Despite substantial clinical interest in the fracture resistance of human dentin, there is little mechanistic information in archival literature that can be usefully used to model such fracture. In fact, although the fracture event indent in, akin to other mineralized tissues like bone, is widely believed to be locally strain-controlled, there has never been any scientific proof to support this belief. The present study seeks to address this issue through the use of a novel set of in vitro experiments in Hanks' balanced salt solution involving a double-notched bend test geometry, which is designed to discern whether the critical failure events involved in the onset of fracture are locally stress- or strain-controlled. Such experiments are further used to characterize the notion of "plasticity" in dentin and the interaction of cracks with the salient microstructural features. It is observed that fracture in dentin is indeed locally strain-controlled and that the presence of dentinal tubules does not substantially affect this process of crack initiation and growth. The results presented are believed to be critical steps in the development ofa micromechanical model for the fracture of human dentin that takes into consideration the influence of both the microstructure and the local failure mode.

The behavior of nanostructured materials/small-volume structures and biologi-cal/bio-implantable materials, so-called "nano" and "bio" materials, is currently much in vogue in materials science. One aspect of this field, which to date has received only limited attention, is their fracture and fatigue properties. In this paper, we examine two topics in this area, namely the premature fatigue failure of silicon-based micron-scale structures for microelectromechanical systems (MEMS), and the fracture properties of mineralized tissue, specifically human bone.

A micro-mechanistic understanding of bone fracture that encompasses how cracks interact with the underlying microstructure and defines their local failure mode is lacking, despite extensive research n the response of bone to a variety of factors like aging, loading, and/or disease.

Mechancial surface treatments are known to be effective at improving the fatigue resistance of metallic alloys at elevated temperatures (~;550-600 aC), even though the near-surface compressive residual stress fields have been annealed out. We have investigated the thermal stability of near-surface microstructures induced by deep rolling and laser-shock peening in an austentic stainless steel (AISI 304) and a titanium alloy (Ti-6Al-4V) using in situ hot-stage transmission electron microscopy. It is found that the improvements in fatigue resistance at elevated temperature are related to the high-temperature stability of the work-hardened near-surface microstructure in each case.

The in vitro fracture toughness of human dention has been reported to be of the order of 3 MPa sqrt m. This result, however is based on a single study for a single orientation, and furthermore involves notched, rather than fatigue precracked, test samples.

Human dentin is known to be susceptible to failure under cyclic loading. Surprisingly, there are few reports that quantify the effect of such loading, considering the fact that a typical tooth experiences a million or so loading cycles annually. In the present study, a systematic investigation is described of the effects of prolonged cyclic loading on human dentin in a simulated physiological environment. In vitro stress-life (S/N) data are discussed in the context of possible mechanisms of fatigue damage and failure.

Ultraviolet resonance Raman spectroscopy (UVRRS) using 244 nm excitation was used to investigate the impact of aging on human dentin. The intensity of a spectroscopic feature from the peptide bonds in the collagen increases with tissue age, similar to a finding reported previously for human cortical bone.