Monitoring screws under tensile load using acoustic emission analysis

The acoustic behaviour of screws under applied load was examined to develop an in-process monitoring system for endangered screw connections. In order to apply load to a screw, a hydraulic tensile testing machine was designed. Signals occurring during tensile tests were digitally recorded and analyzed using an acoustic emission (AE) technique. An increase of the AE-rate at the yield limit and weaker AE-activity during plastic deformation were found. The AE-signals contained dominant frequencies in two narrow frequency ranges, at 590 kHz and 1010 kHz, which made it possible to distinguish between friction processes in the screw thread and abrupt dislocation processes under load. The significance of the results for in-process monitoring of screws is illustrated.