Continuous Fiber Reinforced Ceramic Matrix Composites (CMC) confront significant and undeniable elevated thermal gradient. In this study, a periodic RVE geometrical model was simplified based on the ...XCT scanning images of 2.5D woven CMC, and the model of the thermal structural strength of 2.5D woven CMC under the influence of fine-scale temperature fields was developed. In conjunction with the temperature distribution of the aero-engine turbine working state, ten sets of temperature field conditions were established and the model of thermal structural strength of 2.5D woven CMC under the influence of fine-scale temperature fields was developed. The effects of the overall temperature level, the form of the temperature difference and the size of the temperature difference on the tensile curve, the failure limits, the form of damage evolution, and the distribution of the failure unit at the time of failure of the 2.5D woven CMC were discussed. It was found that the temperature field induced by temperature difference in the thickness direction has a more significant effect on the tensile mechanical properties of the material, which is a key consideration for the structural design process using this material.
This paper reports the use of a fluorinating agent, diethylaminosulfur trifluoride (DAST), to effectively reduce graphene oxide (GO) thin films at a relatively low temperature (50°C). X-ray ...photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray diffraction, and sheet resistance measurement were used to investigate the reduction process. The C/O atomic ratio and sheet resistance of the GO films reduced by DAST were 9.24 and 398Ω/□ respectively, which are comparable to the reduced GO films of similar thickness (6.5μm) produced by hydrazine vapor. GO film reduction by DAST is advantageous in preserving the film intactness, owing to the mild reaction conditions involved. A small amount of fluorine (3–4 at.%) was incorporated into rGO by DAST reduction which was confirmed by XPS and FTIR, which might influence the electrical properties of rGO or render it suitable for further functionalization. Further, our findings suggest that special attention should be paid to the steric hindrance of the hydroxyl groups in GO while adapting organic chemistry methods for GO reduction and functionalization, which could alter reaction outcome significantly and could also be exploited for steric protection.
Graphene‐based materials are useful reinforcing agents to modify the mechanical properties of hydrogels. Here, an approach is presented to covalently incorporate graphene oxide (GO) into hydrogels ...via radical copolymerization to enhance the dispersion and conjugation of GO sheets within the hydrogels. GO is chemically modified to present surface‐grafted methacrylate groups (MeGO). In comparison to GO, higher concentrations of MeGO can be stably dispersed in a pre‐gel solution containing methacrylated gelatin (GelMA) without aggregation or significant increase in viscosity. In addition, the resulting MeGO‐GelMA hydrogels demonstrate a significant increase in fracture strength with increasing MeGO concentration. Interestingly, the rigidity of the hydrogels is not significantly affected by the covalently incorporated GO. Therefore, this approach can be used to enhance the structural integrity and resistance to fracture of the hydrogels without inadvertently affecting their rigidity, which is known to affect the behavior of encapsulated cells. The biocompatibility of MeGO‐GelMA hydrogels is confirmed by measuring the viability and proliferation of the encapsulated fibroblasts. Overall, this study highlights the advantage of covalently incorporating GO into a hydrogel system, and improves the quality of cell‐laden hydrogels.
Methacrylate is chemically grafted on the graphene oxide (GO) surface. Higher concentrations of the resulting methacrylated graphene oxide (MeGO) can be stably dispersed and conjugated within the hydrogels which improved fracture strength as compared with GO. In addition, cells maintain high viability within MeGO‐linked hydrogels. Therefore, covalent incorporation of GO induces proper interfacial bonding between GO and the polymeric network, and ultimately improves the quality of cell‐laden hydrogels.
This paper deals with the evolution of thermal residual stress (TRS) in Ceramic Matrix Composites (CMC) with environmental barrier coating (EBC) for aeroengine applications. In particular, in hot ...section component applications, where the durability requirement is of primary concern, CMC/EBC materials seem to be the most fascinating solution for effective and performant design. In this work, a numerical analysis has been conducted to study the effect of TRS on the damage mechanism in the 2.5D woven CMC coated with EBC. To figure out the heterogeneous and stochastic evolution of TRS in the coatings and substrate, a micromechanical finite element model of the composites containing the coatings was established based on the concept of representative volume elements. Especially, the variation of microstructure characteristics and constituents properties were taken into account in the micromechanical model. A series of simulations were performed to investigate the evolution of TRS during the cooling process after preparation and the thermo-mechanical loading process. Then, the thermal stress in coatings and substrate was calculated, and the influence mechanism of TRS on the failure was analyzed by comparison. The driven mechanism of the delamination crack and vertical crack was discussed based on the TRS results with uncertain characteristics. Based on this, uncertainty quantification can be performed to predict the durability life of the CMC/EBC system.
