The current work wants to analyse and model the fatigue strength of lattice materials built by means of selective laser melting process. Lattice structures are a new class of materials and they provide good strength -weight ratio and good energy absorption properties. Different cell topologies have been analysed: the FCC and the BCC ones and their difference has been studied. The first part of this analysis is characterized by experimental tests, in which the two geometries have been tested with different loading conditions: tensile load with R-1, tensile load with R 01 and compressive load with R 01. The main results show the great influence of plastic strain accumulation in the fatigue life of the component. The damage factor indicates a crack propagation is some struts is expected, also in the run out condition. Once the experimental trends have been extracted, the results have been interpolated with exponential laws, showing good fitting. After that, the numerical part has been based on the re-construction of the real geometries of the specimens scanned previously. This allows to study the defects and the notches of the real geometries. After the STL has been completed, it has been modelled with Abaqus. The results of the FEM have been processed with a Matlab code which permits to evaluate the local equivalent stress amplitude according to the Sines criterion. Finally, the results of the Matlab code have been compared with the experimental results observing the presence of cracks near the notches of the structure and finding some struts failed also in the run – out condition. To complete the work a metallographic analysis has been performed to demonstrate the absence of correlation between the porosity and the crack nucleation, which is due to surface defects.
Questo lavoro vuole analizzare e modellare la resistenza a fatica di materiali lattice realizzati con la tecnica del selective laser melting. Le strutture lattice sono una nuova classe di materiali a forniscono un buon rapporto tra resistenza e peso e buone proprietà di assorbimento energetico. Differenti geometrie sono state analizzate: le quali sono chiamate FCC e le BCC; in questo lavoro sono state analizzate le loro differenze. La prima parte dello studio è caratterizzata dalla sessione di esperimenti, in cui le due geometrie sono state testate con differenti condizioni di carico: carico a trazione con R -1 e R 01, mentre i provini a compressione sono stati caricati solo con R 01. I risultati indicano una grande influenza dell’accumulo di deformazione plastica sulla resistenza a fatica del componente e il fattore di danneggiamento mostra una propagazione di cricche anche nella condizione di run-out. Una volta estratti i risultati dei test, i dati sono stati interpolati con funzioni esponenziali, i quali seguono i trend ipotizzati con una buona approssimazione, in particolare aumentando i punti sperimentali. Dopo ciò inizia la parte numerica, in cui sono stati ricostruiti i provini, tomografati in precedenza. Questo permette uno studio dei difetti e degli intagli delle geometrie reali. Dopo che lo STL è stato completato, è stato modellato con Abaqus. I risultati del FEM sono stati processati con un codice Matlab, che ha permesso di valutare lo sforzo locale equivalente secondo il criterio di Sines. In fine i risultati ottenuti con il codice numerico sono stati confrontati con i valori sperimentali, osservando la presenza di cricche vicino agli intagli della struttura e trovando anche alcuni strut rotti anche nella condizione di run-out. Per completare il lavoro è stata portata a termine anche un’analisi metallografica per dimostrare l’assenza di correlazione tra la forte porosità della struttura e la nucleazione di cricche, la quale in invece è dovuta agli intagli presenti sulla superficie.
Experimental and numerical modelling of fatigue of AlSi10Mg lattice structures obtained by selective laser melting
TRONCONI, FEDERICO
2018/2019
Abstract
The current work wants to analyse and model the fatigue strength of lattice materials built by means of selective laser melting process. Lattice structures are a new class of materials and they provide good strength -weight ratio and good energy absorption properties. Different cell topologies have been analysed: the FCC and the BCC ones and their difference has been studied. The first part of this analysis is characterized by experimental tests, in which the two geometries have been tested with different loading conditions: tensile load with R-1, tensile load with R 01 and compressive load with R 01. The main results show the great influence of plastic strain accumulation in the fatigue life of the component. The damage factor indicates a crack propagation is some struts is expected, also in the run out condition. Once the experimental trends have been extracted, the results have been interpolated with exponential laws, showing good fitting. After that, the numerical part has been based on the re-construction of the real geometries of the specimens scanned previously. This allows to study the defects and the notches of the real geometries. After the STL has been completed, it has been modelled with Abaqus. The results of the FEM have been processed with a Matlab code which permits to evaluate the local equivalent stress amplitude according to the Sines criterion. Finally, the results of the Matlab code have been compared with the experimental results observing the presence of cracks near the notches of the structure and finding some struts failed also in the run – out condition. To complete the work a metallographic analysis has been performed to demonstrate the absence of correlation between the porosity and the crack nucleation, which is due to surface defects.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/151486