Untersuchung der mikro- und makromechanischen Eigenschaften von Zahnschmelz
Final Report Abstract
Enamel’ elastic/inelastic transitions decrease from 17 GPa to 0.4 GPa with increasing hierarchical levels. Similarly, its elastic moduli measures up to 115 GPa for a single nanometer-sized construction unit (single crystallite fiber) but 30 GPa for bulk enamel. These trends are attributed to increased volume of weak phases around each crystallite fiber and rod with additional hierarchical levels as well as increased volume of defects. At the first glance, the number of hierarchical structures brought disadvantages. However, investigation of enamel’s sub-10 µm fracture behaviors indicates otherwise. The presence of weak phases provides pathways to crack-bridges and microcrack formation, both are crack closure mechanisms. The presence of a hierarchical structure allows enamel to form crack bridges and microcracks at different lengths scales: the bridges are from ~85 nm to ~3 µm in width (up to 30 µm in previous studies) and the microcracks are from 100-500 nm to ~6 µm. Such ability to form toughening mechanisms across different length scales from tens of nanometer to tens of micrometer has never been observed in other man-made materials. Enamel has a low crack tip toughness, characterized as KI0=0.5-1.6 MPa√m. The parameters for the cohesive zone at the crack tip were also calculated. The mean crack closure stress at the crack tip was computed as 163-770 MPa with a cohesive zone length and width 1.6-10.1 µm and 24-44 nm. This means that the cohesive zone encompasses multiple crystallite fibers or even multiple enamel rods. 3 possible morphological surroundings at the crack tip are presented as a hypothesis to explain the scattering of KI0 values and the cohesive zone parameters depending on the orientation between the crack propagation direction and crystallite fiber arrangements. The significant different crystallite fiber orientations in interrod regions compared to the intrarod regions have been observed to encumber crack propagation (see above). Besides, the interrod regions were observed to dissipate significant higher inelastic energy compared to the intrarod regions possibly due to higher protein unfolding activities in the organics-rich sheath. When longer time is allowed for deformation, the magnitude in inelastic energy dissipation increment in the interrod region is significantly higher than in the intrarod region. For the testing conditions in this study, a rough calculation shows that up to 10 protein unfoldings could happen for each nm3 of organic matters over a period of 3 minutes. Summarized, the presence of hierarchical structures in enamel reduce its elastic moduli and elastic/inelastic transition with increasing levels but the weak phase is in fact highly beneficial in making enamel damage-tolerant.
Publications
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Determination of the elastic/plastic transition of human enamel by nanoindentation. Dental Materials, 25(11), 1403-1410 (2009)
Ang, S. F.; Scholz, T.; Klocke, A.; Schneider, G. A.
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„AFM and PFM measurements of enamel in order to determine the crack tip toughness and cohesive zone“. MRS Spring Meeting, San Francisco, USA.16 April 2009
Ang, S. F.; Pacher Fernandes, R.; Schneider, G. A.
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„Indentation and uni-axial compression study of enamel’s elastic/plastic behavior from milimeter to nanometer length scale“. MRS Spring Meeting, San Francisco, USA.15 April 2009
Ang, S. F.; Habelitz, S.; Klocke, A.; Swain, M. V.; Schneider, G. A.
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Size-dependent elastic/inelastic behavior of enamel over millimeter and nanometer length scales. Biomaterials, 31(7), 1955-1963 (2010)
Ang, S. F.; Bortel, E. L.; Swain, M. V.; Klocke, A.; Schneider, G. A.
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„Enamel‘s size-dependent elastic/inelastic behavior over millimeter and nanometer length scales“. Materials Science and Engineering Congress 2010, Darmstadt, Germany. 22-24 Aug 2010
Ang, S. F.; Bortel, E. L.; Swain, M. V.; Klocke, A.; Schneider, G. A.
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Characterizing dental enamel’s mechanical properties from milli- to nanometer length scales. PhD thesis, Institute of Advanced Ceramics, Hamburg University of Technology, 2011
Ang, S. F.
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Sub-10-micrometer toughening and crack tip toughness of dental enamel. Journal of the Mechanical Behavior of Biomedical Materials, 4(3), 423-432 (2011)
Ang, S. F.; Schulz, A.; Pacher Fernandes, R.; Schneider, G. A.
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Comparison of Mechanical Behavior of Inter- and Intra-rod Regions in Enamel. Journal of Materials Research 27(2), 448-456 (2012)
Ang, S. F.; Saadatmand, M.; Swain, M. V.; Klocke, A.; Schneider, G. A.
