A critical review of recent research on functionally graded plates

Strengthening mechanisms in particulate metal matrix composites

In this report we present the results from the second part of a study on the influence of fibre length and concentration on the properties of glass reinforced polypropylene laminates. The heat deflection temperature of these laminates is dependent on both fibre length and concentration. A maximum plateau level close to the polypropylene melting point was observed, longer fibres require a lower concentration to attain this plateau value. Elevated temperature stiffness retention was also enhanced by higher fibre concentration and longer fibres. The Cox—Krenchel equations gave a good prediction of the laminate stiffness over the temperature range —50 to 100°C. Both the in-plane and out-of-plane linear coefficients of thermal expansion were strongly dependent on fibre concentration but relatively insensitive to the fibre length. We obtained excellent correlation between experimental values of the in-plane linear coefficients of thermal expansion and theoretical predictions based on the shear lag theory. Out-of-plane linear coefficients of thermal expansion were found to be much larger than predicted by the equations used for continuous fibre reinforced composites. An approach based on the in-plane compression of the matrix due to the restriction of the matrix expansion by the reinforcing fibres was found to give good agreement with the experimental data. Good correlation of the experimental and predicted data was only obtained when the effect of voids was included in the calculations

The influence of fibre length and concentration on the properties of glass fibre reinforced polypropylene: 2. Thermal properties

The local microstructure can be altered significantly by various fibre surface modifications, causing property differences between the interphase region and the bulk matrix. By using tapping mode phase imaging and nanoindentation tests based on the atomic force microscope (AFM), a comparative study of the sized fibre surface topography and modulus as well as the local mechanical property variation in the interphase of E-glass fibre reinforced epoxy resin and E-glass fibre reinforced modified polypropylene (PPm) matrix composites was conducted. The phase imaging AFM was found a highly useful tool for probing the interphase with much detailed information. Nanoindentation experiments indicated the chemical interaction during processing caused by a gradient profile in the modulus across the interphase region of γ-aminopropyltriethoxy silane (γ-APS) and polyurethane (PU)-sized glass fibre reinforced epoxy composite. The interphase with γ-APS/PU sizing is much softer than the PPm matrix, while the interphase with the γ-APS/PP sizing is apparently harder than the matrix, in which the modulus was constant and independent of distance away from the fibre surface. The interphase thickness varied between less than 100 and ≈300 nm depending on the type of sizing and matrix materials. Based on a careful analysis of ‘boundary effect’, nanoindentation with sufficient small indentation force was found to enable measuring of actual interphase properties within 100 nm region close to the fibre surface. Special emphasis is placed on the effects of interphase modulus on mechanical properties and fracture behaviour. The interphase with higher modulus and transcrystalline microstructure provided simultaneous increase in the tensile strength and the impact toughness of the composites.

Characterisation of interphase nanoscale property variations in glass fibre reinforced polypropylene and epoxy resin composites

Several analyses for predicting the longitudinal, alpha(1), and transverse, alpha(2), coefficients of thermal expansion of unidirectional composites were compared with each other, and with experimental data on different graphite fiber reinforced resin, metal, and ceramic matrix composites. Analytical and numerical analyses that accurately accounted for Poisson restraining effects in the transverse direction were in consistently better agreement with experimental data for alpha(2), than the less rigorous analyses. All of the analyses predicted similar values of alpha(1), and were in good agreement with the experimental data. A sensitivity analysis was conducted to determine the relative influence of constituent properties on the predicted values of alpha(1), and alpha(2). As would be expected, the prediction of alpha(1) was most sensitive to longitudinal fiber properties and the prediction of alpha(2) was most sensitive to matrix properties.

Prediction of Coefficients of Thermal Expansion for Unidirectional Composites

Three-dimensional finite-element analyses are used to assess the accuracy of simplified composite micromechanics equations (SME) for hygral, thermal, and mechanical properties of unidirectional composites with orthotropic fibers. The properties predicted by the SME are in reasonably good agreement with those predicted by the three-dimensional finite-element analyses. This correlation demonstrates that the SME can be used with confidence in predicting the hygral, thermal, and mechanical behavior of unidirectional fiber composites.

CC Chamis

Papers

Assessment of Simplified Composite Micromechanics Using Three-Dimensional Finite-Element Analysis