- Walnut Creek, California, United States
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The finite element method, which has been successfully applied to studies of the elastic properties of trabecular bone, is now being used to simulate its failure. These simulations have used a geometrically linear (linear kinematic)... more
The finite element method, which has been successfully applied to studies of the elastic properties of trabecular bone, is now being used to simulate its failure. These simulations have used a geometrically linear (linear kinematic) approximation to the total stiffness matrix; nonlinear terms in the total stiffness matrix have been excluded from the computation in order to achieve efficiency. Because trabecular bone appears to be a slender (i.e., geometrically nonlinear) structure, we studied the validity of the linear kinematic approximation for simulating its failure. Two cases, designed to bracket the extremes of stability behavior, were explored: a single representative spicule of trabecular bone (case 1) and a volume of trabecular bone consisting of relatively low aspect ratio members (case 2). For case 1, geometrically linear (GL) and nonlinear (GNL) analyses were performed with two different materials models: a plastic damage model and a brittle damage model. When GNL terms were included in the total stiffness matrix, we found that load-path bifurcation preceded tissue failure regardless of the form of the damage model. This bifurcation was the result of a complex coupling between material yield and structural instability. The nature of this coupling was highly sensitive to the form of the damage model. None of these behaviors was observed in the linear analyses, where failure was insensitive to the form of the damage model and where structural instabilities were prevented from occurring. For case 2, compressive loading of a volume of trabecular bone, geometric nonlinear effects were pronounced. There was a bifurcation in load response that resulted in large apparent strain to failure. The GL simulations, on the other hand, precluded this bifurcation. We hypothesize that trabecular bone is a geometric nonlinear structure; nonlinear terms must be included in the total stiffness matrix to accurately simulate its failure.
Research Interests: Engineering, Materials Science, Finite element method, Finite Element, Medicine, and 15 moreBiological Sciences, Elasticity, Humans, Bone, Kinematics, Compressive Strength, Bifurcation, Finite element simulation, Finite Element Model, Elastic Properties, Material Model, Aspect Ratio, Biomechanical Phenomena, Medical and Health Sciences, and Fractures Bone
The calculation of the scalar compressive and shear anisotropy factors developed for single crystal refractory compounds has been adapted to the anisotropic elastic stiffness coefficients determined by a number of ultrasonic measurements... more
The calculation of the scalar compressive and shear anisotropy factors developed for single crystal refractory compounds has been adapted to the anisotropic elastic stiffness coefficients determined by a number of ultrasonic measurements of bone based on transverse isotropic symmetry. Later, this work was extended to include the ultrasonic measurements of bone based on orthotropic symmetry. Recently, the five transverse isotropic elastic constants for both wet and dry human dentin were determined using resonant ultrasound spectroscopy. The five transverse isotropic elastic constants for wet bovine enamel and dentin had been calculated based on modeling of ultrasonic wave propagation measurements and related data in the literature. The scalar compressive and shear anisotropy factors have been calculated from both these sets of data and are compared with a representative set from those published previously for both human and bovine bone and both fluoro- and hydroxyl-apatites.
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Most non-traumatic fractures occur in teeth that have been treated, for example restored or endodontically repaired. It is therefore essential to evaluate the structure and mechanical properties of altered forms of dentin. One such... more
Most non-traumatic fractures occur in teeth that have been treated, for example restored or endodontically repaired. It is therefore essential to evaluate the structure and mechanical properties of altered forms of dentin. One such altered dentin is transparent (sclerotic) dentin, which forms gradually with aging. Accordingly, in the present study, we seek to study differences in the structure, i.e., dentinal mineral concentration, mineral crystallite size, and the mechanical properties, i.e., elastic moduli, fracture toughness and fatigue behavior, of normal and transparent root dentin. The mineral concentration, measured by x-ray computed tomography, was found to be significantly higher in transparent dentin, with the majority of the increase being due to the closure of the tubule lumens. Crystallite size, as measured by small angle x-ray scattering, appeared to be slightly reduced in transparent dentin, although the difference was not statistically significant. The elastic proper...
