Deflection Mechanism Research Articles

Phase analysis, mechanical properties and in vitro bioactivity of graphene nanoplatelet-reinforced silicon nitride-calcium phosphate composites

ABSTRACT The aim of this study is to produce highly dense Si3N4 based composites with good mechanical properties and bioactivity. Si3N4 ceramics without using sintering aids, Si3N4-HA and Si3N4-HA-GNP based composites have been produced by spark plasma sintering (SPS) at temperatures of 1525–1550°C. The effect of reinforcement type and content on the densification behavior, phase analysis, microstructural development, mechanical properties, and in-vitro bioactivity behavior of Si3N4 were systematically investigated. Monolithic Si3N4 that contains a high amount of β-Si3N4 phase (~87 wt%) was produced by nearly full densification (~99%). Hydroxyapatite (HA) was used as a starting powder during the preparation of binary and triple composites to provide bioactivity to Si3N4, and after sintering, HA transformed into tricalcium phosphate (β-TCP and α-TCP) polymorphs. The incorporation of GNPs had a positive effect on the stability of β-TCP phases at higher sintering temperatures. The improvement in indentation fracture toughness of the samples with GNP reinforcement was mainly attributable to pull-out and crack deflection mechanisms. In-vitro bioactivity of GNP added composites enhanced with increasing α-TCP content. More calcium phosphate-based particle formation was observed in Si3N4-HA-GNP composites compared to the Si3N4-HA.

Fatigue crack deflection and branching behavior of low carbon steel under mechanically large grain condition

In situ SEM observation of fatigue crack growth behavior of low carbon steel under mechanically large grain condition, and EBSD analysis were carried out to investigate mechanisms of crack deflection and branching and effects of crystal orientation on them. It is found that the crack propagates along the activated slip bands in the present material and the crack deflection can be formed when a new activated slip band is created in a different direction from the growing crack. The crack branching can be formed by the following process: (1) the slip band along the crack cannot be activated further due to the restraint by such as grain boundary or strain hardening, (2) the crack cannot propagate further in the same direction, and (3) the secondary slip plane is activated to form a new slip band behind the crack tip. Not the stress intensity factors Kdef for deflected crack and ΔKeff for crack closure but rather the ΔKeff,tip is the intrinsic fracture mechanics parameter for controlling fatigue crack growth behavior of the material with crack deflection and branching.

Effect of the Introduction of Reactive Fillers and Metakaolin in Waste Clay-Based Materials for Geopolymerization Processes.

In this study, the role of two reactive fillers, specifically a sand from a clay washing process as an alternative to waste glass powder and a commercial metakaolin (MK), into the geopolymerization process of waste clay-based materials was assessed. Three kinds of clayey wastes from mining operations—halloysitic, kaolinitic and smectitic clays—were tested as potential precursor of geopolymeric materials in view of a potential valorisation of these by-products. A mix-design based on the addition of low percentages (20%) of these fillers or MK to improve the mechanical and chemico-physical properties of geopolymeric formulations was evaluated. All the clays were thermally treated at a temperature of 650 °C, while the geopolymeric pastes were cured at room temperature. In particular, the chemical stability in water (pH and ionic conductivity of leachate water, weight loss), the variations in the microstructure (XRD, SEM), and in the mechanical performance (compressive strength) were analysed. The most reactive additive was MK, followed by sand and waste glass at very similar levels—1:1 or 2:1—depending upon the type of the clay but not strictly related to the clay type. The increase of geopolymeric gel densification due to the presence of MK and sand was replaced by a crack deflection mechanism in the case of the WG grains. The worst performance (chemical stability and mechanical properties) was found for the halloysitic clay, while kaolinitic and smectitic clays developed strengths slightly below 30 MPa.

