## 3 Materials and Methods  
An experiment was conducted on a 21-foot section of 6-inch NPS schedule 40 carbon steel pipe. A segmented magnetostrictive (SMS) collar was placed 3 feet from the pipe end and used to generate torsional axisymmetric ultrasonic guided waves in the pipe. Generation of ultrasound using the magnetostrictive effect is described in Ref. [25] 1993. The SMS collar was constructed of copper meandering coils printed on flexible circuit boards with a design frequency of 64 kHz, which were placed over a 2-inch-wide strip of iron cobalt (FeCo) that was bonded to the circumference of the pipe. Olympus UltraWave LRT hardware (OSSA, Waltham, MA, USA) and custom software were used to generate torsional mode ultrasonic guided waves and collect data. This sending setup, and the SMS coil design, is identical to the one used and described in Ref. [11]. Optical FBG sensors were initially collocated with the collar, on the top and bottom of the pipe, and used to receive ultrasonic guided wave energy. The optical fiber sensors were oriented at 45\({}^{\circ}\) relative to the pipe axis and directly attached to the pipe surface using clear Scotch(r) tape. Tape was used because a temporary method for sensor bonding was desired so that the fiber optic sensor could be easily and quickly moved to various locations during experimental testing. Epoxy or superglue (cyanoacrylate) would be a more appropriate long-term choice for permanent sensor installation. The 45\({}^{\circ}\) orientation was previously shown to result in maximum reception of shear horizontal-type guided wave energy in plates [22] 2018. Ref. [22] also gives a theoretical justification for why this angle achieves the highest amplitude response to shear-type guided waves. Two flaws were introduced in the pipe at 13 feet and 10 feet from the pipe end. A schematic of the pipe is illustrated in Figure 2.  
A schematic of the test equipment is shown in Figure 3. A laptop was used for data acquisition and custom software was used to control the UltraWave pulser/receiver hardware. The pulse was sent to the collar to generate ultrasonic guided waves in the pipe. One or more fiber optic sensors were coupled to the pipe and a distributed feedback (DFB) laser was used with a locking system to keep the laser wavelength locked onto the spectral notch of the optical fiber sensor, and a photodetector  
Figure 2: Schematic of the 21-foot pipe specimen. The SMS collar and the initial FBG sensor locations were 3 feet from the pipe end. Flaws were introduced at 13 feet and 10 feet from the pipe end. The side with flaw 2 will be referred to as the right side. Number and locations of the optical fiber sensors vary in the different experiments. was used to convert the light signal into a voltage signal for recording. Different from the benchtop narrow linewidth laser used in our previous work [22], the much more portable and low-cost DFB lasers (A1905 LMI, Avanex, New York, NY, USA) were used in this paper with moderate compromise in signal-to-noise ratio (due to the larger noise of a DFB laser). Furthermore, a two-channel system was used in this work to demonstrate the multiplexing capability of the FBG sensing system (see Figure 3), i.e., one DFB laser and FBG pair works at 1550 nm wavelength while the other pair works at 1545 nm. A dense wavelength division multiplexer (DWDM) was used to optically separate the two channels and the ultrasonic signals received by the two FBG sensors are acquired simultaneously. Additional details describing each single-channel setup are found in Ref. [22] 2018. Although the two-channel system is enough for the demonstration of multiplexing capability, when using this type of optical setup additional lasers could be added for additional simultaneous reception by fiber sensors connected to other channels of the DWDM but it is not the focus of this paper.