averaging to reduce noise, and the results are from the averaging of 128 consecutive signals.
Tsuda (2006) demonstrated FBG sensing of Lamb waves in a cross-ply CFRP using a broadband source and a narrowband tunable laser sources for FBG sensing. Only S0
waves were transmitted using a conventional ultrasonic transducer, as the S0 wave has a
larger in-plane component which could be sensed by the FBG; both propagation through pristine and impact-damaged regions was assessed, with damage-related features observed. Using the average of 512 waveforms, SNRs of over 40 and 70 dB were observed for the broadband laser and the narrowband tunable laser, respectively.
Betz (Betz et al. 2007) used a narrowband source intensity modulation approach for FBG Lamb wave sensing on a thin aluminum plate for damage localization. Three FBGs were placed in a rosette configuration, where the directional properties of FBG allowed for the direction of the incident wave to be calculated. Using two FBG rosettes and PWAS actuators, the localization of a flaw was demonstrated.
Davis (Davis et al. 2014) established that a ratio of wavelength to FBG length must be at least 8.8 to resolve the response of a dynamic strain field with less than 2% loss from the true strain value. A ratio of 4.8 could be used to resolve a dynamic strain field with less
calculating the wavelength of a Lamb wave in a 0.8 mm aluminum plate, and varying the excitation frequency in a pitch-catch configuration in a range where only the fundamental S0 and A0 modes would propagate. The ratios held true across a range of FBG lengths from
0.2 mm to 5 mm. This research provides well-verified rule-of-thumb metrics for designing experiments with FBGs, although the results may not necessarily be valid for higher order Lamb wave modes.
This work shows that in a general sense, FBG sensors are well suited for active SHM using both PWAS and ultrasonic transducer actuators. The signals obtained were clear, with high SNRs, and could generally be used to distinguish damage-related features and localize damage, so long as practical considerations like FBG length were appropriately used. One of the main advantages of pitch-catch active SHM is the use of horizontal acquisition – averaging many signals together to obtain high SNRs. Thus, these positive results are not necessarily transferrable to passive SHM, where horizontal acquisition is infeasible or inappropriate. Thus, suitability for passive SHM, including AE sensing must be shown separately.
6.1.2 Fiber-Optic Acoustic Emission Sensing
AE sensing is an area of great interest for fiber-optic sensors. There is a large market for AE sensors, and conventional AE sensors use piezoelectric sensing elements. The advantages of fiber optics, specifically its multiplexing capabilities have a lot to offer for AE sensors; traditional AE sensors cost hundreds of dollars, and implementation over large structures can be bulky and costly. This is of particular importance true for the aerospace industry and for industries where sensors must be intrinsically safe to not ignite a flammable environment. Despite these advantages and great interest, fiber-optic sensing
has not yet broken into the AE market, largely due to their noise and lower sensitivity than their piezoelectric counterparts. Much of the research into fiber-optic sensing of AE has been aimed at proof of concept demonstrations, and incrementally mitigating the noise issues.
In fiber-optic AE sensing research, there is often a misconception of the definition of an AE event. Generally, claims of AE sensing in the literature fall into three categories: 1. Crack-related, or other damage-induced “true” AE events. A test using a true AE event, or a proven AE calibration method is the best indicator of the functionality of an AE sensor
2. Simulated AE events, such as pencil-lead-break (PLB), glass rod fracture, or even impact. PLB-simulated AE events is most commonly seen. But for various reasons, it is distinctly different than crack-related AE events: lower frequency range (hundreds of kHz), higher amplitude, and a monopole versus dipole source
3. Pitch-catch configurations which are called AE events due to a desire to continue research in AE. Although pitch-catch configurations are a good starting point for developing and testing AE sensors and systems, ultimately the results are not necessarily directly comparable as the waveforms are often higher in magnitude by orders of magnitude
Fiber-optic AE sensing has been an active area of research at least since the early 2000s. For FBG sensing, Perez (Perez, Cui, and Udd 2001) demonstrated the capability for FBG sensing of ultrasonic waves transmitted by piezoceramic active transduction, as well
Read (Read, Foote, and Murray 2001) was able to detect AE during a fatigue test of a large composite specimen made to resemble an aerospace panel. This was done using a surface- bonded EFPI sensor, with a fatigue test performed with intermittent impacts of increasing amplitude. This can be considered highly favorable conditions for AE detection due to the large extent of damage and the composite specimen with higher amplitude AE events. Upon searching, this seems be the first demonstration of true damage-related AE capture by fiber-optic sensors.
Chen (Chen et al. 2004) developed a fiber-optic sensor based on a fused-tapered fiber two-input, two-output (2 x 2) coupler. The principle of operation is that an elastic strain field changes the coupling coefficient through elasto-optic effects, and can be used to track the applied strain. Mechanical strain amplification was used to increase sensor sensitivity; for this sensor, a V-groove was used for strain concentration, and the fiber coupler was bonded via two-points across the groove. Additionally, the tapered region at the center of the 2 x 2 coupler acted as a strain concentrator. The AE sensing was done by PLB only, with the fused-tapered fiber-optic sensor showing a lower signal-to-noise ratio than a conventional piezoelectric AE sensor.
Kageyama (Kageyama et al. 2005) used a principle of a Doppler effect in a curved light waveguide for fiber-optic sensing. Light frequency shifts were tracked in an optical fiber, these shifts being related to motion in the fiber. A full-scale model of a reinforced concrete railway girder was loaded to failure, and the load history tracked by piezoelectric AE sensors and this new fiber-optic sensor. The author reports that the fiber-optic sensor has sensitivity equal or greater than conventional AE sensors. In principle, it is difficult to
compare this work to the rest of the literature since AE events associated with a test to failure of such a large-scale specimen can be very high in amplitude.
de Oliveira (de Oliveira et al. 2008) assessed the response of embedded FBG and EFPI sensors to PLB and impact excitation. The EFPI was more sensitive, but still had limitations with a low SNR. Fu (Fu et al. 2009) used a fused-tapered 2 x 2 coupler sensor inside a silica capillary tube for embedment into CFRP specimens. During a three-point bending test of a composite specimen, AE events could be detected, but not until a piezoelectric AE sensor detected large increases in AE event amplitude, nearing catastrophic failure of the specimen. Tsuda (2009) used an intensity modulation approach with a cantilevered FBG to sense AE events during a pressure test of a curved filament- wound CFRP tank. The AE hits started and ended around the same time during the test for both conventional piezoelectric AE sensors and the FBG sensor, indicating that the FBG performed well compared to the conventional AE sensor. This work underscored the potential for an FBG to serve as mechanical waveguide which has become a new area of research in the last few years. Yu (2016) detected AE events during quasi-static tension of CFRP specimens using an intensity demodulation approach with a pi-FBG and a balanced photodetector.