Effect of TM (TM=Fe, Mn, Cr) alloying on the high temperature properties and strengthening mechanism of Cf/Al composites

In this paper, the Cf/pure Al composite and the Cf/Al–Fe–Mg–Si composite were prepared by pressure infiltration method, and the microstructure and high temperature mechanical properties of these two kinds of composites were studied. The main interface phase of the Cf/pure Al composite is Al4C3, while in the Cf/Al–Fe–Mg–Si composite, a new type of Fe-rich phase containing Al, Fe, Mn, Cr, Si and other elements with a body-centered cubic structure calibrated by TEM diffraction pattens is discovered, whose lattice constant is 1.39 nm. Results of high temperature bending tests and researches of microstructure demonstrated that, the bending strength of the Cf/Al–Fe–Mg–Si composite decreased first at room temperature to 200 °C, due to re-dissolution of nano-precipitates. Afterwards, the bending strength increased under the combined action of the stress relaxation mechanism, the crack deflection mechanism and the high temperature strengthening mechanism of the Fe-rich phase at a temperature range in 200 °C–400 °C, and reached about 880 MPa at 400 °C, which demonstrated better high temperature mechanical properties than that of the Cf/pure Al composite. The results indicated that the formation of high-temperature stable precipitates through TM alloying can effectively increase the ultimate service temperature of Cf/Al composites. The work in this paper is of great significance in the design and preparation of Cf/Al composites with excellent high-temperature properties.

Microstructure study of fractured polar bear hair for toughening, strengthening, stiffening designs via energy dissipation and crack deflection mechanisms in materials

Crack propagation stopping inside the polar bear hair – a microstructure study to design the optimum toughening mechanism in materials.

Efficient multiscale strategy for toughening HfB2 ceramics: A heterogeneous ceramic–metal layered architecture

AbstractA multiscale structural design was innovatively adopted herein to increase the toughness of monolithic HfB2 ceramics. SiC whiskers (SiCw) and graphene oxide (GO) were used as fillers for the HfB2 matrix, whereas a ductile W foil was introduced as an interlayer to synthesize laminated HfB2‐SiCw‐rGO/W ceramics. Monolithic HfB2‐SiCp (particulate) and laminated HfB2‐SiCp/W ceramics were prepared using the same routes and used as controls. Following tape casting and spark plasma sintering at 1800°C, the toughness of the prepared laminated HfB2‐SiCw‐rGO/W samples was increased to 14.2 ± 0.6 MPa·m1/2, with minimal sacrifice in flexural strength (421 ± 16 MPa). Morphological analysis of the fracture surface revealed the synergistic effects of micro‐toughening (including bridging and pullout of whiskers and rGO) and macro‐toughening (including crack deflection, bifurcation, and delamination) mechanisms responsible for improving the fracture toughness of the laminated HfB2‐SiCw‐rGO/W composites.

The Number of Subgrain Boundaries in the Airfoils of Heat-Treated Single-Crystalline Turbine Blades.

In the present study, the dendrites deflection mechanism from the mold walls were subjected to verification regarding its heat-treated turbine rotor blades. The number of macroscopic low-angle boundaries created on the cross-section of the blades’ airfoil near the tip was experimentally determined and compared to the number of low-angle boundaries calculated from a model based on the dendrites deflection mechanism. Based on the Laue patterns and geometrical parameters of airfoils, the number of low-angle boundaries occurring at the upper part of the blades airfoil after heat treatment was calculated. This number for the analyzed group of blades ranged from 5 to 9.

Effects of HfC/PyC core-shell structure nanowires on the microstructure and mechanical properties of Hf1-xZrxC coating

To elevate the mechanical and anti-ablation properties of Hf1-xZrxC coating on C/C composites, HfC/PyC core-shell structure nanowires (HfCnw/PyC) with different PyC layer thickness were synthesized by two steps of CVD. Influences of HfCnw/PyC on the microstructure and mechanical properties of Hf1-xZrxC coating were researched. Toughening mechanism of HfCnw/PyC was also investigated. PyC layer exhibited a lamellar structure and combined well with HfCnw. After incorporating HfCnw/PyC, Hf1-xZrxC coating structure converted from columnar crystal to isometric crystal. HfCnw improved H, E, Kc and bonding strength of Hf1-xZrxC coating, which is ascribed to the nanowire pullout, debonding, bridging and crack deflection mechanism. With the PyC layer thickness increasing, H and E of the coating reduced, Kc and bonding strength of the coating increased. Because of the moderate bonding strength between HfCnw/PyC and coating matrix, lamellar structure of PyC layer and higher Kc of PyC, toughening effectiveness of the core-shell structures gradually enhanced with the PyC layer thickness increasing.

