Ultrasonic measurement and analysis of screw elongation

Abstract: Based on the customers' increasing demands on the precision of the preload in a screw joint Atlas Copco is investigating the opportunity to integrate ultrasonic technologies in their industrial tools in order to be able to measure the screw elongation and thereby preload. The preload in a screw joint is important when trying to optimize the joint in terms of weight and life time. The ultrasonic technology for preload measurements has two large advantages; the technique requires only access to the screw head and it is completely independent of the friction in the joint which enables more accurate measurements of the preload. In ultrasonic preload measurements the time of flight is measured (i.e. time for ultrasonic waves to travel through the screw). The time can be transformed into length and elongation by use of the sound velocity. Of importance in this calculation is to take the so called acoustoelastic effect into account which predicts a lower longitudinal wave velocity with increasing tensile stress. The purpose of this master thesis is to develop a method that can predict screw elongations from ultrasonic measurements. Finite element simulations showed that the acoustoelastic ultrasonic constant only depend on the ratio between the clamp length and the screw diameter up to a certain degree of accuracy. A function of type =∙ ∙ where a, b and c are real-valued constants and = is the clamp length () divided by the screw diameter () fits the data well. However, the ultrasonic measurements showed some deviations from the theoretical predictions.