The aim of this work is to assess the cutting technique of pericardium based on water jet (WJ) technology and to provide a mechanical characterization of the pericardium patches through biaxial stretching tests and finite element simulation of the experiments. The used bovine pericardium (LivaNova®) is stabilized by cross-linking in a solution made of 0.5% glutaraldehyde for 72h. The characterisation is performed with equibiaxial and uniaxial tension tests in quasi static conditions. On the experimental data statistical analyses are performed with one-way ANOVA tests (α=0.05) to understand the anisotropy of the tissue and to point out whether the cutting technologies influence the material response; the deformation state is analysed through DIC technique. Experimental results are replicated in FEBio® with simulations to extrapolate the material constitutive law characteristics. The mechanical parameters that described direction-dependent properties (slopes, power law terms) were identified. The laser cut samples exhibited a slightly different mechanical response w.r.t. WJ ones, inferring that laser heat-affected boundaries may exhibit a slightly higher stiffness of the material; furthermore, DIC analyses revealed that the anisotropy of the tissue affects the strain distribution within the sample, which is not directly detectable from grip displacement data, making it difficult to model the material in a general way; however, simulations were successful and the optimised mixed model parameters have been used on all experimental results to verify the prediction accuracy. A design of experiments approach is used to evaluate the quality of cut: using the WatAJet® micro pure WJ machine, three cut setups are tested varying the principal cut parameters (pressure, velocity, orifice diameter, standoff distance and air pressure) in a factorial design. Three subsequent statistical analyses are performed. An optimal cutting procedure has been identified which involves the use of setup 1 and the following parameters: pressure=3200[bar], velocity=300[mm/min], orifice=80[µm], standoff=3[mm] and Pair=-0.5[bar].
Lo scopo di questa tesi è valutare la tecnologia di taglio a getto d’acqua (WJ) del pericardio ed elaborare una caratterizzazione meccanica delle pezze di pericardio tramite test di trazione biassiale a simulazioni agli elementi finiti. Le pezze di pericardio bovino (LivaNova®) sono stabilizzate in una soluzione 0.5% di glutaraldeide per 72h. La caratterizzazione è svolta con test di trazione biassiali e uniassiali in condizioni quasi statiche. I dati sperimentali ottenuti sono analizzati con test statistici ANOVA ad una via (α=0.05) per studiare l’anisotropia del tessuto e l’effetto che la tecnologia di taglio ha sulla risposta del tessuto stesso. Il campo di deformazioni è studiato attraverso analisi DIC. I risultati sperimentali sono riprodotti con simulazioni numeriche in FEBio® per la definizione dei parametri del modello costitutivo del materiale. La caratterizzazione meccanica ha consentito di identificare i parametri direzione-dipendenti (pendenze, legge di potenza). I campioni tagliati a laser hanno mostrato comportamenti leggermente differenti da quelli ottenuti al getto d’acqua, a causa della presenza della zona di bordo termicamente alterata, che ha rigidità maggiore. Inoltre, la DIC ha mostrato che l’anisotropia del tessuto genera una distribuzione di deformazioni non omogenea, difficilmente riproducibile nelle simulazioni, e che non viene rilevata dallo spostamento complessivo; tuttavia, le simulazioni hanno dato buoni risultati, che sono stati verificati per tutte le geometrie analizzate. Per l’ottimizzazione della qualità di taglio al getto d’acqua, svolto in WatAJet®, è stato adottato un piano sperimentale fattoriale con tre setup di taglio e numerosi parametri (pressione, velocità, diametro dell’orifizio distanza di lavoro e pressione dell’aria) che sono stati variati. Sono state svolte in sequenza tre analisi statistiche, che hanno portato al miglior taglio, ottenuto con il setup 1, e i seguenti parametri: pressione=3200[bar], velocità=300[mm/min], diametro dell’orifizio=80[µm], distanza di lavoro=3[mm] e Paria=-0.5[bar].
Mechanical characterisation of bovine pericardium and optimisation of the water jet cutting procedure
Forzinetti, Elisa;Enei, Viviana
2019/2020
Abstract
The aim of this work is to assess the cutting technique of pericardium based on water jet (WJ) technology and to provide a mechanical characterization of the pericardium patches through biaxial stretching tests and finite element simulation of the experiments. The used bovine pericardium (LivaNova®) is stabilized by cross-linking in a solution made of 0.5% glutaraldehyde for 72h. The characterisation is performed with equibiaxial and uniaxial tension tests in quasi static conditions. On the experimental data statistical analyses are performed with one-way ANOVA tests (α=0.05) to understand the anisotropy of the tissue and to point out whether the cutting technologies influence the material response; the deformation state is analysed through DIC technique. Experimental results are replicated in FEBio® with simulations to extrapolate the material constitutive law characteristics. The mechanical parameters that described direction-dependent properties (slopes, power law terms) were identified. The laser cut samples exhibited a slightly different mechanical response w.r.t. WJ ones, inferring that laser heat-affected boundaries may exhibit a slightly higher stiffness of the material; furthermore, DIC analyses revealed that the anisotropy of the tissue affects the strain distribution within the sample, which is not directly detectable from grip displacement data, making it difficult to model the material in a general way; however, simulations were successful and the optimised mixed model parameters have been used on all experimental results to verify the prediction accuracy. A design of experiments approach is used to evaluate the quality of cut: using the WatAJet® micro pure WJ machine, three cut setups are tested varying the principal cut parameters (pressure, velocity, orifice diameter, standoff distance and air pressure) in a factorial design. Three subsequent statistical analyses are performed. An optimal cutting procedure has been identified which involves the use of setup 1 and the following parameters: pressure=3200[bar], velocity=300[mm/min], orifice=80[µm], standoff=3[mm] and Pair=-0.5[bar].| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/175707