Bulk Mechanical Properties Research Articles

Towards linking histological changes to liver viscoelasticity: a hybrid analytical-computational micromechanics approach

Motivated by elastography that utilizes tissue mechanical properties as biomarkers for liver disease, with the eventual objective of quantitatively linking histopathology and bulk mechanical properties, we develop a micromechanical modeling approach to capture the effects of fat and collagen deposition in the liver. Specifically, we utilize computational homogenization to convert the microstructural changes in hepatic lobule to the effective viscoelastic modulus of the liver tissue, i.e. predict the bulk material properties by analyzing the deformation of repeating unit cell. The lipid and collagen deposition is simulated with the help of ad hoc algorithms informed by histological observations. Collagen deposition is directly included in the computational model, while composite material theory is used to convert fat content to the microscopic mechanical properties, which in turn is included in the computational model. The results illustrate the model's ability to capture the effect of both fat and collagen deposition on the viscoelastic moduli and represents a step towards linking histopathological changes in the liver to its bulk mechanical properties, which can eventually provide insights for accurate diagnosis with elastography.

Enhanced Mechanical Properties in Bulk Nanograined Ni with High-Density Fivefold Twins.

Fivefold twins are extensively present in nanoparticles and nanowires, enhancing their performance in physical, chemical, and mechanical properties. However, a deep insight into the correlation between mechanical properties and fivefold twins in bulk nanograined materials is lacking due to synthesis difficulties. Here, a bulk fivefold-twinned nanograined Ni is synthesized via electrodeposition. The fivefold-twinned nanograins typically feature decahedral and icosahedral shapes similar to fivefold-twinned particles. The material exhibits a yield strength of ≈1.7GPa under both compression and tension. Tensile samples achieve an ultimate strength of 2.15GPa with 15% elongation to failure. The plastic deformation is accommodated by partial dislocation sliding on twin boundaries, splitting fivefold twins, and abnormally refining grains. The size dependence of nucleation stress for partial dislocation is responsible for strengthening and strain hardening. The results showcase the potential of incorporating fivefold twins into bulk nanocrystalline materials to tailor mechanical properties and applications across diverse fields.

Nanoliposome functionalized colloidal GelMA inks for 3D printing of scaffolds with multiscale porosity

Bioprinting has enabled the creation of intricate scaffolds that replicate the physical, chemical, and structural characteristics of natural tissues. Recently, hydrogels have been used to fabricate such scaffolds for several biomedical applications and tissue engineering. However, the small pore size of conventional hydrogels impedes cellular migration into and remodeling of scaffolds, diminishing their regenerative potential. Porous scaffolds have been utilized for their improved diffusion of nutrients, dissolved oxygen, and waste products. However, traditional methods of generating porous structures require multiple processing steps, making them incompatible with bioprinting. Recently, we developed a method to generate multi-scale porous structures by foaming hydrogel precursors prior to printing to form colloidal bioinks. Here, to further improve the biological, mechanical, and physical properties, we functionalize colloidal bioinks with nanoliposomes (NLs), one of the most promising methods for bioactive delivery. We assess the impact of the concentration of NL on the characteristics of bioinks made from gelatin methacryloyl (GelMA) and their resulting scaffolds. Anionic liposomes made from rapeseed lecithin of 110 nm were synthesized and found to be stable over several weeks. Increasing concentrations of NL decreased the zeta potential and increased the viscosity of foamed bioinks, improving their rheological properties for printing. Furthermore, the incorporation of NL allowed for precise adjustment of the macropore size and bulk mechanical properties without any chemical interaction or impact on photocrosslinking. The nanofunctionalized foam bioinks, composed exclusively of natural components, demonstrated significant antioxidant activity and were printed into multilayered scaffolds with high printability. The foam-embedded NL showed remarkable biocompatibility with myoblasts, and cell-laden bioinks were able to be successfully bioprinted. Due to their high biocompatibility, tunable mechanical properties, printability, and antioxidant behavior, the nanofunctionalized porous scaffolds have promise for a variety of biomedical applications, including those that require precise delivery of therapeutic substances and tissue engineering.

