Vibrations are common in flexible engineering structures. In most cases, they are undesirable as they waste energy, cause fatigue, and create unwanted sound. All vibrations are inherently nonlinear in nature, meaning that the vibration response is not proportional to the excitation force. However, nonlinear phenomena are mostly overlooked in the industry because scientists lack a specific methodology and software to deal with them properly. As a result, they often favour linear approaches because they keep system behaviour simple, predictable and easy to manage.
Nonlinear phenomena with no linear counterparts
Linear approaches have undoubtedly aided in the development of techniques and tools that help solve complex engineering challenges. However, “The trend for lighter, more flexible, eco-friendly, and less costly structures is spurring engineers to come up with innovative designs and, importantly, reduce safety margins,” explains Gaëtan Kerschen, Professor of Aerospace Engineering in the Aerospace and Mechanical Engineering Department at the University of Liège, Belgium. Such structures are prone to display nonlinear behaviour once they are stretched or rotated. “The uneven friction between the different elements of a battery pack or between engine blades and the casing can produce highly nonlinear dynamics (vibrations) in electric cars and aircraft, respectively,” adds Kerschen. In general, nonlinearity may give rise to complex dynamic phenomena including modal interactions, bifurcations or chaos. Therefore, any attempt to apply traditional linear techniques to capture their dynamics is bound to fail.
Solving key hurdles in the design of safe engineering structures
Capitalising on the success of another project to develop the first software that can accurately quantify the impact of nonlinear vibrations on engineering structures, the EU-funded NI2D project is helping to commercialise the newly developed technology. “Our software assists test and simulation engineers in assessing the importance of nonlinear phenomena present in structures by analysing traditional test data in innovative ways,’’ explains Kerschen. Engineers can identify the source of nonlinear vibration and then create new models describing this behaviour for design purposes. He continues: “Nonlinearity then turns from an unknown and potentially dangerous phenomenon to a well-understood, predictable and acceptable dynamical event. This allows engineers to push their designs to new heights and helps overcome fears regarding the nonlinear vibration realm.” NI2D’s unique feature is that it can embrace both time series and finite element models. Compared to existing commercial software, it also allows updating nonlinear models as well as supports modal analysis (the study of the dynamic properties of systems in the frequency domain). Combining nonlinear time responses, vibration modes, resonance frequencies and bifurcations, it provides a clear and immediate view on the impact of nonlinearities on the dynamical properties of the structure for predictive and design purposes.
Nonlinearities can compromise the structural integrity and performance of products. The major risk is that product development is interrupted when certification requirements are not fulfilled (such as in the case of satellites). Regarding aircraft and car components, the accelerated wear caused by nonlinearities leads to increased maintenance costs and a negative environmental impact. NI2D offers the opportunity to assess the impact of nonlinearities in the early design phase, providing companies with high-fidelity models of the nonlinear connections in their structure. What’s more, NI2D substantially shortens the duration of the test campaign and improves product time to market. Kerschen concludes: “Conveying a rich set of nonlinear interactions, NI2D accelerates validation of promising new designs, eliminating the need to revert to conservative designs.”
NI2D, engineering structures, nonlinear vibration, vibration analysis, car, aircraft, satellite