Development and application of experimental methodologies for the mechanical characterization of non-linear materials exploited in the production of loudspeakers

La caratterizzazione di materiali utilizzati nella produzione di altoparlanti necessita dello sviluppo di tecniche investigative che si adattano alle specificità di ciascuna tipologia di materiale in esame. In questa tesi vengono quindi proposti approcci diversi in base al materiale e al tipo di caratterizzazione impiegata, ovvero statica/transitoria e dinamica. Viene presentata una nuova caratterizzazione dinamica della carta basata su un metodo ibrido. Infatti, grazie all’utilizzo combinato del metodo agli elementi finiti e dell’analisi modale sperimentale, è stato possibile valutare le proprietà dinamiche meccaniche del materiale in termini di modulo conservativo e dissipativo all’interno di un ampio range di frequenze, In particolare, nel range di frequenze acustiche è stata individuata una relazione lineari tra le caratteristiche meccaniche della carta e la frequenza d’eccitazione. Per quanto riguarda la gomma, è stato possibile analizzare la deformazione tridimensionale dei campioni sottoposti a carico uniassiale tramite lo sviluppo di un opportuno setup sperimentale. Ciò ha permesso di descrivere il comportamento viscoelastico del materiale medianti le leggi costitutive dei modelli generalizzati di Maxwell e di Kelvin-Voigt. Inoltre, la misura della deformazione tridimensionale ha permesso di identificare i limiti in termini di deformazione entro i quali le ipotesi di incomprimibilità e isotropia del materiale sono verificate. La caratterizzazione dinamica ha sfruttato i dati ottenuti mediante prove di analisi dinamica termica al fine di sviluppare un nuovo approccio per la costruzione delle mastercurve del modulo conservativo e di quello dissipativo. I risultati sperimentali sono stati descritti mediante il modello di Havriliak-Negami, il quale è risultato essere un potente strumento per la determinazione dei fattori di shift necessari all’ottenimento della mastercurve in un ampio range di frequenze. Le varie metodologie sviluppate hanno permesso di descrivere le proprietà meccaniche dei materiali in esame tramite modelli parametrici. I parametri così individuati possono essere facilmente impiegabili nelle simulazioni numeriche degli altoparlanti.The mechanical characterization of materials exploited in the production of loudspeakers requires the development of investigative techniques that adapt to the specificities of each type of test material. The proposed approaches were diversified depending on the concerned material and the nature of the characterization, i.e. static or transient versus dynamic. A new hybrid dynamic characterization of paper was presented. Thanks to the combined employment of a Finite Element Method together with the Experimental Modal Analysis, the dynamic mechanical properties of the material, i.e. the storage and the loss moduli, can be evaluated in a wide range of frequency. A linear relationship between mechanical properties of paper and frequency was found within the acoustic range. The proposed experimental setup for the transient test on rubbery materials aimed to the investigation of the three-dimensional strain state of the sample subjected to uniaxial load. The viscoelastic behavior of the material was then described through the constitutive laws provided by the generalized Maxwell model and the generalized Kelvin-Voigt model. The three-dimensional strain measurement allowed the identification of the limits in terms of applied strain within which the hypothesis of isotropic and incompressible material is verified. The dynamical characterization was performed by means of a new approach for the construction of the mastercurves of both storage and loss modulus starting from Dynamic Mechanical Thermal Analysis data. The experimental results were described by the Havriliak-Negami model, which represents a powerful tool for determining the shift factors needed to achieved a reliable mastercurve within a wide range of frequency. The developed methodologies allowed to describe the mechanical properties of the investigated materials by means of parametric models. The complete set of identified parameters are readily employable for the purpose of numerical simulation of the loudspeaker

Sensor calibration of polymeric Hopkinson bars for dynamic testing of soft materials

