Initial Hertzian characteristics of the human tooth as a damage tolerant bio-ceramic/bio-composite bi-layer

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. \u3c 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\u27s response to Hertzian contact testing has allowed for investigation of and further insight into the microstructure-sensitive properties providing teeth with their damage tolerance

A statistical approach for fracture property realization and macroscopic failure analysis of brittle materials

Lacking the energy dissipative mechanics such as plastic deformation to rebalance localized stresses, similar to their ductile counterparts, brittle material fracture mechanics is associated with catastrophic failure of purely brittle and quasi-brittle materials at immeasurable and measurable deformation scales respectively. This failure, in the form macroscale sharp cracks, is highly dependent on the composition of the material microstructure. Further, the complexity of this relationship and the resulting crack patterns is exacerbated under highly dynamic loading conditions. A robust brittle material model must account for the multiscale inhomogeneity as well as the probabilistic distribution of the constituents which cause material heterogeneity and influence the complex mechanisms of dynamic fracture responses of the material. Continuum-based homogenization is carried out via finite element-based micromechanical analysis of a material neighbor which gives is geometrically described as a sampling windows (i.e., statistical volume elements). These volume elements are well-defined such that they are representative of the material while propagating material randomness from the inherent microscale defects. Homogenization yields spatially defined elastic and fracture related effective properties, utilized to statistically characterize the material in terms of these properties. This spatial characterization is made possible by performing homogenization at prescribed spatial locations which collectively comprise a non-uniform spatial grid which allows the mapping of each effective material properties to an associated spatial location. Through stochastic decomposition of the derived empirical covariance of the sampled effective material property, the Karhunen-Loeve method is used to generate realizations of a continuous and spatially-correlated random field approximation that preserve the statistics of the material from which it is derived. Aspects of modeling both isotropic and anisotropic brittle materials, from a statistical viewpoint, are investigated to determine how each influences the macroscale fracture response of these materials under highly dynamic conditions. The effects of modeling a material both explicitly by representations of discrete multiscale constituents and/or implicitly by continuum representation of material properties is studies to determine how each model influences the resulting material fracture response. For the implicit material representations, both a statistical white noise (i.e., Weibull-based spatially-uncorrelated) and colored noise (i.e., Karhunen-Loeve spatially-correlated model) random fields are employed herein

Stress-induced permeability evolution in coal: Laboratory testing and numerical simulations

Mining operations produce a multiscale network of fractures in the coal seams. Permeability evolution in rocks is important for coal bed methane (CBM) and shale gas exploitation as well as for greenhouse gas storage. Therefore, this work presents laboratory tests and a coupled model using PFC3D and FLAC3D to simulate the stress induced permeability evolution in coal samples. Basic mechanical properties are determined via lab testing. The spatial distributions of different components inside the reconstructed samples produce a significant heterogeneity based on CT technique. A newly developed experimental system is employed to perform 3-dimensional loading and to measure the flow rate simultaneously. The evolution process is described by 5 distinct phases in terms of permeability and deformation. Triaxial tests are simulated with PFC3D using a novel flexible wall boundary method. Gas seepage simulations are performed with FLAC3D. Relations between hydraulic properties and fracture data are established. Permeability and volumetric strain show good nonlinear exponential relation after a newly introduced expansion point. Piecewise relations fit the whole process, the expansion point can be treated as critical point. The structural characteristics of the samples influence this relation before and after the expansion point significantly

Fracture in compression of brittle solids

The fracture of brittle solids in monotonic compression is reviewed from both the mechanistic and phenomenological points of view. The fundamental theoretical developments based on the extension of pre-existing cracks in general multiaxial stress fields are recognized as explaining extrinsic behavior where a single crack is responsible for the final failure. In contrast, shear faulting in compression is recognized to be the result of an evolutionary localization process involving en echelon action of cracks and is termed intrinsic

Damage of woven composite under tensile and shear stress using infrared thermography and micrographic cuts

Infrared thermography was used to study damage developing in woven fabrics. Two different experiments were performed, a ±45° tensile test and a rail shear test. These two different types of tests show different damage scenarios, even if the shear stress/strain curves are similar. The ±45° tension test shows matrix hardening and matrix cracking whereas the rail shear test shows only matrix hardening. The infrared thermography was used to perform an energy balance, which enabled the visualization of the portion of dissipated energy caused by matrix cracking. The results showed that when the resin is subjected to pure shear, a larger amount of energy is stored by the material, whereas when the resin is subjected to hydrostatic pressure, the main part of mechanical energy is dissipated as heat

