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.     

Transverse elastic moduli of unidirectional fiber composites with interfacial debonding

A mathematical model is developed to predict the transverse elastic moduli of unidirectional fiber composites. Two cases of fiber/matrix interfacial bondings are investigated: perfect bonding and complete debonding,i.e., no transferring of tensile stress on surfaces where the fiber and matrix materials are separated. Fibers are assumed to exhibit transverse isotropy, and the elastic interactions among fibers are also considered. In the analysis of the completely debonded case, the cavity formation model is adopted and the original fiber and surrounding cavities are replaced by an imaginary anisotropic inclusion. Closed form solutions of effective elastic moduli are obtained for transverse tensile and compressive moduli, and transverse shear moduli. The limiting case of fiber-like voids is also studied. Numerical results are presented for two metal matrix composites, where the optimal wetting between the fiber and matrix is difficult to be obtained and hence the interfacial bonding and transverse property have been major concerns. Formerly Visiting Associate Professor at The University of Delaware     

A systematic investigation of elastic moduli of Wc-Co alloys

Measurements of Young’s modulus, shear modulus, compression modulus, and Poisson’s ratio on WC-(1 ~ 30) wt pct Co alloys were carried out by dynamic resonance method. The effects of volume fraction of the WC phase, the carbon content, and the WC particle-size on the elastic moduli were investigated in some detail. The result shows that the various elastic moduli of the alloys depend solely on volume fractions of the constituent phases and they invariably fall within Hashin and Shtrikman’s bounds. Moreover, the moduli are found to be approximated by Paul’s “strength of materials” type formulas. It is concluded that elastic behavior of the alloys can be essentially predicted on the basis of a simplified geometrical model of WC particles embedded in continuous cobalt matrix.     

The high-temperature elastic properties of palladium single crystals

A thin-line acoustic method has been utilized to measure the single-crystal elastic moduli of palladium in the temperature range 30–1200 K. The single-crystal constants behave, at high temperatures, in a classical manner in contrast to a large modulus defect observed in polycrystalline palladium. The latter effect has been interpreted as a result of grain boundary relaxation.     

不同弹性模量的基台-种植体组合对周围成骨性能的影响

利用有限元分析软件计算了不同静力作用下的多种基台-种植体周围骨组织的应力分布.模拟结果显示, 基台-种植体组合中Ti6Al4V钛合金-聚醚醚酮(TC4-PEEK)相对于其他实验组其应力集中程度现象可以有效降低, 周围骨组织的应力分布较为均匀, 最大应力值为40~60 MPa.在轴向加载条件下, 不同基台-种植体系统中PEEK种植体的应力水平较小, 而周围骨组织应力水平较大; 在斜向45°加载条件下, 相对于其他两种基台-种植体系统, TC4-PEEK的应力水平更低, 其周围骨组织中的皮质骨承受的最大应力值为55 MPa, 松质骨承受的最大应力值为5 MPa, 综合来看的应力水平最小, 有助于骨沉积和成骨量增加, 从而有效提高种植体的界面稳定性.      

不同弹性模量的基台--种植体组合对周围成骨性能的影响

利用有限元分析软件计算了不同静力作用下的多种基台-种植体周围骨组织的应力分布.模拟结果显示,基台-种植体组合中Ti6Al4V钛合金-聚醚醚酮(TC4-PEEK)相对于其他实验组其应力集中程度现象可以有效降低,周围骨组织的应力分布较为均匀,最大应力值为40～60 MPa.在轴向加载条件下,不同基台-种植体系统中PEEK种植体的应力水平较小,而周围骨组织应力水平较大;在斜向45°加载条件下,相对于其他两种基台-种植体系统,TC4-PEEK的应力水平更低,其周围骨组织中的皮质骨承受的最大应力值为55 MPa,松质骨承受的最大应力值为5 MPa,综合来看的应力水平最小,有助于骨沉积和成骨量增加,从而有效提高种植体的界面稳定性.     

Influence of interfacial reactions on the fiber-matrix interfacial shear strength in sapphire fiber-reinforced Nial(Yb) composites

The influence of microstructure of the fiber-matrix interface on the interfacial shear strength, measured using a fiber-pushout technique, has been examined in a sapphire-fiber-reinforced NiAl(Yb) matrix composite under the following conditions: (1) as-fabricated powder metallurgy (PM) composites, (2) PM composites after solid-state heat treatment (HT), and (3) PM com-posites after directional solidification (DS). The fiber-pushout stress-displacement behavior con-sisted of an initial “pseudoelastic” region, wherein the stress increased linearly with displacement, followed by an “inelastic” region, where the slope of the stress-displacement plot decreased until a maximum stress was reached, and the subsequent gradual stress decreased to a “fric-tional” stress. Energy-dispersive spectroscopy (EDS) and X-ray analyses showed that the inter-facial region in the PM NiAl(Yb) composites was comprised of Yb₂O₃,O-rich NiAl and some spinel oxide (Yb₃Al₅O₁₂), whereas the interfacial region in the HT and DS composites was comprised mainly of Yb₃Al₅O₁₂. A reaction mechanism has been proposed to explain the pres-ence of interfacial species observed in the sapphire-NiAl(Yb) composite. The extent of inter-facial chemical reactions and severity of fiber surface degradation increased progressively in this order: PM < HT < DS. Chemical interactions between the fiber and the NiAl(Yb) matrix resulted in chemical bonding and higher interfacial shear strength compared to sapphire-NiAl composites without Yb. Unlike the sapphire-NiAl system, the frictional shear stress in the sap-phire-NiAl(Yb) composites was strongly dependent on the processing conditions. Formerly Research Associate, Department of Chemical Engineering, Cleveland State University     

Determination of the elastic moduli of a directionally solidified nickel-based TaC eutectic

Measurements of ultrasonic wave velocities in a polycrystalline directionally solidified nickel based eutectic alloy are used to evaluate the three independent single crystal elastic moduli at temperatures between 298 to 925 K. The C_u and C₄₄ moduli are obtained directly from high frequency wave propagation along the D.S. axis, corresponding to <100>. Evaluation of C₁₂ requires measurements at lower frequencies to obtain (C₁₁ −E _<100>). The elastic anisotropy and temperature dependence of the elastic moduli are almost identical to those reported for pure Ni, indicating that neither TaC fiber reinforcement nor γ′ precipitate have strong effects on elastic properties of Ni based turbine blade alloys.