Research

         Elena Pierro - Research Interests

APPLIED MECHANICS LABORATORY
In the Laboratory of Applied Mechanics both experimental and numerical investigations are carried out, regarding the mechanical vibrations of macro- and micro- structures, the study of the mechanical properties of unconventional materials, the contact mechanics and the vehicle dynamics.
                           
Viscoelastic Materials: damping mechanisms
  • Viscoelastic materials are widely employed in different engineering areas, however it is still not so easy to properly assess their mechanical properties, in terms of viscoelastic - frequency dependent modulus. Standard experimental procedures are utilized in this direction (e.g. DMA - Dynamic mechanical analysis), but such techniques still present some complexities, and this is why possible alternative approaches would be desirable. For example, the experimental investigation of a viscoelastic beam dynamics would be challenging, especially for the intrinsic simplicity of this kind of test. In this direction, a deep understanding of damping mechanisms in viscoelastic beams results to be a quite important task to better predict their dynamics. With the aim to elucidate damping properties in such structures, an analytical study of the transversal vibrations of a viscoelastic beam has been developed. By means of a dimensional analysis, some key parameters have been defined, which depend on the material properties and the beam geometry. In this way, by properly tuning such disclosed parameters, e.g. the dimensionless beam length once the material is chosen, it is possible to enhance or suppress some resonant peaks, one at a time or more simultaneously. This is a remarkable possibility to efficiently control damping in these structures, and the results of this research may help in elucidating experimental procedures for the characterization of viscoelastic materials.

  • The acceleration modulus at a certain beam section (on the left), decreased in correspondence of the first resonance peak, for some numerical values of the dimensionless beam length a, related to discriminant values D (on the right) close to zero.
        • References: 
        • 1. E. Pierro, Damping Control in Viscoelastic Beam Dynamics, Journal of Vibration and Control, https://doi.org/10.1177/1077546320903195, 2020.
          2. E. Pierro, Viscoelastic beam dynamics: theoretical analysis on damping mechanisms, COMPDYN2019, Computational Methods in Structural Dynamics and Earthquake Engineering, Crete, Greece, 24-26 June 2019.
          3. E. Pierro, G. Carbone, "A new technique for the characterization of viscoelastic materials: Theory, experiments and comparison with DMA", Journal of Sound and Vibration, 515(10):116462, 2021 
  • Vibro-acoustical coupled systems
  • Noise and vibrations are often considered as phenomena affecting and disturbing the normal operating conditions of a wide range of industrial products. As a consequence, the research community has spent many years facing with such “problems” and has developed several techniques in order to prevent, mitigate, and monitor the sound emission and the structural vibration levels. Besides the "aerodynamic" sound produced by turbulences or unsteady flows, it is quite often that due to the fluid-structure interaction, the two phenomena are related to each other, in the so called “structure-borne” sound, i.e. the vibration of a solid body results in the generation of sound energy, as well as a noise source could let a body in vibration. When this interaction is so strong to couple the fluid response to the structural one and vice versa, i.e. they are vibro-acoustically coupled, the system dynamics becomes more complex. This is why a great deal of research has been required on this topic: indeed, just in the last decade this kind of coupled dynamics has been implemented in the more advanced commercial tools of testing and simulation, actually requiring deeper investigations. Regardless of the sources at the origin of the noise and vibrations phenomena, this first approach is strictly related to the human tolerances to the sound emissions, being hearing damage irreversible, and to the fatigue problems of the structures as well. In this direction, all the industries must respect both European Directives, whose objective is to define a common approach to avoid, prevent and reduce, the effects of environmental noise exposure on human being, and internal standards which often consist in targets or maximum levels to be not exceeded.
  • Atomic Force Microscope
  • dAFM are employed in many applications, e.g. to study biological targets as cells, proteins, DNA. They consist of an oscillating microcantilever which holds a sharp nanoscale tip that intermittently interacts, close to its first resonance frequency, with the sample.The micro-cantilever tip often needs to operate in a liquid environment to extract the required information from the sample. However, tip dynamics is strongly affected by the presence of the liquid itself, so that understanding the actual microcantilever response in such conditions, has become one of the most challenging problems the researchers are trying to face. A deep knowledge of the degree of interaction between the cantilever dynamics and the fluid is extremely important to avoid misleading information. Because of the micro-scale size of the cantilever, thermal noise due to Brownian forcing of liquid particles cannot be neglected and therefore proper insights about this effect are required. This is why different numerical approaches have been presented in literature, which only approximatively describe the liquid - cantilever interaction. In this context it has been presented an analytical heuristic formulation of the force the liquid exerts on the cantilever, which can be successfully utilized to investigate the AFM cantilever dynamics under the action of both linear and non linear forces. It has been shown that the liquid response consists of three terms: (i) a viscous term, (ii) a velocity-diffusive term, and (iii) an inertial term. The novelty of the model is mainly represented by the velocity-diffusive term, that to the best of our knowledge has never been taken into account before. We show indeed, that neglecting this term leads to large errors in the estimation of the cantilever response and , hence, of its thermal response, which is often used to calibrate the instrument.
  • The influence of the velocity-diffusive term on the cantilever thermal power spectrum.
  • Contact Mechanics
  • Real surfaces are affected by roughness at different lenght scales, and very often a rough surface can be described as a self-affine fractal. In this framework, the influence of fractal dimension on adhesion is investigated. It is found that at high loads the influence of the fractal dimension Df is not negligible. However at small loads the influence of Df is less important in agreement with the predictions of some contact mechanics theories.
    Moreover, the adhesion properties of biological fibrillar attachment systems is studied. In particular, it has been recently experimentally proved that mushroom-shaped micro-pillars have a significantly enhanced adhesion, if compared to simple flat micro-punches. We have carried out a theoretical investigation aimed to clarify the physical mechanisms that determine such a different behaviour. The performance of the two geometries have been compared, in terms of the pull-off force needed to detach the pillar from the substrate. In the flat punch case, we have shown that in almost every practical applications, the detachment occurs because of crack propagation from the edge towards the center of the contact. For the mushroom pillar, on the contrary, the separation at the edge is inhibited by the presence of the terminal plate, and the detachment is a consequence of the propagation of inner defects at the interface. In particular, the presence of the plate eliminates the square root stress singularity (which is a characteristic of the flat punch interfacial stress distribution), therefore it is possible to assert that the plate does not give any significant contribution in supporting the load, but it stabilizes defects at plate-substrate interface. Our pull-off force calculations are in good agreement with some available experimental outcomes, and confirm that mushroom-shaped pillars outperform the flat punch in terms of adhesion strength.
  • The mushroom shaped pillars.




Scuola di Ingegneria (SI - UniBas) - Università degli Studi della Basilicata