Sviluppo di tecniche per l'analisi peridinamica della frattura duttile in problemi termomeccanici fortemente accoppiati

LAUREA MAGISTRALEQuesta tesi di laurea propone lo sviluppo di nuove tecniche basate sulla teoria peridinamica per l'analisi della frattura duttile e della frammentazione in problemi termomeccanici fortemente accoppiati. La peridinamica è una formulazione della meccanica del continuo, alternativa a quella classica, basata sull'ipotesi che i punti dei corpi deformabili interagiscano tra loro attraverso forze non locali non dipendenti da derivate nello spazio. Ciò permette la definizione e l'insorgenza di cricche e fratture senza bisogno di trattare le derivate spaziali in corrispondenza di discontinuità e senza definire leggi empiriche di crescita delle cricche in funzione di parametri come lo stress intensity factor. Viene dedicata particolare attenzione alla definizione teorica e all'implementazione numerica dei fenomeni di accoppiamento termomeccanico, che portano ad un ulteriore sviluppo della tecnica di integrazione nel tempo esplicita multirate. Si descrive anche la procedura implementativa sviluppata che ha permesso una migliore gestione dei legami non locali nel calcolo parallelo. Nel lavoro di tesi vengono studiati e sviluppati modelli elastici, elastoplastici, viscoelastici e di diffusione termica. I modelli trattati vengono analizzati numericamente per validarli e studiarne le caratteristiche di convergenza e stabilità. Il problema della frattura duttile viene affrontato implementando un metodo energeticamente consistente, detto del micropotenziale critico. La formulazione dei vari modelli è stata fatta in maniera tale da poter definire proprietà caratteristiche dei materiali in funzione della temperatura. Le conoscenze acquisite e sviluppate sono state applicate in due casi studio. Il primo caso studio presenta l'analisi della frattura duttile di un provino di acciaio sottoposto ad alte velocità di deformazione; i risultati numerici ottenuti sono stati confrontati con dati sperimentali di temperatura e deformazione. Il secondo caso studio riporta l'analisi esplorativa del fenomeno di danneggiamento in un meteorite soggetto agli alti carichi termici e di pressione dovuti all'impatto con l'atmosfera. Si riportano analisi sulle tempistiche di calcolo al variare della dimensioni del problema e sull'efficienza dello scalamento multiprocessore del software usato.The development of new techniques based on peridynamics theory for the analysis of ductile fracture and fragmentation in strongly coupled thermomechanical problems is submitted in this thesis. Peridynamics is a formulation of continuum mechanics based on the hypothesis that the points of the deformable bodies interact with each other through non-local forces that does not dependent on spatial derivatives. The definition and the onset of cracks and fractures is permitted without the need to treat the spatial derivatives near discontinuities and without defining empirical laws of crack growth in function of parameters, such as stress intensity factor. Special attention is given to the theoretical definition and the numerical implementation of thermomechanical coupling phenomena, which lead to a further development of the explicit time integration multirate technique. The developed implementation that allows better management of non-local bonds in parallel calculation has also been described. Elastic, elastoplastic, viscoelastic and thermal diffusion models are studied and developed in the thesis work. These models are analyzed numerically to validate them and study their convergence and stability characteristics. The problem of the ductile fracture is faced by implementing an energetically consistent method, called the critical micropotential model. The formulation of the peridynamic models was made in such a way as to be able to define characteristic properties of the materials as a function of temperature. The acquired and developed knowledge have been applied in two case studies. The first case study reports the analysis of the ductile fracture of a steel specimen subjected to high deformation rates; the numerical results obtained are compared with experimental temperature and deformation data. The second case study reports the analysis of the phenomenon of damage in a meteorite subject to high thermal and pressure loads due to the impact with the atmosphere. Calculation timing of analyses with different numerical sizes, and a study on the efficiency of multiprocessor scaling of the software used are also reported

Fault Injection for Synthetic Data Generation in Aircraft: A Simulation-Based Approach

The safety of aircraft heavily depends on the in-tegrity of the Landing Gear System (LGS). However, gathering real-world fault data to support effective Prognostic and Health Management (PHM) practices presents significant challenges. This work proposes a novel methodology for generating synthetic fault data using a multi-physics Simscape model of a landing gear deployment/retraction mechanism. The model incorporates specialized fault blocks designed to replicate various hydraulic failure modes, aiming to broaden the pool of fault data covering the most common failures. This approach promises to enhance maintenance strategies and facilitate the development of hybrid Model-Based and Data-Driven solutions. Ultimately, the results of this study will be used to understand the physics within the landing gear better and gather the necessary data to create an effective Digital Twin for predictive maintenance

