The purpose of this work was to analyze the biomechanics of degeneration, damage, and failure in biological soft tissues both experimentally and computationally to provide insight into tendon or ligament tearing, tendo-achilles lengthening and lumbar spine dysfunction. For soft tissue tearing, experimental studies for calculating fracture toughness were performed and determined that tendons and ligaments are able to completely resist tear propagation. For tendo-achilles lengthening, a damage model was developed to mimic the behavior of the lengthening that occurs as a result of the percutaneous triple hemisection technique. The model provided insight for predicting the amount of lengthening that occurs during the procedure. For lumbar spine dysfunction, a finite element model was validated against experimental testing and simulated using boundary conditions representing physiological loading. The model was able to predict how biomechanical changes can lead to pain and how the prevalence of Schmorl's nodes can be predicted. For each of the situations, the best verification and validation methods were selected and are presented throughout the research to demonstrate the predictive capabilities and limitations of the work. Results of these studies are presented along with how those results influence the clinical endeavors associated with the degeneration, damage and failure of soft tissues.

BACKGROUND: Polymer-infiltrated zirconia ceramic, benefiting from the synergistic effect of the ceramic matrix providing strength and the polymer enhancing toughness, has the potential to mimic the structure of natural teeth in its optical and mechanical properties. OBJECTIVE: To determine the effect of additives and various sintering temperatures on the optical and mechanical properties of zirconia ceramic matrix composites. MATERIALS AND METHODS: Groups consisted of unmodified zirconia powder, and zirconia modified with porcelain and porogens to form the porous ceramic matrix. Three types of Tosoh zirconia powder, TZ-3YSB-E, Zpex, and Zpex Smile, were used to fabricate porous blocks. Zirconia powder and porcelain powder were ball-milled separately. Zirconia powder was dry pressed and then cold isostatic pressed. The blocks were sintered at 1000 and 1150 ºC and sectioned into discs (n=10). For zirconia with additives groups, 10% of Titankeramik and 5% of PEG8000 were mixed to zirconia powder using a high-speed mixer. The zirconia blocks were pressed and sintered at 1000, 1150, 1200 and 1300 ℃, and sectioned into discs (n=10). Porous discs were treated with a 10% wt solution of 10-MDP for 4 hours and then dried in a vacuum oven for 24 hours. TEGDMA-UDMA resin monomers were infiltrated into discs and cured at 90°C under pressure. Polymer-infiltrated ceramics specimens were polished to 1.5 mm in thickness. Optical properties were determined with an X-rite spectrophotometer. Biaxial flexural strength and Vickers indentation tests were performed using an Instron universal mechanical tester. Vickers hardness and indentation fracture toughness values were calculated by measuring the indent dimensions under FESEM, in addition to microstructure assessment. Statistical analyses were performed using computer software, Microsoft Excel 2016 and JMP Pro 15. RESULTS: This study revealed that the type of zirconia powder utilized for the fabrication of porous ceramics for polymer-infiltration structures did not significantly influence their optical properties. Mean values of fully sintered zirconia showed significantly higher biaxial flexural strength (628.5-1277.4 MPa) than polymer-infiltrated groups (105.4-433.6 MPa), with P-3Y1150 achieving the highest value. Higher pre-sintering temperature from 1000 ℃ to 1150 ℃ led to enhanced biaxial flexural strength for polymer-infiltrated pure zirconia specimens, with values rising from 126.5-158.2 MPa to 243.4-433.6 MPa. Adding porcelain and porogens did not significantly affect the optical or specific mechanical properties, such as biaxial flexural strength and Vickers hardness, despite increasing the sintering temperature to 1300 ℃. Nevertheless, a significant increase in indentation fracture toughness was noted with ZPTKPEG1200 (7.65±0.55 MPa·m1/2) and ZPTKPEG1300 (7.09±0.61 MPa·m1/2), values that were markedly higher than those in all control groups of fully sintered zirconia (p<0.001). Sintering temperature was found to be a key determinant in influencing the ceramic matrix's microstructure, porosity, and density, as well as the biaxial flexural strength, Vickers hardness, and indentation fracture toughness of polymer-infiltrated zirconia. While changes in temperature did not affect optical properties, and polymer infiltration did not enhance all attributes, it did substantially elevate the indentation fracture toughness in mixed zirconia samples with additives, offering a potential area for further research. CONCLUSION: The mechanical properties of polymer-infiltrated ceramics responded significantly to the sintering temperature and the type of zirconia powder utilized, most notably in the 3Y-TZSB-E group. A notable increased indentation fracture toughness was discernible when Zpex powder, mixed with additives, was subject to polymer infiltration and sintered at temperatures between 1200-1300 °C. Even though polymer infiltration and additive incorporation did not uniformly enhance all properties, a noticeable improvement in fracture toughness was observed. These findings open the door to future research, especially in potential applications of dental restorative materials that demand superior fracture toughness.