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ABSTRACTThe goal of this study was to develop a three–dimensional imaging method for studies of deformation in low-density materials during loading, and to implement finite element solutions of the elastic equations based on the images.... more
ABSTRACTThe goal of this study was to develop a three–dimensional imaging method for studies of deformation in low-density materials during loading, and to implement finite element solutions of the elastic equations based on the images. Specimens of silica–reinforced polysiloxane foam pads, 15 mm in diameter by 1 mm thick, were used for this study. The nominal pore density was 50%, and the pores approximated interconnected spheres. The specimens were imaged with microtomography at ∼16µm resolution. A rotating stage with micrometer driven compression allowed imaging of the foams during deformation with precise registration of the images. A finite element mesh, generated from the image voxels, was used to calculate the mechanical properties of the structure, and the results were compared with conventional mechanical testing. The foam exhibited significant nonlinear behavior with compressive loading. The finite-element calculations from the images, which were in excellent agreement wit...
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We present experimental measurements of the accessible pore fraction in ceramic matrix composites during consolidation by vapor phase infiltration. For two topologically distinct filament architectures, the accessible pore fraction... more
We present experimental measurements of the accessible pore fraction in ceramic matrix composites during consolidation by vapor phase infiltration. For two topologically distinct filament architectures, the accessible pore fraction decreased during consolidation with a power law decay and a critical scaling exponent of 0.41 (R2= 0.97). A three-dimensional analysis of the percolating pores revealed that the structures became topologically equivalent and simply connected near the critical density.
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Research Interests: Materials Science, Dentistry, Atomic Force Microscopy, Medicine, Elasticity, and 15 moreHumans, Collagen, Compressive Strength, Dental Caries, Dentine, Composite Material, Hardness, Demineralization, Dentin, Dental Bonding, Apatites, Dentin bonding, Electron Probe Microanalysis, Hydrostatic Pressure, and Mechanical Property
An atomic force microscope was used to measure the hardness and elasticity of fully-hydrated peritubular and intertubular human dentin. The standard silicon nitride AFM tip and silicon cantilever assembly were replaced with a diamond tip... more
An atomic force microscope was used to measure the hardness and elasticity of fully-hydrated peritubular and intertubular human dentin. The standard silicon nitride AFM tip and silicon cantilever assembly were replaced with a diamond tip and stainless steel cantilever having significantly higher stiffness. Hardness was measured as the ratio of the applied force to the projected indentation area for indentations with depths from 10–20 nm. The sample stiffness was measured by imaging specimens in a force-modulated mode. Hardness values of 2.3 ± 0.3 GPa and 0.5 ± 0.1 GPa were measured for the peritubular and intertubular dentin, respectively. Stiffness imaging revealed that the elastic modulus of the peritubular dentin was spatially homogeneous; whereas, there was considerable spatial variation in the elasticity of the intertubular dentin. The atomic force microscope can be used to measure the mechanical properties of fully hydrated calcified tissues at the submicron level of spatial r...
Research Interests: Mechanical Engineering, Biomechanical Engineering, Materials Science, Biomedical Engineering, Nanoindentation, and 15 moreAtomic Force Microscopy, Medicine, Elasticity, Humans, Silicon Nitride, Composite Material, Hardness, Diamond, Cantilever, Indentation, Dentin, Elastic Modulus, Atomic Force Microscope, Pilot Projects, and Reference Values
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X-ray tomographic microscopy (XTM), a three-dimensional X-ray imaging technique, has been used to quantitatively map mineral concentrations in carious dentin. Data analysis from the XTM study indicates that variations in the mineral... more
X-ray tomographic microscopy (XTM), a three-dimensional X-ray imaging technique, has been used to quantitatively map mineral concentrations in carious dentin. Data analysis from the XTM study indicates that variations in the mineral concentration surrounding the caries can be imaged in three dimensions with a spatial resolution that is sufficient to detect calcified and enlarged tubule spaces in the lesion. A three-dimensional image of the subsurface lesion indicates that lesion penetration is along the direction of the tubules. The mineral concentration in the uninfected dentin was measured by the XTM to be 1.29 +/- 0.14 g/cm3 based upon the tabulated X-ray attenuation coefficients for apatite. This value is in excellent agreement with averaged estimates for the mineral concentration in dentin (1.4 g/cm3). Furthermore, the mineral concentration determined using XTM varies from 2.25 g/cm3 in the remineralized dentin to as low as 0.55 +/- 0.17 g/cm3 in the demineralized tissue. The h...
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Although human dentin is known to be susceptible to failure under repetitive cyclic fatigue loading, there are few reports in the literature that reliably quantify this phenomenon.
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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... more
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.
