On the complexity of failure: a functional approach to vibration analysis

This dissertation addresses the phenomenon of complex failure in the context of rotating mechanical systems. Specifically, complex failure refers to the irreversible alterations of the load distribution, leading to a heterogeneous degradation of system components. In general complex failures cannot be attributed to a single component damage nor the operational environment alone. Rather these emerge as a combination of factors that undermine system functionality altogether. Complex failures stretch the modelling possibilities from a physics and an empirical perspective to a system-wide scale, limiting the accuracy of their assessment.

This dissertation offers a framework for the study of complex failures. This framework elaborates from the theory of material degradation and extends to the changes in load distribution among system components. Such changes are understood to obey to fundamentally different principles than those used in the design. The framework is guided according to four main elements: the mechanisms and dynamics of complex failures, and the methods and instruments necessary to the assessment of the corresponding failure behaviour.

The principles of failure behaviour and the functional approach to vibration monitoring are illustrated in two mechanical sub-systems: roller bearings and rotor blade systems. The rolling bearing case is developed through a comparative study of three different systems displaying bearing damage, a lab set-up, a wind turbine generator and a train axle box, for which a comparison of different types of damage and operational environment is performed. The rotor blade case is developed through a representing the operational excitations and boundary conditions of a rotating blade. Finally, a design framework for vibration monitoring systems in accordance with the monitoring needs for complex failure is presented.