Often, in the field of structural dynamics, engineers use predictive models to verify a structure’s dynamic response to external excitations, specifically when designing new structures that implement bolted connections. This can be attributed to the fact that modeling of bolted flanges has remained an appalling challenge. One notorious case is with bolted flanges employed in aero-engine castings. These flanges have been shown to exhibit a non-linear response when subjected to forced vibrations. As of now, models in use for structural dynamic analysis of such flanges are accurate, nonetheless, they are cumbersome when it comes to the design of new structures.
To date, three types of model have been reported, namely: thin layer elements, distributed contact elements and lump models, as potential solutions to the problem of aero-engine flange modeling. Each of these approaches has their own pros and cons. Regardless, the lump models have yielded promising results, however, currently in use lump models do not account for spigot connections. Generally, the models used either require model updating from modal testing, or they require a large number of nonlinear elements.
In this view, Professor Kamran Behdinan and his student Marc-Antoine Beaudoin at University of Toronto, Toronto, Canada, presented a study where they proposed a novel lump model for the nonlinear dynamic analysis of bolted flanges. To be precise, their novel approach could account for the nonlinear phenomena of partial clearance and friction, and had analytical parameters related to the flange geometry. Their work is currently published in the research journal, Mechanical Systems and Signal Processing.
The study commenced with the design of a test structure, i.e. a simplified and reduced scale version of a typical aero-engine connection. The section was composed of two pipes connected by a bolted flange. The researchers focused on the structure’s first bending mode because it was most influenced by the joint nonlinearities. Altogether, the model was implemented in finite element analysis and compared with an experiment where a test structure was excited by an electromechanical shaker.
The authors observed that the novel lump model was able to predict the structural response better than a traditional linear method; yet it did not require model updating, and it used a very limited number of nonlinear elements. Additionally, the team highlighted that inclusion of spigots increased the natural frequency of the first bending mode. Moreover, by observing the variations of contact status in a distributed contact element model, it was found that the approximation of bi-linear stiffness was valid for higher excitation levels, while being over simplifying for smaller excitations.
In summary, a new lump model was built for the nonlinear dynamic analysis of aero-engine bolted flanges. Ideally, the model’s constitutive equations were formulated to include the effect of partial clearance, friction, and a spigot connection. As a result, the lump model demonstrated promising results for aero-engine bolted flange modeling. Overall, based on facts enlisted, the researchers concluded that the model was well suited for the early design phase of new aero-engine casings.
Marc-Antoine Beaudoin, Kamran Behdinan. Analytical lump model for the nonlinear dynamic response of bolted flanges in aero-engine casings. Mechanical Systems and Signal Processing, volume 115 (2019) page 14–28.Go To Mechanical Systems and Signal Processing