The mechanical performances of cellular structures have been widely studied in literature in terms of static strength and uniaxial fatigue, while a lack of knowledge is evident about their dynamic behavior. The aim of this thesis work is the definition of a testing procedure to numerically and experimentally study the fatigue life of metamaterials subjected to deterministic and random vibrations. Three different cells topologies have been investigated: the face-centered cubic (FCC), diamond and the triply periodic minimal surfaces (TPMS) gyroid. Fatigue tests under constant and random signals have been carried out to identify the fatigue strength of lattice specimens. The dynamic behavior has been predicted with Finite Element (FE) analyses, matching with a high accuracy the natural frequency and frequency response function (FRF) of the tested samples. The well definition of the FE model, based on a local approach, has allowed to foresee the state of stress in the lattice structure, identifying the stresses which lead the metamaterial to failure. By combining the numerical results with the experimental number of cycles to failure, all the points lay on the S-N curve of the bulk material. Therefore, this curve can be used for fatigue design of metamaterials. Random vibrations experiments have been performed on FCC and Diamond lattice specimens. The maximum acceleration has been acquired during the tests, to be implemented in a numerical code to predict the fatigue damage with the Rainflow algorithm and Miner’s rule. Most of the specimens failed with a damage about one, confirming the use of the bulk S-N curve. These results are considered as a solid base to develop in-depth future works within this subject, giving the necessary instruments to perform the tests and interpret the results.
Le performance meccaniche dei metamateriali sono sempre state studiate in termini di resistenza statica e fatica uniassiale, a discapito del loro comportamento dinamico che risulta ancora poco conosciuto. L’obiettivo di questa tesi è di definire un test per studiare numericamente e sperimentalmente la vita a fatica di metamateriali sollecitati con vibrazioni deterministiche e random. Tre diverse topologie di celle sono al centro dell’analisi: la cubica a facce centrate (FCC), la Diamond e la cella a minime superfici (triply periodic minimal surfaces, TPMS) gyroid. Per valutare la vita a fatica dei provini lattice, sono state eseguite delle prove con un segnale deterministico, e random. Il comportamento dinamico è stato predetto tramite un’analisi ad elementi finiti, identificando correttamente le frequenze naturali e la funzione di risposta in frequenza (FRF) dei provini testati. La bontà del modello ad elementi finiti, basato su un approccio locale, ha permesso di prevedere lo stato di sforzo nella struttura lattice, identificando lo sforzo che porta il metamateriale a rottura. Combinando i risultati numerici con il numero di cicli a rottura ottenuto sperimentalmente, tutti i punti identificati giacciono sulle curve S-N del materiale di base. Quindi, queste curve possono essere usate per progettare a fatica questi metamateriali. Successivamente, i provini FCC e Diamond sono stati testati tramite vibrazioni random. Per prevedere il danno a fatica, l’accelerazione massima è stata registrata per tutta la durata del test, ed implementata in un codice che utilizza l’algoritmo di Rainflow e la regola di Miner per calcolare il danno. La maggior parte dei provini ha raggiunto la rottura con un danno circa unitario, confermando l’utilizzo delle curve S-N del materiale di base. Questi risultati devono essere considerati come una solida base per più approfonditi lavori futuri, fornendo gli strumenti necessari per fare i test e valutare i risultati.
Estimation of fatigue life in the lattice structure of a heat exchanger from operational vibrations
Corneo, Giuseppe Marco
2019/2020
Abstract
The mechanical performances of cellular structures have been widely studied in literature in terms of static strength and uniaxial fatigue, while a lack of knowledge is evident about their dynamic behavior. The aim of this thesis work is the definition of a testing procedure to numerically and experimentally study the fatigue life of metamaterials subjected to deterministic and random vibrations. Three different cells topologies have been investigated: the face-centered cubic (FCC), diamond and the triply periodic minimal surfaces (TPMS) gyroid. Fatigue tests under constant and random signals have been carried out to identify the fatigue strength of lattice specimens. The dynamic behavior has been predicted with Finite Element (FE) analyses, matching with a high accuracy the natural frequency and frequency response function (FRF) of the tested samples. The well definition of the FE model, based on a local approach, has allowed to foresee the state of stress in the lattice structure, identifying the stresses which lead the metamaterial to failure. By combining the numerical results with the experimental number of cycles to failure, all the points lay on the S-N curve of the bulk material. Therefore, this curve can be used for fatigue design of metamaterials. Random vibrations experiments have been performed on FCC and Diamond lattice specimens. The maximum acceleration has been acquired during the tests, to be implemented in a numerical code to predict the fatigue damage with the Rainflow algorithm and Miner’s rule. Most of the specimens failed with a damage about one, confirming the use of the bulk S-N curve. These results are considered as a solid base to develop in-depth future works within this subject, giving the necessary instruments to perform the tests and interpret the results.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/174299