Einflüsse auf das Vorspannkraftrelaxationsverhalten von Schraubenverbindungen im Leichtbau

The central design criterion of a highly stressed bolted joint connection is its pre-load. Over the course of the operational lifetime of the connection this force is reduced by the damage mechanism of preload relaxation, which may lead to a loss of function of the connection. Using temperature sensitive materials like aluminium for clamped parts and nut thread parts considerably increases the risk of preload relaxation effects, especially if the steel bolts are highly utilized. Until now there is no design advice on how to reduce the preload relaxation nor a way to estimate the preload loss of such bolted joint connections.
The aim of this work is the experimental determination of practice relevant factors which have an influence on preload relaxation behaviour. Most of the examined specimen use a screw made of steel, an aluminium clamped part, and an aluminium nut thread part. To determine and quantify the influencing factors on the preload relaxation behaviour a series of tests were carried out. The determination of preload is based on the bolt elongation, which was measured with a tactile sensor. Additionally, the local deformation of selected clamped parts was measured with a novel optical-mechanical measurement system to determine the local relaxation behaviour.
The influence of the parameters temperature, assembly preload, clamping length, outer diameter of clamped part and nut thread part, thread engagement, surface roughness as well as the choice of material and alloy of screw, clamped part and nut thread part were systematically examined throughout the experiments. The parameters temperature, thread engagement and assembly preload have the highest influence on preload relaxation. The single contributions to the total preload loss show that all parts of the bolted joint connection have plastic material deformations, which results in a reduced preload. The main loss can be seen at the clamped part and, depending on the thread engagement, in the nut thread part. Design advice was derived from the experimental data to perform an easy and practically applicable optimization of thermally loaded bolted joint connections in lightweight constructions.