INVESTIGATIONS ON HARDFACING CHARACTERISTICS OF NI-BASED ALLOY ON ASS 304 USING PLASMA TRANSFERRED ARC WELDING AT DIFFERENT SPEEDS

AISI 304 stainless steel is one of the most commonly used austenitic steel in various industries such as petro-chemical, nuclear plant, food, bio-medical, due to its desirable bulk mechanical properties such as high toughness, corrosion resistance, etc. However, it lacks the surface properties such as wear resistance and hardness which is critically required in severe working conditions. Hardfacing is an effective method to increase the surface properties, without disturbing the bulk mechanical characteristics. Among other coating techniques such as MIG, FCAW, laser cladding and so on, Plasma transferred arc welding can be considered superior in terms of bond strength, controlled dilution and high deposition rate. Due to the formation of high hardness and wear resistance coating quality, single layered Ni-Cr-B-Si alloy is coated over AISI 304 using plasma transferred arc welding process in present work. Among various process parameters, travel speed directly affects the rate of heat input and rate of overlay deposition which determines the cooling rate which influences microstructure, coating thickness and dilution. In this work, effect of travel speed was observed on coating microstructure, microhardness, wear resistance and dilution. Slurry abrasive wear test was performed to evaluate the wear resistance of the coating, by measuring the weight loss at various travel speed. Wear resistance was observed to be improved by a factor of about 4 as compared to the substrate material which shows a decreasing trend as we increase the travel speed. The microhardness of the coating was observed to go as high as 500 HV for certain travel speed. Dilution was observed to be ranging between 18% to 32% which increases with travel speed because of low heat input at higher travel speed. The microstructure investigation shows formation of a dendritic structure of borides and carbides, which was concluded to be the reason behind the obtained high hardness. Formation of Ni- γ phase was also observed on the top layer of the coating microstructure.

Robust, Antifouling, and Hydrophilic Particle-Based Double-Network Hydrogel-PVDF Interpenetrating Microfiltration Membrane.

Poly(vinylidene fluoride) (PVDF) membranes with highly hydrophilic and antifouling properties are desirable for oily wastewater treatment. Herein, we report (1) a strategy of bulk modification of PVDF by in situ integration of PVDF and a particle-based double-network (PDN) hydrogel, poly-2-acrylamido-2-methylpropanesulfonate/polyacrylamide (PAMPS/PAAm), via a strong PDN and PVDF interpenetrating polymer network (PDN-PVDF IPN) to obtain a PVDF/PDN solution and (2) the subsequent casting of it into a microfiltration membrane via spray-assisted non-solvent-induced phase separation (SANIPS). The IPN structure modulates the surface segregation behavior of the highly hydrophilic and robust PDN hydrogel in the process of SANIPS, endowing the resulting PVDF/PDN membrane with excellent bulk mechanical properties and much enhanced wettability and thereby high oil/water emulsion separation efficiency and antifouling performance. Moreover, the PVDF/PDN membrane presented good chemical stability upon soaking in strongly acidic and alkaline solutions for an extensive time. Our work expands the research in phase-inversion-based antifouling oil/water separation materials.

Uncovering Reduction Mechanisms of Fluoroethylene Carbonate

Electric vehicles sales have increased exponentially in the United States, and the White House has announced a goal of 50% of all new vehicle sales to be electric by 2030. This growing electrification of the transportation sector to mitigate climate change requires safe, energy dense, and power dense batteries. Particularly, lithium metal batteries (LMBs) are a promising technology due to lithium’s high theoretical specific capacity and low reduction potential, which can result in batteries with energy densities surpassing current state of the art lithium-ion batteries.1 However, the highly reductive lithium metal anode reacts with the electrolyte during cycling, resulting in capacity loss. These reactions form a solid electrolyte interphase (SEI), and electrolyte design is a key strategy to improve the cycling stability and Coulombic efficiency of LMBs. Many successful electrolyte engineering strategies have involved tuning the SEI to be mechanically and chemically stable to prevent further reactions.In this work, we develop a novel approach to studying radical intermediates formed during electrolyte reduction, focusing on fluoroethylene carbonate (FEC), a highly effective electrolyte solvent in LMBs.2 Although prior research has illuminated key reduction products of FEC2,3, including lithium fluoride (LiF), there remains a gap in understanding regarding the mechanism by which FEC and other similar fluorinated solvents are reduced to form an SEI. Furthermore, recent research has demonstrated that a preformed SEI comprised solely of LiF breaks down during cycling, illustrating that bulk chemical and mechanical properties of individual SEI components may not translate to their function within the SEI.4 We employ electron paramagnetic resonance (EPR) spectroscopy to study radical reaction intermediates during FEC reduction. We reduce FEC in the presence of a spin trap to stabilize these radical intermediates for characterization. EPR with spin traps enabled us to identify two distinct radical species formed during FEC reduction, which we hypothesize are precursors to cross-linked polymer networks that are key to a high performing SEI. Radical species were identified with a combination of density functional theory (DFT) calculations and nuclear magnetic resonance (NMR) spectroscopy. Ultimately, this approach affords insight into molecular mechanisms to form an effective SEI which can both accommodate volume expansion and inhibit parasitic electrolyte consumption. This will guide electrolyte design for high Coulombic efficiency LMBs in the future. K. G. Gallagher et al., Energy Environ. Sci., 7, 1555–1563 (2014).X.-Q. Zhang, X.-B. Cheng, X. Chen, C. Yan, and Q. Zhang, Advanced Functional Materials, 27, 1605989 (2017).Y. Jin et al., J. Am. Chem. Soc., 139, 14992–15004 (2017).M. He, R. Guo, G. M. Hobold, H. Gao, and B. M. Gallant, Proceedings of the National Academy of Sciences, 117, 73–79 (2020).

