Fatigue assessment of a FSAE car rear upright by a closed form solution of the critical plane method

Material fatigue is extensively discussed and researched within scientific and industrial communities. Fatigue damage poses a significant challenge for both metallic and non-metallic components, often resulting in unexpected failures of in-service parts. Within multiaxial fatigue assessment, critical plane methods have gained importance due to their ability to characterize a component's critical location and detect early crack propagation. However, the conventional approach to calculate critical plane factors is time-consuming, making it primarily suitable for research purposes or when critical regions are already known. In many real-world scenarios, identifying the critical area of a component is difficult due to complex geometries, varying loads, or time limitations. This challenge becomes particularly crucial after topological optimization of components and in the context of lightweight design. Recently, the authors proposed an efficient method for evaluating critical plane factors in closed form, applicable to all cases that necessitate the maximization of specific parameters based on stress and strain components or their combination. This paper presents and validates the proposed methodology, with reference to a rear upright of a FSAE car, which is characterized by a complex geometry and is subjected to non-proportional loading conditions. The efficient algorithm demonstrated a substantial reduction in computation time compared to the standard plane scanning method, while maintaining solution accuracy

Schede (Palazzo Dotto de\u2019 Dauli, ora Da Rio - Palazzo Dotto, ora Vigodarzere - Palazzo Savonarola - Palazzo Maffetti, ora Manzoni - Palazzo Trento, poi Papafava, poi Vigodarzere, ora Papafava - Palazzo Buzzacarini - Palazzo Mussato - Loggia del Consiglio o della Gran Guardia - Palazzo Belloni - Palazzo Obizzi - Palazzo Abriani - Palazzo del Capitanio - Palazzo Dondi dall\u2019Orologio, via Dondi - Palazzo Capodilista, ora Emo Capodilista)

Immissione e compilazione schede nel database sulla "Grande decorazione profana" del Sei e Settecento all'interno dei palazzi di Padova per il Centro di ricerca \u201cRossana Bossaglia\u201d in occasione del PRIN 2010-2011 \u2013 La pittura profana dall\u2019et\ue0 barocca all\u2019et\ue0 neoclassica nell\u2019Italia settentrionale, con particolare attenzione alla presenza dei pittori veneti e veneziani

Analysis of the interaction and propagation of multiple cracks in weldments.

This thesis is the result of a five-months work at the Fraunhofer LBF research centre. This thesis explores the recent field of fracture mechanics about the study of interaction and coalescence´of multiple cracks. The work will be focused on propagation of multiple cracks in a butt welded joint specimen intended for heavy industrial application. The thesis is based on previous research carried forward by Fraunhofer LBF group for Numerical Method and Component Design and all the groups belonging to the IBESS project. The study takes place on short cracks regime in which the use of elastic material hypothesis through the stress intensity factor loses its validity. Instead, an elastic plastic material behaviour is necessary with the introduction of the J-integral parameter, calculated through the use of FEM simulations of interacting and coalescing cracks’ models. This leads to the definition of two factors necessary for a more accurate study of the propagation of fatigue cracks, the interaction and coalescence factors. A research is conducted to compare the crack growth between experimental results and the IBESS computational algorithm. Also, an improvement of the coalescence factor is required, to solve a singularity problem in the function. The work showed that the simulations with heat affected zone material behaviour leads to results closer to reality respect the base material behaviour. Furthermore, the new implemented coalescence formula solved the problem of singularity at the beginning of coalescence, maintaining the behaviour of the function close to the experimental results

