Browsing by Subject "Fracture"

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Analytical and Experimental Nanomechanical Approaches to Understanding the Ductile-to-Brittle Transition
(2015-10) Hintsala, Eric
This dissertation presents progress towards understanding the ductile-to-brittle transition (DBT) using a mixture of nanomechanical experiments and an analytical model. The key concept is dislocation shielding of crack tips, which is occurs due to a dislocation back stress. In order to properly evaluate the role of these interactions, in-situ experiments are ideal by reducing the number of interacting dislocations and allowing direct observation of cracking behavior and the dislocations themselves. First, in-situ transmission electron microscope (TEM) compression experiments of plasma-synthesized silicon nanocubes (NCs) are presented which shows plastic strains greater than 50% in a semi-brittle material. The mechanical properties are discussed and plasticity mechanisms are identified using post-mortem imaging with a combination of dark field and high-resolution imaging. This observations help to develop a back stress model which is used to fit the hardening regime. This represents the first study of its kind where back stresses are used in a discrete manner to match hardening rates. However, the important measurable quantities for evaluating the DBT include fracture toughness values and energetic activation parameters for cracking and plasticity. In order to do this, a new method for doing in-situ fracture experiments is explored. This method is pre-notched three point bending experiments, which were fabricated by focused ion beam (FIB) milling. Two different materials are evaluated: a model ductile material, Nitronic 50, an austenitic steel alloy, and a model brittle material, silicon. These experiments are performed in-situ scanning electron microscope (SEM) and TEM and explore different aspects including electron backscatter diffraction (EBSD) to track deformation in SEM scale experiments, pre-notching using a converged TEM beam to produce sharper notches better replicating natural cracks, etching procedures to reduce residual FIB damage and elevated temperature experiments. Lastly, an analytical method to predict DBTs is presented which can account for effects of strain rate, temperature and impurity presence. The model is tested by pre-existing data on macroscopic compact tension specimens of single crystal Fe-3%Si. Next, application of the model to nano/micro scale fracture toughness experiments is explored and the large number of confounding variables is discussed in detail. A first attempt at fitting is also presented.
Assessing the Accuracy and Reliability of Root Crack and Fracture Detection in Teeth Using Sweep Imaging with Fourier Transform (SWIFT) Magnetic Resonance Imaging (MRI)
(2017-08) Schuurmans, Tyler
Introduction: Magnetic Resonance Imaging (MRI) has the potential to aid in determining the presence and extent of cracks/fractures in teeth due to more advantageous contrast, without ionizing radiation. An MRI technique called Sweep Imaging with Fourier Transform (SWIFT) has overcome many of the inherent difficulties of conventional MRI with detecting fast-relaxing signals from densely mineralized dental tissues. The objectives of this in vitro investigation were to develop MRI criteria for root crack/fracture identification in teeth and to establish intra- and inter-rater reliabilities and corresponding sensitivity and specificity values for the detection of tooth-root cracks/fractures in SWIFT MRI and limited field of view (FOV) CBCT. Materials and Methods: MRI-based criteria for crack/fracture appearance was developed by an MRI physicist and 6 dentists, including 3 endodontists and 1 Oral and Maxillofacial (OMF) radiologist. Twenty-nine human adult teeth previously extracted following clinical diagnosis by a board-certified endodontist of a root crack/fracture were frequency-matched to 29 non-cracked controls. Crack/fracture status confirmation was performed with magnified visual inspection, transillumination and vital staining. Samples were scanned with two 3D imaging modalities: 1) SWIFT MRI (10 teeth/scan) via a custom oral radiofrequency (RF) coil and a 90cm, 4-T magnet; 2) Limited FOV CBCT (1 tooth/scan) via a Carestream (CS) 9000 (Rochester, NY). Following a training period, a blinded 4-member panel (3 endodontists, 1 OMF radiologist) evaluated the images with a proportion randomly re-tested to establish intra-rater reliability. Overall observer agreement was measured using Cohen’s kappa and levels of agreement judged using the criteria of Landis and Koch. Sensitivity and specificity were computed with 95% confidence interval (CI); statistical significance was set at 0.05. Results: MRI-based crack/fracture criteria were defined as 1-2 sharply-delineated, high-signal (bright/white) line shape(s) that must be visible on multiple contiguous image slices. The line