Colloquia

Summary

This thesis focusses on the development of an analytical model which quantifies damage propagation during vibration fatigue testing in composite structures. In stead of conventional fatigue parameters, such as stiffness- and frequency decline, the decrease in phase angle between the excitation is used as the primary measurement parameter. Previous conducted research shows that the phase near the resonance is sensitive for small changes in the effective stiffness, making early damage easier detectable. 

The developed model describes the tested specimen as a simplified Single-Degree-of-Freedom system, where damage is modelled as a decrease in the effective stiffness due to the delamination. By deriving a relationship between the phase, stiffness, and geometric damage parameter, it is possible to directly link the phase drop from the vibration fatigue test to the damage propagation. As a result, the phase is no longer used as a quantitative indicator, but as a qualitative parameter. 

From the results it becomes clear that damage can be identified with the analytical model, but the sensitivity is heavily influenced by assumptions in the model. Especially the chosen loss-factor proves to de a determining factor in the models sensitivity. This introduces uncertainties in de in the total damage predictions, while the damage trend is shown consistently.

In addition it has been investigated if the heat generation during the fatigue vibration test can be described analytically. By modelling the hysteretic dissipation with the Jenkins element, a proof of concept is delivered at which the mechanical energy losses are linked to the heat generation. Although this thermomechanical description is simplified, it shows potential to physically connect the phase degradation to the temperature development.