Robert Ritchie

Professor and Chair, Materials Science and Engineering Associated Faculty Scientist, Lawrence Berkeley National Laboratory Affiliate, UCB/UCSF Graduate Group in Bioengineering 216 Hearst Memorial Mining Building mailcode: MC 1760 (510) 486-5798 fax: (510) 486-4881 This e-mail address is being protected from spambots. You need JavaScript enabled to view it http://www.lbl.gov/Ritchie Membership effective July 1995

Research Interests

Mechanics and mechanisms of the failure of biomaterials, with application to damage tolerance and life prediction in prosthetic devices.

Research Summary

The objective of our current studies in biomaterials, which are funded through an NIH grant with UCSF, is to examine the mechanical properties of teeth, specifically human dentin, and in particular to characterize the damage tolerance of this material as a basis for developing life-prediction methodologies. To date, we have characterized the in vitro fracture toughness of human dentin using precracked, three-point bend samples and found it to be substantially lower than that reported in the literature, where testing with notched samples had led to erroneously high values due to the absence of a sharp crack. Our measured values give a toughness for dentin in simulated body fluid of 1.8 MPam, which is only three times greater that of window pane glass! This value is mildly sensitive to orientation, with respect to where the crack lies in relation to the long axis of the cylindrical tubules which run from the dentin-enamel junction to the soft, interior pulp. Currently, studies are focused on discerning the micromechanisms of fracture in dentin, including testing to determine whether it is locally stress- or strain-controlled and to define the relationship between the crack trajectories and the microstructure.

More importantly, we have performed an extensive characterization of the behavior of human dentin under repetitive cyclic loading. Although it is known that human dentin is susceptible to fatigue failure, there are few reports in the literature that reliably quantify this phenomenon. Our work has centered on determining the stress/life (S/N) characteristics in simulated body fluid over a range of cyclic frequencies, and in determining, from in situ monitoring of the stiffness loss, the rates of fatigue-crack growth, da/dN, as a function of the stress-intensity range, DK (a fracture-mechanics parameter which defines the intensity of the stress and displacement fields at the crack tip). Such S/N and da/dN-DK data have been used to develop a framework for a fracture-mechanics based methodology for the prediction of the fatigue life in teeth. Based on our current results, we can make quantitative predictions about the effect of small incipient flaws of specific dimensions in affecting the useful life of human teeth.

With respect to implant materials, our current work has been centered on the superelastic/shape-memory alloy Nitinol (a near equiatomic Ni and Ti alloy), which is used extensively in the manufacture of endovascular stents, catheters, and dental components. To date, we have focused on the fatigue and fatigue-crack growth characteristics of Nitinol, in particular how the in situ phase transformations relate to the actual micromechanisms of damage and crack advance (under the sponsorship of NDC, Corp.), and in collaboration with Panos Papadopoulos of the UCB Mechanical Engineering Department on the superelastic constitutive behavior of Nitinol under multiaxial (tension plus torsion) loading (under NSF Sponsorship). We are currently extending the latter work to an in situ study of the superelastic transformation under multiaxial loading using the micro X-ray diffraction facilities at LBNL's Advanced Light Source in order to understand the crystallography of the deformation mechanisms.

Selected Publications