J U L Y 2 0 2 1 • M A T E R I A L S E V A L U A T I O N 725 increased, signal amplitude increased under the same testing conditions. This increase can be attributed to the following factors: the coil-conforming mode had little effect on the generation of Lamb waves and the geometry of the gripper resulted in better functionality for current pads on pipes with a diameter of #114 mm. Another interpretation of this state- ment is demonstrated in Figure 13, which indicates that the maximum amplitude linearly decreased as the curvature of the test sample increased. As discussed in the introduction, the tubular components in power plants throughout their life suffer from defects such as corrosion, cracks, and stress corrosion cracks. In this part, the proposed modular gripper was used to detect simulated defects in a stainless steel pipe sample. To this end, two artifi- cial defects were introduced on the pipe with the OD of 88.9 mm. A surface longitudinal crack and partial corrosion with a depth of 1 ± 0.1 mm (30% of the pipe thickness) were simulated. The crack is 25 mm long and approximately 0.5 mm wide. The circular shape corrosion, on the other hand, has a diameter of 25 mm (see Figure 14). To better evaluate the performance of a pair of trans- ducers embedded in the pitch-catch configuration, two grippers were attached to the robotic arms separately to mimic the movements and performance of the LTI robot. The EMAT sensors (transmitter and receiver) were embedded on a finger of each gripper. Figure 14 shows the placement of the EMAT and the defect locations. As seen in Figure 14, the spiral coils are located on each side of the partial cracks and corrosion (all the sensors are in line with the defect in the center). The crack position was selected to be parallel to the wave propagation to simulate the worst-case scenario and have the minimum effect on wave propagation. Figure 15 shows pitch-catch signals taken when the pair of EMAT sensors is located 10 cm away from the center of the defects. In this figure, the signals in the presence of the defects are compared with the signal response of an intact pipe. A reduction in signal amplitude and velocity are clearly observed when the defect lies between the transmitter and receiver. The results show that to detect the defects using the inte- grated gripper-sensing system, one can monitor the changes 15 0.6 0.8 1 1.2 1.4 1.6 1.8 2 20 κ (1/m) 25 30 Figure 13. Relationship between signal amplitude and sample curvature. Figure 14. Experimental setup for EMAT testing of two grippers on the pipe with artificial defects: (a) partial crack (b) partial corrosion. (a) (b) Crack Corrosion 2 Time (μs) 1 0 0 50 100 150 –1 –2 No defect Crack 2 Time (μs) 1 0 0 50 100 150 –1 No defect Corrosion Figure 15. Experimental A-scan from the defect and intact areas: (a) partial crack (b) partial corrosion. (a) (b) Max amplitude (V) ×104 Amplitude (V) ×104 Amplitude (V) ×104

726 M A T E R I A L S E V A L U A T I O N • J U L Y 2 0 2 1 in amplitude and velocity of the transmission of the Lamb ultrasound waves caused by the presence of defects. Further investigation is needed to evaluate the sensitivity of the designed EMATs to size and shape of cracks and corrosion. Conclusion The objective of this study was to design and test a modular robotic gripper with embedded EMATs as the main compo- nent of a versatile LTI robot. By integrating a couplant-free ultrasound sensing and friction-based mechanical component in a single robot, it eliminated the need for smooth surfaces and simple pipe geometries. Moreover, the proposed system removes the need for point-by-point scanning of tubular surfaces for crack and corrosion detection. The proposed modular robotic gripper embedded with EMATs consisted of three major mechanical components, including base plate, arch, and fingers. The designed gripper was attached to a robotic manipulator and its performance was tested on pipes with different outer diameters. The sensing process was accomplished using Lamb waves. To this end, the S0 mode was applied to evaluate the performance of the gripper. Preliminary tests were conducted to investigate the effect of curvature on the properties of the Lamb waves. The obtained results revealed that conforming of the coils with the sample surface did not necessarily increase signal amplitude. Therefore, a nonconformed configuration was applied in the gripper to inspect pipes with different curvatures. The received time-domain waveform generated by the EMATs indicated that group velocities in the conformed and nonconformed configura- tions were almost consistent, while there existed an inversely proportional relationship between sample curvature and group velocity. From the experimental results, it could be inferred that directly transmitted and received signals were successfully acquired in the time domain. An increase in sample diameter led to an increase in signal amplitude. Moreover, it could be verified that the robotic ultrasonic system integrated with EMATs gener- ated Lamb waves with acceptable SNR levels for inspecting tubular components. Finally, the sensing system’s efficacy in terms of cracks and corrosion detection was considered via experimental measurements on artificially induced defects. For the developed modular robotic gripper system, two sets of tests were carried out on both defect-free pipes and pipe with simu- lated defects. Two grippers were used to detect a surface crack and partial corrosion with a depth of 1 ± 0.1 mm. Reduction in amplitude and wave velocity were shown to be effective damage- sensitive features. Although the robotic manipulator was a good choice to test the mechanism and performance of the proposed gripper, the performance of the modular gripper should be considered using the LTI robot. Furthermore, gripper pads that can enable the fingers to move up vertical tubes and not just along horizontal samples should be tested in order to deploy the designed gripper in LTI robots. The authors will focus on this aspect in future studies. ACKNOWLEDGMENTS The authors would like to thank DOE-NETL for its financial support. This research effort was funded under Award No. DE-FE0031649. REFERENCES ASTM, 2017, ASTM-E1774-96: Standard Guide for Electromagnetic Acoustic Transducers (EMATs), ASTM International, West Conshohocken, PA, https:/doi.org/10.1520/E1774-17 Chattopadhyay, P., S. Ghoshal, A. Majumder, and H. 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