Loss of Preload in Bolted Joints: Comparative Analysis between Length Measurement by Ultrasonic Method and Checking Torque Method 2017-36-0087

The loss of preload in bolted joints is the main cause of quality problems, reworks or even joint failures during work. Several measurement methods are presented in the literature, but the difference of precision between each one or even the right criteria to select the most appropriate method for each project is not clearly defined. This study evaluated the differences between two of the main losses of preload measurements methods currently used in the market: clamp load through elongation measurement by ultrasonic method and checking torque method. For this, a load cell was used to measure the clamp load after the tightening torque and joint relaxation. Since the checking torque method is one of the most frequently used method of the torque auditing, it is also presented a torque x angle curve analysis for different torque points that are normally displayed by the measuring equipment. The samples were tested with a pre-defined assembly torque ensuring tightening only in the elastic (linear) region and a cardboard gasket that contains cellulose fiber was added to the system in order to potentiate the joint relaxation. In this study, the main differences, advantages and disadvantages of each method were presented. The results obtained were statistically evaluated comparing both methods with the values obtained by the load cell. Only the ultrasonic measurement method obtained results with statistical similarity.