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Many fractures occur in teeth that have been altered, forexample restored or endodontically repaired. It is therefore essential toevaluate the structure and mechanical properties of these altereddentins. One such altered form of dentin is... more
Many fractures occur in teeth that have been altered, forexample restored or endodontically repaired. It is therefore essential toevaluate the structure and mechanical properties of these altereddentins. One such altered form of dentin is transparent (sometimes calledsclerotic) dentin, which forms gradually with aging. The present studyfocuses on differences in the structure and mechanical properties ofnormal versus transparent dentin. The mineral
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Samples of single and poly-crystal aluminum were shocked to incipient spallation fracture and recovered on the LLNL light gas gun. Previously, we analyzed the void structure in these samples using 3D x-ray tomography. Here, we extend this... more
Samples of single and poly-crystal aluminum were shocked to incipient spallation fracture and recovered on the LLNL light gas gun. Previously, we analyzed the void structure in these samples using 3D x-ray tomography. Here, we extend this analysis to sub-micron length scales using ultra-small-angle scattering (USAXS and USANS). The USANS data overlaps in length-scale with the tomography data. The data
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Incipient spallation damage was studied in Aluminum non-destructively using 3D X-ray tomography. Two samples were laser shocked and recovered using a weaker and stronger laser pulses. The X-ray tomography was performed at SSRL with a... more
Incipient spallation damage was studied in Aluminum non-destructively using 3D X-ray tomography. Two samples were laser shocked and recovered using a weaker and stronger laser pulses. The X-ray tomography was performed at SSRL with a resolution of five microns. The 3D distribution of damage in both samples was clearly evident about a well defined spall plane. The results were analyzed
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Research Interests: Materials Science, Direct Numerical Simulation, Strain Rate, High strain rate testing, Dislocations, and 15 moreNucleation and Growth, Polarizable continuum model, Computer Program, Free Surface, Dislocation Density, Grain Boundary, Single Crystal, Microstructures, Three Dimensional, Plastic deformation, Dynamic Fracture, Shock Wave, Material Model, Experimental Data, and Cumulant
Research Interests: Chemistry, Biomedical Engineering, Wound Healing, Orthopaedic, Medicine, and 15 moreSynchrotron Radiation, Bone Regeneration, Thrombin, Controlled release, Animals, Male, Clinical Sciences, Mechanical Testing, Radius, Ulna, Three Dimensional, Bone remodeling, Rabbits, Microspheres, and Biomechanical Phenomena
Human dentin is susceptible to failure under repetitive cyclic-fatigue loading. This investigation seeks to address the paucity of data that reliably quantify this phenomenon. Specifically, the effect of alternating vs. mean stresses,... more
Human dentin is susceptible to failure under repetitive cyclic-fatigue loading. This investigation seeks to address the paucity of data that reliably quantify this phenomenon. Specifically, the effect of alternating vs. mean stresses, characterized by the stress- or load-ratio R (ratio of minimum-to-maximum stress), was investigated for three R values (−1, 0.1, and 0.5). Dentin was observed to be prone to fatigue failure under cyclic stresses, with susceptibility varying, depending upon the stress level. The “stress-life” ( S/N) data obtained are discussed in the context of constant-life diagrams for fatigue failure. The results provide the first fatigue data for human dentin under tension-compression loading and serve to map out safe and unsafe regimes for failure over a wide range of in vitro fatigue lives (< 103 to > 106 cycles).
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Research Interests: Engineering, Materials Science, Biomedical Engineering, Microstructure, Fracture, and 15 moreScanning Electron Microscopy, Medicine, Biological Sciences, Humans, Biomedical Materials, Composite Material, Crack Initiation, Fractography, Mechanical Stress, Microstructures, Dentin, Biocompatible Materials, Bending Test, Failure Mode, and Sprains and strains
Most current dentin bonding procedures use acid etchants to partially demineralize the dentin structure and provide pathways for resin infiltration. This study determined the recession rates of peritubular dentin and intertubular dentin... more
Most current dentin bonding procedures use acid etchants to partially demineralize the dentin structure and provide pathways for resin infiltration. This study determined the recession rates of peritubular dentin and intertubular dentin as a function of pH during demineralization in citric acid solutions (0.0005-2.5M) and the effects of dehydration and rehydration on the partially demineralized dentin. Polished dentin disks were prepared with an internal reference layer and were studied at specific intervals for citric acid etching between pH 1 and 3.4 in an atomic force microscope. Peritubular dentin etched rapidly and linearly with time until it could no longer be measured. The intertubular surface began etching at nearly the same rate, but then recession slowed for all concentrations and stabilized after recession of less than 1 microm for all but the pH 1 solution. The decrease in recession was attributed to the limitation of contraction of the demineralized collagen scaffold as long as it remained hydrated. Dehydration following etching resulted in significant collapse of the surface, changes in roughness, and a slight decrease in tubule diameter for samples etched for 30 min. Measurements could not be made of the collapse for low pH samples, because shrinkage stresses disrupted the integrity of the reference layer. On rehydration, the dehydrated surfaces underwent an expansion up to the level seen after etching and tubule diameters returned to the etched values. These results indicate that the collapse of demineralized matrix is almost totally recoverable on rehydration.
