Studio del comportamento meccanico dell'osso corticale

Questo lavoro è incentrato sullo studio del comportamento meccanico dell’osso, ed in particolare a frattura, al fine di individuare elementi chiave della struttura ossea da riprodurre in un materiale bio-ispirato ex novo. L’osso è generalmente considerato un composito, caratterizzato da una struttura gerarchica a più livelli, dove ogni componente gioca un ruolo fondamentale nel determinare la risposta meccanica. Pertanto, per poter riprodurre le sue caratteristiche è necessario studiare attentamente la struttura e come questa influenza le performance finali. In questo lavoro sono state eseguite prove sperimentali su provini di osso corticale, prelevati dalla diafisi di un femore bovino. I campioni sono stati adeguatamente conservati in soluzione salina al fine di preservare le caratteristiche del materiale, intrinsecamente legate alla sua igroscopicità. Le prove sono state eseguite in accordo alle normative ASTM per materiali metallici e plastici, seguendo l’approccio più comunemente usato in letteratura. I risultati ottenuti dalle prove trovano riscontro con quanto presente in letteratura. Particolarmente utili ai fini dello studio del legame tra struttura e proprietà sono risultate le osservazioni al microscopio, che hanno consentito di individuare i vari componenti microstrutturali e i meccanismi di danneggiamento del materiale stesso

Cortical Bone as a Biomimetic Model for the Design of New Composites

AbstractComposite materials are widely used to build structures for their great mechanical performance combined with a low weight. However, the relatively low toughness of some composite materials is often a limitation as it can cause sudden failure. At present, there is a need for new lightweight materials with a good combination of strength and toughness, to be used for a variety of structural applications. Strength and toughness are the key requirements for structural materials. However, they are often mutually exclusive. Examples of effective design solutions can be found in natural materials, showing an optimal strength-toughness balance. Such materials can be a good source of inspiration for the design of new smart materials, by following a biomimetic approach. Among natural materials, bone tissue is an intriguing one. Bone combines few meagre constituents, hydroxyapatite and collagen, as building blocks to build up a complex hierarchical structure, reaching remarkable mechanical properties and a large amplification in toughness not observed in synthetic counterparts. For this reason, bone can be considered as a biomimetic model material that many researchers have recently tried to mimic adopting different techniques. In this study, we take inspiration from bone to design and manufacture new FRC (fiber-reinforced composite) materials inspired by the microstructure of cortical bone, with the aim of mimicking some toughening mechanisms and improving the toughness of conventional composites. We focus on the microstructural level, since the fundamental toughening mechanisms occur at the microscale, and we mimic the main features involved in the fracture process in our new design. The choice of the key features to be mimicked in the biomimetic material design process is guided by a previous experimental campaign performed on bovine cortical bone. Here we describe the design of a new bio-inspired material and an experimental campaign to assess the mechanical performance and the failure modes. The results of the tests allow us to confirm the promising mechanical characteristics of such material, compared to our previous design solutions and to similar classic structural composites (e.g. laminates). Moreover, the failure modes show many similarities with some of the toughening mechanisms occurring in cortical bone, confirming the key role, played by the mimicked bone-inspired microstructural features, in determining and enhancing the fracture toughness of the composites

VALUTAZIONE DEL DANNO DA IMPATTO IN PANNELLI SANDWICH CON LA TECNICA TERMOGRAFICA

Oggetto di questo studio sono dei pannelli sandwich per applicazioni automobilistiche, costituiti da pelli esterne in acciaio separate da poliolefina. Lo scopo del lavoro è lo studio del loro comportamento quando sono soggetti a impatti a bassa velocità. In particolare, si vogliono valutare le modalità e l’entità del danneggiamento subito, al variare della geometria e delle caratteristiche meccaniche dei componenti dei sandwich. Il lavoro è distinto in una parte sperimentale e in una numerica. Quella sperimentale consiste nella valutazione del danneggiamento, tramite la tecnica termografica, dei pannelli sottoposti a impatto. La seconda parte, numerica, sviluppa dei modelli a elementi finiti che simulano i pannelli durante la fase di impatto e le successive prove di trazione. Questi modelli numerici permettono di determinare parametri direttamente confrontabili con le grandezze termografiche

thermographic applications for the rapid estimation of fatigue limit

Abstract The AIAS group studying Energetic Methods for Experimental Analysis, MEAS, is performing round robin experimental tests for the rapid determination of fatigue limit on steels by different thermographic techniques. This work is part of the project and describes the experimental activity performed at Politecnico di Milano, based on stepwise cyclic tests. Thermograms are processed in terms of: 1) amplitude of the first order harmonic, in-phase with respect to the loading signal; 2) amplitude of the second harmonic, out-of-phase; 3) slope of the thermal signal with respect to the number of cycles. These values typically show bilinear trends, allowing to define a breakup point and a corresponding stress which is the thermographic estimation of the fatigue limit. The paper presents and discusses the results of tests with different stress ratios, i.e. fully reversed cycling with R=-1 and tensile-tensile cycling with R=0.1