The discovery of carbon nanotubes in 1990s popularized a new area of research in materials science called Nanoscience. In the following decades, several carbon based nanoparticles were discovered or engineered and with the discovery of Graphene nanoplatelets (GNP) in 2010, carbon based nanoparticles were propelled as the most promising class of nanoparticles. High mechanical strength and stiffness, excellent electrical and thermal conductivity, and high strength to weight ratios are some of the unique abilities of CNTs and GNPs which allow their use in a wide array of applications from aerospace materials to electronic devices. In the current work presented herein, CNTs and GNPs are added to polymeric materials to create a nanocomposite material. The effects of this nanoparticle addition (a.k.a reinforcement) on the mechanical properties of the nanocomposite polymer materials are studied. Specifically, efforts are focused on studying fracture toughness, a material property that describes the material's ability to resist crack growth. Relative to the conventional metals used in structures, epoxy-based composites have poor fracture toughness. This has long been a weak link when using epoxy composites for structural applications and therefore several efforts are being made to improve their fracture toughness. In the first, second and third chapters, the enhancement of fracture toughness brought about by the addition of carbon nanotubes (CNTs) and graphene nanoplatelets (GNPs) was investigated. CNT-Epoxy and GNP-Epoxy Compact Tension (CT) samples were fabricated with 0.1% and 0.5% nanofiller weight concentrations. The potential synergistic effects of dual nanofiller reinforcements were also explored using CNT/GNP-Epoxy CT samples at a 1:3, 3:1 and 1:1 ratio of CNT:GNP. Displacement controlled CT tests were conducted according to ASTM D5045 test procedure and the critical stress intensity factor, $K_{IC}$, and the critical fracture energy, $G_{IC}$, were calculated for all the material systems. Significant enhancements relative to neat epoxy were observed in reinforced epoxies. Fracture surfaces were analyzed via scanning electron microscopy. Instances of CNT pullouts on the fracture surface were observed, indicating the occurrence of crack bridging. Furthermore, increased surface roughness, an indicator of crack deflection, was observed along with some crack bifurcations in the GNP-Epoxy samples. In the fourth chapter of Part I, the influence of pre-crack characteristics on the Mode-I fracture toughness of epoxy is investigated. Pre-crack characteristics such as pre-crack length, crack front shape, crack thickness and crack plane profile are evaluated and their influence on the peak load, fracture displacement, and the critical stress intensity factor, $K_{IC}$ is studied. A new method of razor blade tapping was used, which utilized a guillotine-style razor tapping device to initiate the pre-crack and through-thickness compression to arrest it. A new approach of quantitatively characterizing the crack front shape using a two-parameter function is introduced. Surface features present on the pre-crack surface are classified and their effects on the post crack initiation behavior of the sample are analyzed. This study aims to identify and increase the understanding of the various factors that cause variation in the fracture toughness data of polymeric materials, thus leading to more informed engineering design decisions and evaluations. Chapters six and seven of Part II investigate the SHM capabilities of dispersed MWCNTs in mock, inert, and active energetics. In these experimental investigations, the strain and damage sensing abilities of multi-walled carbon nanotube (MWCNT) networks embedded in the binder phase of polymer bonded energetics (PBEs) are evaluated. PBEs are a special class of particulate composite materials that consist of energetic crystals bound by a polymer matrix, wherein the polymer matrix serves to diminish the sensitivity of the energetic phase to accidental mechanical stimuli. The structural health monitoring (SHM) approach presented in this work exploits the piezoresistive properties