33 research outputs found
Mechanistic Fracture Criteria For The Failure Of Human Cortical Bone
m thick) . Permeating this lamellar structure are the secondary osteons 9 (up to 200--300 m diameter): large vascular channels (up to 50--90 m diameter) oriented roughly in the growth direction of the bone and surrounded by circumferential lamellar rings. The difficulty in understanding the mechanisms of fracture in bone lies in determining the relative importance of these microstructural hierarchies on crack initiation, subsequent crack propagation and consequent unstable fracture, and in separating their effects on the critical fracture events. A vital distinction in the definition of the local (precursor) fracture events that cause macroscopic failure is whether they are locally stressor strain-controlled. Brittle fracture is invariably stress-controlled, for example in structural steels at low temperatures, where cleavage fracture is instigated by the precursor cracking of carbide particles or inclusions . Ductile fracture, conversely, is strain-controlled, as in th
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On the fracture of human dentin: Is it stress- or strain-controlled?
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
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Effect of orientation on the in vitro fracture toughness of dentin: The role of toughening mechanisms
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
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Failure by fracture and fatigue in "NANO" and "BIO" materials
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
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On the fracture of human dentin: Is it stress- or strain-controlled?
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
Re-evaluating the toughness of human cortical bone
Data for fracture in human humeral cortical bone are re-analyzed to assess the validity for this material of linear-elastic fracture mechanics (LEFM), which is the standard method of analyzing toughness and one basis for analyzing clinical data relating to bone quality. A nonlinear fracture model, which is based on representing the damage zone in the bone by a cohesive model, is calibrated against a number of sets of test data for normal (not diseased or aged) human cortical bone taken from cadavers. The data consist of load vs. load-point displacement measurements from standard compact–tension fracture tests. Conventional LEFM is unable to account for the shape of the load–displacement curves, but the nonlinear model overcomes this deficiency. Calibration of the nonlinear model against one data curve leads to predictions of the peak load and the displacement to peak load for two other data curves that are, for this limited test set, more accurate than those made using LEFM. Furthermore, prior observations of damage mechanisms in bone are incompatible with the modeling assumption of LEFM that all nonlinearity is confined to a zone much smaller than the specimen and the crack length. The predictions of the cohesive model and the prior observations concur that the length of the nonlinear zone in human cortical bone varies in the range 3–10 mm, which is comparable to or larger than naturally-occurring bones and the specimens used to test them. We infer that LEFM is not an accurate model for cortical bone. The fracture toughness of bone deduced via LEFM from test data will not generally be a material constant, but will take different values for different crack lengths and test configurations. The accuracy of using LEFM or single-parameter fracture toughness for analyzing the significance of data from clinical studies is called into question. The nonlinear cohesive zone model is proposed to be a more accurate model of bone and the traction-displacement or cohesive law is hypothesized to be a material property. The cohesive law contains a more complete representation of the mechanics of material failure than the single-parameter fracture toughness and may therefore provide a superior measure of bone quality, e.g., for assessing the efficacy of therapy for osteoporosis