shape(s) must present as: single entities, or parallel pairs in close proximity, or pairs in close proximity exhibiting convergence or divergence extending from the external boundary of the tooth to the pulpal cavity. Intra-rater reliability for MRI was fair-to-almost perfect (κ=0.38-1.00) and for CBCT was moderate-to-almost perfect (κ=0.66-1.00). Inter-rater reliability for MRI was fair (κ=0.21; 95%CI:0.10-0.31; p< 0.001) and for CBCT was moderate (κ=0.45; 95%CI:0.34-0.56; p<0.001). Sensitivity: MRI=0.59 (95% CI:0.39-0.76; p=0.46); CBCT=0.59 (95% CI:0.59-0.76; p=0.46). Specificity: MRI=0.83 (95% CI:0.64-0.94; p<0.01); CBCT=0.90 (95% CI:0.73-0.98; p<0.01). Conclusions: Education and training for both imaging modalities is needed to improve reliabilities for the identification of tooth-root crack/fractures. Despite the advantages of increased contrast and absence of artifact from radio-dense materials in MRI, comparable measures of sensitivity and specificity (in relation to CBCT) suggest quality MRI improvements are needed, specifically in image acquisition and post-processing parameters. Given the early stage of technology development and multiple available pathways to optimize MR imaging of teeth, there may be a use for SWIFT MRI in detecting cracks and fractures in teeth.
Cavity Expansion and Fracture Propagation in Dunnville Sandstone
(2015-11-30) Sharpe, Jacob D
Fracture initiation of a pressurized circular cavity in Dunnville Sandstone was observed using a cavity expansion apparatus and particle tracking technique called digital image correlation. Different far-field stress configurations were used to determine its effect on the fracture initiation and propagation. The results of the tests show fracture initiation as a percent of the borehole breakdown (peak) pressure was determined to decrease as the far-field stress increased. In addition, the crack opening displacement at the borehole wall measured at peak pressure increased as the far-field stress increased.
Chemo-mechanical Modeling of Stress Corrosion Cracking of High Density Polyethylene in Bleach Solutions
(2020-10) Tripathi, Anu
High density polyethylene (HDPE) is increasingly being used in infrastructure applications with a design service lifetime of several decades. In many cases, the member is exposed to a corrosive environment, such as in pipes carrying potable water, where the dissolved bleach selectively attacks the loosely packed amorphous phase of the polymer. The failure mode of HDPE transitions from a ductile to a brittle mode as the corrosion level increases. This leads to subcritical crack propagation that deteriorates the load capacity and long-term behavior of HDPE structure exposed to a chlorinated environment. In this study, we develop a coupled chemo-mechanical model to simulate stress corrosion cracking (SCC) of HDPE members in a bleach solution. The mechanical response of the polymer is described by a constitutive model to considers the individual deformation and damage mechanisms of the amorphous and crystalline phases. The model accounts for the intermolecular deformation and homogeneous void growth in the crystalline and amorphous phases, along with entangled network resistance and craze damage in the amorphous phase. The embrittlement due to corrosion is captured by relating the amorphous phase parameters to the polymer molecular weight which decreases with corrosion level. The proposed model is calibrated using uniaxial tensile tests at different deformation rates, crystallinities, and corrosion levels. The model is used to simulate the double-edge notched (DEN) tension specimens at different corrosion levels. The constitutive model can capture the rate-dependent elasto-viscoplastic behavior of HDPE under the unexposed condition as well as the brittle failure behavior after exposure to a highly corrosive environment. The decrease in the molecular weight of HDPE due to exposure to bleach environment is captured by a reduced-order corrosion kinetics model. The selective diffusion and chemical reaction of bleach into the amorphous phase lead to polymer chain scission that reduces the molecular weight. The corrosion kinetics model describes this diffusion-chemical reaction of bleach and expresses the extent of chain scission as a function of the bleach concentration. The proposed material constitutive model and the diffusion-reaction model are combined in a single finite element (FE) code to investigate the SCC behavior of double edge notched HDPE specimens. The simulation yields the stress-life curves which qualitatively match the measured stress-life data of polymer pipes. The stress-life curve is shown to exhibit different regimes corresponding to distinct failure mechanisms, as indicated by the stress and strain distributions in the specimen. The simulations also provide the fracture kinetics under different environments, which can be used to predict the service life of an HDPE specimen with any geometry and applied load.