Investigating Reduction Intermediates of Fluoroethylene Carbonate

Lithium-ion batteries are integral to the clean energy transition due to the important role of electric vehicles in lowering transportation sector emissions. As electrification continues to play a key role in mitigating climate change, there is a need for lithium-ion batteries with energy densities exceeding 500 Wh/kg. Among energy storage technologies, lithium metal batteries (LMBs) offer great potential due to lithium’s high theoretical specific capacity and low electrochemical potential, enabling batteries with energy densities surpassing current state of the art lithium-ion batteries.1 However, the highly reductive lithium metal anode reacts with the electrolyte during cycling, resulting in capacity loss and forming a solid electrolyte interphase (SEI). Electrolyte engineering has emerged as a successful strategy in tuning these reactions to form mechanically and chemically stable SEI, while preventing further parasitic reactions between the lithium metal anode and electrolyte.In this work, we investigate the reduction mechanism of fluoroethylene carbonate (FEC), a highly effective electrolyte solvent in LMBs.2 We probe the mechanism by which FEC and other similar fluorinated solvents are reduced to form an SEI in order to identify key reduction intermediates that underpin the high performance of these electrolytes and their interphases. In the case of FEC, LiF is commonly identified as a crucial SEI component that results in a stable SEI, but recent research has demonstrated that a preformed SEI comprised solely of LiF breaks down during cycling, illustrating that bulk chemical and mechanical properties of individual SEI components may not translate to their function within the SEI.3 In this work, we employ electron paramagnetic resonance (EPR) spectroscopy to study radical reaction intermediates formed during FEC reduction within an electrochemical cell. We identify these reaction intermediates by employing a spin trap, which stabilizes these intermediates and enables their characterization at room temperature. Using this approach, we have identified two distinct radical species formed during FEC reduction. We further analyze these species with a combination of density functional theory (DFT) calculations and nuclear magnetic resonance (NMR) spectroscopy. Ultimately, this framework will expand the tools available to the field of electrolyte engineering to design stable and effective interphases for lithium metal batteries, enabling the next generation of electrolyte design.References K. G. Gallagher et al., Energy Environ. Sci., 7, 1555–1563 (2014).X.-Q. Zhang, X.-B. Cheng, X. Chen, C. Yan, and Q. Zhang, Advanced Functional Materials, 27, 1605989 (2017).M. He, R. Guo, G. M. Hobold, H. Gao, and B. M. Gallant, Proceedings of the National Academy of Sciences, 117, 73–79 (2020).

Mechanical Analysis of Lithium-Ion Batteries via Acoustic Impedance Spectroscopy

Current state-of-charge (SOC) and state-of-health (SOH) estimation techniques for commercial Li-ion batteries (LIB) rely almost exclusively on voltage measurements and current integration, which produce large errors. Recently, acoustic techniques, primarily ultrasonic time-of-flight (US-ToF) have been used to reduce these errors, though cost, size, and voltage limitations prevent widespread use. Here, we introduce a practical alternative acoustic technique based on acoustic impedance spectroscopy (AIS). Using thin, cheap piezoelectric resonators, we can identify the acoustic resonances of commercial LIBs by measuring the complex electrical impedance of the resonator. In-operando AIS shows that these resonances are dependent on the bulk mechanical properties of the LIB, which change reversibly with SOC. Moreover, these mechanical changes induce unique responses in the resonance frequencies based on the modal structure of each resonance. Utilizing these changes, we can obtain a multi-dimensional trace of the SOC and SOH of a LIB. The application of AIS to LIBs represents a major step forward in practical acoustic characterization techniques.