of the distributed MWCNT networks. Major challenges faced during such implementation include the low binder concentrations of PBEs, presence of conductive/non-conductive particulate phases, high degree of heterogeneity in the PBE microstructure, and achieving the optimal MWCNT dispersion. In chapter seven, Ammonium Perchlorate (AP) crystals as the oxidizer, Aluminum grains as the metallic fuel, and Polydimethylsiloxane (PDMS) as the binder are used as the constituents for fabricating PBEs. To study the effect of each constituent on the MWCNT network's SHM abilities, various materials systems are comprehensively studied: MWCNT/PDMS (nBinder) materials are first evaluated to study the binder's electromechanical response, followed by AP/MWCNT/PDMS (inert nPBE) to assess the impact of AP addition, and finally, AP/AL/MWCNT/PDMS (active nPBE-AL) to evaluate the impact of adding conductive aluminum grains. Compression samples (ASTM D695) were fabricated and subjected to monotonic compression. Electrical resistance is recorded in conjunction with the mechanical test via an LCR meter. Gauge factors relating the change in normalized resistance to applied strain are calculated to quantify the electromechanical response. MWCNT dispersions, and mechanical failure modes are analyzed via scanning electron microscopy (SEM) imaging of the fracture surfaces. Correlations between the electrical behavior in response to the mechanical behavior are presented, and possible mechanisms that influence the electromechanical behavior are discussed. The results presented herein demonstrate the successful ability of MWCNT networks as structural health monitoring sensors capable of real-time strain and damage assessment of polymer bonded energetics. / Doctor of Philosophy / The discovery of carbon nanotubes in 1990s popularized a new area of research in materials science called Nanoscience. Carbon nanotubes (CNTs) are one of several forms of Carbon, meaning a differently structured carbon molecule in the same physical state similar to diamonds, graphite, and coal. In the following decades, several carbon based nanoparticles were discovered or engineered and with the discovery of Graphene (GNP) in 2010, carbon based nanoparticles were propelled as the most promising class of nanoparticles. High mechanical strength and stiffness, excellent electrical and thermal conductivity, and high strength to weight ratios are some of the unique abilities of CNTs and GNPs which allow their use in a wide array of applications from aerospace materials to electronic devices. In the current work presented herein, CNTs and GNPs are added to polymeric materials to create a nanocomposite material, where the term "composite" refers to a material prepared with two or more constituent materials. The effects of this nanoparticle addition (a.k.a reinforcement) on the mechanical properties of the nanocomposite polymer materials are studied. Specifically, efforts are focused on studying fracture toughness, a material property that describes the material's ability to resist crack growth. Fracture toughness is a critical material property often associated with material and structural failures, and as such it is very important for safe and reliable engineering design of structures, components, and materials. Moving from a single function (i.e. mechanical enhancement) to a more multi-functional role, taking advantage of the excellent electrical and mechanical abilities of CNTs, a structural health monitoring system is developed for use in polymer bonded energetics (eg. solid rocket propellants). When a material undergoes mechanical deformation or damage, the measured electrical properties of the material undergo some change as well. Using sensor networks built with multiple CNTs dispersed within a polymeric material, a whole structure can be made into an effective sensor where by simply monitoring the electrical properties, the extent of material deformation and damage can be known. Such a system is geared towards providing early warning of impending catastrophic material failures thus directly improving the safety during material handling and operations.