A coupled lattice and nite element model for fracture in composite concrete pavements
(2012-11) Tompkins, Derek Michael
Recent research in the United States has focused on the design, construction, and performance of composite concrete pavements - i.e., two heterogeneous concrete layers placed soon after the other using "wet-on-wet" techniques. While these pavements offer many benefits, they also introduce some uncertainties, including the possibility of thermally, hygrally, or mechanically induced fracture and separation at interface of the concrete layers. Despite over 40 years of experience in Europe that has yet to observe debonding in composite concrete pavements, debonding remains a commonly held concern among pavement engineers in the United States. To complement field evidence from Europe in addressing debonding concerns, this dissertation describes the development of a computational tool specifically designed for the simulation of a composite pavement under thermal, hygral, and mechanical loads. This simulation would be difficult using an exclusively continuum approach such as finite element methods in view of the heterogeneity of the pavement materials and the associated lack of smoothness in the crack propagation path. Given that the problem involves both heterogeneous media and the interface between the pavement layers, in this thesis the simulations are instead conducted using three-dimensional lattice modeling with emphasis on the potential for mixed-mode fracture at the interface. This discrete approach is coupled with a finite element model for plate behavior away from the potential cracking zone. The intricacies of that coupling are discussed and illustrated through numerical tests and examples.
Determinants of oral medication compliance in osteoporosis: the role of medication beliefs.
(2009-01) Schousboe, John T.
As medical technology has advanced and the age structure of the population has increased, much of the focus of medical therapy has shifted from treating acute symptomatic illness to prevention and management of chronic illness. Medications play a central role in the management of many chronic conditions (such as hypertension, hyperlipidemia, non-insulin dependent diabetes, and osteoporosis) that are risk factors for a variety of adverse disease outcomes. Non-compliance to medications has been shown to be a significant impediment to more comprehensive control of these conditions and avoidance of associated adverse health outcomes, and remains poorly understood. Beliefs regarding the potential benefits and harms associated with medications may be an important determinant of medication non-adherence. The relationship between these beliefs and adherence, adjusted for the perceived threat of the target condition and other variables that influence adherence, has not been explicated. The aims of this cross-sectional study of persons prescribed oral bisphosphonate medication to prevent osteoporotic fractures are as follows. Estimate the associations of perceived necessity of and concerns about medication and three aspects of medication use behavior Non-persistence due to side effects Non-persistence for reasons other than side effects Non-compliance (missed doses) Estimate associations of perceived need for medication Susceptibility to and severity of fractures Patient-provider relationship quality Objective indicators of fracture risk Estimate the associations of patient-provider relationship quality and self-reported adherence Directly Indirectly through other variables A conceptual framework is presented of medication persistence and compliance with fracture prevention medication. Four medication attitude variables, perceived necessity of fracture prevention medication, concerns about the long-term safety of and dependence upon medication, medication use self-efficacy, and perceived medication cost burden were postulated to be predictors of fracture prevention medication use behavior. These attitudinal variables were postulated to be mediating variables between fracture prevention medication use behavior and other predictors, such as perceived susceptibility to and severity of fractures, the patient-provider relationship quality, and objective indicators of fracture risk. Medication persistence and compliance was assessed by self-report, and medication beliefs by a mailed survey. Non-persistence was defined as stopping fracture prevention medication for more than one month, and non-compliance as missing one or more doses over the past 4 weeks. A multivariate path model, consisting of six regression equations, was used to estimate this model. Perceived need for fracture prevention medication was moderately strongly associated with non-persistence due to side effects and for other reasons, but not with non-compliance. Concern about medications was associated with non-persistence due to side effects and with non-compliance, but not with non-persistence for other reasons. Medication use self-efficacy was strongly associated with non-compliance and modestly with non-persistence for other reasons, but not with non-persistence due to side effects. The patient-provider relationship quality was modestly associated with non-persistence, but this effect was indirect through perceived need for fracture prevention medication, concern about medications, and medication use self-efficacy. Documentation of a prevalent vertebral fracture was associated with a higher perceived need for fracture prevention medication, and indirectly with mildly lower self-reported non-persistence. Providers can leverage the trust patients have in them to modestly improve persistence and compliance with fracture prevention medication by assessing whether or not the patient's perceived need for fracture prevention medication is congruent with their actual fracture risk and soliciting and addressing concerns about medications. Wider use of lateral spine imaging to identify clinically unrecognized vertebral fractures may also encourage persistence with fracture prevention medication among that subset of older men and women at highest risk of fracture, and for whom the benefits of fracture prevention medication have been extensively demonstrated.
