Choose an application

• Reference Manager
• EndNote
• RefWorks (Direct export to RefWorks)

Finite element methods.

Choose an application

• Reference Manager
• EndNote
• RefWorks (Direct export to RefWorks)

Sandstone is the most common kind of natural stone used for historic buildings in Central Europe. During the past century a dramatic increase in different types of damage to historic buildings, monuments and sculptures made from natural stone has been observed. The present work deals with theoretical aspects of strength loss, fracture processes and degradation during the decay processes.

Ermüdung --- weathering model --- Lebensdauer --- finite element methods --- durability --- Verwitterungsmodell --- FEMsandstone --- Sandstein --- fatigue behaviour

Choose an application

• Reference Manager
• EndNote
• RefWorks (Direct export to RefWorks)

The bone-cartilage interface plays an important biomechanical role by transmitting forces across the joint. This highly complex region undergoes notable structural and mechanical changes with aging, closely associated with the development of diseases such as osteoarthritis and incomplete growth plate fusion. Despite its significant clinical relevance, there is a lack of comprehensive data on how aging impacts the microarchitecture and biomechanical properties of subchondral bone. Consequently, studying the structure-mechanics relationship as well as the impact of aging on this one compared to the metaphyseal bone part is important.

This thesis investigates how age-related alterations in bone microstructure and stiffness influence the mechanical transfer in the proximal rat tibiae under load application. To this end, a computational study using micro-structural finite element analysis (micro-FE) was conducted alongside experimental analyses.

A first analysis of the impact of bone stiffness on bone mechanics demonstrates a clear impact on its overall mechanical behavior, though to a lesser extent in the subchondral region compared to the metaphyseal region. This variation in behavior between the two regions underscores the importance of examining specific sub-regions of the bone individually, rather than treating the bone as a homogeneous entity, to capture its different mechanical responses. 

Further analysis of microstructural influence, in relation to aging, highlighted the importance of bone bridges in facilitating the transmission of mechanical forces throughout the bone, rather than forcing them to concentrate in the subchondral region when few bone bridges are present, as in young bones. Additionally, a significant degree of inter-sample variability was observed in the mechanical behavior of young bones, a phenomenon notably absent in older bones.

The impact of the presence or absence of a growth plate had a more pronounced effect on the mechanical behavior of young bones compared to older ones, given the higher amount of bone bridges connecting the subchondral to the metaphyseal regions in older samples. Simulating a pathological growth plate in young bones, characterized by a high value of Young's modulus, revealed deformation patterns that closely resembled those in old bones, suggesting that such changes might mimic age-related alterations in bone mechanics.

Although the computational analysis provided valuable insights into the redistribution of strains within the bone structure, it did not enable the assessment of failure loads. To address this limitation, experimental processes were developed to specifically determine failure loads in both old and young samples. 

Overall, this study provides valuable insights into the relationship between bone microstructure and its underlying mechanical behavior as it ages, in the proximal tibia. It demonstrated how specific bone structures influence the mechanical properties in the subchondral trabecular, subchondral cortical, metaphyseal trabecular, and metaphyseal cortical bone regions, enhancing our understanding of how mechanical forces affect bone during the aging process and associated pathologies. L'interface os-cartilage dans les articulations joue un rôle biomécanique important en transmettant les forces à travers l'articulation. Cette région très complexe subit des changements structurels et mécaniques notables avec le vieillissement, étroitement associés au développement de maladies telles que l'arthrose et la fusion incomplète de la plaque de croissance. Malgré son importance clinique, très peu de données concernant l'impact du vieillissement sur la microarchitecture et les propriétés biomécaniques de l'os sous-chondral sont recensées. Par conséquent, il est important d'étudier la relation structure-comportement mécanique ainsi que l'impact du vieillissement sur l'os sous-chondral par rapport à d'autres sites osseux.

Cette thèse étudie comment les altérations de la microstructure osseuse liées à l'âge influencent le transfert mécanique dans le tibia proximal du rat sous l'application d'une charge. À cette fin, une étude computationnelle utilisant l'analyse micro-structurale par éléments finis (micro-FE) a été menée parallèlement à des analyses expérimentales.

L'analyse des propriétés mécaniques de l'os a démontré un impact clair sur son comportement mécanique global, bien que dans une moindre mesure dans la région sous-chondrale par rapport à la région métaphysaire. Cette variation de comportement entre les deux régions souligne l'importance d'examiner individuellement des sous-régions spécifiques de l'os, plutôt que de traiter l'os comme une entité homogène, afin de saisir ses différentes réponses mécaniques. 

Une analyse plus poussée de l'influence de la microstructure, en relation avec le vieillissement, a mis en évidence l'importance des ponts osseux reliant la partie sous-chondrale à la région métaphysaire afin de faciliter la transmission des forces mécaniques dans l'ensemble de l'os, plutôt que de les forcer à se concentrer dans la région sous-chondrale lorsque peu de ponts sont présents, comme dans les os jeunes. En outre, un degré significatif de variabilité inter-échantillons a été observé dans le comportement mécanique des os jeunes, un phénomène notablement absent dans les os plus âgés.

