parametric finite elements model of slm additive manufacturing process

Abstract An obstacle to the diffusion of additive technology is the difficulty of predicting the residual stresses introduced during the fabrication process. This problem has a considerable practical interest as evidenced by the abundant literature on residual stresses and distortion induced by the SLM (Selective Laser Melting) and EBAM (Electron Beam Additive Manufacturing). The purpose of this paper is to evaluate the effect of different process parameters on the heat distribution and residual stresses in components made with SLM technique. Three aspects are developed and illustrated: a) thermomechanical modeling of the growth process, based on Finite Elements (FE), which considers changes in the behavior of the material (powder→liquid→solid) through the finite element "birth" and "death" technique that enables the progressive activation of the elements as the component grows; b) sensitivity analysis of the model to the physical characteristics of the material (conductivity, specific heat capacity, Young's modulus). This is an important aspect allowing to focus on the most significant parameters to be determined experimentally with high reliability; c) evaluation of the effects of different process parameters (laser power, scan speed, overlap between adjacent paths) on the process. The article illustrates the theoretical thermal model and the detail of the strategy used in the FE analysis. The most influential characteristics of the material are highlighted and, finally, general criteria for choosing the optimal combination of process parameters to limit the residual stresses are provided

dynamic modeling of wind turbines how to model flexibility into multibody modelling

Abstract This work is part of a research activity inserted into "Smart Optimazed Fault Tolerant WIND Turbines (SOFTWIND)" project of PRIN 2015, funded by the Italian Ministry of the University and Research (MIUR). The need to define a robust multibody modelling procedure to realistically characterize the dynamical behavior of a generic wind turbine and to have a reduced computational burden has pushed the authors to adopt a freeware software called Nrel-FAST, that is universally considered to be a reference in the field of aeroelastic wind turbine simulations. The lightness of this software is paid in terms of modelling simplicity, which makes the modelling of wind turbines with unconventional support structures (i.e. that con not directly outlined as a fixed-beam) difficult. In this paper, some methodologies to overcome this obstacle are presented, including the use of a more powerful multibody software which, on the other hand, entails higher simulation times. In particular, the authors present a methodology based on structure stiffness-matrix reconstruction that allows, under appropriate hypothesis, to reduce a complex wind turbine support frame to a simple fixed beam so that the simulations can be done directly in FAST environment, with low computational times. The results obtained from these different approaches are compared using as test-case a small wind turbine property of University of Perugia (UniPG)

evaluation of fatigue damage with an energy criterion of simple implementation

Abstract Many theoretical methods for multiaxial fatigue life prediction are present in literature, most of them based on their effectiveness on knowledge of the entire stress time history. This represents the great applicative limit. The incapacity to study real situations, not only deterministic one, let the authors to develop a simple and rigorous criterion, which helps the designer who works in this area. The criterion is presented focusing the attention on the basic premise, highlighting its applicability, its practicality and its computational power. To do that, the Authors take into account the deterministic or random character of the individual constraint components and their degree of correlation. In order to verify the method, simulations of multiaxial loads conditions, developed in the time domain, will be carried out with various correlation levels between the stress components on which the method will be applied

correction formula approach to evaluate fatigue damage induced by non gaussian stress state

Abstract In the present paper the authors define an original analytical expression of a corrective coefficient to evaluate fatigue damage induced by a non-Gaussian stress state affected by high Kurtosis (values higher than 5) and by zero Skewness. This approach starts from a previous activity in which the authors solved an analogous problem but for light non-Gaussian stress states (Kurtosis value less than 5). The proposed procedure assumes to know the fatigue damage induced by Gaussian equivalent stress state time domain process. This characteristic allows the proposed procedure to be easily adopted inside the so-called Frequency Domain Fatigue Methods but in parallel with the statistical analysis of the system time domain response (Kurtosis and Skewness evaluation). Interesting considerations about its applicability will be proposed as concerns the non-Gaussianity and non-Stationarity of the inputs when the system is a flexible component excited in its frequency range

dynamic modeling of wind turbines experimental tuning of a multibody model

Abstract This work is part of a research project funded by the Italian Ministry of the University and Research (MIUR), under the call for "National Interest Research Projects 2015 (PRIN 2015)", titled "Smart Optimized Fault Tolerant WIND turbines (SOFTWIND)". Within this project, the research unit of the University of Perugia (UniPG) aims to develop dynamic modeling and simulation methodologies and fatigue behavior evaluation ones for wind turbine as a whole. The development of these methodologies will be aimed at predicting the life of generic wind turbines, also providing important and fundamental parameters for optimizing their control, aimed at reducing the failures of these machines. In the present paper, a small turbine, developed at the Department of Engineering of the University of Perugia, will be analyzed. The multibody modeling technique adopted and the experimental activity conducted in the wind tunnel of UniPG, needed for the tuning of the model, will be described. The analysis of both model behavior and experimental data has allowed for the definition of a robust multibody modeling technique that adopts a freeware code (NREL - FAST), universally considered to be a reference in this field. The goodness of the model guarantees the capabilities of the simulation environment to analyze the real load scenario and the fatigue behavior of this kind of device

fatigue life extimation of a military aircraft structure subjected to random loads

Abstract The suspension system of external stores of a military aircraft has reached the fatigue life limit estimated during the design phase, however, some elements suggest that the duration of the structure has been largely underestimated. This work aims to re-evaluate the fatigue life of the system and the potential extension of its use following an experimental numerical approach where, starting from a series of experimental flights in which the structure was instrumented, the forces acting on the system were calculated. Therefore, a methodology was developed to limit the damage calculation time using a hybrid approach that exploits the advantages deriving from the low computational burden typical of the methods in the frequency domain coupled with the Rainflow Counting precision. The study, although penalized by a series of conservative hypotheses, allowed to estimate a residual life equal to past life. It also provided important feedback on the field of application of advanced techniques for estimating the fatigue life of aeronautical structures subjected to random loading stories

on field durability tests of mechanical systems the use of the fatigue damage spectrum

