Nature represents an exceptional source of inspiration because it has refined and updated itself over the centuries. Biomimetics, alternatively called biomimicry or bionics, has been exploited in the current work in order to steal from nature some attractive functions and reproduce them in a man-made material. More specifically, bone hierarchical organization has been investigated. Bone is a sensational biological composite structure, since it is characterized by a fracture toughness 3-5 order of magnitude higher than the one of its main constituents, collagen and hydroxyapatite. At a micrometric level the prevalent structures of cortical bone are the Haversian systems (also called osteons), consisting in longitudinally oriented cylindrical features, made up by overlapped concentric lamellae surrounded by an interstitial matrix. By means of this controlled organization, many toughening mechanisms are exploited in order to make tortuous the crack path and thus increase the energy level required for the crack growth. Since composite materials are generally depicted by high modulus and strength but low fracture toughness, the particular toughening mechanisms of bone are mimicked in order to improve this weakness. Moreover, bone is a multifunctional biological structure, able to effectively self-repair in case a damage occurs. Haversian canal, responsible for housing the vascular system which carries nutrients and blood to the whole bone, is thus introduced in the material design with the purpose of including this new function. The current work is a development of a larger project and aims at introducing those features in order to obtain increased fracture properties and lay the foundations for a full exploitation of self-healing capability.The new geometry consists of tubes organized in three rows, spaced by an inter-osteon layer and separated by horizontal plies. After having designed it, material manufacturing has been optimized and a series of laminates has been produced through "prepreg and autoclave" process. Finally, flexural properties and translaminar fracture toughness have been tested. Results, compared with previous laminates, are promising when normalized by material density. In fact, tubes addition ensures, due to holes exploitation, both the possibility of material functionalization and a great density decrease with respect to a solid material. Normalized mechanical properties are thus increased in longitudinal direction, but worsened in transverse, while fracture toughness has been boosted by the successful replication of some toughening mechanisms similar to those observed in bone Haversian structure.
La natura rappresenta una straordinaria fonte di ispirazione, essendosi fortemente perfezionata ed adattata nel corso dei secoli. La biomimetica, in alternativa chiamata bionica, è stata utilizzata nel presente lavoro al fine di appropriarsi di alcuni meccanismi rilevanti dalla natura e riprodurli in un materiale artificale. Più specificatamente, è stata analizzata l'organizzazione gerarchica dell'osso, il quale rappresenta una eccezionale struttura biologica composita, essendo caratterizzata da una tenacità a frattura 3-5 ordini di grandezza superiore rispetto ai suoi principali costituenti, collagene e idrossiapatite. A livello micrometrico le strutture prevalenti dell'osso corticale sono i sistemi Haversiani (denominati anche osteoni), i quali sono composti da elementi cilindrici orientati longitudinalmente, costituiti da lamelle concentriche sovrapposte immerse nella matrice interstiziale. Attraverso tale disposizione, molti meccanismi di tenacizzazione possono essere attivati al fine di deviare il percorso della cricca ed aumentare l'energia necessaria per la propagazione. Poichè i materiali compositi sono generalmente caratterizzati da un alto modulo elastico e resistenza ma bassa tenacità a frattura, i particolari meccanismi di tenacizzazione dell'osso sono stati imitati per migliorare tale proprietà. Inoltre, l'osso rappresenta una struttura biologica multifunzionale, capace di autoripararsi in caso di danneggiamento. Il canale di Havers, contenente il sistema vascolare che trasporta sangue e nutrienti in tutto l'osso, è stato perciò introdotto nella progettazione del materiale allo scopo di aggiungere questa nuova funzionalità. Il presente lavoro è lo sviluppo di un progetto già iniziato e si pone l'obiettivo di introdurre tali funzioni al fine di migliorare la tenacità a frattura e gettare le basi per la completa realizzazione del meccanismo di autoriparazione. La nuova geometria è composta da tre righe di tubi allineati, separate da strati orizzontali di preimpregnati, in cui i tubi sono distanziati da uno strato inter-osteonico in modo alternato. Dopo aver definito la geometria, la fase di produzione del materiale è stata ottimizzata e una serie di laminati prodotti per mezzo del processo "prepreg e autoclave". Infine, le proprietà flessionali e la tenacità translaminare a frattura sono state testate sperimentalmente. I risultati, rispetto a laminati precedenti, risultano essere promettenti se normalizzati rispetto alla densità. Infatti, aver incluso dei fori permette sia la possibilità di funzionalizzare il materiale, sia di ottenere una densità minore rispetto ad uno pieno. Le proprietà meccaniche sono state migliorate in direzione longitudinale, ma peggiorate in quella trasversale. La tenacità a frattura è stata aumentata attraverso la corretta imitazione dei meccanismi di tenacizzazione osservati nella struttura Haversiana.
Design and manufacturing of a bio-inspired composite structure
CAVALLI, RICCARDO
2018/2019
Abstract
Nature represents an exceptional source of inspiration because it has refined and updated itself over the centuries. Biomimetics, alternatively called biomimicry or bionics, has been exploited in the current work in order to steal from nature some attractive functions and reproduce them in a man-made material. More specifically, bone hierarchical organization has been investigated. Bone is a sensational biological composite structure, since it is characterized by a fracture toughness 3-5 order of magnitude higher than the one of its main constituents, collagen and hydroxyapatite. At a micrometric level the prevalent structures of cortical bone are the Haversian systems (also called osteons), consisting in longitudinally oriented cylindrical features, made up by overlapped concentric lamellae surrounded by an interstitial matrix. By means of this controlled organization, many toughening mechanisms are exploited in order to make tortuous the crack path and thus increase the energy level required for the crack growth. Since composite materials are generally depicted by high modulus and strength but low fracture toughness, the particular toughening mechanisms of bone are mimicked in order to improve this weakness. Moreover, bone is a multifunctional biological structure, able to effectively self-repair in case a damage occurs. Haversian canal, responsible for housing the vascular system which carries nutrients and blood to the whole bone, is thus introduced in the material design with the purpose of including this new function. The current work is a development of a larger project and aims at introducing those features in order to obtain increased fracture properties and lay the foundations for a full exploitation of self-healing capability.The new geometry consists of tubes organized in three rows, spaced by an inter-osteon layer and separated by horizontal plies. After having designed it, material manufacturing has been optimized and a series of laminates has been produced through "prepreg and autoclave" process. Finally, flexural properties and translaminar fracture toughness have been tested. Results, compared with previous laminates, are promising when normalized by material density. In fact, tubes addition ensures, due to holes exploitation, both the possibility of material functionalization and a great density decrease with respect to a solid material. Normalized mechanical properties are thus increased in longitudinal direction, but worsened in transverse, while fracture toughness has been boosted by the successful replication of some toughening mechanisms similar to those observed in bone Haversian structure.File | Dimensione | Formato | |
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https://hdl.handle.net/10589/153206