This dissertation describes the development and application of various mechanical characterization techniques to four types of polymer composite materials. The composite nature of these materials ranges from molecular to macro-scale, as do the size scales probed by the techniques chosen. The two main goals of this work are to evaluate the suitability of existing characterization methods to new composite materials (and augment the methods as needed), and to use these methods to determine optimal composite system parameters to maximize the desired mechanical response. Chapter 2 employs nondestructive ultrasonic spectroscopy for characterizing the stiffness response of both micron-scale woven composites and macro-scale glass-polymer-glass laminates. Both traditional wavespeed measurement as well as aspects of resonant ultrasonic spectroscopy are applied to determine material parameters. The laminates are also examined in Chapter 3, which utilizes both large-scale and small-scale quasi-static and dynamic puncture tests to elucidate the size- and rate-dependence of dynamic behavior. Because of limitations encountered with these methods, a smaller-scale, more fundamental test is developed and applied which focuses solely on the deformation and delamination of the polymer. These two processes, which account for the vast majority of energy absorbed during a puncture event, can be evaluated in terms of self-similar process zone propagation process models. Identifying and optimizing the relevant model parameters can promote the design of systems with maximum energy absorption. Exploratory work on nanocomposite systems is presented in Chapter 4. The polymer matrix from the laminated systems of the previous chapter is used to produce nanosilica composites. A range of techniques are employed to determine the level of dispersion and the mechanical reinforcement provided. The final project presented investigates copolycarbonates, or molecular composites, that have been developed to lessen the detrimental effect of aging on mechanical properties. Mechanical and thermal measurements can elucidate the effect of structure, specifically molecular mobility, on susceptibility to physical aging. The differences in molecular mobility contribute to differences in energy absorption by plastic deformation and damage, which is required for material toughness. Thus, understanding the molecular structure allows for determination of an optimal structure or copolymer concentration to maximize fracture toughness.

As a preliminary starting point for the present study, physical and mechanical properties of polypropylene nanocomposites (PPNCs) for samples received from Queen's University Belfast have been evaluated. Subsequently, polymer/clay nanocomposite material has been produced at Bradford. Mixing and processing routes have been explored, and mechanical properties for the different compounded samples have been studied. Clay intercalation structure has received particular attention to support the ultimate objective of optimising tensile and fracture behaviour of isotropic and die-drawn PPNCs. Solid-state molecular orientation has been introduced to PPNCs by the die-drawing process. Tensile stress-strain measurements with video-extensometry and tensile fracture of double edge-notched tensile specimens have been used to evaluate the Young¿s modulus at three different strain rates and the total work of fracture toughness at three different notch lengths. The polymer composite was analyzed by differential scanning calorimetry, thermogravimetric analysis, polarizing optical microscopy, wide angle x-ray diffraction, and transmission electron microscopy. 3% and 5% clay systems at various compatibilizer (PPMA) loadings were prepared by three different mixing routes for the isotropic sheets, produced by compression moulding, and tensile bars, produced by injection moulding process. Die-drawn oriented tensile bars were drawn to draw ratio of 2, 3 and 4. The results from the Queen's University Belfast samples showed a decrement in tensile strength at yield. This might be explained by poor bonding, which refers to poor dispersion. Voids that can be supported by intercalated PP/clay phases might be responsible for improvement of elongation at break. The use of PPMA and an intensive mixing regime with a two-step master batch process overcame the compatibility issue and achieved around 40% and 50% increase in modulus