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Modal Complexity Factors as indexes for modal parameter identification in operational modal analysis of coupled dynamic systems
(Elsevier, 2024-11-28) Ibarrola Chamizo, Javier; Agirre Olabide, Iker; Merino Olagüe, Mikel; Aginaga García, Jokin; Ingeniería; Ingeniaritza; Institute of Smart Cities - ISC; Gobierno de Navarra / Nafarroako Gobernua
Vibration analysis seeks to extract the modal parameters of a mechanical system by means of experimental measurements. Natural frequencies, damping ratios and mode shapes are identified from the measurements data from experimental or operational modal analysis. Modal shapes can show real or complex values. The degree of complexity of a modal shape can be measured by the Modal Complexity Factors (MCF). Among others, modal complexity can be due to non-uniformly distributed damping. In complex mechanical systems like a robot, complex modes are expected due to its active and non distributed damping. In turn, in a metallic workpiece real modes are expected. In the robotic machining of thin workpieces, both the robot and the workpiece constitute a coupled dynamic system, operating within the same frequency range. This work proposes the use of MCFs as indexes to determine if each mode corresponds to the workpiece or the robot. Experimental results of an operational modal analysis show a lower mode complexity for the workpiece modes and a higher complexity for the robot frequencies. MCFs show a good performance in separating modes of such coupled systems due to the different damping nature of the robot and the workpiece.
Open Access
Compression and torsion testing for elastic moduli and Poisson's ratio characterization in silicone rubber samples with varying shape factors
(Elsevier, 2025-05-19) Cortazar-Noguerol, Julen; Cortés, Fernando; Agirre Olabide, Iker; Elejabarrieta, María Jesús; Ingeniería; Ingeniaritza
Elastomeric materials, such as silicone rubber, are widely used in engineering applications due to their high deformability and viscoelastic properties. Under quasistatic regime and small deformations their behavior can be considered purely elastic and can be characterized by the elastic modulus, shear modulus, and Poisson's ratio, which are interrelated in isotropic materials. Although standard methodologies exist for determining these properties, experimental measurements are known to be affected by the geometry of the tested samples. The influence of sample geometry on compressive modulus measurements is well understood, however, its effect on shear modulus measurements is less explored. This study investigates how the dimensions of cylindrical samples influence the experimental determination of both the compressive and shear moduli and, consequently, Poisson's ratio. Compression and torsion tests are performed on silicone rubber samples of varying diameters and lengths using a dynamic mechanical analyzer and a rheometer respectively. The results confirm that both the compressive and shear moduli are affected by sample geometry, leading to unrealistic values of Poisson's ratio. To account for these effects, a correction model is proposed for shear modulus measurements, complementing existing corrections for compressive tests. The model successfully describes experimental trends and provides a more reliable estimation of Poisson's ratio, aligning with theoretical expectations for nearly incompressible elastomers. These findings emphasize the importance of considering geometric effects in compressive and torsion tests and provide a framework for improving the accuracy of mechanical characterization in elastomeric materials.

Agirre Olabide, Iker

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