Influence of residual stresses on the fatigue life of welded joints

SOMMARIO Per ottenere elevati standard di qualità e sicurezza in un componente strutturale, occorre una notevole familiarità con il processo di produzione. Di fatto, i processi di produzione tendono a generare difetti quali tensioni residue, imperfezioni interne e superficiali, e considerando anche le variazioni geometriche intrinseche, come intagli, cricche o difetti in generale, è spesso complesso caratterizzare con precisione la resistenza strutturale di tali parti. In questo contesto, le tensioni residue giocano un ruolo importante, specialmente nelle strutture saldate. La stima delle tensioni residue è tipicamente effettuata tramite metodi sperimentali e numerici. Entrambi presentano punti di forza e svantaggi che ne richiedono l'uso combinato in modo da ottenere una valutazione coerente e significativa delle tensioni residue. La presente tesi di dottorato si inserisce in tale ambito. Il lavoro presentato nel seguito propone uno studio sulla generazione delle autotensioni e analizza il loro effetto sulla vita a fatica di giunti saldati tubo-piastra caricati in torsione e in flessione. Nella prima parte, viene valutata la capacità di diversi metodi termici nel riprodurre la distribuzione della temperatura intorno al cordone di saldatura, utilizzando simulazioni agli elementi finiti. I risultati delle simulazioni sono stati successivamente confrontati con misure sperimentali della temperatura superficiale calcolata in prossimità al cordone di saldatura. I modelli termici considerati presentano diversi livelli di complessità, partendo da quello base, che implementa una temperatura iniziale costante degli elementi appartenenti al cordone di saldatura, al più completo e diffuso modello a doppia ellissoide di Goldak. Lo studio mostra come i diversi metodi termici impiegati, presentino comportamenti termici simili in prossimità del cordone di saldatura. In secondo luogo è stato ottenuto un buon accordo tra le misure sperimentali e quelle numeriche utilizzando un metodo semplificato (cioè che richiede l'impostazione di un solo parametro). Il seguente modello può essere utilizzato per riprodurre efficacemente la storia termica durante un processo di saldatura. Nella seconda parte, viene presentato lo studio sulle autotensioni per un giunto saldato tubo-piastra in acciaio al carbonio S355JR. Per ottenere una conoscenza più ad ampio raggio sono state utilizzate sia simulazioni numeriche che prove sperimentali. Il software Ansys è stato utilizzato per effettuare simulazioni termo-strutturali disaccoppiate e valutare così le sollecitazioni, deformazioni e la temperatura in ogni nodo del modello. Per simulare correttamente il processo di saldatura agli elementi finiti sono state implementate proprietà del materiale elasto-plastiche e dipendenti dalla temperatura mentre, per riprodurre il processo di solidificazione del cordone, sono stati analizzati due diversi approcci numerici. I risultati ottenuti sono stati discussi e confrontati con dati sperimentali di deformazioni radiali rilassate misurate vicino al cordone di saldatura e ottenute tramite una tecnica di foratura incrementale. Nella terza parte del lavoro è stata studiata l'influenza delle autotensioni sulla vita a fatica dei giunti saldati. Nell'analisi sono stati considerati fattori critici quali discontinuità geometriche e la microstruttura eterogenea del materiale. Sono state eseguite prove sperimentali sui giunti saldati in condizioni as-welded e rilassati, sottoposti a carichi di torsione e flessione. I risultati sperimentali hanno mostrato come le tensioni residue abbiano un'influenza principalmente sui componenti caricati in torsione. In questo contesto il modello termo-strutturale presentato nei capitoli precedenti è stato usato per valutare il campo completo delle sollecitazioni residue all'interno del provino, così da poterle mappare e includerle come condizioni iniziali nei modelli numerici destinati al calcolo dei fattori di danno a fatica. Infine, l'efficacia dei modelli numerici nel descrivere il danneggiamento a fatica è stata valutata tramite confronto con risultati sperimentali. Nell'ultima parte di questo lavoro viene presentato uno studio numerico preliminare su un provino con intaglio acuto. Il lavoro si pone lo scopo di riprodurre nel provino intagliato condizioni di stress residuo paragonabili a quelle riscontrate sulla sezione critica del giunto saldato. Infatti, è possibile ottenere un gradiente di stress simile a quello ottenuto dopo il processo di saldatura variando il raggio e l'angolo di apertura dell'intaglio. L'uso di una geometria semplificata permette un'analisi più accessibile e migliora la