L'impact de la présence ou de l'absence d'une plaque de croissance a eu un effet plus prononcé sur le comportement mécanique des os jeunes que sur celui des os plus âgés, étant donné la plus grande quantité de ponts reliant les régions sous-chondrales aux régions métaphysaires dans les échantillons plus âgés. La simulation d'une plaque de croissance pathologique, caractérisée par un module de Young plus élevé dans des échantillons jeunes, a révélé des modèles de déformation qui ressemblaient étroitement à ceux des os âgés, ce qui suggère que de tels changements pourraient imiter les altérations de la mécanique osseuse liées à l'âge.

Bien que l'analyse computationnelle ait fourni des indications précieuses sur la redistribution des contraintes dans la structure osseuse, elle n'a pas permis d'évaluer les charges de rupture. Pour remédier à cette limitation, des processus expérimentaux ont été mis au point afin de déterminer spécifiquement les charges de rupture dans les échantillons jeunes et vieux. 

Dans l'ensemble, cette étude fournit des informations précieuses concernant la relation entre la microstructure osseuse et son comportement mécanique sous-jacent au cours du vieillissement, en particulier au niveau de l'interface os-cartilage. Elle a démontré comment des structures osseuses spécifiques influencent les propriétés mécaniques dans les quatre zones définies, appelées trabéculaire sous-chondrale, corticale sous-chondrale, trabéculaire métaphysaire et corticale métaphysaire, améliorant ainsi notre compréhension de la manière dont les forces mécaniques affectent l'os au cours du processus de vieillissement et des pathologies qui en découlent.

Finite Element Methods --- Mechanical Testing --- Bone-Cartilage Interface --- Mechanical Properties --- Biomechanics --- Microstructure --- Ingénierie, informatique & technologie > Science des matériaux & ingénierie

Choose an application

• Reference Manager
• EndNote
• RefWorks (Direct export to RefWorks)

Hydrology --- Finite element method --- Mathematics --- Natural resources --- Water --- Analysis --- Applications of finite element methods --- Aquatic sciences --- Earth sciences --- Hydrography --- Congresses --- Mathematics&delete&

Choose an application

• Reference Manager
• EndNote
• RefWorks (Direct export to RefWorks)

The microscopic structure of cortical bone reveals a complex architecture composed of osteons that contributes to bone fracture resistance. Osteons are formed from concentric lamellae that protect blood vessels at the center. Osteons are bordered by a tiny layer called cement line. Biologically, the cement lines are more mineralized than the surrounding bone and rich in osteopontin, a non collagenous protein. Their thickness varies between 1 and 5 µm and they appear during the remodeling of bone tissue, separating the new bone from the old. These thin structures play a predominant role in biomechanics by effectively dissipating energy, by deviating or stopping microcracks, thus increasing the bone's resistance to fracture. Despite their importance, the mechanical behavior of cement lines remains poorly understood.

Local mechanical properties of cement line can be investigated by experimental techniques such as nanoindentation. However, the interpretation of the nanoindentation results is challenging as the cement lines are surrounded by osteon and interstitial bone, and they are very thin. Nanoindentation techniques face experimental limitations due to the size of the probed surface, which may be larger than the extremely fine dimension of the cement lines. Therefore, simulations can be used to better understand indentation results. The main objective of this master's thesis is to explore these local mechanical properties at bone interfaces through nanoindentation simulations conducted by the finite element method. This study determines the impact of the bone internal interfaces on local mechanical responses, evaluating the variability of indentation measurements induced by the positioning of the indenter, and thus proposes recommendations for supporting a better interpretation of experimental data.

Initially, pilot tests were conducted using the ANSYS Workbench software, where the model size, mesh sensitivity, and other simulation parameters were examined. The comparison of the results from two-dimensional and three-dimensional simulations showed an underestimation of measurements in two-dimensional modeling. Keeping this in mind, 2D models could still be used to perform parametric studies, especially for comparing relative properties and not absolute values.

The thesis was structured in two main phases. Initially, the material properties of 3 bone regions of different mineral content and age were analyzed. A spatial resolved analysis was performed, and it demonstrated a significant local influence of bone interfaces, causing substantial deviations in the measured tissue modulus with respect to the nominal value for a position in the middle of the cement line, with this deviation increasing as the indenter approached interfaces with surrounding tissues, thus highlighting the importance of the placement of the indenter tip within the cement line. Subsequently, the inclination of these bone interfaces was explored to assess its impact on the measured mechanical behavior. This confirmed that the sample sectioning process is of great importance in the reliability of the results.

In conclusion, thin bone interfaces have shown a significant influence on the mechanical properties that can be measured experimentally, suggesting several improvements in data interpretation. One possible improvement is the use of backscattered electron imaging post-nanoindentation to determine the exact position of the indents and to consider only those falling in the middle of the cement line, hence reducing the influence of surrounding tissues. This study has also opened new research perspectives, including the exploration of the influence of porosity, the study of surface roughness induced by polishing, and the direction of collagen fibers.

Nanoindentation --- Finite Element Methods --- Cement Line --- Interface --- Osteonal Bone --- Interstitial Bone --- Effective Young’s Modulus --- Ingénierie, informatique & technologie > Science des matériaux & ingénierie