Thermal Residual Stresses in Particulate Composites and Its Toughening Effect

In this paper, an accurate formula for calculating the thermal residual stress field in a particle-reinforced composite are presented. Numerical examples are given to show r-variations of the thermal residual stresses. The increase in fracture toughness of matrix predicted by the thermal residual stress field is compared well with the experimentally measured increase.

Shear Strength Measurements in LY-12 Aluminium Alloy During Shock Loading

Lateral stress of LY-12 alummium alloy under plate impact shock loading was measured. Based on the measured data, the Hugoniot relation and shear strength were obtained. The result has demonstrated that the shear strenath of the tested material increases remarkably with the increasing longitudinal stress. This means that the assumption of constant shear strength usually adopted in shock stress calculation is not suitable for the present material.

Tensile properties of thermally exposed aluminium borate whisker reinforced 6061 aluminium alloy composite

The effect of thermal exposure on the tensile properties of aluminium borate whisker reinforced 6061 aluminium alloy composite was studied. The interfacial reaction was investigated by TEM and the mechanical properties were studied using tensile tests. The results indicated that the interfacial reaction had an influence on the mechanical properties of the composite, so that the maxima of Young’s modulus and ultimate tensile strength of the composite after exposure at 500?C for 10 h were obtained for the optimum degree of interfacial reaction. The yield strength,however, was not only affected by the interfacial state but also by many other factors.

Implication of scaling hierarchy associated with nonequilibrium: field and particulate

The advent of nanotechnology has necessitated a better understanding of how material microstructure changes at the atomic level would affect the macroscopic properties that control the performance. Such a challenge has uncovered many phenomena that were not previously understood and taken for granted. Among them are the basic foundation of dislocation theories which are now known to be inadequate. Simplifying assumptions invoked at the macroscale may not be applicable at the micro- and/or nanoscale. There are implications of scaling hierrachy associated with in-homegeneity and nonequilibrium. of physical systems. What is taken to be homogeneous and equilibrium at the macroscale may not be so when the physical size of the material is reduced to microns. These fundamental issues cannot be dispensed at will for the sake of convenience because they could alter the outcome of predictions. Even more unsatisfying is the lack of consistency in modeling physical systems. This could translate to the inability for identifying the relevant manufacturing parameters and rendering the end product unpractical because of high cost. Advanced composite and ceramic materials are cases in point. Discussed are potential pitfalls for applying models at both the atomic and continuum levels. No encouragement is made to unravel the truth of nature. Let it be partiuclates, a smooth continuum or a combination of both. The present trend of development in scaling tends to seek for different characteristic lengths of material microstructures with or without the influence of time effects. Much will be learned from atomistic simulation models to show how results could differ as boundary conditions and scales are changed. Quantum mechanics, continuum and cosmological models provide evidence that no general approach is in sight. Of immediate interest is perhaps the establishment of greater precision in terminology so as to better communicate results involving multiscale physical events.

Thermal mismatch stress in SiC whisker reinforced aluminium composite: new measurement method by X-ray diffraction

A new X-ray diffraction method for characterising thermal mismatch stress (TMS) in SiC_w–Al composite has been developed. The TMS and thermal mismatch strain (TMSN) in SiC whiskers are considered to be axis symmetrical, and can be calculated by measuring the lattice distortion of the whiskers. Not only the average TMS in whiskers and matrix can be obtained, but the TMS components along longitudinal and radial directions in the SiC whiskers can also be deduced. Experimental results indicate that the TMS in SiC whiskers is compressive, and tensile in the aluminium matrix. The TMS and TMSN components along the longitudinal direction in the SiC whiskers are greater than those along the radial direction for a SiC_w–Al composite quenched at 500°C.

Ab initio investigation of the elasticity and stability of aluminium

On the basis of the pseudopotential plane-wave (PP-PW) method in combination with the local density functional theory (LDFT), complete stress-strain curves for the uniaxial loading and uniaxial deformation along the [001] and [111] directions, and the biaxial proportional extension along [010] and [001] for aluminium are obtained. During the uniaxial loading, certain general behaviours of the energy versus the stretch and the load versus the stretch are confirmed; in each case, there exist three special unstressed structures: f.c.c., b.c.c., and f.c.t. for [001]; f.c.c., s.c., and b.c.c. for [111]. Using stability criteria, we find that all of these states are unstable, and always occur together with shear instability, except the natural f.c.c. structure. A Pain transformation from the stable f.c.c. structure to the stable b.c.c. configuration cannot be obtained by uniaxial compression along any equivalent [001] and [111] direction. The tensile strengths are similar for the two directions. For the higher energy barrier of the [111] direction, the compressive strength is greater than that for the [001] direction. With increase in the ratio of the biaxial proportional extension, the stress and tensile strength increase; however, the critical strain does not change significantly. Our results add to the existing ab initio database for use in fitting and testing interatomic potentials.