Preliminary Evidence for Cell Membrane Amelioration in Children with Cystic Fibrosis by 5-MTHF and Vitamin B12 Supplementation: A Single Arm Trial

Cystic fibrosis (CF) is one of the most common fatal autosomal recessive disorders in the Caucasian population caused by mutations of gene for the cystic fibrosis transmembrane conductance regulator (CFTR). New experimental therapeutic strategies for CF propose a diet supplementation to affect the plasma membrane fluidity and to modulate amplified inflammatory response. The objective of this study was to evaluate the efficacy of 5-methyltetrahydrofolate (5-MTHF) and vitamin B12 supplementation for ameliorating cell plasma membrane features in pediatric patients with cystic fibrosis.A single arm trial was conducted from April 2004 to March 2006 in an Italian CF care centre. 31 children with CF aged from 3 to 8 years old were enrolled. Exclusion criteria were diabetes, chronic infections of the airways and regular antibiotics intake. Children with CF were supplemented for 24 weeks with 5-methyltetrahydrofolate (5-MTHF, 7.5 mg /day) and vitamin B12 (0.5 mg/day). Red blood cells (RBCs) were used to investigate plasma membrane, since RBCs share lipid, protein composition and organization with other cell types. We evaluated RBCs membrane lipid composition, membrane protein oxidative damage, cation content, cation transport pathways, plasma and RBCs folate levels and plasma homocysteine levels at baseline and after 24 weeks of 5-MTHF and vitamin B12 supplementation. In CF children, 5-MTHF and vitamin B12 supplementation (i) increased plasma and RBC folate levels; (ii) decreased plasma homocysteine levels; (iii) modified RBC membrane phospholipid fatty acid composition; (iv) increased RBC K(+) content; (v) reduced RBC membrane oxidative damage and HSP70 membrane association.5-MTHF and vitamin B12 supplementation might ameliorate RBC membrane features of children with CF.ClinicalTrials.gov NCT00730509