Fatigue analysis and fracture toughness of graphene reinforced carbon fibre polymer composites

AbstractThree‐point flexural fatigue behaviour and mode I interlaminar fracture toughness of the carbon fibre reinforced polymer composites (CFRPCs) modified by the addition of graphene nanoplatelet (GnP) were investigated. CFRPCs without any addition of nanoparticles were also tested as control samples. Fatigue life was characterized by the two parameter Weibull distribution function and predicted using the combined Weibull and Sigmoidal model. It was found that incorporation of very small amount (0.1%) of GnP in the epoxy polymer matrix improved mean and predicted fatigue life up to 155% and 190% (0.5 failure probability), respectively. The critical mode I interlaminar fracture toughness of the GnP reinforced composites was found up to 40% higher than the control samples. The improvement achieved by incorporating GnP is discussed in terms of crack deflection mechanism by nanoparticles and scanning electron micrograph images of the fracture specimens that revealed strong interfacial bonding in the nanoreinforced specimens.

Microstructural development and mechanical performance of CaSiO3–Ca3(PO4)2 bioceramics following the addition of CaSiO3–Ca3(PO4)2–MgCa(SiO3)2 eutectic glass

This study aims to investigate the effects of adding CaSiO3–Ca3(PO4)2–MgCa(SiO3)2 eutectic glass on the sinterability, microstructure, and mechanical properties of CaSiO3–Ca3(PO4)2 bioceramics. The CaSiO3–Ca3(PO4)2 (80/20 wt%) bioceramics with 30 vol% CaSiO3–Ca3(PO4)2–MgCa(SiO3)2 eutectic glass sintered at 1245 °C for 4 h showed a biaxial flexural strength and fracture toughness of 228 MPa and 1.7 MPa∙m1/2, respectively, which were considerably higher than those of pure CaSiO3–Ca3(PO4)2 bioceramics (165 MPa, 0.8 MPa∙m1/2) fabricated under the same conditions. In contrast, the elastic modulus of these new composites decreased to 93 GPa, which is similar to that of the human cortical bone. X-ray diffraction and scanning electron microscopy observation indicated that the main phase after sintering was β-CaSiO3 with an elongated morphology. These results indicate the applicability of CaSiO3–Ca3(PO4)2–MgCa(SiO3)2 eutectic glass as an “active agent” for the formation of rod-like β-CaSiO3 crystals, which improve the flexural strength and toughness by a crack deflection and bridging mechanism. The favourable mechanical properties and adjustable microstructure of these new bioactive and degradable bioceramics make them ideal candidates for bone tissue engineering applications.

Mechanical and dielectric properties of functionalized boron nitride nanosheets/silicon nitride composites

Uniformly dispersed boron nitride nanosheets (BNNSs) reinforced silicon nitride (Si3N4) composites were prepared by surface modification assisted flocculation combined with SPS sintering. In order to improve the dispersibility of the BNNSs in the composites, the liquid phase stripped BNNSs are surface functionalized by a two-step covalently modification. The amino-modified BNNSs (NH2-BNNSs) and Si3N4 powders have opposite surface potential, mixed evenly by electrostatic interaction during flocculation. The results showed that mechanical properties of Si3N4 composites were obviously enhanced by adding NH2-BNNSs. The fracture toughness and bending strength of Si3N4 composites added 0.75 wt% NH2-BNNSs were increased by 34% and 28%, respectively, compared with monolithic Si3N4. Toughening mechanisms are synergistic action of the torn, pull-out or bridging of BNNSs and crack deflection mechanisms with microstructural analyzes. The dielectric properties of the Si3N4 ceramics are also improved after the addition of NH2-BNNSs.

Energy-absorbing wood composite for improved damage tolerance inspired by mollusc shells

The crossed lamellar structure (CLS) found in mollusc shells is an excellent example for nature’s ability to form complex hierarchical microstructures with a remarkable balance between strength and toughness. The CLS has become the subject of numerous studies focusing on the replication of the unique microstructure using synthetic composites. The present study proposes a wood composite replicating the CLS’ middle layer microstructure and investigates the mechanical properties using three-point bending tests. The morphology of the failure mechanisms is recorded using digital microscopy and the experimental data are compared to those from ply- and solid woods. The results show a successful replication of the dominating failure mechanisms of crack deflection and crack bridging. While strength decreased significantly by ∼60%, toughness increased remarkable by ∼70% compared to plywood and was in the range of solid wood. The small data scattering from the wooden CLS samples compared to solid wood further hints on a stable failure mechanism and uniform energy-absorption. The results document that wood can be used to design an energy-absorbing composite based on the CLS-inspired ductile microstructure.

