Beitrag zur Auslegungsrechnung von Leichtbauschraubenverbindungen im Hinblick auf die Vorspannkraftrelaxation

A loss of preload occurs in every bolted joint after tightening, which is referred to as “preload relaxation”. The magnitude of this loss is mainly influenced by the material of the screw and the clamped parts, which are in the flow of forces, as well as the prevailing temperature. As the sufficient height of preload is of great importance for the load bearing capacity of a bolted joint, a method of predicting the preload relaxation has been searched for a long time. Due to dissatisfactory predictive accuracy, missing transferability to different designs or the high effort for calculation and parameter determination, previous approaches have mostly turned out to be impractical and have thus not been established. In this thesis an analytical (equation-based) calculation model was therefore developed, for which all parameters, besides general material properties, can be derived from only one simple preload measurement, which are necessary to describe the loss of preload. At the same time all individual contributions of preload relaxation as well as the real resiliencies are considered. The calculation is based on a joint, where lightweight materials are clamped with steel bolts and nuts. Thus, the loss of preload mainly occurs from the time- and load-dependent plastic deformation of the clamped parts. It is shown that this method is suitable to predict continuously measured preload developments very well. The prediction of relative residual preloads of other test setups with different relative resiliencies, changed fasteners and different ambient temperatures is also practically sufficient. Moreover, the underlying material law can be varied, if fiber-reinforced plastics are used instead of lightweight metals. Furthermore, general statements on the influencing parameters on preload relaxation are given, based on the large experimental basis.