Bone toughness and crack propagation: An experimental study

Bone is a topic of great interest for researchers, such as biologists or engineers, both interested in understanding the structurerelated properties of bone and how they are affected by aging, disease and therapies. In particular, a topic of common interest between medicine and engineering is the fracture behavior of bone. Indeed, a thorough understanding of the mechanical behavior of bone is helpful to predict the fracture risk, but it can also provide the basis for the design of de novo biomimetic materials. In this paper, we show the initial results of an experimental study of the mechanical behavior of bovine bone, with a special focus on fracture toughness. The latter is evaluated under tensile and bending loading, by following the ASTM adopted for metals. Finally, we perform microscopic observations to better understand the fracture behavior and correlate it with the microscopic structure

Understanding the structure-property relationship in cortical bone to design a biomimetic composite

Bone is a hot topic for researchers, interested in understanding the structure-related properties of the tissue and the effect of aging, disease and therapies on that. A thorough understanding of the mechanical behavior of bone can be helpful to medical doctors to predict the fracture risk, but it can also serve as a guideline for engineers for the design of de novo biomimetic materials. In this paper, we show a complete characterization of cortical bone under static loading (i.e. tensile, compressive, three-point bending) and we carried out tests in presence of a crack to determine the fracture toughness. We performed all the tests on wet samples of cortical bone, taken from bovine femurs, by following the ASTM standards designed for metals and plastics. We also performed microscopic observations, to get an insight into the structure-property relationship. We noted that the mechanical response of bone is strictly related to the microstructure, which varies depending on the anatomical position. This confirms that the structure of bone is optimized, by nature, to withstand the different types of loads generally occurring in different body areas. The same approach could be followed for a proper biomimetic design of new composites

REVISIONE DEI MODELLI NUMERICI ALLA MACROSCALA PER LO STUDIO DEL FENOMENO DI INFRAGILIMENTO DA IDROGENO

Il fenomeno di infragilimento da idrogeno, noto da diversi anni, rimane un argomento di grande interesse scientifico. Di recente, in letteratura l’attenzione si è spostata prevalentemente sullo sviluppo di modelli numerici in grado di riprodurre il fenomeno o di chiarire e/o interpretare alcuni aspetti che lo caratterizzano. Diverse sono le scale dimensionali considerate: scala atomistica, nano, meso, macroscala. I modelli sviluppati alla macroscala sono prevalentemente modelli ad elementi finiti di tipo coesivo il cui scopo è la valutazione della resistenza meccanica di un materiale sotto effetto combinato di sollecitazione meccanica applicata e ambiente aggressivo, in particolare ricco di idrogeno atomico. Il presente lavoro propone una revisione dei modelli numerici alla macroscala presenti in letteratura al fine di riassumere lo stato dell’arte, di valutare pro e contro dei modelli presenti e individuare quali possano essere gli sviluppi futuri per l’ottimizzazione di un approccio numerico a questa scala

A review of thermographic techniques for damage investigation in composites

The aim of this work is a review of scientific results in the literature, related to the application of thermographic techniques to composite materials. Thermography is the analysis of the surface temperature of a body by infrared rays detection via a thermal-camera. The use of this technique is mainly based on the modification of the surface temperature of a material, when it is stimulated by means of a thermal or mechanical external source. The presence of defects, in fact, induces a localized variation in its temperature distribution and, then, the measured values of the surface temperature can be used to localize and evaluate the dimensions and the evolution of defects. In the past, many applications of thermography were proposed on homogeneous materials, but only recently this technique has also been extended to composites. In this work several applications of thermography to fibres reinforced plastics are presented. Thermographic measurements are performed on the surface of the specimens, while undergoing static and dynamic tensile loading. The joint analysis of thermal and mechanical data allows one to assess the damage evolution and to study the damage phenomenon from both mechanical and energetic viewpoints. In particular, one of the main issues is to obtain information about the fatigue behaviour of composite materials, by following an approach successfully applied to homogenous materials. This approach is based on the application of infrared thermography on specimens subjected to static or stepwise dynamic loadings and on the definition of a damage stress, D, that is correlated to the fatigue strength of the material. A wide series of experimental fatigue tests has been carried out to verify if the value of the damage stress, D, is correlated with the fatigue strength of the material. The agreement between the different values is good, showing the reliability of the presented thermographic techniques, to the study of composite damage and their fatigue behaviour

A multiscale XFEM approach to investigate the fracture behavior of bio-inspired composite materials

In the setting of emerging approaches for material design, we investigate the use of the extended finite element method (XFEM) to predict the behavior of a newly designed bone-inspired fiber-reinforced composite and to elucidate the role of the characteristic microstructural features and interfaces on the overall fracture behavior. The outcome of the simulations, showing a good agreement with the experimental results, reveals the fundamental role played by the heterogeneous microstructure in altering the stress field, reducing the stress concentration at the crack tip, and the crucial role of the interface region (i.e. cement line) in fostering the activation of characteristic toughening mechanisms, thus increasing the overall flaw tolerance of the composite

numerical investigation of wire clamp contact for a drawing machine

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