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Incipient spallation fracture damage was characterized in aluminum non-destructively using 3D X-ray tomography. Polycrystalline samples of 5mm thickness and 0.2 mm grain size were shocked at impact velocities between 150m/s and 210m/s... more
Incipient spallation fracture damage was characterized in aluminum non-destructively using 3D X-ray tomography. Polycrystalline samples of 5mm thickness and 0.2 mm grain size were shocked at impact velocities between 150m/s and 210m/s using 1.5 ...
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Research Interests: Materials Science, Biomaterials, Aging, Transmission Electron Microscopy, Nanotechnology, and 15 moreAtomic Force Microscopy, Dental Materials, Medicine, Multidisciplinary, Humans, High Resolution Transmission Electron Microscopy, Hardness, Aged, Adult, X ray diffraction, Age Factors, Dentin, Tooth, Biocompatible Materials, and Ions
Research Interests: Materials Science, Biomaterials, Aging, Minerals, Small Angle X Ray Scattering, and 15 moreMedicine, Multidisciplinary, Humans, Hardness, Aged, Tooth Root, Crystallite Size, Time Factors, Microstructures, Dentin, Fracture Toughness, Elastic Properties, X ray Computed Tomography, Tensile Strength, and Mechanical Property
Research Interests: Materials Science, Biomaterials, Fracture Mechanics, Fatigue, Medicine, and 15 moreMultidisciplinary, Fatigue crack growth, Elasticity, Crack propagation, Elephants, Animals, Power Law, Hardness, Frequency Dependence, Mechanical Stress, Fracture Mechanic, Growth rate, Dentin, Fatigue Crack Propagation, and Biomechanical Phenomena
Research Interests: Materials Science, Biomaterials, Fracture, Fatigue crack growth, Elasticity, and 15 moreCrack propagation, Humans, Computer Simulation, Compressive Strength, Hardness, Bridging, Cadaver, Crack Initiation, Adult, Bone fracture, Growth rate, Cortical Bone, Fracture Toughness, Biomechanical Phenomena, and In Vitro Techniques
Research Interests: Materials Science, Biomaterials, Fracture Mechanics, Medicine, Multidisciplinary, and 15 moreElasticity, Elephants, Computer Simulation, Compressive Strength, Animals, Composite Material, Hardness, Mechanical Stress, Fracture Mechanic, Dentin, Dental Stress Analysis, Fracture Toughness, Weight Bearing, Biomechanical Phenomena, and Tooth fractures
Research Interests: Materials Science, Biomaterials, Fracture, Linear Elasticity, Medicine, and 15 moreMultidisciplinary, Fatigue crack growth, Humans, Computer Simulation, Compressive Strength, Life Prediction, Cyclic Loading, Cadaver, Mechanical Stress, Humerus, Adult, Fracture Mechanic, Growth rate, Cortical Bone, and In Vitro Techniques
A generalized, self-consistent model of cylindrical inclusions in a homogeneous and isotropic matrix phase was used to study the effects of tubule orientation on the elastic properties of dentine. Closed-form expressions for the five... more
A generalized, self-consistent model of cylindrical inclusions in a homogeneous and isotropic matrix phase was used to study the effects of tubule orientation on the elastic properties of dentine. Closed-form expressions for the five independent elastic constants of dentine were derived in terms of tubule concentration, and the Young&amp;amp;#39;s moduli and Poisson ratios of peri- and intertubular dentine. An atomic-force microscope indentation technique determined the Young&amp;amp;#39;s moduli of the peri- and intertubular dentine as approx. 30 and 15 GPa, respectively. Over the natural variation in tubule density found in dentine, there was only a slight variation in the axial and transverse shear moduli with position in the tooth, and there was no measurable effect of tubule orientation. It was concluded that tubule orientation has no appreciable effect on the elastic behaviour of normal dentine, and that the elastic properties of healthy dentine can be modelled as an isotropic continuum with a Young&amp;amp;#39;s modulus of approx. 16 GPa and a shear modulus of 6.2 GPa.