Key Engineering Materials Vols. 353-358

Abstract: By incorporating the Taylor-based nonlocal theory of plasticity, the finite element method (FEM) is applied to investigate the effect of particle size on the deformation behavior of the metal matrix composites. In the simulation, the two-dimensional plane strain and random distribution multi-particles model are used. It is shown that, at a fixed particle volume fraction, there is a close relationship between the particle size and the deformation behavior of the composites. The yield strength and plastic work hardening rate of the composites increase with decreasing particle size. The predicted stress-strain behaviors of the composites are qualitative agreement with the experimental results.

Abstract: To improve the strength-toughness and heat-resistance of die cast magnesium alloy, the ZA85 alloy has been prepared by adding proper amount of spherical quasicrystals containing master alloy. The results indicate that the solidification microstructure is inlaid with quasicrystal particles, which is a new phase i(Mg45Zn47Y5Mn3), co-existing with the α-Mg, Φ(Al2Mg5Zn2) phase and τ(Mg32(AlZn)49) phase. Mechanical properties of ZA85 alloys have been dramatically improved owing to the obvious grain refinement of the matrix microstructure. While adding 1.7 wt % spherical quasicrystal containing master alloy, macro-hardness of ZA85 magnesium alloy reaches 75HB; meanwhile, impact toughness reaches the peak value 18.4Jcm-2, which is about 202% of ZA85 base alloy. Due to the pinning effect of quasicrystal particles on grain boundaries, the strengthened ZA85 alloys would have excellent heat-resistance.

Abstract: Curved panels can bear more lateral load than flat plates because they can transmit the external load along curved surface in addition to load-carrying capacity by the bending stiffness. For curved panels, however, there is a critical point of the lateral load that structure can endure before it buckles. On the other hand, composites are known to have more advantages in specific strength and stiffness than conventional metal materials. The present paper proposes a semi-analytical method to predict the initial buckling loads of slightly curved panels composed of thin orthotropic composite layers under general boundary conditions. Based on the Donnell type theory, the potential strain energy is evaluated as a sum of stretching energy, stretching-bending coupling energy and bending energy, and the external work done by uniform external pressure is included in the functional. The eigenvalue equation is derived by the Ritz method to yield such initial buckling load parameters as eigenvalues. Numerical examples include a list of buckling loads and the corresponding buckling patterns for typical panels with simply supported and clamped edges.

Abstract: SiCw/Al-18Si composites were prepared by squeeze casting technique. SiCw/Al-18Si composites were remelted before solidification. The effects of volume fraction of SiC whisker on solidification behavior of SiCw/Al-18Si composites were investigated by means of differential scanning calorimetry (DSC) technique and microstructure observation. DSC results indicated that the start solidification temperature and primary silicon peak temperature decreased gradually and the degree of supercooling increased with the increasing of SiC whisker content. SiC whisker and Sr decreased the average size of Si phases and improved the mechanical properties of the composites.

Abstract: Extrusion experiments have been carried out on Bi2O3-coated Al18B4O33 whisker reinforced aluminum matrix composite. The effects of the extrusion temperature on the microstructures, the formability and the mechanical properties of ABOw reinforced aluminum (ABOw/Al) composites with and without Bi2O3 coating were investigated. The results show that the steady-state extrusion load and the probability of whisker fracture of the extruded two composites decrease with the extrusion temperature increasing. The mechanical properties of the composites attain highest values at extrusion temperature of 673K.

Abstract: The effect of temperature on strain softening behavior of composites with small misaligned whiskers is investigated. The results show that the temperature affects the matrix and whisker mechanical behavior and corresponding composite deformation behavior. With increasing temperature, the whisker rotation angle increases, but their breakage decreases. Meanwhile elevating temperature not only reduces the matrix flow stress and work hardening rate, but also decreases load transfer from the matrix to the whiskers and stress induced by the whisker rotation and breakage.It is found that during hot compression, strain softening behavior of composites decreases as temperature increases.

Abstract: Thermal barrier coating (TBC) was carried out on Ni based super-alloy specimens, and edge indent tests were carried out at 1073K under different displacement rate. In addition, the specimens were heated and held at 1273K for long time, and the edge-indent tests of the specimens were performed both at 1273K and at room temperature. The results showed that the delamination load Pd and the delamination energy Ed measured at 1073K after short time holding were larger than those measured at room temperature, and the Pd and Ed slightly decreased with increasing displacement rate at 1073K while they slightly increased at 293K. The Pd and Ed at 1273K increased with increasing holding time and reached a maximum at 1400ks. With further increase in holding time, the Pd and Ed largely decreased. The Pd and Ed measured at 1273K were smaller than those at room temperature.

Abstract: The residual stresses induced in functionally graded medium (FGM) with inhomogeneity cooling down from the processing temperature are determined with concentric cylinder model and analytical solutions of the inhomogeneous governing equations for displacement components. The analytical solutions derived here are general for power-law variations of the elastic moduli of the FGM. With a power exponent, analytical expressions for the residual stresses of FGM with inhomogeneity can be obtained. By changing the power exponent and the coefficient of the power terms, the solutions obtained here could be applied to different properties of FGM with inhomogeneity. The results show that the huge difference exists between FGM with inhomogeneity and homogeneous medium with inhomogeneity. The variations of FGM and inhomogeneity size have a great deal of effect on the residual stresses in FGM.

Abstract: Electrical resistance change method has been applied to monitor a delamination crack of a thin CFRP laminate. For a thick CFRP laminate, multiple delamination cracks are made with many matrix cracks, and the electric current in the thick CFRP laminate may not flow in the thickness direction due to the strong orthotropic electrical conductivity. The present study employs an electric impedance change method for the identification of damage location and dimension of the damaged area; applicability of the method is investigated experimentally using thick beam-type specimens fabricated from cross-ply laminates of 36 plies. After making the damage, electrical impedance was decreased. A residual stress relief model was proposed to explain the decrease. From the measured electrical impedance changes, the relationships between the electrical impedance changes and damages are obtained by means of response surfaces. The response surfaces estimated the damage location and dimension of the damaged area exactly even for the thick CFRP laminates. The electrical impedance change method can be used as an appropriate sensor for measurement of residual stress relief due to damages of thick CFRP laminates.