Durability of high density polyethylene for potable hot water applications: crack propagation.
(2012-09) Singh, Gyanender P.
Polyethylene (PE) pipes, are used for water delivery, are susceptible to oxidation. As a result of oxidation PE becomes brittle and brittle pipes/tubes crack under the influence of tensile loads. These cracks initially propagate slowly and later on grow quickly becoming unstable. The focus of this study is slow crack growth in high density polyethylene (HDPE). Crack propagation experiments were conducted to determine the dependence of crack growth on degradation and stress levels. HDPE samples, with 0.3mm thickness, were exposed to 80°C chlorinated water (5-8 ppm) for up to 65 days. Thin samples were selected to ensure uniform degradation through the thickness. Although the brittleness of the polymer can be evaluated using strain-at-failure, the drawback of this method is that it destroys the sample. The Carbonyl Index (CI) obtained by Fourier Transform Infrared (FTIR) spectroscopy was established as a nondestructive measure of the degradation level. CI ranged from 35 to 93. A higher value of carbonyl index represents a greater extent of degradation. The relationship between CI and loss of mechanical performance was validated by strain-at-failure. Crack propagation tests were conducted were conducted on degraded polymer samples at constant load. The load (stress level) ranged from 5.1 to 9.2 MPa. In all 5 samples were tested. It was found that the crack propagation rate ranged from 6.31 x 10-10 to 1.26 x 10-2 m/s while the stress intensity factor ranged from 0.91 to 4.07 MPa√m. For a single degradation level, regardless of stress, the data when converted to log scale, and fit with the linear elastic fracture mechanics (LEFM) relationship = CKn. As the degradation increased the crack propagation rate increased such that all data were fit by the relationship = C(CI)Kn such that the exponential parameter ‘n’ was a constant for all the samples regardless of the level of degradation. The LEFM model fit to the data was best for moderate and high levels of degradation corresponding to CI of 55 and 90. Scanning Electron Microscopy (SEM) images show minimal deformation in the region around the crack tip, and ductile fibril stretching in the process zone. While the polymer had become brittle upon oxidation, there is local ductility in the process zone. An LEFM approach is typically applied to brittle materials, while the SEM results show that crack propagation is a combination of brittle and ductile behavior. Future studies should consider other modeling approaches that allow for ductile behavior in the process zone.
The Effect of a Screw Access Channel on the Fracture Resistance of Cement-Retained Metal- Ceramic Implant Crowns
(2018-05) McMillan, Dane
Purpose: To determine if the presence of a screw-access channel results in a statistically significant difference in fracture strength of molar and premolar cemented metal-ceramic implant crowns. Materials and Methods: Twenty abutment-crown specimens were fabricated and tested in this study. Ten metal-ceramic premolar crowns and 10 metal-ceramic molar crowns were fabricated on their respective custom abutments. Of the 10 specimens in both the premolar and molar groups, five were fabricated as the experimental group with a screw access channel while the other five had their occlusal surface intact and served as the control group. Polytetrafluoroethylene tape was inserted in the screw access channel of the custom abutment for all specimens and the crowns were cemented using resin-modified glass ionomer cement. Following cementation, the screw access channels of the 10 specimens in the two experimental groups were filled with composite resin. Specimens were individually mounted onto a custom fixture with an implant analog and tested on a servohydraulic testing machine at a crosshead speed of 0.5 mm/min at least 24 hours after cementation. Specimens were tested until failure with the highest force at the point of failure being recorded. Results: The mean axial load at failure for the molar crown without a screw access channel was 2032 N while the molar crown with an access channel was 1505 N. Comparatively, the mean axial load at failure for the premolar crown without a screw access channel was 1338 N while the premolar crown with an access channel was 964.5 N . Using a one-way ANOVA, the presence of a screw access channel in molar restorations led to a significant decrease in the axial load needed to fracture the restorations (P<.05); however, the screw access channel did not significantly affect the premolar restorations (P=.12). Molar restorations also required significantly more axial load to fracture relative to premolar restorations in specimens without a screw access channel (P<.01) and those with a screw access channel (P<.05). Conclusions: Metal-ceramic molar crowns, with or without a screw access channel, required a significantly higher axial load to fail than metal-ceramic premolar crowns. The presence of a screw access channel resulted in a significantly lower axial load force required for failure in the molar restorations; however, the difference was not statistically significant in the premolar restorations.