for 3% and 5% clay systems respectively. This improvement of the two systems was reduced after drawing to around 15% and 25% compared with drawn PP. The work of fracture is increased either by adding nanoclay or by drawing to low draw ratio, or both. At moderate and high draw ratios, PPNCs may undergo either an increase in the size of microvoids at low clay loading or coalescence of microvoids at high clay loading, eventually leading to an earlier failure than with neat PP. The adoption of PPMA loading using an appropriate mixing route and clay loading can create a balance between the PPMA stiffness effect and the degree of bonding between clay particles and isotropic or oriented polymer molecules. Spherulites size, d-spacing of silicate layers, and nanoparticles distribution of intercalated microtactoids with possible semi-exfoliated particles have been suggested to optimize the final PPNCs property. / SABIC

OBJECTIVES: This study examined mechanical, chemical and microstructural properties of multilayer zirconia materials that are composed of layers of different forms of zirconia with varying translucency. Their resistance to low temperature degradation and their properties were compared to each other, and to monolithic zirconia. METHODOLOGY: “ZirCAD Prime” from Ivoclar Vivadent, “AxZir XT Multilayer Dental Zirconia” from Axsys Dental Solutions, and “inCoris ZI” from Dentsply Sirona, were examined. Twenty specimens were created from each material, half of which were aged. Specimens were examined for the following: Three point bending flexural strength, grain size, microhardness, indentation fracture toughness, warp and elemental composition. RESULTS: inCoris ZI had significantly higher flexural strength than ZirCAD Prime, which in turn had significantly higher strength than AxZir XT. The flexural strength values were 1113.55MPa, 857.21MPa and 625.77MPa, respectively. Grain size patterns were noted in multilayer specimens; more translucent layers had significantly larger grain sizes. AxZir XT’s incisal most layer average grain size was 0.988μm, whereas ZirCAD Prime’s was 1.172μm. The dentin most layer of AxZir XT average grain size was 0.529μm whereas ZirCAD Prime’s was 0.470μm. Microhardness results showed few significant differences between layers. The highest microhardness was found in AxZir XT’s incisal most layer, after aging, with a value of 13.502 GPa. The lowest was found in the aged inCoris ZI specimen, with a value of 10.775 GPa. In the ZirCAD Prime, fracture toughness was highest in the dentin most layer with a value of 8.88 MPa m¹/², compared to its incisal most layer that had a value of 4.92 MPa m¹/². This pattern was not seen in AxZir XT, where the dentin most layer had a value of 8.36 MPa m¹/², and the incisal most layer had a value of 6.40 MPa m¹/². Hydrothermal aging had detrimental and significant impacts on fracture toughness of all materials. Elemental composition analysis revealed predictable levels of elements or molecules in ZirCAD Prime. and inCoris ZI, but not within the AxZir XT. 5Y levels were seen in ZirCAD Prime’s incisal layer, and 2.5-3Y in the dentin most layer. inCoris ZI had constant levels of 3Y, but AxZir XT had no distinct level of Yttria in its layers. CONCLUSIONS: 1. Flexural Strength of multilayer materials was significantly lower than monolithic zirconia. 2. Grain sizes appeared largest in translucent incisal-most layers, with significant differences between them and the opaque dentin-most layers. 3. The elemental composition analysis showed an expected level of 3 mol% Yttria in the inCoris ZI with varying amounts by layer in the ZirCAD Prime from 3Y (cervical) to 5Y (incisal), but there was no clear gradation in the AxZir XT. 4. Some significant differences were seen between the materials and their layers in the microhardness tests. inCoris ZI had significantly lower values than both ZirCAD Prime and AxZir XT. The highest values were found within AxZir Xt. 5. Fracture toughness was significantly higher in the dentin-most layer compared to the incisal most-layer of ZirCAD Prime but not in AxZir XT. 6. LTD significantly decreased some fracture toughness test values. inCoris ZI, AxZir XT’s 1st incisal layer and ZirCAD Prime’s 2nd transition layer had significant decreases in fracture toughness after aging. 7. LTD had no impact on flexural strength or microhardness values. 8. LTD significantly decreased grain size of inCoris ZI.