The influence of the angle between the secondary and primary cylinders on force coefficients and flow field characteristics at a low Reynolds number

Viscous flow around two unequal cylinders is simulated by solving the non-dimensional Navier–Stokes equations. A total of 13 two-dimensional numerical cases are carried out in this study for the flow fields around two adjacent cylinders at Re= 200. The ratio of d/D is equal to 0.25 (d and D are the diameters of the secondary and primary cylinders, respectively), the distance between two cylinders is kept as a constant (G = D), and the angle between them ranges from θ = 0° to θ = 180°. Firstly, six types of wake patterns are proposed and detailedly analyzed for the first time in this study. It is observed that the vortex shedding pattern is dependent on the position of the secondary cylinder. Secondly, this paper systematically analyzes the gap flow features between two cylinders for various angles, including the gap-flow strength, the deflection direction and the deflection mechanism. Additionally, this study also discusses in detail the variation characteristics of the drag coefficient, the lift coefficient, the Strouhal number and the time-averaged pressure. It is found that the drag and lift coefficients share a similar variation regularity, while the Strouhal number presents an opposite tendency with the change of the angle.

Mirror Angle Tuning of Electromagnetic Micro-Mirrors With Oscillation Compensation

Underdamped feature and hysteresis existing in electromagnetic scanning micro-mirrors (EMSMs) often lead to oscillation and undesired positioning error of deflection angle. In this paper, a model-based robust scheme is proposed for the angle tuning of mirror plate of the EMSM. To build a proper model for describing the behavior of EMSM, which is also convenient for the design of tuning strategy, a data-driven model composed of linear dynamic submodel as well as hysteresis submodel is developed. In the data-driven model, the deflection mechanism is described by a linear dynamic submodel while the hysteresis in EMSM is depicted by a nonsmooth hysteresis basis function submodel. Then, in order to compensate for the internal oscillation caused by both underdamping factor and hysteresis, a pseudo-feedforward oscillation compensator (PFOC) not relying on the inverse model is proposed. Then, the design procedure of the angle tuning strategy with the PFOC is presented. Afterward, the robust design of the angle tuning scheme is also studied to overcome the effect of compensation residual. Subsequently, the proposed scheme is applied to the angle tuning of EMSM.

4D synchrotron X-ray microtomography of fracture in nuclear graphite after neutron irradiation and radiolytic oxidation

Herein, the first study is presented using 4D synchrotron X-ray microtomography to capture all stages of crack development in neutron irradiated and radiolytically oxidised nuclear graphite. Employing a novel loading setup, specimens of Gilsocarbon graphite, both unirradiated and irradiated at 301 °C to 19.7 × 1020 neutrons/cm2 (∼2.6 displacements/atom (dpa)), were loaded to generate a crack. All stages of the fracture process were then captured using synchrotron X-ray imaging. Reconstructed tomographic images and 3D segmented crack volumes have been used to observe and analyse the irradiation-induced evolution of the graphite microstructure as well as corresponding changes in the crack initiation, propagation, and arrest behaviour of graphite after neutron irradiation. Close examination of the applied stress-strain curves highlights the suppression of micro-crack-based damage accumulation in irradiated graphite. Moreover, as well as the crack-bridging and deflection mechanisms characteristic of unirradiated graphite, crack arrest in the irradiated graphite is shown to be significantly influenced by crack tip blunting. This change is associated with the growth of the open pore structure of graphite, specifically the enlargement and increased frequency of macro-pores, resulting from the simultaneous radiolytic oxidation of the graphite microstructure during neutron irradiation.

