Colloquia

Summary

This work investigates the localised and nonlinear behaviour inherent to joints of assembled systems. The primary objective is to develop a framework allowing the investigation of the nonlinear dynamic behaviour of joints, such that the forced vibration response of built-up systems can be predicted already in the early stages of product development.

Tests were performed on a typical space hardware subsystem to detect and characterise any nonlinear dynamic behaviour. Using this subsystem-level information, an attempt was made at increasing the correlation between model predictions and test results of the built-up space structure. As analysis was done using a linear model, nonlinear effects measured in the tests cannot be included. This is because commercially available finite element packages, such as NASTRAN and Abaqus, do not allow for simulation of nonlinear structures in the frequency domain. Therefore, a framework was developed for simulating nonlinear forced vibrations from within the standard finite element environment. The mode shapes of a nonlinear system, linearised around the equilibrium state, were used for a static, step-wise loading of the structure for increasing deflection levels. For each step in the obtained nonlinear static response, the system was linearised around the deflection state, as to obtain the quasi-linear forced vibration response.

The linear model of the built-up space structure showed suboptimal correlation with measurements. For including damping at known interface locations, as is suggested by the subsystem-level tests, a limited effect on correlation is found. As the structure proved to be too complex for further investigation in a linear model, it was decided to put no further effort in improving it. Instead, the numerical framework that was developed, is applied to a simple lap joint assembly with nonlinear contact interactions. It is demonstrated that the proposed method is able to successfully estimate a nonlinear relation between vibration amplitude and natural frequency, which in this case is due to a changing contact patch in the different deflection states. In addition, the analysis shows how a rather simple joint already gives rise to significant nonlinear behaviour. Further research includes the extension of the framework for more complex systems with closely-spaced modes, and verification of the predictions by experiments.