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Item type:Publication, Access status: without files , Fracture behavior of hydroxyapatite nanofibers in dental enamel under micropillar compression(Elsevier, 2012-11-10); ; ; ; Understanding the structure-property relations of load-bearing natural materials can provide design strategies for the generation of bio-inspired materials showing enhanced mechanical properties. Dental enamel is a highly damage-tolerant biological nanocomposite having a high mineral content. Its hierarchical structure consists of hydroxyapatite nanofibers enveloped by proteins on the smallest length scale. In this work, we show the fracture behavior of nanofibers in enamel by conducting micropillar compression tests. The evaluation and the result of the mechanical properties are discussed.Publicationtype: Journal Article90 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Hierarchical flexural strength of enamel : transition from brittle to damage-tolerant behaviour(Royal Society of London, 2011-10-26); ; ; ; Hard, biological materials are generally hierarchically structured from the nano- to the macroscale in a somewhat self-similar manner consisting of mineral units surrounded by a soft protein shell. Considerable efforts are underway to mimic such materials because of their structurally optimized mechanical functionality of being hard and stiff as well as damage-tolerant. However, it is unclear how different hierarchical levels interact to achieve this performance. In this study, we consider dental enamel as a representative, biological hierarchical structure and determine its flexural strength and elastic modulus at three levels of hierarchy using focused ion beam (FIB) prepared cantilevers of micrometre size. The results are compared and analysed using a theoretical model proposed by Jäger and Fratzl and developed by Gao and co-workers. Both properties decrease with increasing hierarchical dimension along with a switch in mechanical behaviour from linear-elastic to elastic-inelastic. We found Gao's model matched the results very well. This journal is © 2011 The Royal Society.Publicationtype: Journal Article173 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Crack arrest within teeth at the dentinoenamel junction caused by elastic modulus mismatch(Elsevier, 2010-05); ; ; ; Enamel and dentin compose the crowns of human teeth. They are joined at the dentinoenamel junction (DEJ) which is a very strong and well-bonded interface unlikely to fail within healthy teeth despite the formation of multiple cracks within enamel during a lifetime of exposure to masticatory forces. These cracks commonly are arrested when reaching the DEJ. The phenomenon of crack arrest at the DEJ is described in many publications but there is little consensus on the underlying cause and mechanism. Explanations range from the DEJ having a larger toughness than both enamel and dentin up to the assumption that not the DEJ itself causes crack arrest but the so-called mantle dentin, a thin material layer close to the DEJ that is somewhat softer than the bulk dentin. In this study we conducted 3-point bending experiments with bending bars consisting of the DEJ and surrounding enamel and dentin to investigate crack propagation and arrest within the DEJ region. Calculated stress intensities around crack tips were found to be highly influenced by the elastic modulus mismatch between enamel and dentin and hence, the phenomenon of crack arrest at the DEJ could be explained accordingly via this elastic modulus mismatch.Publicationtype: Journal ArticleScopus© Citations 71 750 979 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Mixed-mode stress intensity factors for kink cracks with finite kink length loaded in tension and bending(Elsevier, 2010-05); ; ; ; Fracture toughness resistance curves describe a material’s resistance against crack propagation. These curves are often used to characterize biomaterials like bone, nacre or dentin as these materials commonly exhibit a pronounced increase in fracture toughness with crack extension due to co-acting mechanisms as crack bridging, crack deflection and microcracking. The knowledge of appropriate stress intensity factors which depend on sample and crack geometry is essential for determining these curves. For the dental biomaterials enamel and dentin it was observed that under bending and tensile loading crack propagation occurs under certain constant angles to the initial notch direction during testing procedures used for fracture resistance curves determination. For this special crack geometry (kink crack of finite length in a finite body) appropriate geometric function solutions are missing. Hence, we present in this study new mixed-mode stress intensity factors for kink cracks with finite kink length within samples of finite dimensions for two loading cases (tension and bending)which were derived from a combination of mixed-mode stress intensity factors of kink cracks with infinitely small kinks and of slant cracks. These results were further applied to determine fracture resistance curves of enamel and dentin by testing single edge notched bending(SENB) specimens. It was found that kink cracks with finite kink length exhibit identical stress fields as slant cracks as soon as the kink length exceeds 0,15 times the initial straight crack or notch length. The usage of stress intensity factor solutions for infinitely small kink cracks for the determination of dentin fracture resistance curves (as was done by other researchers) leads to an overestimation of dentin’s fracture resistance of up to 30%.Publicationtype: Journal ArticleScopus© Citations 30 590 641 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , On the mechanical properties of hierarchically structured biological materialsMany biological materials are hierarchically structured which means that they are designed from the nano- to the macroscale in a sometimes self-similar way. There are lots of papers published including very detailed descriptions of these structures at all length scales – however, investigations of mechanical properties are most often focused on either nano-indentation or bulk mechanical testing characterizing properties at the smallest or largest size scale. Interestingly, there are hardly any investigations that systematically interconnect mechanical properties of different length scales. Nevertheless there are often conclusions drawn like the one that “biological materials exhibit their excellent mechanical properties due to their hierarchical structuring”. Thus, we think there is a gap and discrepancy between the detection and description of biological structures and the correlated determination and interpretation of their mechanical properties. Hence, in this paper we order hierarchically structured biological materials with high mineral content according to their hierarchical levels and attribute measured mechanical properties to them. This offers the possibility to gain insight into the mechanical properties on different hierarchical levels even though the entire biological materials were tested. On the other hand we use data of one material, namely enamel, where mechanical properties were measured on every length scale. This kind of data analysis allows to show how a theoretical model developed by Huajian Gao and co-workers can be used to get closer insights into experimental data of hierarchically structured materials.Publicationtype: Journal ArticleScopus© Citations 130 971 1276 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , The fracture behaviour of dental enamel(Elsevier, 2009-01); ; ; ; Enamel is the hardest tissue in the human body covering the crowns of teeth. Whereas the underlying dental material dentin is very well characterised in terms of mechanical and fracture properties, available data for enamel are quite limited and are apart from the most recent investigation mainly based on indentation studies. Within the current study, stable crack-growth experiments in bovine enamel have been performed, to measure fracture resistance curves for enamel. Single edge notched bending specimens (SENB) prepared out of bovine incisors were tested in 3-point bending and subsequently analysed using optical and environmental scanning electron microscopy. Cracks propagated primarily within the protein-rich rod sheaths and crack propagation occurred under an inclined angle to initial notch direction not only due to enamel rod and hydroxyapatite crystallite orientation but potentially also due to protein shearing. Determined mode I fracture resistance curves ranged from 0,8 – 1,5 MPa*m1/2 at the beginning of crack propagation up to 4,4 MPa*m1/2 at 500 m crack extension; corresponding mode II values ranged from 0,3 to 1,5 MPa*m1/2.Publicationtype: Journal ArticleScopus© Citations 129 1870 1239