Validation of assessment methods for creep crack growth rates in irradiated components from Nimonic PE16 Tie-Bar tests

Creep crack growth is a life-limiting failure mechanism in high-temperature metallic components. The creep crack growth response is linked to the underlying creep deformation and failure properties, such as creep ductility. Irradiation effects such as helium (He) embrittlement in high-temperature reactor components trigger decreases in creep ductility. Creep crack growth (CCG) data obtained from unirradiated and irradiated Nimonic PE16 tie-bars confirm the accelerated crack growth rates under neutron irradiated conditions. The exact same data were also used to validate the analytical defect assessment procedure for transient creep crack growth in Code Case N-934 of ASME BPVC Section XI Division 2, developed from unirradiated material data. To do so, the mechanics-based creep crack growth law of Nikbin-Smith-Webster (NSW) is adopted to estimate the crack growth rates in unirradiated PE16, and then adjusted through changes in bulk mechanical properties in the irradiated case to estimate the changes in crack growth rates. The estimated ȧ-C∗ trend was employed as part of the analytical defect assessment procedure for transient creep crack growth in Code Case N-934 of ASME BPVC Section XI Division 2. Evaluation of results via the procedure and the estimated trends confirm the validity of the approach to predict increases in creep crack growth in neutron irradiated components, as observed in experimental data. The approach yields moderately conservative predictions for crack growth rates, and accurately captures the relative increase in creep crack growth rates for the irradiated case. The predictions were independently validated by predicting the C*-values expected in the loading configuration for a given recorded crack growth rate ȧ. The validation of the defect assessment method introduced in Code Case N-934 against both unirradiated and irradiated PE16 CCG data provides a practical path to its implementation in metallic components across high-temperature reactor designs. This is particularly useful where crack growth data in irradiated material may not be readily available. The technique also allows for propagating uncertainties in crack growth predictions which stems from compounding effects such as variability in unirradiated properties, degree of irradiation and corresponding embrittling effects, crack size and load perturbations. The relevance of mechanical properties accurately depicting aspects of the He embrittlement process for practical applications in plant and the importance of creep crack growth testing are discussed.

The role of annealing and grain boundary controls on the mechanical properties of limestones and marbles

Chemical and mechanical processes are coupled in many geological and geochemical environments. Reactive processes anneal defects and restructure grain boundaries, modifying their elastic properties, levels of internal friction, wave propagation rates and fracture behaviors. The nature of these changes is, however, contingent on the initial state of the rock. In this study, impulse excitation was used to measure changes in mechanical properties as a function of dry and steam heating time at 300 °C for three carbonate rocks: Carrara marble, Carthage marble (Burlington Limestone), and Texas Cream limestone (Edwards limestone), with initial porosities of about 1 %, 3 %, and 27 %, respectively. Frequency-dependent phenomena along with mineral recrystallization were observed. This was coupled with small-angle X-ray and neutron scattering analysis to determine the relationship between changes in bulk mechanical and microstructural properties. An observed decrease in both the Young's and shear moduli in the experimental limestones as a function of heating time reflects and quantifies a reduction in the stiffness of the rock due to annealing. The internal friction of the samples first increases then decreases with reaction time, reflecting defect annealing, but this was balanced against grain boundary dissolution apparently driven by condensation of steam in the confined grain-boundary environment. This suggests an increase in the boiling point under confinement leading to dissolution, increased porosity and widening of the grain boundaries. In addition, comparison of small-angle X-ray and neutron scattering results suggests that it is inappropriate to assume that pores are empty for quantitative analysis of typical small-angle scattering samples.

On the influence of structural and chemical properties on the elastic modulus of woven bone under healing.