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 SiC
f
/SiC
m
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.
A constitutive model of SiC/SiC composites is developed which considers fiber failure and broken fibers’ load carrying capability. To obtain the in situ properties of SiC fibers, tensile tests are ...performed on the heat-treated fibers. A more universal fiber strength model is developed to describe the strength distribution of SiC fibers. In the constitutive model of SiC/SiC composites, the stress distribution of broken fibers is analyzed. To validate the in situ fiber strength distribution and the constitutive model, a tensile test is performed on SiC/SiC minicomposites. The predicted strength and stress-strain response of SiC/SiC minicomposites are in good agreement with the experimental results. The numerical calculations show that fiber failure nearly has no effect on the nonlinearity of SiC/SiC composites although the strength of composites will increase to infinity without fiber failure, and that the strength of the composites will decrease greatly if broken fibers do not carry the load.
The transverse constitutive relationship of warp yarns and weft yarns was established separately. A new analytical expression considering the coupling effect of fiber/matrix interface debonding ...damage was proposed to simulate the transverse elastic modulus of warp yarns. The validation using the finite element method shows that the analytical expression has good accuracy. The transverse cracking of single weft yarn and weft yarn attached to warp yarn was simulated with the finite element method, and the transverse constitutive relationship in these two cases was simulated. The transverse constitutive relationship of single weft yarn shows obvious brittleness, while the transverse constitutive relationship of weft yarn attached to warp yarn shows obvious toughness. The transverse constitutive relationship of warp yarns and weft yarns established in this paper can contribute to establish the three-dimensional constitutive relationship of yarns in the multi-scale modeling of the mechanical behavior of plain-woven ceramic matrix composites.
The size effect of the strength for SiC/SiC composites must be considered when measuring the strength of this material using a specimen. However, the size effect and its micro-mechanisms are still ...unclear at present. The size effect is studied using minicomposites through experimental investigation and theoretical analysis. The strengths of specimens with different lengths are measured and it is found that the size effect of the strength for SiC/SiC composites is not remarkable. To explain this phenomenon, the distribution of fiber flaws and the stress distribution of broken fibers are analyzed. It is found that owing to the fiber/matrix interfacial shear stress, broken fibers still have the load carrying capacity and then the effected region of each fiber flaw is limited. This mechanism decreases the sensitivity of SiC/SiC composites to the size effect. A fracture model of SiC/SiC composites is developed according to this mechanism. The predicted strength, stress–strain response and fracture surface match well with the experimental results.
The effect of the pyrolytic carbon (PyC) interface thickness on the heat-stability of Cansas-II SiC
f
/SiC composites under Ar up to 1 500 °C was studied in detail. After the heat treatment at 1 500 ...°C for 50 h, the interface bonding strength of the thin interface (about 200 nm) decreases from 74.4 to 20.1 MPa (73.0%), while that of the thick interface (about 2 µm) declines from 7.3 to 3.2 MPa (52.7%). At the same time, the decline fraction of strength of the composites with the thin interface is 12.1%, less than that with the thick interface (42.0%). The fiber strength also decreases after heat treatment, which may be due to the significant growth of
β
-SiC grains and critical defects. The different heat-stability of the interface with the thin and thick thickness might be related to the inconsistency of the degree of the graphitization of PyC. Compared with the composites with the thick interface, the composites with the thin interface remained higher tensile strength after heat treatment due to the better interface bonding strength. The interface with strong bonding strength could protect the fiber by postponing the decomposition of amorphous phases SiC
x
O
y
and hindering the generation of fiber defects.
The high entropy alloy coating is considered as one of the most promising methods to improve the high-temperature oxidation resistance of titanium aluminum alloys due to its fine mechanical property ...and thermal stability. However, the high entropy alloy coating prepared so far has poor coating quality, and low coating forming efficiency and there are certain requirements for the substrate alloys. To this end, a NiCoCrAl high entropy alloy coating was developed using the double glow plasma alloying technique on the TiAl alloy. The morphology, phase structure, and high temperature oxidation resistance of the coating were comprehensively studied. The results indicate that a uniform and dense coating with a single solid solution of face-centered cube phase formed on the alloy surface. Furthermore, isothermal oxidation test was performed, and the oxidation failure process of the TiAl substrate and the NiCoCrAl-coated sample with different oxidation times was analyzed. It is found that the coating efficiently prevented the TiAl alloy from degeneration for up to 100 h at 900 °C.
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