PSI - Issue 64

Lukasz Scislo et al. / Procedia Structural Integrity 64 (2024) 2246–2253 Lukasz Scislo et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction 1.1. Dynamic measurements

Dynamic measurements are a preferable choice in determining, improving and optimising the dynamic characteristics of manufactured structures. It is especially beneficial in mechanical and aeronautical engineering (Dawood et al., 2022) but also has numerous applications for civil and building structures (Scislo and Szczepanik Scislo, 2023), material science (Scislo, 2023), biomechanical problems (Al-Baghdadi et al., 2021), understanding acoustical phenomena (Hase, 2020), transportation (Scislo and Andruszkiewicz, 2023), heritage elements evaluation (Fioriti et al., 2022), and many others. Dynamic measurements give the data to confirm the simulation of new elements and structures, allow the monitoring of the transportation of sensitive objects, observe the effects of earthquakes and civil engineering works, test new materials for future implementation, improve quality control and foresee problems in the future. Moreover, natural frequencies, Frequency Response Functions (FRFs), and mode shapes are good ways to check FEA models' validity. 1.2. Introduction to measurement equipment The typical measurement setup in a laboratory environment should have three constituent parts. Take a simple single input and single output case as an example. The first part is responsible for generating the excitation force and applying it to the test structure; the second part is to measure and acquire the response data, and the third part provides signal processing capacity to derive FRF. A basic understanding of structural dynamics is necessary for successful modal testing. Specifically, it is essential to understand the relationships between frequency response functions and their individual modal parameters. This understanding is of value in both the measurement and analysis phases of the survey. Knowing the various forms and trends of frequency response functions will increase accuracy during the measurement phase. 1.2.1. Excitation techniques One of the main steps in the measurement process involves selecting an excitation function (e.g., random noise) and an excitation system (e.g., a shaker) that best suits the application. The choice of excitation can make the difference between a good measurement and a poor one. Excitation selection should be approached from both the type of function desired and the type of excitation system available because they are interrelated. Excitation selection, depending on measurement connections, can be grouped as follows: • Laboratory test conditions: electrodynamic shakers, modal hammers, force cells, ball shooting systems, etc. • Operational excitations: road conditions in case of vehicle analysis, flight conditions for planes, wind excitation, excitations from ground motion, excitation from CO2 circulations, etc. • Specific test conditions: loudspeaker, gunshot, explosions, beam damping etc. The excitation function is the mathematical signal used for the input. The excitation system is the physical mechanism used to prove the signal. Generally, the excitation function's choice dictates the excitation system's choice. A genuinely random or burst random function requires a shaker system for implementation. In general, the reverse is also true. Choosing a hammer for the excitation system dictates an impulsive excitation function. Excitation functions fall into four general categories: steady-state, random, periodic and transient. The best choice of excitation function depends on several factors: available signal processing equipment, structure characteristics, general measurement considerations and, of course, the excitation system. The dynamics of the structure are also important when choosing the excitation function. The level of nonlinearities can be measured and characterised effectively with sine sweeps or chirps, but a random function may be needed to estimate the best-linearised model of a nonlinear system. The amount of damping and the density of the modes within the structure can also dictate the use of specific excitation functions. If modes are closely coupled and/or lightly damped, an excitation function that can be implemented in a leakage-free manner (e.g. burst random)