Experimental Study on Nonlinear Random Excitation

Experimental design methods are instruments for directing useful, time-effective and efficient experiments and an accurate strategy in experimental activities lead to successful results. In the present paper is explained an experimental campaign focused on the random vibrations of circular cylindrical shells under thermal gradients across the shell thickness and broadband random loading to identify a particular phenomenon called synchronicity: the investigation is fully experimental. Nuclear, aerospace and automotive are some of the engineering fields involved in this subject, and in these real environments non-deterministic excitations can be coupled with a thermal load; extreme thermal conditions can cause differences of the temperature inside and outside the shell, e.g. thermal ex-changers. Due to the importance of the subject, the literature on shell vibration is extremely wide, it is not analyzed here for the sake of brevity; however, it is to note that the number of papers containing experimental results is not large. Under a random forcing, a system generally expects a random response, the statistical properties of the random response are correlated with the forcing through the transfer function in the case of linear systems, or more complicated relationships in the case of nonlinear systems. However, in some particular conditions (e.g. internal resonances, parametric resonances, ...) the presence of nonlinearity in the systems can give rise to a surprising phenomenon, said synchronicity or entrainment. In this work a shell subjected to a random base excitation is analyzed experimentally, the excitation is random (flat or limited frequency band), and takes advantage of previous setup and experimental techniques [3–5] developed by the present research team. The phenomenon of synchronicity is clearly observed for some particular thermal conditions: a strong transfer of energy from a broadband excitation signal to an almost harmonic response is experimentally observed, confirming the general findings of refs. [1, 2].