Weight reduction in lightweight structures of dynamically loaded systems by new energy dissipative elements in bolted joints

In order to reduce vibration amplitudes, joint damping is being investigated at the MFPA Weimar and Fraunhofer IWM. It is intended to replace damping, often done by a frequency shift of naturals frequencies via added masses, and thus contribute to lightweight construction. To characterise the energy dissipation ability of joints, fretting wear tests with various parameter settings have been performed.
In fretting wear tests, two material samples are set in relative motion with a constant normal force. Friction in the joint creates a friction-force-hysteresis, the area of which reflects the energy dissipation. The larger the hysteresis area, the greater the energy dissipation. Based on the experimental observations, a new material model is formulated to capture energy dissipation starting form micro slipping up to macro-slip situations in joints.
Input parameters for the constitutive law are material pairing, the contact force, the frequency spectrum and the surface roughness. The constitutive law is defined in a finite element method (FEM) in intermediate elements between two friction bodies, where energy dissipation can be simulated for complex geometries. The numerical calculations are compared with validation experiments.
Bolted joints are investigated as an application. The pressure distribution in the connection depends on the distance of the considered point to the bolt. According to distance, the constitutive law is variously implemented in the joint. Bolted joints exhibit both micro-slip (near the bolt shank) and macro-slip as relative motions, resulting in different friction states in the joint. Despite the displacement, the function must be maintained, which is why materials with low fretting wear are used. Figure 1 illustrates the relationship.