Effect of a screw-access channel on the fracture resistance of monolithic zirconia crowns
(2019-06) Loeb, Mitchell
Purpose This in-vitro study was designed from a clinical case and investigated the effect of the presence of a screw assess channel, created either during the milling phase or following cementation, on the fracture strength of a monolithic zirconia cement-retained implant-supported fixed prosthesis (ISFP). Material & Methods A definitive cast from a clinical case restoring a mandibular right first molar implant was utilized to fabricate three different styles of monolithic zirconia cement-retained ISFP. Group 1 had no screw-access channel (CR), Group 2 had a screw-access channel milled in the green phase (MA), and Group 3 had a screw-access channel created by hand preparation after cementation (HA). With 3 groups and 5 samples in each group, there were a total of 15 monolithic zirconia crowns fabricated on custom abutments in preparation for testing. The maximum force required for crown fracture was measured using a universal testing machine. Results The mean loads to fracture from highest to lowest were: the milled screw-access channel group (MA), followed by the hand-made screw-access channel group (HA), and lastly by the cement-retained group without a screw-access channel (CR). One-way ANOVA analysis indicated the fracture strength of the MA was statistically significantly different than the cement-retained samples (P<0.05). No statistically significant differences were found between the milled screw-access channel and the access channel created by hand. Conclusion The presence of a screw-access channel, whether milled or prepared by hand, does not negatively affect the fracture strength of a monolithic zirconia ISFP.
Explicit Crack Modeling based Approach for Structural Integrity Assessment of Brittle and Quasi-Brittle Structures
(2015-02) Singh, Gyanender
There is considerable variation in the fracture properties of brittle and quasi-brittle materials. Due to this large variation, probabilistic models are employed for estimating failure of brittle components/structures. However, due to limitations and shortcomings in the models, the predictions are not accurate. The shortcomings include: inability to handle stress concentrations, dependence of empirical constants on loading conditions, incorrect size-effect predictions and limited applications of the model. Although higher design margins can accommodate the inaccuracy in predictions, the cost of manufacturing increases. The work presented herein is directed towards addressing these issues. An approach based on explicit crack modeling (ECM) for accurately estimating failure in brittle/quasi-brittle components and structures is presented. Factors which govern fracture in a structure (fracture energy, strength of the material, damage behavior of the material, heterogeneity in the material microstructure) are incorporated in the ECM approach. The approach was validated by predicting the failure probability of L-shaped specimens at varying load levels followed by comparison of the predictions with published data. The study showed that the predictions from the ECM approach were not only in good agreement with the published data but were also more accurate than the Weibull model based predictions. The ECM approach can also predict size effect--the dependence of fracture properties and their statistical variation on the size of the specimen. This capability was demonstrated through failure prediction of specimens in tensile and flexural tests. Specimens of different sizes were considered and the predicted fracture properties were in good agreement with those obtained experimentally. The ECM approach for estimating failure of components/structures subjected to complex physical conditions was illustrated through the failure estimation of nuclear reactor graphite components. For modeling stresses in the graphite components subjected to high temperature and neutron irradiation, a constitutive model for evaluating the stresses was constructed and implemented through a user material (UMAT) subroutine in finite element software Abaqus. UMAT was integrated with Extended Finite Element (XFEM) technique for modeling irradiation-induced failure of the components under in-reactor conditions. Component lifetime as well as crack initiation and propagation details were predicted. This type of detailed failure information has the potential to improve design guidelines and standards of brittle components/structures.