comprensione dei processi che avvengono all'interno del materiale in presenza di autotensioni e carichi affaticanti. ABSTRACT A deep knowledge of the production process is needed, in order to achieve quality and safety requirements in a structural component. As a matter of fact, manufacturing processes can introduce defects such as residual stresses, internal and superficial imperfections. Together with the inherent geometric variations, such as notches, cracks or defects in general, it is often difficult to precisely characterise the structural strength of such parts. In this context, residual stresses play an important role, especially in welded structures. The evaluation of residual stresses is typically performed using both experimental and numerical methods. Both present strengths and drawbacks which demand their combined usage to achieve a consistent and meaningful evaluation of the residual stresses. Within this scope, this PhD thesis presents an evaluation of residual stresses in a pipe-to-plate welded joint and studied their influence on the fatigue life of torsionally and bending loaded components. In the first part, the finite element method was used to assess the capability of different thermal methods used to simulate a single pass of the gas metal arc welding process in reproducing the temperature distribution around the weld bead. Results of the simulations were compared to experimental measurements of the surface temperature close to the weld region. The considered thermal techniques adopted different levels of complexity, from the basic implementation of a constant initial temperature assigned to a given material volume, to the more comprehensive and widespread Goldak's double-ellipsoid model. The study shows that, close to the weld seam, very similar thermal behaviours can be achieved by employing each one of the analysed methods. Secondly, considering the constant initial temperature method, the comparison between experimental measurements and numerical simulations showed a fairly good agreement, suggesting that a relatively simple method (i.e., requiring the setting of only one parameter) can be used to efficiently reproduce the thermal history of a welding process. In the second part, the study of residual stresses for a S355JR carbon steel pipe-to-plate welded joint is presented. Numerical simulations and experimental tests were both employed in order to gain wide-ranging knowledge. Numerical simulations were performed with the software Ansys through uncoupled thermal-structural simulations in order to evaluate the stresses, strains and temperature at each node of the finite element model for each phase of the simulation. Temperature-dependent elastic-plastic material properties were adopted in combination with the \textit{element birth \& death} method to simulate the welding process. Two different numerical approaches were implemented for reproducing the weld seam solidification process. The obtained results were discussed and compared with experimental data, in terms of relaxed radial strains measured nearby the seam weld, due to a material removal procedure. The third part investigates the influence of residual stresses on the fatigue life of the welded joints. Influencing factors such as geometric discontinuities and the material heterogeneous microstructure were considered. Experimental tests on as-welded and stress relieved specimens with fully reversed torsion and bending loading conditions were carried out. Experimental results showed how residual stresses exhibited an influence mainly on torsionally loaded components. Numerically, the uncoupled thermal-structural finite element simulation presented in the chapters before was used to assess the complete residual stress field within the specimens. Secondly, residual stresses were mapped and included as initial condition in numerical models intended for fatigue damage factors calculation. Finally, experimental results were then used to corroborate numerical models and verify their efficacy in assessing fatigue endurance. In the last part of this work, a preliminary numerical study of a notched specimen geometry is presented. The work attempts to reproduce residual stress conditions comparable to those found on the welded joint critical notch section on notched specimens thus in order to explain the results observed on welded specimens. Indeed, by varying notch radius and opening angle of a cylindrical specimen, it is possible to obtain a stress gradient similar to that obtained after the welding process at the weld notches. The use of simplified geometry allows easier analysis and a possible improved understanding of the processes taking place within the material