Woven bone, a heterogeneous and temporary tissue in bone regeneration, is remodeled by osteoblastic and osteoclastic activity and shaped by mechanical stress to restore healthy tissue properties. Characterizing this tissue at different length scales is crucial for developing micromechanical models that optimize mechanical parameters, thereby controlling regeneration and preventing non-unions. This study examines the temporal evolution of the mechanical properties of bone distraction callus using nanoindentation, ash analysis, micro-CT for trabecular microarchitecture, and Raman spectroscopy for mineral quality. It also establishes single- and two-parameter power laws based on experimental data to predict tissue-level and bulk mechanical properties. At the macro-scale, the tissue exhibited a considerable increase in bone fraction, controlled by the widening of trabeculae. The Raman mineral-to-matrix ratios increased to cortical levels during regeneration, but the local elastic modulus remained lower. During healing, the tissue underwent changes in ash fraction and in the percentages of Calcium and Phosphorus. Six statistically significant power laws were identified based on the ash fraction, bone fraction, and chemical and Raman parameters. The microarchitecture of woven bone plays a more significant role than its chemical composition in determining the apparent elastic modulus of the tissue. Raman parameters were demonstrated to provide more significant power laws correlations with the micro-scale elastic modulus than mineral content from ash analysis.

Densification mechanism and corrosion properties of h-BN/MgAl2O4 ceramics prepared by hot-pressed sintering

The dense h-BN/MgAl2O4 composites without additives were fabricated by hot-pressed sintering. The effect of sintering temperature and MgAl2O4 on the bulk density, mechanical properties and corrosion resistance were systematically investigated. The results showed that the collaboration of sintering temperature and MgAl2O4 dominated the densification behavior due to the enhanced pore filling effects and solid diffusion. The improved densification and induced microstructure development contributed to enhanced mechanical properties. Additionally, the residual MgAl2O4 is more conducive to forming plate-like complex oxide with molten steel after BN oxidation and B2O3 volatilization. Consequently, the plate-like complex oxide can effectively hinder direct contact between molten steel and the composite, which determines the corrosion resistance of the h-BN/MgAl2O4 composite. This study highlights the enormous potential of h-BN/MgAl2O4 composites for future use as side-sealing materials.

The effect of aging on the mechanical properties of bulk molding compound with different fiber lengths

The effect of aging on the mechanical properties of bulk molding compound with different fiber lengths

Plasma/Ozone Induced PolyNaSS Graft-Polymerization onto PEEK Biomaterial for Bio-integrated Orthopedic Implants

Owing to its superior bulk mechanical properties, poly (ether ether ketone) (PEEK) has gained popularity over the past 15 years as a metal substitute in biomedical implants. Low surface energy is a fundamental issue with PEEK implants. This low surface energy caused by a moderately hydrophobic surface may be able to inhibit cellular adherence and result in the development of an inflammatory response, which may result in cell necrosis and apoptosis. In this work, plasma and ozone treatments have been utilized to surface activate PEEK and graft ionic bioactive polymer polyNaSS (poly (sodium styrene sulfonate)) successfully on the surface to promote cellular attachment and biomineralization. The main goal of our research has been to find a stable green process for surface modification of PEEK by plasma/ozone approaches to increase PolyNaSS grafting efficiency and biomineralization. To further the field of bioactive orthopedic and dental implant technology, this research attempts to address a significant constraint of PEEK implants while preserving their favorable mechanical properties.

A study of fatigue property enhancement of 1045 steel processed by surface mechanical rolling treatment with an emphasis on residual stress influence

Several factors contribute to the observed bulk mechanical properties enhanced by surface mechanical rolling treatment (SMRT), and it is important to distinguish the influences of each major factor. A commonly used engineering carbon steel, 1045 steel, was chosen for the current study because the material does not have stress-induced phase transformation. A fully reversed strain-controlled gradual decreasing amplitude loading spectrum (ENV) was employed to release the residual stresses induced by SMRT. Fatigue experiments were conducted on the 1045 steel specimens with and without ENV to assess the influences of grain refinement and the residual stress on fatigue. It was found that SMRT refined and recombined the microstructures in the surface layer through deformation and fragmentation of brittle cementite, and extrusion of ductile ferrite between adjacent cementite layers. In the high cycle fatigue regime, both the SMRT induced residual stresses and the refined/recombined microstructures contributed to the enhancement of fatigue strength and fatigue ductility but the gradient microstructure contributed significantly more than do the residual stresses. Unlike stainless steels processed by SMRT, the carbon steel does not display a kink point in the strain-life fatigue curve, and all the crack initiations were found to occur on the specimen surface.