Modeling and Simulation of Liquid Pulsed Particulate Fluidized Beds

Pulsed fluidization is of considerable interest in process engineering for improving fluidization quality. Quantitative understanding of the pulsed two-phase flow behaviors is very important for proper design and optimum operation of such contactors. The

An incremental micromechanical scheme for nonlinear particulate composites

A general incremental micromechanical scheme for the nonlinear behavior of particulate composites is presented in this paper. The advantage of this scheme is that it can reflect partly the effects of the third invariant of the stress on the overall mechanical behavior of nonlinear composites. The difficulty involved is the determination of the effective compliance tensors of the anisotropic multiphase composites. This is completed by making use of the generalized self-consistent Mori-Tanaka method which was recently developed by Dai et al. (Polymer Composites 19(1998) 506-513; Acta Mechanica Solida 18 (1998) 199-208). Comparison with existing theoretical and numerical results demonstrates that the present incremental scheme is quite satisfactory. Based on this incremental scheme, the overall mechanical behavior of a hard-particle reinforced metal matrix composite with progressive particle debonding damage is investigated.

Particulate Size Effects in the Particle-Reinforced Metal-Matrix Composites

The influences of I,article size on the mechanical properties of the particulate metal matrix composite;are obviously displayed in the experimental observations. However, the phenomenon can not be predicted directly using the conventional elastic-plastic theory. It is because that no length scale parameters are involved in the conventional theory. In the present research, using the strain gradient plasticity theory, a systematic research of the particle size effect in the particulate metal matrix composite is carried out. The roles of many composite factors, such as: the particle size, the Young's modulus of the particle, the particle aspect ratio and volume fraction, as well as the plastic strain hardening exponent of the matrix material, are studied in detail. In order to obtain a general understanding for the composite behavior, two kinds of particle shapes, ellipsoid and cylinder, are considered to check the strength dependence of the smooth or non-smooth particle surface. Finally, the prediction results will be applied to the several experiments about the ceramic particle-reinforced metal-matrix composites. The material length scale parameter is predicted.

Failure of SiC particulate-reinforced metal matrix composites induced by laser thermal shock

Thermal failure of SiC particulate-reinforced 6061 aluminum alloy composites induced by both laser thermal shock and mechanical load has been investigated. The specimens with a single-edge notch were mechanically polished to 0.25 mm in thickness. The notched-tip region of the specimen is subjected to laser beam rapid heating. In the test, a pulsed Nd:glass laser beam is used with duration 1.0 ms or 250 mu s, intensity 15 or 70 kW/cm(2), and spot size 5.0 mm in diameter. Threshold intensity was tested and fracture behavior was studied. The crack-tip process zone development and the microcrack formation were macroscopically and microscopically observed. It was found that in these materials, the initial crack occurred in the notched-tip region, wherein the initial crack was induced by either void nucleation, growth, and subsequent coalescence of the matrix materials or separation of the SiC particulate-matrix interface. It was further found that the process of the crack propagation occurred by the fracture of the SiC particulates.

On the influence zone and the prediction of tensile strength of particulate polymeric composites

An intended numerical investigation is carried out. The results indicate that, even if a perfect adhesive bond is preserved between the particles and matrix materials, the two-phase element cell model is unable to predict the strength increment of the particulate polymeric composites (PPC). To explore the main reinforcing mechanism, additional microscopic experiment is performed. An ''influence zone'' was observed around each particle which is measured about 2 to 10 micrometers in thickness for a glass-polyethylene mixture. Then, an improved computational model is presented to include the ''influence zone'' effect and several mechanical behaviors of PPC are well simulated through this new model.

Micromechanics analysis of particulate-reinforced composites and their failure mechanisms

Stress fields and failure mechanisms have been investigated in composites with particles either surface treated or untreated under uniaxial tension. Previous experimental observation of failure mechanisms in a composite with untreated particles showed that tensile cracks occurred mostly at the polar region of the particle and grew into interfacial debonding. In a composite with surface-treated particles, however, shear yielding and shear cracking proceeded along the interphase-matrix interface at the polar area of the matrix and thus may improve the mechanical behaviour of the material. The finite element calculations showed that octahedral shear stress at the polar and longitudinal areas of the particle treated by coupling agents is much larger than that of materials with untreated particles, and the shear stress distribution around the interface is sensitive to the interphase property. The results suggest that a th ree-phase model can describe the composites with surface-treated fillers.

Molecular dynamics studies on the local disordering of σ3 and σ11 grain boundaries in aluminium bicrystal

Molecular dynamics (MD) simulations using Morse interaction potential are performed in studies of [110] symmetrical tilt grain boundary (GB) structures with mis-orientation angles 50.5 degrees(Sigma 11), 129.5 degrees(Sigma 11), 70.5 degrees(Sigma 3) and 109.5 degrees(Sigma 3) at various tempratures. The GB structures are found to start local disordering at about 0.5T(m)(T-m is the melting point of aluminium) for 50.5 degrees(Sigma 11), 0.32T(m) for 129.5 degrees(Sigma 11) and 0.38T(m) for 70.5 degrees(Sigma 3), respectively. These results agree with conclusions deduced from the anelastic measurements. But, for twin-boundary structure 109.5 degrees(Sigma 3), this disordering has not been found even when temperature increases up to 0.9T(m).

Interface cracking and particulate size effect in particle-reinforced metal-matrix composites

The mechanical behaviors of the ceramic particle-reinforced metal matrix composites are modeled based on the conventional theory of mechanism-based strain gradient plasticity presented by Huang et al. Two cases of interface features with and without the effects of interface cracking will be analyzed, respectively. Through comparing the result based on the interface cracking model with experimental result, the effectiveness of the present model can be evaluated. Simultaneously, the length parameters included in the strain gradient plasticity theory can be obtained.