A Modeling Method of Electromagnetic Micromirror in Random Noisy Environment

In this paper, a data-driven modeling method for electromagnetically actuated micromirror in random noisy environment is proposed. In this method, the electromagnetic micromirror is considered as a dynamic system with preceded hysteresis. Then, a linear dynamic submodel is used to describe the angular deflection mechanism, while a preceded rate-dependent hysteresis submodel is used to depict the hysteresis phenomenon inherent in the electromagnetic driver. By considering the influence of random noise on the micromirror, an on-line modeling scheme with varying weighting factors is studied to handle the data contaminated by random noises. Subsequently, the convergence of proposed modeling method is analyzed. Finally, the experimental results of the proposed modeling scheme for an electromagnetically actuated micromirror are presented.

Finite Element Analysis of the Crack Deflection in Fiber Reinforced Ceramic Matrix Composites with Multilayer Interphase Using Virtual Crack Closure Technique

Crack deflection along the interphase for fiber reinforced ceramic matrix composites (CMCs) is an important condition upon which the toughening mechanisms depend. The multilayer interphase is designed and developed to enhance this deflection mechanism. Combined with the virtual crack closure technique, a finite element model was proposed to predict the competition between crack deflection and penetration in multilayer interphase of CMCs. The model was used to analysis the propagation of primary matrix crack in a SiCf/SiCm composite with (PyC/SiC)n multilayer interphase. The effects of the number of sublayers, thicknesses of sublayers and thermal residual stress (TRS) on the energy release rate and the crack deflection mechanisms were studied. Results show that the multilayer interphase increases the ability to deflect the matrix crack at interfaces between sublayers. Moreover, the number of sublayers shows a larger effect than the thicknesses of the sublayers. The influence of TRS is much complex and needs to be evaluated accordingly. The research provides an analysis tool for promoting the toughening design of CMCs.

When Stigma Doesn’t Transfer: Stigma Deflection and Occupational Stratification in the Sharing Economy

AbstractResearch has suggested that when an occupation is stigmatized, new occupational members will assume the stigma of incumbents because stigma transfers. Yet, current research does not account for shifts in the modern workforce that are changing the nature of many stigmatized occupations. We argue that these changes raise questions about whether stigma will transfer to new occupational members. Drawing from a study of Uber’s entry into Toronto, Canada, we reveal the process by which stigma transfer can be avoided by new occupational members. We show how categorical ambiguity during entry enabled two sets of activities: creating categorical distinctiveness and showcasing identity discrepancies. These activities acted as mechanisms of stigma deflection by distancing Uber drivers from the taint associated with taxi drivers. However, this further entrenched the taint facing incumbents and stratified the occupation along a stigma faultline. We offer implications for research on stigma, market entry, and the sharing economy.

Prolonged yield platform in bioinspired three dimensional carbon materials derived from crack deflection

Brittle fracture is one of the most severe problems in carbon materials, which limits their applications in energy storage and conversion devices. Three dimensional (3D) porous carbon materials with honeycomb structure are extracted from the bio-waste, peeled pomelo material (PPM). The tensile test shows that both the ultimate tensile strength and elongation increases with increasing sintering temperature. Pronounced enhancement in the mechanical stability is revealed from the prolonged yield platform (maximum elongation, 9%), which is further rationalized in terms of the crack deflection mechanism identified from the corresponding fracture morphology. The enhancing mechanism can be utilized to design carbon materials with enhanced mechanical stability, and further improve the cycle life of carbon material applied in battery.

The effect of Cr2O3 additions on sinterability and mechanical properties of liquid-phase sintered SiC ceramics

In this study, the effect of using Cr2O3 with Al2O3 and Y2O3 additives on physical and mechanical properties of SiC-matrix composites was investigated. The samples were sintered at 1900 °C for 1.5 h in argon atmosphere. The phases, microstructure and chemical composition of the samples were studied using X-ray diffraction analysis (XRD), scanning electron microscopy (FESEM) and X-ray diffraction spectroscopy (EDX). The results showed that addition of Cr2O3 nanoparticles up to 5 wt % results in densification improvement of samples. Due to weak interface of SiC grains and grain boundary phase, crack propagation path is mainly grain boundaries. The highest density, elastic modulus, hardness and indentation fracture resistance were respectively 97.45%, 403 GPa, 28.19 GPa and 6.2 Mpa.m1/2, attributed to specimen containing 5 wt % Cr2O3. Microscopic studies showed that, formation of microcracks, crack deflection, crack bridging and crack branching mechanisms are the most important mechanisms in indentation fracture resistance improvement of composites.