Abstract: In the present paper the authors, starting from a previously proposed method for the combination and the synthesis of equivalent load conditions (by only managing PSD representations of the load conditions), developed a new approach based on the concept of Fatigue Damage Spectrum and on the system dynamics. The proposed approach was then validated by a durability test case, in which two different acceleration motion based load conditions, a norm load condition (by using laboratory test) and an operative one (by using acceleration measurements acquired during an experimental activity conducted on a transport vehicle) were compared

Multibody Models for the Analysis of a Fall From Height: Accident, Suicide, or Murder?

The final subject position is often the only evidence in the case of the fall of a human being from a given height. Foreseeing the body trajectory and the respective driving force may not be trivial due to the possibility of rotations and to an unknown initial position and momentum of the subject. This article illustrates how multibody models can be used for this aim, with specific reference to an actual case, where a worker fell into a stair well, prior to stair mounting, and he was found in an unexpected posture. The aim of the analysis was establishing if this worker was dead in that same place, if he had been pushed, and which was his initial position. A multibody model of the subject has been built (“numerical android”), given his stature and his known mass. Multiple simulations have been performed, following a design of experiments where various initial positions and velocity as well as pushing forces have been considered, while the objective function to be minimized was the deviation of the numerical android position from the actual worker position. At the end of the analysis, it was possible to point how a very limited set of conditions, all including the application of an external pushing force (or initial speed), could produce the given final posture with an error on the distance function equal to 0.39 m. The full analysis gives a demonstration of the potentiality of multibody models as a tool for the analysis of falls in forensic inquiries

Ottimizzazione del processo di riscaldo per tempra di grandi fucinati mediante analisi FEM

In questo lavoro è presentata la simulazione del processo di riscaldo per austenitizzazione di grandi fucinatiin acciaio destinati alla tempra differenziale. In ogni trattamento termico, le grandezze controllabili a livelloindustriale sono il tempo e la temperatura. È quindi agendo su queste che si può ottimizzare il processo perottenere le proprietà e le microstrutture desiderate per l’acciaio esaminato, e in definitiva massimizzare laqualità del pezzo finale. I prodotti analizzati sono i cilindri di laminazione fucinati, la cui produzione richiede unattento ciclo termico successivo alla deformazione plastica. Infatti, dopo un ciclo termico preliminare di ricotturacon lo scopo di facilitare le lavorazioni di sgrossatura alle macchine utensili, questi pezzi vengono sottopostiad una tempra differenziale per incrementare la durezza dello strato di lavoro, il cui spessore varia in funzionedelle specifiche richieste dal cliente. L’ottimizzazione di un processo può essere implementata utilizzando leggiempiriche ed il proprio know-how in prove sperimentali. La simulazione numerica rappresenta uno strumentodi previsione che potrebbe favorire un approccio più sistematico e scientifico alla risoluzione, in fase diprogetto, di tali problematiche industriali, specialmente quando il rapporto di scala tra il provino sperimentaleed il componente reale è molto elevato, come nel caso oggetto del presente lavoro. Lo strumento numericotipicamente adottato per simulare tale processo è la modellazione e l’analisi agli elementi finiti (FEM). Avendocome possibili parametri il livello di suddivisione del modello (meshatura), le caratteristiche chimico-fisiche delmateriale ed i parametri di scambio termico (entrambe differenziabili topologicamente all’interno del modellostesso) l’approccio FEA consente di simulare sia la forgiatura che i trattamenti termici ed in particolare le fasi diriscaldo, mantenimento e raffreddamento. I risultati ottenibili sono il campo termico, la distribuzione delle variefasi ed i valori di tensioni residue derivanti dal trattamento termico stesso. I risultati sono monitorabili nell’interoarco temporale del processo. Questo tipo di analisi agli elementi finiti consente di prevedere le proprietà delpezzo nell’intero arco temporale del processo: una volta caratterizzato il materiale ed i forni di riscaldo, si haa disposizione uno strumento capace di prevedere in maniera affidabile i risultati di un trattamento termico. Ilsoftware utilizzato in questo studio è il codice FEM commerciale Forge

Analisi del processo di deformazione a caldo e dell’evoluzione microstrutturale di un acciaio al 3% Cr mediante prove di torsione

Lo studio dell’evoluzione della microstruttura durante il processo di deformazione plastica di acciai rivestefondamentale importanza. In questo lavoro è stato analizzato il comportamento a deformazione a caldo di unacciaio 3% Cr tramite prove di torsione eseguite nell’intervallo di temperatura 1000?1200°C e con e.compresatra 0.01 e 1.00s-1. L’analisi condotta ha consentito di determinare le costanti della legge di potenza che regolail processo di deformazione a caldo (A0 , Q ed n ) tramite le quali è possibile ottimizzare i processi industriali.L’evoluzione microstrutturale durante la deformazione a caldo è stata valutata in termini di grado di percentualedi fase ricristallizzata ed è analizzata attraverso microscopia elettronica a trasmissione (TEM) e microscopiaad orientazione di immagine-diffrazione da retrodiffusione elettronica (OIM?EBSD). In particolare attraversoquesta ultima tecnica viene stimata la frazione volumetrica di ricristallizzato in termini di parametro GOS (GrainOrientation Spread), che meglio sembra interpretare l’evoluzione microstrutturale dopo deformazione a caldo inmicrostrutture martensitiche