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Item type:Publication, Access status: with files , Hierarchical microcrack model for materials exemplified at enamelThis article investigates the mechanical properties of a material with hierarchically arranged microcracks.Publicationtype: Journal ArticleScopus© Citations 7 306 455 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Automated control of stable crack growth for R-curve and rapid v-K-curve measurements(2016-10); ; ; ; A semi-automated technique for controlled crack growth in brittle materials and the simultaneous measurement of R-curve and v-K-curve (subcritical crack growth) is presented. The experimental technique and the evaluation procedure are described in detail. By using bending tests in 3-point and 4-point bending, the technical effort is comparatively low, as the preparation of bending bars is relatively simple. Some experimental results for glass and silicon nitride illustrate the capability of this new method.Publicationtype: Conference Presentation64 25 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Erratum: Intergranular nanostructure effects on strength and toughness of Si3N4 (Journal of the American Ceramic Society (2015) 98:5 (1650-1657) DOI: 10.1111/jace.13484)(Wiley-Blackwell, 2015-09-11); ; ; ; Publicationtype: Journal Article56 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Intergranular nanostructure effects on strength and toughness of Si3N4(Wiley-Blackwell, 2015-04-21); ; ; ; Strength, toughness, microstructure, and atomic adsorption arrangement in silicon nitrides with MgO and RE2O3 additions (RE = La, Gd, Y, Lu) were examined. Mechanical properties were high for La, Gd, and equal La-Lu additions, but surprisingly were progressively lower for Y- and Lu-doped samples. The lower strength and toughness were associated with fewer visible crack deflections and grain bridges. Detailed microstructural analysis of the Lu-doped material revealed a complex intergranular nanostructure with variable Lu content and Si3N4 nanocrystals. Furthermore, the Lu-rich areas showed an extra Lu-adsorption site on the Si3N4 prismatic planes not previously observed in other studies. This inhomogeneous structure was attributed to grain growth impingement and higher viscosity of the Lu-doped oxynitride glass that slows homogenization. The Y-doped material with nearly identical glass viscosity demonstrates intermediate behavior. Finally, substituting half of the Lu2O3 with La2O3 resulted in a homogenous intergranular structure, attributed to a lower viscosity of the oxynitride glass phase, and high mechanical properties. Overall, care must be taken when adapting Si3N4 processing parameters for the smaller ionic radius rare earth dopants such as Lu and Y.Publicationtype: Journal Article87 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Biomechanical adaptation of the bone-periodontal ligament (PDL)-tooth fibrous joint as a consequence of disease(Elsevier, 2014-11-08); ; ; ; In this study, an in vivo ligature-induced periodontitis rat model was used to investigate temporal changes to the solid and fluid phases of the joint by correlating shifts in joint biomechanics to adaptive changes in soft and hard tissue morphology and functional space. After 6 and 12 weeks of ligation, coronal regions showed a significant decrease in alveolar crest height, increased expression of TNF-α, and degradation of attachment fibers as indicated by decreased collagen birefringence. Cyclical compression to peak loads of 5-15. N at speeds of 0.2-2.0. mm/min followed by load relaxation tests showed decreased stiffness and reactionary load rate values, load relaxation, and load recoverability, of ligated joints. Shifts in joint stiffness and reactionary load rate increased with time while shifts in joint relaxation and recoverability decreased between control and ligated groups, complementing measurements of increased tooth displacement as evaluated through digital image correlation. Shifts in functional space between control and ligated joints were significantly increased at the interradicular (δ10-25. μm) and distal coronal (δ20-45. μm) regions. Histology revealed time-dependent increases in nuclei elongation within PDL cells and collagen fiber alignment, uncrimping, and directionality, in 12-week ligated joints compared to random orientation in 6-week ligated joints and to controls. We propose that altered strains from tooth hypermobility could cause varying degrees of solid-to-fluid compaction, alter dampening characteristics of the joint, and potentiate increased adaptation at the risk of joint failure. © 2013.Publicationtype: Journal ArticleScopus© Citations 15 25 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Automated control of stable crack growth for R-curve measurements in brittle materialsStable crack advance is required for a reliable crack growth resistance (R-Curve) measurement. In bending experiments, manual control of the mechanical load and observation of the growing crack is still being done by the operator. This work presents an approach to partly and fully automated R-curve measurements, where stable crack