Incorporation of Fatigue Detail Classification of Steel Bridges into the Minnesota Department of Transportation Database
(Minnesota Department of Transportation, Research Services Section, 2007-06) Lindberg, Adam; Schultz, Arturo
This report addresses the fatigue and fracture susceptibility of the Minnesota Department of Transportation (Mn/DOT) steel bridge inventory and provides a framework for enumerating fracture and fatigue sensitive details present in steel bridges. It also provides a method for rating the details in terms of their overall frequency and consequence of cracking. The research examines eighteen details identified as possible cracking locations, and a composite rank number is computed based on the details it possesses. The report includes a collection of case studies on cracking due to fatigue and fracture, as well as a timeline of changes to the American Association of State Highway and Transportation Officials Specifications for Highway Bridges and the Minnesota Standard Specifications for Highway Construction. Frequency of occurrence of cracking was defined on the basis of a national survey of state Departments of Transportation, from which the experiences regarding steel bridge cracking were collected from fifteen states. The results are organized in a comprehensive table correlating geometric constraints to rank numbers. The report concludes with a program that calculates a composite rank number for each bridge based upon distribution and rank of the individual details present in the bridge.
The Influence of Cutback Design on the Fracture Strength of Zirconia Crowns with Minimal Porcelain Veneering
(2022-12) Rainville, Louis
The purpose of this study is to compare designs of partially veneered 3Y-TZP zirconia crowns, quantify the fracture and ultimate compressive loads and define the failure patterns. A premolar was prepared with a 1mm reduction. The die was duplicated, 30 specimens were fabricated and divided in 3 groups (n=10): Group A – Control (PFZ), Group B – Experimental #1 (Veneer) and Group C – Experimental #2 (Window). Zirconia copings were fabricated with manually layered porcelain. The crowns were cemented onto the dies using resin cement and the specimens were tested under axial loading. The mean FL were 1538.40N (A), 2025.33N (B) and 2266.03N (C), and the mean UCL were 1554.42N (A), 2032.85N (B) and 2279.39N (C). 26/29 specimens underwent core fracture, and 3/29 specimens experienced adhesive veneer fracture. In conclusion, there were statistical differences between the control and the experimental groups, but not between the experimental groups.
Size effect on fatigue crack growth of a quasibrittle material
(2013-01) Manning, Jonathan
The Paris-Erdogan law describes the rate of fatigue crack growth as a function of the amplitude of the applied stress intensity factor. This equation, however, does not include a dependence of the crack growth on the structure size, which has been observed experimentally for concrete. The size effect on the fatigue crack growth is derived based on two hypotheses: (1) the scaling of the critical energy dissipation for fatigue crack growth has the same form as that of fracture energy for monotonic loading; (2) the difference in transitional sizes between the fatigue and monotonic loading is purely due to the difference in the fracture process zone (FPZ) size. The size-dependent fatigue crack growth law is verified experimentally through size effect tests on Berea sandstone. Using digital image correlation, it is shown that the FPZ length is approximately 7 mm and 11 mm for monotonic and cyclic loading, respectively. Optimal fitting resulted in transitional sizes of 34 mm and 54 mm for monotonic loading and cyclic loading, respectively, which shows a proportional relationship between the FPZ length and the transitional size.