Split Hopkinson pressure bar (SHPB) testing is one of the most common techniques for the estimation of the constitutive behaviour of metallic materials. In this paper, the characterisation of soft rubber-like materials has been addressed by means of polymeric bars thanks to their reduced mechanical impedance. Due to their visco-elastic nature, polymeric bars are more sensitive to temperature changes than metallic bars, and due to their low conductance, the strain gauges used to measure the propagating wave in an SHPB may be exposed to significant heating. Consequently, a calibration procedure has been proposed to estimate quantitatively the temperature influence on strain gauge output. Furthermore, the calibration is used to determine the elastic modulus of the polymeric bars, which is an important parameter for the synchronisation of the propagation waves measured in the input and output bar strain gate stations, and for the correct determination of stress and strain evolution within the specimen. An example of the application has been reported in order to demonstrate the effectiveness of the technique. Different tests at different strain rates have been carried out on samples made of nytrile butadyene rubber (NBR) from the same injection moulding batch. Thanks to the correct synchronisation of the measured propagation waves measured by the strain gauges and applying the calibrated coefficients, the mechanical behaviour of the NBR material is obtained in terms of strain-rate-strain and stress-strain engineering curve

Assessment of effort and pain after compak sporting competition using three different over&under shotguns

The recoil of a shotgun can result in both an increase in perceived exertion and pain by the shooter and the occurrence of muscle fatigue phenomena. This represents a critical aspect of the disciplines of clay pigeon shooting where it is expected to hit a large number of targets by means of a rearm, which is typically the over/under shotgun. Therefore, this study aims to evaluate the e ects of the recoil on the shooter and how the use of di erent weapons can modify these e ects. Both qualitative and quantitative tests allowed to assess e ort and pain perceived by the shooter (Rate of Perceived Exertion, RPE, and Visual Analog Scale, VAS) and his muscular force variation (Hand Grip Test, HGT, and Isometric Maximum Voluntary Contraction, IMVC) between rest condition and after the competition. 8 shooters (age 26.8±4.6 years; BMI 22.9±1.5 kg/m2) with at least 4 years of shooting experience were recruited. Each shooter has red with three di erent shotguns having di erent technical characteristics (balance and ergonomics) during three Compak Sporting competition. The reproducibility of rest conditions during the three competition days was assessed by Intraclass Correlation Coe cient (ICC). Subsequently, Wilcoxon signed-rank test was used to compare the results of each test before and after the shooting session. Moreover, to determine any dependence on the technical features of the shotgun on the shooter’s fatigue, an analysis of variance (ANOVA) was applied to highlight any di erences between the three tested weapons. Signi cance was set a priori at p<0.05 for all tests. Thanks to the proposed experimental protocol, signi cant di erences were observed between rest condition and after Compak Sporting competition on HGT (p=0.036) and ET (p=4.5·10-5) for muscle fatigue and VAS (p=1.5·10-5) and RPE (p=1.4·10-3) for self- perceived fatigue. Signi cant di erences in muscle fatigue levels were found when comparing the three types of gun, especially with regard to the upper part of the non- dominant shooter’s body

Comfort assessment in the use of shotgun for skeet shooting: an EMG based approach

In the last decades the improvements of sportive performance are due to the consistent innovation in equipment, clothing and training. Hence for what regards the equipment, comfort is becoming everyday more interesting as it strongly influences the performance and the risks of injuries. Is therefore, proposed in this research a methodology for an assessment of the comfort using frequency domain indexes, that are fatigue dependent, extrapolated from the surface electromyographic signal. The use of mean and median frequency of the surface electromyography were found to be useful indicators of fatigue in dynamic condition hence fatigue can be used as one of the variables concurring in the assessment of comfort. The methodology proposed could be used for both improvement of the performances of the athlete and either for quantifying the comfort in the interaction between the athlete and his sport equipment

Purified collagen I oriented membrane for tendon repair: an ex vivo morphological study.

Purpose: Injured tendons have limited repair ability after full-thickness lesions. Tendon regeneration properties and adverse reactions were assessed ex vivo in an experimental animal model using a new collagen I membrane. Methods: This multilamellar membrane obtained from purified equine Achilles tendon is characterized by oriented collagen I fibers and has been shown to sustain cell growth and orientation in vitro. The central third of the patellar tendon (PT) of 10 New Zealand White rabbits was sectioned and grafted with the collagen membrane; the contralateral PT was cut longitudinally (sham-operated controls). Animals were euthanized 1 or 6 months from surgery and tendons subjected to histological and Synchrotron Radiation-based Computed Microtomography (SRµCT) examination and 3D structure analysis. Results: Histological and SRuCT findings showed satisfactory graft integration with native tendon; histological examination also showed ongoing angiogenesis. Adverse side-effects (inflammation, rejection, calcification) were never observed. Conclusion: The multilamellar collagen I membrane can be considered as an effective tool for tendon defect repair and tendon augmentation