The mechanical and thermal properties of natural short latania fiber (SLF)-reinforced poly(propylene)/ethylene-propylene-diene-monomer (SLF/PP/EPDM) bio-composites reinforced with nano-clays (NCs), pistachio shell powders (PSPs), and/or date seed particles (DSPs) were studied using experiments and machine learning (ML) predictions. This dissertation embraces three related investigations: (1) an assessment of maleated polypropylene (MAPP) coupling agent on mechanical and thermal behavior of SLF/PP/EPDM composites, (2) heat deflection temperature (HDT) of bio-nano-composites using experiments and ML predictions, and (3) fracture toughness ML predictions of short fiber, nano- and micro-particle reinforced composites. The first project (Chapter 2) investigates the influence of MAPP on tensile, bending, Charpy impact and HDT of SLF/PP/EPDM composites containing various SLF contents. The second project (Chapter 3) introduces two new bio-powderditives (DSP and PSP) and characterizes the HDT of PP/EPDM composites using experiments and K-Nearest Neighbor Regressor (KNNR) ML predictions. The composites contain various contents of SLF (0, 5, 10, 20, and 30wt%), NCs (0, 1, 3, 5wt%), micro-sized PSPs (0, 1, 3, 5wt%) and micro-sized DSPs (0, 1, 3, 5wt%). The third project (Chapter 4) characterizes the fracture toughness of the same composite series used in the second project, by applying Charpy impact tests, finite element analysis, and a ML approach using the Decision Tree Regressor (DTR) and Adaptive Boosting Regressor (ABR). 2wt% MAPP addition enhanced the composite tensile/flexural moduli and strength up to 9% compared with the composites with zero MAPP. In addition, energy impact absorption was profoundly increased (up to78%) and HDT (up to 4 Co) was improved upon MAPP addition to the composites. SLF, NC, DSP and PSP could separately and conjointly increase HDT and fracture toughness values. The KNNR ML approach could accurately predict the composite’s HDT values and, Decision Tree Regressor (DTR) and Adaptive Boosting Regressor ML algorithms worked well with fracture toughness predictions. Pictures taken through a transmission electron microscope, scanning electron microscope and X-Ray proved the NC dispersion and exfoliation as one of the factors in HDT and fracture toughness improvements.

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Yes / Super-fine stainless wire (SSW) can not only form widely distributed enhancing, toughening and conductive network in reactive powder concrete (RPC) at low dosage level, but also improve weak interface area and refine cracks due to its micron scale diameter and large specific surface. In addition, the crack resistance zone generated by SSWs and RPC matrix together has potential to further enhance the fracture properties of composites. Therefore, fracture and self-sensing characteristics of SSW reinforced RPC composites were investigated in this paper. Experimental results indicated that adding 1.5 vol. % of SSW leads to 183.1% increase in the initial cracking load of RPC specimens under three-point bending load. Based on two parameter fracture model calculations, an increase of 203.4% in fracture toughness as well as an increase of 113.3% in crack tip opening displacement of the composites reinforced with 1.5% SSWs are achieved. According to double-K fracture model calculations, the initiation fracture toughness and unstable fracture toughness of the composites are enhanced by 185.2% and 179.2%, respectively. The increment for fracture energy of the composites reaches up to 1017.1% because of the emergence of blunt and tortuous cracks. The mixed mode Ⅰ-Ⅱ fracture toughness of the composites is increased by 177.1% under four-point shearing load. The initial angle of mixed mode Ⅰ-Ⅱ cracks of the composites decreases with the increase of SSW content. The initiation and propagation of cracks in the composites can be monitored by their change in electrical resistivity. The excellent fracture toughness of the composites is of great significance for the improvement of structure safety in serviceability limit states, and the self-sensing ability of the composites can also provide early warning for the degradation of structure safety. / National Key Research and Development Program of China (2018YFC0705601), the National Science Foundation of China (51578110), China Postdoctoral Science Fundation (2019M651116) and the Fundamental Research Funds for the Central Universities in China (DUT18GJ203).