Determination of load condition of standard screws during tensile tests using acoustic emission analysis

Abstract

We examined the acoustic behaviour of screws under applied load to develop an in-process monitoring system for endangered screw connections. To apply load to a screw, a hydraulic tensile testing machine was designed. Signals occurring during tensile tests were digitally recorded and analysed using acoustic emission (AE) technique. We established an increase of fhe AE- rate at the yield limit and weaker A€-activity during the plastic deformation. The A€-signals contained dominant frequencies in two narrow frequency ranges, namely at 590 kHz and 1010 kHz and therefore we were capable of distinguishing between friction processes in the screw thread and abrupt dislocation processes under load. The significance of the results for in-process moniforlng of screws is illustrated. I. INTRODUCTION Carrying through a special technique for in-process re- pairing of a leaky flange in a pipe line a special sub- stance is pressed from the outside into the flange. A higher pressure is necessary than exists inside of the pipeline, so that the flange screws or flange bolts are more stressed than usual. Generalising this special problem a method for in-process monitoring of screws being loaded in a critical range is needed. We decided to use the phenomen of acoustic emission, i.e. the emission of sound waves (mostly in the ultrasonic range) by a solid as a response to mechanical strength, expecting that structural changes in the condition of a screw express themselves as acoustic signals. Several authors report examinations of steel specimens during tensile tests according to the AE behaviour, especially the AE-rate (l-81. They established an increase of the AE-rate in the region of the elastic limit, with an amplitude strongly depending on the material and some other parameters. But there are other studies, e.g. (9), in which the AE-rate and the elastic limit could not be brought into connection. In this paper we present the results of our experiments with common industrial screws of German standard DIN 931 studying the AE- rate as well as the frequency distribution depending on the load condition. Especially, we were taking into consideration the applicability of our results for the in- process monitoring of screws.