Composites Engineering最新文献

The joining of aluminium/Nicalon composite has been investigated by diffusion bonding under pressure using metal interlayers. Bond strengths have been measured using a simple shear jig and the joint microstructures characterised by electron microscopy and electron-probe microanalysis. Using interlayers of copper and copper-silver alloy, joints were readily formed at 550 and 510°C, respectively, as a result of the formation of eutectic liquid which helped to disrupt the oxide film on the aluminium matrix and promote metal diffusion across the joint interface. In the case of copper, however, oxidation at the edge of the interlayer limited the bonded area to ∼80%. Using an interlayer of aluminium alloy (2124), a temperature of 500°C was sufficient to produce a satisfactory joint with ∼100% bonded area, in this case the aluminium oxide film being disrupted by reaction with magnesium in the alloy interlayer. The shear strength of all bonded specimens was ∼50 MPa, with failure occurring mainly through the adjacent composite rather than at the joint interface.

The dynamic characteristics of NITINOL-reinforced composite plates are controlled by heating sets of NITINOL fibers embedded inside these plates. The activation of the shape memory effect of these NITINOL fibers increases the elastic energy, enchances the stiffness of the composite plates and modifies their modal characteristics. One of the objectives of the resulting modal modification is to shift the modes of vibration of the plates away from the excitation frequencies in order to avoid undesirable resonances. In this way, the modal characteristics can be tailored in response to the external disturbances acting on the plates. The classical finite element approach is used to form the equations of motion of the assembly of NITINOL-reinforced plate elements and the appropriate boundary conditions are then applied. The solution of the eigenvalues of the resulting homogeneous equations gives the natural frequencies of the NITINOLreinforced plate as influenced by the properties of the composite matrix and the NITINOL fibers. It is important to note that these properties are influenced by the temperature distribution inside the composite plate which is developed by virtue of activating and de-activating the NITINOL fibers. Emphasis is placed on the effect of intentional electrical heating of a selected subset of the NITINOL fibers on the overall dynamics of the plates. The effect of the associated thermal energy propagating through the composite on the unintentional thermal activation of additional subsets of the NITINOL fibers is accounted for. Such an effect is not only significant, but also essential to the thorough understanding of the operation of the NITINOL-reinforced plates.

Unidirectionally reinforced [90]₈ specimens of an SCS-6/Ti-24A1-11Nb (at.%) composite (35 vol.% fiber) in three different gage-section widths were thermally cycled in air between 150 and 815°C for 500 cycles. During thermal cycling, matrix cracks initiated at the composite surface and propagated into the composite normal to the fiber direction. However, near the stress-free edges of all specimens, a region void of cracks existed which extended an average of 5.4 fiber diameters into the width of the composite. This cracking pattern was attributed to the presence of a thermally induced cyclic tensile residual stress which dissipates to zero near the stress-free edge of the composite. The finite element method was employed to determine how the fiber-matrix interface shear resistance influences the development of these residual stresses. Using coulomb friction as a measure of shear resistance, the matrix residual stresses in the fiber direction had a peak value of 500 MPa. A frictional coefficient range of 0.18–0.22 was found to give between 95% and 99% of this peak value within 5.4 fiber diameters from the edge. Thermal cycling of the model between 150 and 815°C provided evidence that the resultant cyclic stresses were tensile in nature and were suggested as the probable cause of the periodic surface cracks. The reduction in post-cycling transverse strength with increasing gage-section width indicated that the smaller-width specimens exhibited less damage per cross-sectional area than the wider specimens.

Two- and three-dimensional linearly elastic glass/epoxy and carbon/epoxy laminates of the type [(O_m/90_n/+ θ_p/-θ_q)_s]_M containing periodically distributed matrix cracks have been analysed by aid of the finite element method. The presented finite element model enables modelling of several important thick and thin ply stacking sequences like cross-plies, angle plies and quasi-isotropic laminates. Due to periodicity it suffices to model a representative volume element of the laminate. The boundaries of this unit cell represent prospective crack surfaces. In this way varying crack configurations and crack densities could be simulated. By application of periodic boundary conditions the stiffness tensors for laminates containing different crack configurations were calculated. The results are presented in the form of reduced engineering stiffness parameters as functions of matrix crack densities for a thick quasi-isotropic [(0°/90°/+45°/−45°)_s]_M glass/epoxy laminate, a thick [(0°/90°/+55°/−55°)_s]_M carbon/epoxy laminate and a thin (0°/+45°/−45°)_s glass/epoxy laminate. Comparisons are made to an approximate analytic model developed previously. An excellent agreement between the analytic predictions and the finite element results was found for all cases under consideration.

The thermal expansion response of an ALTEX-aluminium continuous fiber reinforced metal matrix composite produced by gas pressure infiltration was measured experimentally by dilatometry. Theoretical predictions for the composite's thermal expansion behavior were made with simple analytical estimates and with a unit cell model employing the Finite Element method. By correlating experimental and computational results, improved data on the fibers' coefficient of thermal expansion were obtained.

Two silicate matrix composites, Pyrex/Nicalon and BaO-MgO-Al₂O₃-SiO₂ (BMAS)/ Tyranno, have been used to study composite stability with respect to time at temperature, and under applied stress. Samples aged in an oxidizing atmosphere have been tested in flexure at room temperature, and also by fibre “push-down” to investigate the interfacial properties. Tensile tests have been carried out from room temperature up to 1200°C on the BMAS material, and it was found that a steady degradation in strength occurred from 500 to 1100°C, with a small but significant increase up to 1200°C. Creep experiments have been performed on both the Pyrex and BMAS materials, it was found that Pyrex has a creeping matrix and elastic fibres below the matrix softening point, whereas the BMAS composite showed creep in both components, though at long times the creep rate was shown to be fibre controlled. A simple model for the development of strain with time is reported and used to obtain values for the creep rate of both the matrix and fibres. Activation energies were calculated for the creep processes in both matrix and fibres. The values obtained were: Pyrex, 256 kJ mol⁻¹, BMAS matrix, 300 kJ mol⁻¹ and the Tyranno fibres, 495 kJ mol⁻¹.

A special purpose finite element method for design of multiple-row composite joints under generalized in-plane loading is presented. The stress analysis is carried out in two steps. In the first step (source analysis), the distribution of applied load within the joint and between the fasteners is determined. In the second step (target analysis), a series of detailed stress analyses of regions containing single bolt holes is performed to determine the stress distribution around the bolt holes. Far field stress distributions obtained in the source analysis are automatically transferred to the boundaries of the target models. The failure analysis includes evaluation of the failure modes (net-section, bearing and shear-out) according to simple point stress criteria. Experiments were performed using single-bolt specimens to validate the target and failure analyses for a graphite/epoxy material system. A complete analysis of a multiple-row glass-fiber/polyester joint is carried out. Computed results show good agreement with experimental data.