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Acoustic emission (AE) signals from weld defects, incomplete penetration (IP), slag inclusion (SI), and porosity (PR), in longitudinal seam welding of UOE steel pipes were evaluated by using an envelope analysis system and waveform analysis system. In test results, the location accuracy of the envelope and the waveform systems during the loading tests were a few 10 mm and a few mm, respectively. AE activity and intensity and waveform type could identify the welding defect types. Three types of AE spherical radiation patterns (AERPs), a monopole mode, a tensile fracture of dipole mode, and a tensile and shear mixed of dipole mode, were observed during the test. Information from cross section observation of the IP by SEM and AERP suggested that activity and intensity of AE signals from welding defects could depend on both stress concentration of the defect and brittleness of the microstructure around the defect.

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