Uncovering Reduction Mechanisms of Fluoroethylene Carbonate

Energy storage plays an integral role in climate change mitigation, given that many promising alternatives to fossil fuel-based technologies rely on intermittent energy sources like the sun or wind. Particularly, lithium metal batteries (LMBs) are a promising technology due to lithium’s high theoretical specific capacity and low reduction potential, which can result in batteries with energy densities surpassing current state of the art lithium-ion batteries.1 Electrolyte design is a key strategy to improve the cycling stability and Coulombic efficiency of LMBs, where the highly reductive lithium metal anode reacts with the electrolyte during cycling, resulting in capacity loss. These reactions form a solid electrolyte interphase (SEI), and many successful electrolyte engineering strategies have involved tuning the SEI to be mechanically and chemically stable to prevent further reactions.In this research, we focus on fluoroethylene carbonate (FEC), a highly effective electrolyte solvent in LMBs.2 Although prior research has illuminated key reduction products of FEC2,3, including lithium fluoride (LiF), there remains a gap in understanding regarding the mechanism by which FEC and other similar fluorinated solvents are reduced to form an SEI. Furthermore, recent research has demonstrated that a preformed SEI comprised solely of LiF breaks down during cycling, illustrating that bulk chemical and mechanical properties of SEI components may not translate to their function within the SEI.4 In this work, we employ electron paramagnetic resonance (EPR) spectroscopy to study intermediate generation during FEC reduction. We pursued both a chemical reduction using lithium naphthalenide and an electrochemical approach to reduce FEC. The use of EPR with spin traps enables insight into the radical species formed during FEC reduction, which we hypothesize are precursors to cross-linked polymer networks that are key to a high performing SEI. Here, we employed conventional post-mortem SEI analysis like X-ray Photoelectron Spectroscopy (XPS) in addition to EPR to clarify reduction mechanisms of FEC and form a framework for studying electrolyte reduction intermediates. Ultimately, this approach affords insight into electrolyte breakdown mechanisms to form an effective SEI which can both accommodate volume expansion and inhibit parasitic electrolyte consumption. This will guide electrolyte design for high Coulombic efficiency LMBs in the future.References K. G. Gallagher et al., Energy Environ. Sci., 7, 1555–1563 (2014).X.-Q. Zhang, X.-B. Cheng, X. Chen, C. Yan, and Q. Zhang, Advanced Functional Materials, 27, 1605989 (2017).Y. Jin et al., J. Am. Chem. Soc., 139, 14992–15004 (2017).M. He, R. Guo, G. M. Hobold, H. Gao, and B. M. Gallant, Proceedings of the National Academy of Sciences, 117, 73–79 (2020).

Influence of Remote Laser Cutting on Magnetic Loss and Mechanical Properties in 0.2 mm Silicon Steel

Energy loss due to eddy current generation is a significant factor in reduced efficiency of electrical machines, therefore motor cores with thinner sheets are required due to their effect in reducing eddy currents. However, reducing the thickness of laminations is challenging due to the high tooling costs in blanking due to the small tolerances required, whilst other methods such as conventional laser cutting are too slow to be commercially viable. In this paper the influence of remote laser cutting on thin electrical steel sheets (Cogent/Tata Steel NO20 3.2% Si) has been investigated on the tensile strain to fracture, fatigue life, edge hardness, and energy loss in an AC magnetisation cycle for two laser power levels. The laser power has found to have no statistically significant influence on the bulk mechanical properties of the material, but significantly affects the measured specific loss. The latter was associated with the increased edge hardness at higher laser powers. Based on the parameters selected it is highly questionable whether RLC offers a viable method for lamination cutting, significant refinement of the laser parameters and/or the use an alternative laser such as an ultra-fast pulsed laser is required to be competitive with existing methods. Additionally, it is shown that energy loss increases linearly with respect to length of the cut edge - to the minimum cut spacing tested of 6 mm. These results can be used to optimise laser parameters to reduce the degradation of magnetic properties by decreasing laser power where possible.

