Acoustic emission sensors to monitor early onset of necking during uniaxial tension

Elastic waves are generated and propagate when a material undergoes plastic deformation and can be detected by acoustic emission (AE). In this work, AE measurements are obtained during a uniaxial tension (UT) test using a custom-made sensor employing piezoelectric crystals. The UT tests are performed on an MTS machine with two AE sensors clamped on each end of the specimen gage section. A low pass Butterworth filter is designed to attenuate the high frequency noise from the AE signals. Also, full-field strain measurements on the specimen surface are acquired using the 2-D digital image correlation (DIC) method. A typical result from a UT test reveals, as the plastic deformation increases, the AE signals from each sensor increase until they reach a maximum value followed by a drop of signal until the specimen fractures. It is found through interrogation of the DIC images that the maximum amplitude from the AE signals corresponds to the early onset of localized necking. The goal of this work is to implement the UT findings in an actual forming process (e.g., cup drawing) and monitor the event in real time using closed loop control to achieve improved formability.