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

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

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

Fatigue behaviour of a GFRP laminate by thermographic measurements

Composite materials are widely used to build structural components, thanks to their mechanical properties. Those are generally considered ‘engineering materials’, since they are tailored to meet specific requirements. Due to their use for structural components, it is important to know their mechanical behaviour, especially under cyclic loads. At present, there is a common interest, among researchers, to study the mechanical behaviour of composites, by means of both traditional and innovative techniques, with the final purpose of making previsions regarding their service life. In fact, due to their composite nature, they behave in a different mode compared to homogeneous materials. This study is focused on a glass fibre-reinforced plastic (GFRP); the aim of this work is to study its fatigue behaviour, from both the mechanical and the thermal points of view. The main reason is that there is a lack of knowledge, in the literature, about the fatigue of composites. In this study, a GFR laminate was characterized under static and dynamic loading conditions; during the experimental tests, thermal measurements were carried out by means of an IR-thermal camera. Temperature measurements were done during the static tests, whereas in the dynamic tests the dissipated energy was measured, by using the dissipation method (D-mode). Then, various criteria for fatigue life estimation were applied fitting the experimental data. Since different thermographic techniques have been used to estimate the fatigue behaviour, a final comparison between the experimental data and the predicted fatigue behaviour is proposed and discussed, showing a good agreement

Bone-Inspired Materials by Design: Toughness Amplification Observed Using 3D Printing and Testing

Inspired by the fact that nature provides multifunctional composites by using universal building blocks, the authors design and test synthetic composites with a pattern inspired by the microstructure of cortical bone. Using a high-resolution multimaterial 3D printer, the authors are able to manufacture samples and investigate their fracture behavior in mechanical tests. The authors’ results demonstrate that the bone-inspired design is critical for toughness amplification and balance with material strength. The failure modes of the authors’ synthetic composites show similarities with the cortical bone, like crack deflection and branching, constrained microcracking, and fibril bridging. The authors’ results confirm that our design is eligible to reproduce the fracture and toughening mechanism of bone

A new finite element based parameter to predict bone fracture

Dual Energy X-Ray Absorptiometry (DXA) is currently the most widely adopted non-invasive clinical technique to assess bone mineral density and bone mineral content in human research and represents the primary tool for the diagnosis of osteoporosis. DXA measures areal bone mineral density, BMD, which does not account for the three-dimensional structure of the vertebrae and for the distribution of bone mass. The result is that longitudinal DXA can only predict about 70% of vertebral fractures. This study proposes a complementary tool, based on Finite Element (FE) models, to improve the DXA accuracy. Bone is simulated as elastic and inhomogeneous material, with stiffness distribution derived from DXA greyscale images of density. The numerical procedure simulates a compressive load on each vertebra to evaluate the local minimum principal strain values. From these values, both the local average and the maximum strains are computed over the cross sections and along the height of the analysed bone region, to provide a parameter, named Strain Index of Bone (SIB), which could be considered as a bone fragility index. The procedure is initially validated on 33 cylindrical trabecular bone samples obtained from porcine lumbar vertebrae, experimentally tested under static compressive loading. Comparing the experimental mechanical parameters with the SIB, we could find a higher correlation of the ultimate stress, \u3c3ULT, with the SIB values (R2adj = 0.63) than that observed with the conventional DXA-based clinical parameters, i.e. Bone Mineral Density, BMD (R2adj = 0.34) and Trabecular Bone Score, TBS (R2adj = -0.03). The paper finally presents a few case studies of numerical simulations carried out on human lumbar vertebrae. If our results are confirmed in prospective studies, SIB could be used-together with BMD and TBS-to improve the fracture risk assessment and support the clinical decision to assume specific drugs for metabolic bone diseases

Investigation of the Effect of Internal Pores Distribution on the Elastic Properties of Closed-Cell Aluminum Foam: A Comparison with Cancellous Bone

Closed-cell aluminum foams belong to the class of cellular solid materials, which have wide application in automotive and aerospace industries. Improving the mechanical properties and modifying the manufacturing process of such materials is always on demand. It has been shown that the mechanical properties of cellular materials are highly depending on geometrical arrangement, mechanical properties of solid constituents and the relative density of these materials. In this study, using a manufacturing process of foaming by expansion of a blowing agent, we prepared two types of closed-cell aluminum foams with isotropic distribution of cells along length and foams with gradient of pores along its length. We hypothesized that such variation of pores can induce microstructural directionality along the length of foam samples and improve their mechanical properties. For this aim, we studied the microstructural properties by micro-CT imaging and found their relation to macroscopic mechanical properties of foam samples by conducting monotonic compression tests. We compared these results with the one of the bovine femur trabecular bone as they show a dominant microstructural anisotropy due to alignment with the maximum strength direction in body. We also conducted numerical analyses and validated them for the elastic part based on our experimental work. Our results showed that gradient variation in porosity in closed-cell aluminum foams have a minor effect on their macroscopic mechanical properties. Although using such materials in sandwich panel structures, the strength of the material slightly increased. In addition, parameters of a power law model for the description of mechanical properties of foam sample and their relative density and properties of the solid compartment were characterized. The presented results are considered as a preliminary study for improvement of mechanical properties of closed-cell aluminum foams