MEASURING FAIR AND SUSTAINABLE WELL-BEING FOR ECONOMIC POLICY PLANNING AND EVALUATION

openL'Indice del Benessere Equo e Sostenibile (BES) è un importante strumento di valutazione che va oltre il tradizionale focus sul PIL, valutando l'impatto economico e sociale delle politiche. Esso considera una vasta gamma di indicatori che misurano il benessere delle persone, la qualità dell'ambiente e la sostenibilità a lungo termine. L'implementazione del BES può avere un impatto significativo sulle decisioni politiche ed economiche, promuovendo politiche più inclusive, sostenibili e orientate al benessere delle persone e del pianeta.The Well-being, Equity, and Sustainability Index (BES) is an important evaluation tool that goes beyond the traditional focus on GDP, assessing the economic and social impact of policies. It considers a wide range of indicators that measure people's well-being, environmental quality, and long-term sustainability. The implementation of the BES can have a significant impact on political and economic decisions, promoting more inclusive, sustainable, and people- and planet-centered policies

Evaluation of Heat Sources for the Simulation of the Temperature Distribution in Gas Metal Arc Welded Joints

Residual stresses can affect both the static strength and the fatigue endurance of welded joints. Residual stresses can be assessed by numerical simulation; however, the simulation of the welding process is a complex task that requires knowledge of several parameters, many of which can only be estimated with some uncertainty. The reduction in the number of these parameters can lead to a more feasible and efficient study. In this work, the finite element method is used to assess the capability of different thermal methods used to simulate a single pass of the gas metal arc welding process in reproducing the temperature distribution around the weld. Results of the simulations are compared to experimental measurements of the surface temperature close to the welding area. The thermal techniques analyzed adopt different levels of complexity, from the basic implementation of a constant initial temperature assigned to a given material volume, to the more comprehensive and widespread Goldak’s double-ellipsoid model. The study shows that, close to the weld seam, very similar thermal behaviors can be achieved by employing each one of the analyzed methods. Secondly, considering the constant initial temperature method, the comparison between experimental measurements and numerical simulations showed a fairly good agreement, suggesting that a relatively simple method (i.e., requiring the setting of only one parameter) can be used to efficiently reproduce the thermal history of a welding process

Evaluation of residual stresses in a tube-to-plate welded joint

A deep understanding of the manufacturing process is needed in order to achieve safety and quality requirements for parts and components; to this regard, residual stresses play an important role in welded structures. Residual stresses are mainly caused by the extremely severe thermal cycle to which the welded metal and base material are subjected to during welding process and their knowledge leads to a better static and fatigue assessment of welded joints. This work deals with the study of residual stresses for a tube to plate T-joint, made of S355JR carbon steel. The work was carried out by both numerical simulations and experimental tests. The numerical simulations were performed by Ansys FE code through a structural-thermal full transient analysis to evaluate stress, strain and temperature in each node at each step of the simulation. The “birth and death” method was employed, together with temperature-dependent material properties.A2Danda3D simulation were performed, in order to evaluate possible differences due to the welding process. Numerical results were compared to some preliminary measurements obtained through an incremental cut made on the plate

Fatigue life assessment of welded joints under sequences of bending and torsion loading blocks of different lengths

In this work, the nominal stress concept, the notch stress approach and two critical plane approaches are used to analyse the fatigue endurance of a pipe-to-plate welded joint subjected to complex loading histories. Both the pipe and the plate were made of S355JR steel. Starting from already known fatigue endurance curves obtained for the same specimens under pure bending and pure torsion, a first series of tests was conducted, in which specimens were loaded in bending for a given fraction of the estimated life and then in torsion until failure. A similar series of tests was then carried out by changing the loading order: specimens were firstly loaded in torsion for a given fraction of the estimated endurance and then in bending until failure. The whole test campaign was repeated for two different fractions of the estimated life, that is, 0.3 and 0.45, respectively. After that, additional three series of tests were carried out, in which the specimens were subjected to consecutive sequences of bending and torsion blocks of different lengths (short, medium and long, respectively); the relative length of the bending and torsion block in each series was determined in order to produce the same damage. The experimental results, in terms of total damage at failure, were analysed using the Palmgren–Miner hypothesis. For all the assessment methods, the characteristic endurance curves were firstly calibrated on the basis of finite element (FE) analyses and of the experimental results obtained under pure bending and pure torsion loadings. The observed damage at failure resulted always greater than 0.5 for all the employed methods and greater than 1 for most of the tests. The different methods gave similar results, with the critical plane methods giving a slightly more stable damage at failure and a correct determination of the failure location. For all the methods, the damage at failure slightly reduces as the block length shortens