Nonlinearity in estimating bolt tension from vibrations

Many technical installations are held together by critical bolted joints. A critical bolted joint can have many appearances, from bolted flanges with many bolts to smaller lap-joints with only a few bolts. Critical imply that failure is not accepted, as it would lead to dangerous situations and most likely very costly repairs. Such joints are found in wind turbines, pressurized pipelines, large machinery etc. To ensure safe operation, regular checking and documentation of bolt tension is required, which is both costly and time consuming, and traditional tightening techniques are not very accurate. Recent work has shown potential in estimating bolt tension by using vibrations actively. e.g. by analyzing the vibrational response in a bolt after a hammer impact. Tests of this have been carried out with good results for setups with a single bolt. However, most often a bolted joint consists of multiple bolts holding at least two parts together. The more parts and bolts, the more sources to nonlinear effects from frictional contacts and potential vibrational coupling. In this work, we present the current status of the method with new results from a multi-bolt setup, and raise the question of which potential nonlinear effects need to be considered to apply the method more generally for multi-bolt joints.