One of the key aspects in our work was the realization that the prismatic interlayers provide a significant amount of strength, as discussed in the introduction. These prismatic layers occur from nacre growth interruptions causes by low temperature or sustenance. Given that nutrition level and high and low temperature either slow or alter the growth process for nacre; we might expect these three conditions to result in prismatic layers with differing structure and properties as they have different physiological origins. For example, consider the growth interruption in a wild specimen illustrated in Fig.6.6. A typical interruption starts with the formation of block-like calcite, followed by a mesolayer with high organic content, and then columnar or spherulitic aragonite before steady-state tiled nacre forms again. The literature tends to refer to these growth interruptions as prismatic layers, even though it is realized the structure varies. Their overall structure has been best detailed by the Meyers research group [37].
Initial characterization involved indent locations directly on a tablet and at the tablet-tablet interface and assessing the modulus-depth profiles for differences. It is expected that small penetration depths will result in different values that converge to a consistent value at larger depths. These profiles will also be compared to random indent locations to determine which method is most appropriate to obtaining reliable and repeatable data. It is expected that random locations will provide the most representative data and setting up the indents in a 55 array with 200mm spacing would best accomplish this.
Given that the aim is to first assess the material properties of the different layers and then combinations of layers we will be less concerned with small penetration depths where indent location is expected to cause significant variability. Figure6.7 illustrates this in a preliminary set of indents by the PI on farm raised nacre with a fixed 55 array. Here, the nacre layer approximately 800mm thick and the circles represent the average value for the 25 indents. Early on there is a significant distribution of modulus values that tightens up with penetration depth. The average value though remains relatively constant Fig. 6.5 Glass slides
(depicted byarrows) embedded in abalone shell (From Lopez et al. [41])
Fig. 6.6 Growth interruption layer in a wild abalone specimen from the PI’s survey. Three separate prismatic-based layers are identified
42 M. Sullivan and B.C. Prorok
even at low penetration depths. Even though the value of modulus is constant here, it is not necessarily exclusively representative of nacre. This sample was sectioned from a farm-raised abalone shell by procedures focused on keeping the nacre tablets on the surface normal to the indent axis. Thus, there is some ambiguity on how well supported the sample is as well as affects from the structural variability natural organisms possess. We are confident that our implantation and recovery technique will mitigate these issues and yield accurate elastic modulus and Poisson’s ratio for nacre. Our main point in presenting this data is the reasonable consistency of results after 100–200 nm of penetration depth.
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0 10 20 30 40 50 60 70 80
a 90 b
Modulus (GPa)
Displacement Into Surface (nm)
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
0 200 400 600 800 1000 0 200 400 600 800 1000
Hardness (GPa)
Displacement Into Surface (nm)
Fig. 6.7 Preliminary results of indentation testing on a farm raised specimen, elastic modulus (a) and hardness (b). The modulus appears relatively constant with indent depth while hardness varies
6 New Insight into the Toughening Mechanisms of Nacre 43
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44 M. Sullivan and B.C. Prorok
Chapter 7
Thermal Analyses of Dental Ceramic Restorations
Barry Hojjatie, W. Bartholomew, and H. Garmestani
Abstract Previous studies have shown that development of compressive stresses on the surface of materials such as glass, ceramic and steel will improve resistance to crack initiation and propagation in these materials. The objective of this study was to develop computational models in MATALB and ANSYS to predict temperature and stress distributions in dental ceramic materials subjected to thermal tempering as a function of material properties and thermal tempering parameters.
In an initial study we obtained the cooling profiles corresponding to a transient heat transfer problem for bi-layered ceramic disks and compared the results obtained from the MATLAB models with the FEA models developed in ANSYS.
In subsequent analyses we performed computational thermal stress analysis of the bi-layered disks and various dental restorations using ANSYS. Validity of the analytical thermal models was established by comparing the results from one dimensional model with those from the two-dimensional models as well as by comparison of the corresponding results with those from the FEA models. A good agreement between the analytical thermal models using MATLAB and computational models from ANSYS were obtained. The results of this study show that relative geometric dimensions of the layered structures and the rate of cooling have a significant influence on the transient stress distribution within the restorations that may result in premature failure of these structures.
Keywords Dental ceramics • Disks • Analytical thermal models • Transient • Temperature distribution