Size-dependent elastic/inelastic behavior of enamel over millimeter and nanometer length scales

Abstract

The microstructure of enamel like most biological tissues has a hierarchical structure which determines their mechanical behavior. However, current studies of the mechanical behavior of enamel lack a systematic investigation of these hierarchical length scales. In this study, we performed macroscopic uni-axial compression tests and the spherical indentation with different indenter radii to probe enamel's elastic/inelastic transition over four hierarchical length scales, namely: 'bulk enamel' (mm), 'multiple-rod' (10's microm), 'intra-rod' (100's nm with multiple crystallites) and finally 'single-crystallite' (10's nm with an area of approximately one hydroxyapatite crystallite). The enamel's elastic/inelastic transitions were observed at 0.4-17 GPa depending on the length scale and were compared with the values of synthetic hydroxyapatite crystallites. The elastic limit of a material is important as it provides insights into the deformability of the material before fracture. At the smallest investigated length scale (contact radius approximately 20 nm), elastic limit is followed by plastic deformation. At the largest investigated length scale (contact size approximately 2 mm), only elastic then micro-crack induced response was observed. A map of elastic/inelastic regions of enamel from millimeter to nanometer length scale is presented. Possible underlying mechanisms are also discussed.

Spatial distribution of the human enamel fracture toughness with aging

“…Each prism spans a surface that is nearly perpendicular from the dentino-enamel junction (DEJ) to the tooth, and the prisms are separated from each other by a thin layer of protein-based organic matrix (Habelitz et al, 2001). The mechanical properties of enamel have been discussed for decades due to their importance in the clinical treatment and development of tooth-like restorative materials (Ang et al, 2012;An et al, 2012). Enamel has generally been considered a brittle material, similar to ceramic (Robinson et al, 2004).…”

“…The fracture toughness, which describes a material's ability to resist the propagation of an existing crack under a particular state of stress (Park et al, 2008a), is an important parameter in the evaluation of the fracture behavior of enamel. As a hierarchical material, a profound understanding of the structure-behavior relationships of enamel necessitates its mechanical characterization at all hierarchical levels (Ang et al, 2012). However, due to the limited volume of tissue available for examination, conventional tests for fracture toughness, such as the single-edge notched beam (SENB) technique, cannot reflect the changes in the enamel at the micro-scale.…”

Mechanical Properties of Enamel Nanocomposite

“…The dentinal fibers terminate into the junction where the HAP crystallites start showing the enamel nanocomposite characteristics [4]. Therefore, considering the huge importance of the DEJ, it is not surprising that many researchers reported nanohardness ( ) and Young's modulus ( ) of enamel nanocomposite [5][6][7][8][9][10][11][12][13][14][15][16][17][18] wherein occasionally attempts were also made to apply the "Rule of Mixtures" [19][20][21] to predict Young's modulus data. Typically, the tooth enamel nanocomposite showed nanohardness ( ) and Young's modulus ( ) in the range of 3 to 5 GPa and 80 to 120 GPa, respectively [5-8, 14, 15].…”

Multiaxial Strength and Stress Forming Behavior of Four Light-Curable Dental Composites

“…The results can therefore be used on small cavities in small teeth as well as bigger cavities in bigger teeth, as long as aspect ratios are similar. But in fact the tooth behavior is highly orthotropic, individually and spatially distributed, as well as size-dependent as was mentioned by Ang et al (2010). The simplifications were done on literature data for validated finite-element models by Barak et al (2009).…”