Date of Award

Summer 2001

Document Type

Thesis

Degree Name

Master of Science in Biomedical Engineering - (M.S.)

Department

Biomedical Engineering Committee

First Advisor

Van P. Thompson

Second Advisor

David S. Kristol

Third Advisor

Peter Engler

Abstract

The objective of this thesis was to investigate the Hertzian Contact response of human teeth and how the response behavior is related to that of model ceramic bi-layers (high modulus brittle ceramics on compliant substrates). Hertzian Contact is a blunt indentation method, which uses a hard spherical indenter to apply a normal compressive load. Clinical variables of masticatory (occlusal) force and cuspal curvature identify closely with the independent Hertzian variables of contact load and sphere radius [1]. This method offers insight into the role of microstructure and microstructure- sensitive mechanical properties. It was hypothesized that tooth damage modes and patterns observed would mimic those seen in comparable dental ceramic bi-layer structures, with the same dependence on coating thickness.

Under Hertzian Contact testing teeth exhibited both classic ceramic surface ring cracking as well as quasi-plastic behavior of heterogeneous ceramics in the vicinity of the indenter. Radial cracking was seen beneath the indenter originating at the dento-enamel junction and extending upward toward the surface as seen in thin (i.e. < 1.0 mm) dental ceramic layers. The amount of ring cracking and quasiplasticity was inversely related to loading rate, indicating visco-elastic behavior. The highly viscoelastic response found for enamel (supported by dentin) has never been identified. The mechanisms of this response are believed to lie within enamel microstructure and nano-structure. At a low loading rate, the amount of surface indentation strain was inversely related to enamel thickness. At a high loading rate, the amount of surface indentation strain was either unrelated to or modestly related to enamel thickness depending upon the load applied. At the high loading rate, enamel exhibited elastic behavior until higher loads induced radial and ring cracks with little quasi-plastic response. These findings infer that teeth accommodate and tolerate damage better at higher load rates due to increased elastic behavior. It appears that at higher loading rates enamel microstructure and nano-structure are better able to influence the response to loading, which results in the observance of elastic behavior and a diminishihg influence of enamel thickness. Analysis of the tooth's response to Hertzian contact testing has allowed for investigation of and further insight into the microstructure-sensitive properties providing teeth with their damage tolerance.

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