Bridge toughening enhancement in double-notched MoSi2/Nb model composites

Single-ply composites containing both laminate and continuous Nb fiber reinforcement coated with A1₂O₃ debond coatings in an MoSi₂ matrix are used as model systems for investigating bridge toughening concepts for various precrack configurations. When cracks are introduced symmetrically on either side of the ductile phase with zero crack offset spacing (S = 0), a minimum amount of energy is expended in plastic deformation and the local rupture process in the metal, as measured by the area of the force displacement curve in tension. For asymmetric precracks introduced on either side of the ductile reinforcement, as the offset spacing,S, was varied from 1 to 20R (R being the ductile phase half-thickness), the overall extension continuously increased within the bridging ligament. The effective ligament gage length was nearly equal to the crack spacing in the limiting case of a weak interface. However, the ductile Nb phase developed a Nb₅Si₃ reaction layer on its surface which was strongly bonded to the Nb and was found to undergo periodic cracking, leading to numerous shear bands within the ductile phase. This unique and previously unreported mode of metal deformation in shear loading has been analyzed using a simple geometric model. The results indicate that the profusion of shear bands is the primary source of toughening enhancement in the case of asymmetric crack geometry, which was not recognized in prior work of this type. This article is based on a presentation made at the “High Temperature Fracture Mechanisms in Advanced Materials” symposium as a part of the 1994 Fall meeting of TMS, October 2-6, 1994, in Rosemont, Illinois, under the auspices of the ASM/SMD Flow and Fracture Committee.     

Effective elastic response of two-phase composites

We present finite element analyses of the overall elastic properties of two-phase composites as a function of the shape, concentration and spatial distribution of the reinforcement. The analyses of the geometrical effects of constituent phases on the overall elastic moduli have been carried out within the context of axisymmetric and plane strain unit cell formulations. In most calculations, the phases are taken to be isotropic and linear elastic, and the interface perfectly bonded. These finite element results are compared with available analytical results for a variety of geometrical arrangements of the constituent phases of the composites. Computations which predict the effective elastic properties for the limiting case where all the reinforcing particles fracture are carried out. The competition between stiffening due to reinforcement and increased compliance due to cracking of the reinforcement is evaluated for metallic, ceramic and intermetallic matrices with brittle reinforcements, and the numerical results are compared with analytical solutions. The paper also includes discussions of the role of thermal residual stresses in influencing apparent initial moduli and the effects of interfacial compliance on the stress distribution within the reinforcement.     

Effective elastic moduli of fiber-matrix interphases in high-temperature composites

This article describes a theoretical model and an experimental method for determination of interphasial elastic moduli in high-temperature composites. The interphasial moduli are calculated from the ultrasonically measured composite modulivia inversion of multiphase micromechanical models. Explicit equations are obtained for determination of interphasial stiffnesses for an interphase model with spring boundary conditions and multiphase fiber. The results are compared with the exact multiphase representation. The method was applied to ceramic and intermetallic matrix composites reinforced with SiC SCS-6 fibers. In both composites, the fiber-matrix interphases include approximately 3-μm-thick carbon-rich coatings on the outer surface of the SiC shell. Although the same fiber is used in both composite systems, experimental results indicate that the effective interphasial moduli in these two composite systems are very different. The interphasial moduli in intermetallic matrix composites are much greater than those in ceramic matrix composites. After taking the interphase microstructure into account, we found that the interphasial moduli measured for the intermetallic matrix composites are very close to the estimated bulk moduli of the pyrolytic carbon with SiC particle inclusions. Our analysis shows that the lower effective interphasial moduli in the reaction-bonded Si₃N₄ (RBSN) ceramic matrix composites are due to imperfect contact between the interphasial carbon and the porous matrix and to thermal tension forces which slightly unclamp the interphase. Thus, measured interphase effective moduli give information on the quality of mechanical contact between fiber and matrix. Possible errors in the interphasial moduli determined are analyzed and the results show that these errors are below 10 pct. In addition, the use of the measured interphasial moduli for assessment of interphasial damage and interphase reactions is discussed.     

石墨烯/聚酰亚胺介电复合材料的制备与性能

采用密闭氧化法制备氧化石墨,经过超声分散剥离得到氧化石墨烯(graphene oxide,GO),然后以聚乙烯吡咯烷酮为保护剂、维生素C(Vc)为还原剂,在N,N-二甲基甲酰胺溶液中还原氧化石墨烯,得到还原状态的石墨烯(reduced graphene oxide,rGO),最后采用原位复合、流涎成膜方法制备石墨烯/聚酰亚胺(rGO/PI)复合薄膜。利用傅里叶变换红外光谱仪(Fourier transform infrared spectroscopy,FTIR)、扫描电镜(SEM)、透射电镜(TEM)、X射线光电子能谱仪(X-ray photoelectron spectrograph,XPS)、热重分析/差式扫描量热分析(TGA/DSC)等多种表征方法对rGO和rGO/PI复合薄膜的形貌、结构和热稳定性进行分析,用宽频介电谱仪测试复合薄膜的介电性能。结果表明,rGO是一种高效的导电填料,极少的添加量即可显著提高PI的介电常数,且对PI的其它性能影响较小。rGO/PI复合薄膜的介电常数随rGO填料含量的变化规律符合逾渗阈值模型,逾渗阈值f0为0.461%,临界指数q为0.523。当rGO含量(质量分数)为0.7%时,薄膜的介电常数为35.1,是纯PI的8.4倍。     

Effect of dielectric gaps on the conductance of resistance composites

Conclusions Mathematical expressions are given describing the contact resistance of the dielectric gaps in resistance composites and its temperature dependence. It is shown that thermally stable composites can be obtained by ensuring that the thermally induced changes in the resistance of their conducting particles are counter balanced by those of their gaps. Theoretical relationships between the CTE of the binder and conducting components are established which are necessary for the formulation of thermally compensated systems.Translated from Poroshkovaya Metallurgiya, No. 4(208), pp. 85–89, April, 1980.