Flash X-ray Tomography of Kolsky Bar Experiments

Kolsky bar is commonly used to characterize materials at high strain rates from 102 to 104/s. Insitu visualization of specimen fracture process is vital to understand its dynamic mechanical behaviors. In this dissertation, compression Kolsky bar is combined with a high speed multichannel flash X-ray tomography methodology to capture snapshots of the dynamic in-situ 3D specimen volume information to visualize the dynamic damage and fracture process. This method is capable to produce sub-millimeter-resolution tomography reconstructions of dynamic fracture process from very limited number (4 in this research) of projections. This method enables a precise and repeatable control over the loading history of the specimen and the flash X-ray projection time, thus a correlation between the stress-strain/force-displacement response and the reconstructed volume can be clearly defined. The 4-channel flash X-ray setup is built for low geometric unsharpness (0.15 mm) to improve reconstruction resolution by placing intraoral size phosphor storage plate detectors close to the specimen without interference from unnecessary exposures. For each 2D projection, raw image scanned from detector was aligned according to the positions of the Kolsky bars in the projection via edge detection. Dark current was measured for each experiment and imposed to the reconstruction log input. The log inputs with and without the normalization to the un-deformed state were used. To reduce number of artifacts, Algebraic Reconstruction Technique was used to iteratively update the reconstruction. Three different static phantoms were imaged and reconstructed, where the result shows the image processing inputs are correct and the relative locations of X-ray beams, specimen and phosphor storage plate (PSP) detectors are within the tolerance. Three sets of preliminary Kolsky bar experiments were conducted, where the results indicate that a good tomography reconstruction requires a sufficient feature-signal-to-noise ratio in the 2D projection and a small number of cracks inside the specimen for reduced number of artifacts. Applying this technique, dynamic spheroconical indentation experiments on two types of machinable ceramics (Macor and Mykroy/Mycalex 550) and uniaxial compression on 3D printed sandstone around 100/s were conducted. For indentation experiments, the post-peak-force reconstructions show that the Macor specimen fractures due to two major cracks - one parallel and the other one oblique to the loading direction, while the M/M specimen breaks due to only one major crack parallel to the loading. Such finding matches with the cross-sectional microstructure images, where the particle is randomized distributed in Macor, while the particle is highly directional in M/M. For 3D printed sandstone, it is found that the adhesive infiltrant coated layer apparently affects its mechanical behaviors: the specimen with higher percentage of coated region has higher strength. Under ~100/s loading, the 3D printed sandstone exhibits a brittle behavior, and the reconstructions show that the cylindrical specimen fractures due to cracks separating the side coated layer. Such cracks are initialed near one of specimen-bar interfaces. Under 0.001/s loading, however, the sandstone exhibits a ductile behavior, and the crack is initialed from the center of the specimen. Based on the reconstruction results, the limitations and several potential improvements of the flash X-ray tomography are discussed

Fatigue of Micro Molded Materials - Aluminum Bronze and Yttria Stabilized Zirconia

Custom built setups were developed to investigate micro samples during quasistatic and cyclic testing in tension, compression and bending. Micro molded CuAl10Ni5Fe4-samples showed similar fatigue behavior compared to macroscopic samples due to both the sample size and microstructure being scaled down with the manufacturing process. Results from cyclic three-point bending tests on micro molded 3Y-TZP suggested that a minimum crack extension is necessary to develop cyclically degradable shielding

Studies on strain localization, ductile fracture and damage in structural metals

One of the most important limit states in structural metals is ductile fracture, and the prediction of ductile fracture is of great importance in many engineering applications. The overall objective of the research reported in this dissertation is to advance the understanding and modeling of ductile fracture in metals. This research addresses three main issues: micromechanical modeling of ductile fracture, the development of a micromechanics-based ductile fracture model and its numerical implementation, and a numerical investigation of geometry and damage induced strain localization based on a nonlocal formulation. It has long been recognized that stress triaxiality is a key parameter affecting initiation of ductile fracture. More recently, shear stress has been identified as another important parameter, in addition to stress triaxiality, that influences the process of ductile fracture. In this research, a micromechanics-based model is proposed for predicting initiation of ductile fracture that couples both stress triaxiality and shear stress. The new model is based on a combination of the existing Rice-Tracey and modified maximum shear stress models. The new model is applied to construct the fracture locus of different types of metal alloys and is used to predict fracture initiation by numerical tools. The predicted results are in good agreement with experimental data reported in literature that covers a wide range of triaxialities and shear stress. Another portion of this research, within the framework of micromechanics, investigated the effect of combined normal and shear stress components on micro-void evolution and material behavior. This work involved finite element modeling of a cubic unit cell associated with a spherical void. The results show that the void growth process and macroscopic stress-strain response is highly dependent on the shear stress component. At different ranges of triaxialities, and with different void growth and coalescence mechanisms, shear stress has an important effect on the ductile fracture process. Numerical modeling of strain localization in ductile metals based on standard continuum mechanics exhibits non-convergent mesh sensitivity. This issue is addressed in the final portion of this research. A one-dimensional model based on the nonlocal theory is proposed to analyze geometry-induced strain localization, i.e., necking in structural metals. A nonlocal continuum damage model using the same enhanced continuum law is developed to deal with the damage induced strain localization in metals. Both models provide encouraging performance in eliminating the non-convergent mesh sensitivity problem. Such improved strain localization modeling techniques show potential to be useful for further exploration of ductile fracture phenomena.Civil, Architectural, and Environmental Engineerin

Detecting Delamination in Carbon Fiber Composites Using Piezoresistive Nanocomposites

Carbon fiber prepreg composites are utilized successfully as structural materials for different lightweight aerospace applications. Delamination is a critical failure mode in these composite materials. As composite plies separate from each other, the composite loses some of its ability for supporting expected loads. Therefore, detection of delamination at right time is a foremost significance. This study presents a new way for detecting delamination in composite plates using piezoresistive nanocomposites. This new procedure is setup and studied through both experimental and computational investigations. In this research, nanocomposites with 5% coarse graphene platelets are fabricated for detecting delamination. 8-ply carbon fiber prepreg composite samples are fabricated by placing a Teflon film between layers of prepreg. Piezoresistive nanocomposites are attached on top of prepreg laminate samples using epoxy resin. The change in electrical resistivity of these nanocomposites due to the induced strain from flexural test (three point bend test) on delaminated and neat composite laminates are monitored to demonstrate the delamination detection and neat composite laminates are monitored to demonstrate the delamination detection method. A non-linear finite element model is developed using Abaqus software suite to compliment the mechanical testing. Virtual Crack Closure Technique (VCCT) is used to model a delamination in the composite sample. Experimental results and the simulations in this study indicate that piezoresistive nanocomposites can be used for detecting delamination in carbon fiber composite materials