growth is achieved solely via computer control. The experimental setup in conjunction with an intelligent control algorithm leads to reliable results, even for brittle materials like alumina ceramics, silicon nitride, and glass. Furthermore, it allows for a novel type of measurement, because the device detects any kind of energy release in the specimen, actually also without visible crack extension. The setup has been used successfully for about 3 years. The operating principle is explained and some of the results are presented exemplarily. The method is realized in 4-point-bending, but can be implemented also for other types of specimen and loading to automatically achieve stable crack growth. © 2012 Society for Experimental Mechanics.Publicationtype: Journal Article86 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Mechanical characterization of highly porous β-Si3N4 ceramics fabricated via partial sintering & starch addition(Elsevier, 2012-12-21); ; ; ; In this study, porous β-Si3N4 ceramics containing limited amount of Sm2O3 and CaO as sintering aids were produced by addition of potato starch (10 and 20vol.%) and partial sintering. Two different Si3N4 powders, α- and β-, were used as starting materials. Scanning electron microscopy investigations revealed that development of elongated β-Si3N4 grains were much more pronounced when α-Si3N4 starting powder was used. Even though porosity values of the compositions prepared by using α-Si3N4 (∼57.0-58.4%) is significantly higher than the samples produced by β-Si3N4 (42.6%), no significant change was observed for the bending strength, fracture toughness and Weibull modulus. This indicates that microstructural features have a significant contribution to the mechanical properties of the porous materials in terms of bending strength and fracture toughness. © 2012 Elsevier Ltd.Publicationtype: Journal Article75 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Biomechanics of a bone-periodontal ligament-tooth fibrous joint(2012-12-07); ; ; ; This study investigates bone-tooth association under compression to identify strain amplified sites within the bone-periodontal ligament (PDL)-tooth fibrous joint. Our results indicate that the biomechanical response of the joint is due to a combinatorial response of the constitutive properties of organic, inorganic, and fluid components. Second maxillary molars within intact maxillae (N=8) of 5-month-old rats were loaded with a μ-XCT-compatible in situ loading device at various permutations of displacement rates (0.2, 0.5, 1.0, 1.5, 2.0mm/min) and peak reactionary load responses (5, 10, 15, 20N). Results indicated a nonlinear biomechanical response of the joint, in which the observed reactionary load rates were directly proportional to displacement rates (velocities). No significant differences in peak reactionary load rates at a displacement rate of 0.2mm/min were observed. However, for displacement rates greater than 0.2mm/min, an increasing trend in reactionary rate was observed for every peak reactionary load with significant increases at 2.0mm/min. Regardless of displacement rates, two distinct behaviors were identified with stiffness (S) and reactionary load rate (LR) values at a peak load of 5N (S5N=290-523N/mm) being significantly lower than those at 10N (LR5N=1-10N/s) and higher (S10N-20N=380-684N/mm; LR10N-20N=1-19N/s). Digital image correlation revealed the possibility of a screw-like motion of the tooth into the PDL-space, i.e., predominant vertical displacement of 35μm at 5N, followed by a slight increase to 40μm at 10N and 50μm at 20N of the tooth and potential tooth rotation at loads above 10N. Narrowed and widened PDL spaces as a result of tooth displacement indicated areas of increased apparent strains within the complex. We propose that such highly strained regions are "hot spots" that can potentiate local tissue adaptation under physiological loading and adverse tissue adaptation under pathological loading conditions. © 2012.Publicationtype: Journal ArticleScopus© Citations 34 58 - Some of the metrics are blocked by yourconsent settings
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: with files , Computer-controlled stable crack growth as a reliable and fast method to determine subcritical crack growthSubcritical crack growth of synthetic fused silica glass is investigated using the Single-Edge-V-Notched-Beam (SEVNB) method. The measurement was performed in a very stiff four-point bending device, which is equipped with a computer-aided control system. This technique enables several loading cycles of controlled crack growth with one measurement and sample. The control system is based on an online compliance measurement that enables an automatic measurement routine without much operation of the user and without the necessity of the optical observation of the crack. The crack velocity as a function of the applied stress intensity factor was determined by analysing the measured specimen compliance. The presented method allows determining a large number of v-KI curves under identical conditions by measuring only one sample.Publicationtype: PreprintScopus© Citations 6 1132 663