Experimental and numerical studies on particle and friction dampers

DOTTORATOLo smorzamento a particelle ed ad attrito sono tecnologie di smorzamento passivo che condividono numerosi punti di forza: sono resistenti agli ambienti difficili, funzionano su un'ampia gamma di frequenze, possono essere poco ingombranti. Queste tecnologie sono inoltre caratterizzate da un comportamento estremamente non lineare che ne rende problematica la modellazione e la progettazione. In questa dissertazione, gli smorzatori a particelle ed ad attrito sono stati studiati sia sperimentalmente che numericamente. Gli esperimenti condotti sugli smorzatori a particelle si sono concentrati sull'analisi della risposta e del comportamento delle particelle per diversi orientamenti di movimento, cavità e gravità, sotto input sinusoidali o casuali a diversi livelli; si è riscontrato che l'orientamento ha una forte influenza sui regimi di moto interno delle particelle e sullo smorzamento ottenibile. Per eseguire analisi numeriche sugli smorzatori a particelle, è stato sviluppato un nuovo risolutore di elementi discreti basato su GPU, accoppiato al codice multicorpo MBDyn e convalidato con l'esperimento sopra menzionato e studi di casi di letteratura. Diverse tecniche di modellazione sono state prese in considerazione e valutate nel tentativo di rendere le simulazioni del metodo degli elementi discreti più accessibili dal punto di vista computazionale; sono stati analizzati l'effetto di un modello di attrito semplificato, i limiti della riduzione della rigidità di contatto per aumentare il passo temporale e il confronto dell'energia dissipata degli smorzatori a particelle nelle simulazioni accoppiate e non accoppiate. Altre campagne sperimentali sono state eseguite con l'obiettivo di caratterizzare i contatti di attrito metallo-metallo. Per identificare i parametri di attrito dei risultati sperimentali, è stata perfezionata e validata numericamente una procedura di identificazione da utilizzare su campioni sperimentali. Le campagne sperimentali hanno portato allo sviluppo di un tribometro con il quale sono stati studiati gli effetti del carico normale, della velocità, della frequenza. Sono stati eseguiti test di resistenza per studiare l'effetto dell'usura sulla risposta dei contatti metallo-metallo; i risultati hanno mostrato una grande variabilità nel coefficiente di attrito medio della risposta dovuto all'usura e al danneggiamento delle superfici. Questo risultato è stato il motivo per cui studi teorici e numerici sugli assorbitori armonici ad attrito e la loro ottimizzazione robusta rispetto alle incertezze epistemiche nella forza di attrito sono stati implementati. È stato riscontrato che la risposta in frequenza degli assorbitori armonici ad attrito mostra punti invarianti e un minimo nella risposta massima rispetto alla forza di attrito, allo stesso modo del coefficiente di attrito lineare per gli assorbitori armonici lineari. Usando queste premesse, le equazioni dell’assorbitore armonico ad attrito sono state adimensionalizzate con nuovi parametri adimensionali; le curve principali che mettono in relazione le prestazioni e i parametri adimensionali sono state calcolate numericamente. È stato analizzato un semplice caso di studio e i suoi parametri sono stati ottimizzati con un approccio robusto che ha considerato un'elevata incertezza sulla forza di attrito con due strategie di ottimizzazione.Particle and friction damping are passive damping technologies that share numerous strengths: they are resistant to harsh environments, work on a broad range of frequencies, can be fitted in a small volume. These technologies are also characterized by an extremely non-linear behavior that makes modeling and designing them troublesome. In this dissertation, particle and friction dampers were studied both experimentally and numerically. The experiments performed on particle dampers were focused on analyzing the response and the particles' behavior for different orientations of motion, cavity, and gravity, under sine or random inputs at different levels; it was found that orientation has a strong influence on the inner motion regimes of the particles and the achievable damping. To perform numerical analyses on particle dampers at the particle level, a new GPU-based discrete element solver was developed, coupled to the multibody code MBDyn, and validated with the above-mentioned experiment and literature case studies. Different modeling techniques were considered and evaluated in an attempt to make discrete element method simulations more computationally affordable; the effect of a simplified friction model, the limits of reducing the contact stiffness to increase the time-step, and the comparison of the dissipated energy of particle dampers in coupled and uncoupled simulations were analyzed. Other experimental campaigns were performed with the objective of characterizing metal-to-metal frictional contacts. To identify the friction parameters of the experimental results, an identification procedure to be used with experimental samples was perfected and numerically validated. The experimental campaigns resulted in the development of a tribometer with which the effect of normal load, velocity, frequency were investigated. Endurance tests were carried out to study the effect of wear on the response of metal-to-metal contacts; the results showed great variability in the mean friction coefficient of the response due to wear and damage of the surfaces. This result was the reason for implementing theoretical and numerical studies on friction tuned mass dampers, and their robust optimization with respect to epistemic uncertainties in the stiction force. It was found that friction tuned mass dampers' frequency response shows invariant points and a minimum in the maximum response with respect to the stiction force, in the same vein as the linear damping coefficient for linear tuned mass dampers. Using these premises, the friction tuned mass damper equations were nondimensionalized with new nondimensional parameters; master curves that relate performance and the nondimensional parameters were numerically computed. A simple case study was analyzed and its parameters optimized with a robust approach that considered a high uncertainty on the stiction force on two optimization strategies.DIPARTIMENTO DI SCIENZE E TECNOLOGIE AEROSPAZIALI34DI LANDRO, LUCA ANGELOMASARATI, PIERANGEL

Efficient Discrete Element Modeling of Particle Dampers

Particle dampers’ dissipative characteristics can be difficult to predict because of their highly non-linear behavior. The application of such devices in deformable vibrating systems can require extensive experimental and numerical analyses; therefore, improving the efficiency when simulating particle dampers would help in this regard. Two techniques often proposed to speed up the simulation, namely the adoption of a simplified frictional moment and the reduction of the contact stiffness, are considered; their effect on the simulation run-time, on the ability of the particle bed to sustain shear deformation, and on the prediction of the dissipation performance is investigated for different numerical case studies. The reduction in contact stiffness is studied in relation to the maximum overlap between particles, as well as the contacts’ duration. These numerical simulations are carried out over a wide range of motion regimes, frequencies, and amplitude levels. Experimental results are considered as well. All the simulations are performed using a GPU-based discrete element simulation tool coupled with the multi-body code MBDyn; the results and execution time are compared with those of other solvers

Efficient Discrete Element Modeling of Particle Dampers

Particle dampers’ dissipative characteristics can be difficult to predict because of their highly non-linear behavior. The application of such devices in deformable vibrating systems can require extensive experimental and numerical analyses; therefore, improving the efficiency when simulating particle dampers would help in this regard. Two techniques often proposed to speed up the simulation, namely the adoption of a simplified frictional moment and the reduction of the contact stiffness, are considered; their effect on the simulation run-time, on the ability of the particle bed to sustain shear deformation, and on the prediction of the dissipation performance is investigated for different numerical case studies. The reduction in contact stiffness is studied in relation to the maximum overlap between particles, as well as the contacts’ duration. These numerical simulations are carried out over a wide range of motion regimes, frequencies, and amplitude levels. Experimental results are considered as well. All the simulations are performed using a GPU-based discrete element simulation tool coupled with the multi-body code MBDyn; the results and execution time are compared with those of other solvers.</jats:p