## 4 Results
### Individual FBG Sensor NDT  
It is clear from Figures 4 and 6 that the ultrasonic-optical approach can be effective for flaw detection. This section assesses the ability of a point-like receiver to reliably detect reflections from a particular flaw location. What is found is that if the flaw is relatively far from the receiving sensor, then detection is possible at all circumferential locations (see Figure 7). However, if the flaw is relatively close to the receiving sensor, then detection is less reliable as it is more dependent on the circumferential  
Figure 5: F-Scan generated using the SMS collar in pulse-echo mode at 3 feet from the pipe end. Flaw indications are shown at 7 feet and 10 feet from the sensor. Pipe end reflections are shown at \(-3\) feet and 18 feet. There is a higher signal-to-noise ratio for this setup. Note also that the dual coils allow for directional control of the received wave, suppressing the reverse waves shown in Figure 4.  
Figure 6: Waveform data (amplitude envelope) for SHM states with flaw growth of 0 to 2% CSA for the flaw located 7 feet from the optical fiber sensor. Waveforms are shown for an excitation frequency of 76 kHz using the SMS collar and a single direct-bonded FBG optical fiber sensor used as a receiver. As the flaw grows, the reflected ultrasonic wave energy from the flaw increases. The optical fiber sensor can clearly be used for flaw sizing. location of the FBG relative to the flaw (see Figure 8). This result will be dependent on the scattering field from the flaw at small sensor-flaw distances, which will vary according to flaw geometry and ultrasonic wavelength. However, at further distances, for circular and many corrosion-like flaws, the reflected wave will be detectable at nearly any circumferential location due to the wrapping of the reflected wave path circumferentially around the pipe. Additionally, sweeping inspection frequency to generate the F-scan further improves the likelihood of detection. Figure 7: F-Scans collected using a direct-bonded FBG receiver collocated with the sending collar at several circumferential locations: (**a**) top; (**b**) right side; (**c**) bottom; and (**d**) left side of the pipe. Since the receiver is relatively far from the flaws, both flaws (red ovals) are clearly detectable at all circumferential locations. Flaw indications are marked by the red ovals. Arrows indicate the axial location of the fiber optic receiver. Distance is indicated from source/receiver position. When the receiver is relatively far from the flaw, i.e., multiple pipe circumferences, flaw detection is reliable at any circumferential location. Note that the indication in (**c**) at 13 feet (rectangle) is due to a reverse wave reflection. Figure 7 shows F-Scans collected with optical fiber sensors collocated with the collar at four distinct circumferential positions (top, right, bottom, and left sides). The axial distance to the two flaws is 7 feet and 10 feet, respectively. Compared to the pipe circumference, this axial distance is relatively large compared to the pipe circumference, at 4 and 5.8 times the circumference, respectively. At this axial distance, sensors at all four circumferential positions detect both flaw reflections. It is believed that detection is possible at all circumferential locations because the scattered wave generated by the  
Figure 8: F-Scans collected using a direct-bonded FBG receiver 6 feet in front of the sending collar at several circumferential locations: (**a**) top; (**b**) right side; (**c**) bottom; and (**d**) left side of the pipe. Since the receiver is relatively close to the flaws, the best flaw detection occurs directly in line with the flaw location, while the detection at other circumferential locations is unreliable. Clear flaw indications are marked by the red ovals and less clear or undetectable flaw indications are marked by dashed red ovals. Arrows indicate the axial location of the fiber optic receiver. Distance is indicated from the sending collar.