Insights into molecular and bulk mechanical properties of glassy carbon through molecular dynamics simulations and mechanical tensile testing

With increasing interest in the use of glassy carbon (GC) for a broad range of application areas, the need for developing a fundamental understanding of its mechanical properties has come to the forefront. Furthermore, recent theoretical and modeling works that highlight the synthesis of GC via the pyrolysis of polymer precursors has explored the possibilities of a revisit to the investigation of their mechanical properties at a fundamental level. Although there are isolated reports on the experimental determination of its elastic modulus, insights into the stress-strain behavior of a GC material under tension and compression obtained through simulations, either at the molecular level or for the bulk materials, are missing. This study fills the gap at the molecular level and investigates the mechanical properties of GC using molecular dynamics (MD) simulations, which model the atomistic-level formation and breaking of bonds using bond-order-based reactive force field formulations. The molecular model considered in this simulation has a characteristic 3D cage-like structure of five-, six-, and seven-membered carbon rings and graphitic domains of a flat graphene-like structure. The GC molecular model was subjected to loading under varying strain rates (0.4, 0.6, 1.25, and 2.5 ns−1) and temperatures (300 K–800 K) in each of the three axes: x, y, and z. The simulations show that the GC nanostructure has distinct stress-strain curves under tension and compression. In tension, MD modeling predicted a mean elastic modulus of 5.71GPa for a single GC nanostructure with some dependency on the strain rate and temperature, whereas, in compression, the elastic modulus was also found to depend on the strain rate and temperature and was predicted to have a mean value of 35 GPa. To validate the simulation results and develop experimental insights into the bulk behavior, mechanical tests were conducted on dog-bone-shaped testing coupons that were subjected to uniaxial tension and loaded until failure. The GC test coupons demonstrated a bulk modulus of 17 ±2.69 GPa in tension, which compares well with those reported in the literature. However, comparing MD simulation outcomes to those of uniaxial mechanical testing reveals that the bulk modulus of GC in tension found experimentally is higher than the modulus of single GC nanostructures predicted by MD modeling, which inherently underestimates the bulk modulus. With regard to failure modes, the MD simulations predicted failure in tension accompanied by the breaking of carbon rings within the molecular structure. In contrast, the mechanical testing demonstrated that failure modes are dominated by brittle failure planes largely due to the amorphous structure of GC.

Influence of surface finishing by grinding on the cavitation erosion resistance of 316L and NiAl-bronze

Improving the mechanical properties of bulk and near-surface materials and alternating the dynamics of cavitating bubbles are two main methods to reduce cavitation erosion damage. Grinding and polishing, as two common surface finishing stages, can alter the cavitation erosion damage on metallic materials by affecting both the bubble dynamics and the mechanical properties of the surface. In this paper, the ultrasonic cavitation erosion resistance of 316L and NiAl-bronze alloys, each with two initial surface conditions of grinding and polishing is investigated, focussing on the material reaction. The mechanisms of material removal are discussed based on surface and subsurface observations to explain the effect of surface finishing on the cavitation erosion damage. After 8.5 h cavitation erosion testing time, a lower erosion mass loss was obtained for both alloys in the ground state as compared to the polished state suggesting that grinding can improve the cavitation erosion resistance of alloys. On the other hand, the polishing process improved the erosion resistance of NiAl-bronze at the early stages of cavitation. This is discussed in light of the two distinct surface characteristics of roughness and hardness, which, in turn, tend to increase and decrease cavitation erosion damage. Planar crystallographic boundaries and intermetallic-matrix interfaces were found to be the most susceptible sites to early erosion damage on the polished 316L and bronze, respectively.

Comparison of Mechanical Properties of Bulk NiAl-Re-Al2O3 Intermetallic Material Manufactured by Laser Powder Bed Fusion and Hot Pressing

AbstractThe low fracture toughness of NiAl at room temperature is one of the critical issues limiting its application in aircraft engines. It has been previously shown that a small addition of rhenium and alumina significantly improves the fracture toughness of hot-pressed NiAl. In this work, NiAl with an admixture of rhenium and alumina was produced by laser powder bed fusion additive technology (LPBF). The purpose was to compare the fracture toughness, bending strength, and microhardness of the NiAl-Re-Al2O3 material produced by LPBF and hot pressing (HP). Our results show that the LPBF material has lower fracture toughness and bending strength compared to its hot-pressed equivalent. Microcracks generated by thermal stresses during the LPBF process were the primary cause of this behavior. To improve the LPBF material, a post-processing by HP was applied. However, the fracture toughness of the (LPBF + HP) material remained at 50% of the KIC of the HP material. This study supports hot pressing as a suitable processing method for NiAl with rhenium and alumina additions. However, a hybrid approach combining LPBF and HP proved to be highly effective on the raw NiAl powder, resulting in superior fracture toughness of the final material compared to that consolidated by singular HP.