Temperature and Rate Effects on the Mechanical Behavior of Tungsten and Its Nanocomposites
(2022-05) Schmalbach, Kevin
Methods to predict material failure frequently rely on large experimental datasets tuned to the properties of one material or are based on computationally expensive modeling. Both approaches are slow and do not adapt well to changes in material chemistry or processing. Development of analytical models with easily measured, physically meaningful parameters are key to alleviating bottlenecks in new materials development. One such model is presented in this work, which relies on easily measured physical parameters, effective stress and activation volume, referred to as activation parameters. This approach requires knowledge of both the temperature and strain rate effects on mechanical properties, which is the primary focus of this dissertation.Nanoindentation is an ideal way to measure these physical parameters due to its ability to measure properties at a length scale appropriate for grain- or phase-dependent deformation mechanisms and the small quantities of material required for testing (~mm3). Newer nanoindentation tests, such as strain rate jump tests, can quickly determine activation parameters, but lack standardized protocols for experiments and analysis. For this work, tungsten (W) was chosen as a model system in which to understand temperature and strain rate effects. This is in part due to its elastic isotropy, which simplifies data analysis, and its relatively experimentally accessible brittle-ductile transition temperature. In the first half of this dissertation, I will describe the use of nanoindentation strain rate jump tests to predict the fracture behavior of macroscale tungsten single crystals. I begin by presenting necessary protocol development for strain rate jump testing and analysis using a new, freely available software package. Included in the software package are a Python-based load function generator and a series of Matlab functions for data analysis. These tools were validated on single crystal tungsten, yielding a strain rate sensitivity nearly identical to that reported in the literature. The techniques were then applied at low temperature (-100 °C) and high temperature (50-300 °C) to measure activation parameters, effective stress and activation volume, as a function of temperature. The activation parameters, in combination with an analytical model for the strain energy release rate, accurately predict the brittle-ductile transition temperature along particular fracture systems in single crystal tungsten. Activation parameters measured from indentation of the (100) surface of single crystal tungsten accurately predict the brittle-ductile transition and fracture toughness along the {100}<011> fracture system of macroscale tungsten single crystals. Use of data from bulk tension of single crystal tungsten from the literature accurately predicts the fracture toughness in the {110}<110> fracture system of macroscale tungsten single crystals. In the second half of this dissertation, I will describe the synthesis and mechanical properties of 3-dimensionally ordered macroporous (3DOM) tungsten and a nanocomposite based on the porous framework. 3DOM tungsten was made with 35-40 nm wide ligaments, which exploit material size effect to have a ligament yield strength of 6.1 GPa at room temperature, approaching the ideal strength of tungsten. Considerable plasticity was observed above 125 °C, implying a brittle-ductile transition around this temperature, consistent with the predictions based on the earlier-presented model. Filling of the 3DOM tungsten framework with a silicon oxycarbide (SiOC) reinforcing phase resulted in a heterogeneous structure containing tungsten, silicon oxycarbide glass, and small domains of free carbon. The failure strength of 3DOM W-SiOC was 1.1 GPa, a factor of 22 greater than the 3DOM W framework. Although an increase in deformability was observed at 225 °C, pillars still failed by abrupt crack propagation. Fracture was only prevented at the next testing temperature of 425 °C. This places the brittle-ductile transition temperature of the W-SiOC composite significantly higher than that of pure tungsten. Additionally, the composite retains high strength even to 425 °C, achieving a yield strength of 400 MPa.
Toughening Thermosetting Resins with Modified Graphene Oxide
(2018-10) He, Siyao
In this thesis we studied the toughening effects of graphene derivatives, which have drawn much attention recently due to their high aspect ratios and outstanding mechanical properties. Graphene-based toughener can toughen resin at extremely low loading levels, which means it is economically viable for price-driven thermosetting resins market. To understand the toughening effect of graphene derivatives in resins, several GO surface modifications were developed to help disperse GO into the resins. The best performing modified GO (mGO) investigated in this work can be homogeneously dispersed into a resin with merely mechanic mixing. To simplify the materials handling and further improve the toughener dispersion, a styrene masterbatch route was developed to avoid the freeze-drying step in the mGO synthesis. The toughening effect of pristine and modified graphene oxide was tested in both unsaturated polyester and vinyl ester resins. The result indicated that GO and its derivatives can toughen UP and VE resins at a loading lower than 0.04 wt.%. Although, these tougheners are highly efficient in terms of required loading, we found that the toughness improvement obtained by adding mGO is insensitive to changes in particle-matrix interfacial strength and toughener loading. To understand this behavior, we studied the inorganic filler interference to mGO toughening, and also how the mGO toughening effect is affected by the physical dimensions of GO size and mGO aggregate size. Sophisticated data analysis involving computerized particle analysis were carried out to characterize the size differences between samples. The results show that the toughening effect of mGO is identical to that of other inorganic fillers, and this toughening effect is independent of filler mechanical properties. Finally, the toughening performance of mGO was tested in glass fiber reinforced composites, which is the target product for UP and VE resins. Both the interlaminar fracture toughness test and Izod impact test showed no improvement in composite toughness after adding mGO. A detailed fractography analysis of failed composite samples indicate that the failure happens between the resin and the glass fiber, which means increasing the fracture toughness of the resin matrix will not likely show any effect on the composite fracture toughness.

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