Kimberly Y. Donaldson

Effective thermal conductivity of uniaxial composite with cylindrically orthotropic carbon fibers and interfacial thermal barrier

The theories of Hasselman et al. and Benveniste et al. were extended to derive expressions for the longitudinal and transverse effective thermal conductivity of an isotropic matrix uniaxially reinforced with dilute concentrations of cylindrically orthotro pic fibers for finite interfacial thermal conductance values. The transverse thermal conduc tivity of the composite was found to be a function of both the radial and tangential compo nents of the transverse thermal conductivity of the fibers. Comparison with the effective thermal conductivity of an isotropic matrix reinforced with isotropic fibers indicates the effective thermal conductivity of cylindrically orthotropic fibers equals the geometric mean of the radial and tangential transverse thermal conductivity. As for isotropic matrices reinforced with isotropic fibers, a finite value for the interfacial thermal conductance in composites reinforced with cylindrically orthotropic fibers leads to a lowering of the com posite transverse thermal conductivity and also introduces a dependence on fiber diameter. Based on experimental data for the transverse thermal conductivity of a uniaxial carbon fiber-reinforced aluminoborosilicate glass matrix composite in nitrogen at atmospheric pressure and in vacuum, estimates of the interfacial thermal conductances indicate that in nitrogen at atmospheric pressure interfacial gaseous heat transfer contributes about one- third of the total interfacial thermal conductance. The theory should be applicable to other isotropic matrices reinforced with cylindrically orthotropic fibers.

Effect of HIPing on the effective thermal conductivity/diffusivity and the interfacial thermal conductance of uniaxial SiC fibre-reinforced RBSN

Hot isostatic pressing (HIPing) was found to increase the thermal diffusivity/conductivity of uniaxial silicon carbide fibre-reinforced reaction-bonded silicon nitride (RBSN) matrix composites, as the result of the densification of the matrix, the increase in the grain size of the silicon carbide and the improved thermal contact between the fibres and the matrix. Transverse to the fibre direction the thermal diffusivity/conductivity was found to be a function of the surrounding gaseous atmosphere due to the access of the gas phase to the fibre-matrix interface, which was facilitated by the existence of an interfacial gap due to the thermal expansion mismatch between the fibres and the matrix. The interfacial conductance was found to exhibit a strong positive temperature dependence as the result of the closure of the interfacial gap with increasing temperature.

Contribution of damage by multiple crack growth to the strain-rate sensitivity of a polycrystalline alumina at elevated temperatures

The strength of a polycrystalline aluminium oxide measured in four-point bending at elevated temperatures over a range of displacement-controlled loading rates was observed to exhibit an unusually high strain-rate dependence. Scanning electron microscopy revealed that at the lower values of strain-rate, failure was accompanied by the formation of a number of additional macrocracks adjacent to the plane of fracture. Separate measurements of specimens deformed to approximately 75% of the fracture strain indicated that, at these lower values of strain rate, the effective Youngs modulus was decreased by as much as a factor of two. It is suggested that the high strain-rate sensitivity was the direct result of a strain-rate dependent decrease in Youngs modulus and associated effect of “strain-softening” for conditions of displacement-rate controlled mechanical loading. The validity of this hypothesis was verified by a fracture-mechanics analysis of a mechanical model subjected to displacement-controlled tensile loading, with elastic behaviour expressed in terms of the effect of cracks on continuum elastic properties.

Effect of fibre orientation on the thermal conductivity of a uniaxial carbon fibre-reinforced aluminoborosilicate glass-matrix composite for various specimen geometries

The development of fibre-reinforced brittle-matrix composites has led to major improvements in such properties as fracture toughness, brittleness and strength [1-6]. Generally, the thermal conductivity of the fibres exceeds that of the matrix, so increases in thermal diffusivity and/or conductivity of the composite are observed as well [7-11]. The thermal conductivity of fibreor whisker-reinforced composites has received a great deal of theoretical interest [11-18]. In general, the thermal conductivity of such composites has been shown to be a function of the thermal conductivity values and the volume fractions of the fibres and matrix, the fibre orientation and the degree of thermal contact at the fibre-matrix interface. The effective thermal conductivity of a uniaxial composite in any direction relative to the fibres can be calculated from the values of the thermal conductivity parallel and perpendicular to the fibre direction [18]. The above theories generally have assumed that the composite is infinite in extent and have not taken into account the possible effect of finite size of the composite specimen. As pointed out by Nye [19] and Carslaw and Jaeger [20], the net direction of heat flow within single crystals with anisotropic thermal conductivity does not necessarily coincide with the direction of the imposed temperature gradient. This should also be relevant for fibre-reinforced composites, which in general are expected to exhibit anisotropic thermal conductivity. Depending on the fibre angle, the presence of insulated boundaries parallel to the imposed temperature gradient can interfere with the net heat flow within the composite. In this respect the recent analysis of Hasselman et al. [21] showed that depending on the orientation of the fibres, the effective thermal conductivity of a uniaxially reinforced composite strip of finite width can be much less than the value for an infinitely large plate. For a composite strip with insulated sides with a temperature gradient imposed along the length of the strip, the effective thermal conductivity along the length of the strip was derived to be [21]

Thermal Conductivity of Uniaxial Coated Cylindrically Orthotropic Fiber-Reinforced Composite with Thermal Barriers

The theories of Hasselman et al. and Benveniste et al. were extended to derive expressions for the thermal conductivity of continuous matrix composites containing dilute volume fractions of uniaxial coated cylindrically orthotropic fibers with interfacial thermal barriers at the fiber-coating and coating-matrix interfaces. The transverse thermal conductivity of this type of composite was found to depend on ten variables: the thermal conductivities of the matrix and coating, the radial and tangential components of the fiber thermal conductivity, the volume fractions of fibers and coating, the absolute dimensions of the fiber and coating, and the interracial thermal conductances at both interfaces.

Observations on the crack-enhanced creep-fracture of a polycrystalline alumina with a glassy grain-boundary phase

An experimental study was conducted of the role of cracks in the creep-rupture behaviour of a polycrystalline alumina with glassy grain-boundary phase at stress regimes and temperatures at which failure is anticipated to occur from pre-existing flaws. Samples were tested without and with artificial flaws in the form of slots or indentation cracks. All three sample types exhibited non-linear creep. The relative rate of increase in creep rate with increasing dimension of slot-depth greatly exceeded the corresponding increase in specimen compliance, as expected for non-linear creep. An analysis of the data indicated that the observed creep behaviour was primarily controlled by crack-enhanced creep with a minor contribution from elastic creep by crack growth. Very poor correlations were found for the stress dependence of the creep rate and time-to-failure. In terms of Monkman-Grant behaviour, good correlations existed between creep rate and time-to-failure, independent of stress and size of the slots or indentation cracks. Because failure originated from pre-existing flaws, the experimental findings of this study suggest the existence of a failure mechanism referred to by the present authors as “crack-enhanced creep fracture”.

Thermal Properties and Thermal Stresses in Delaminated Ceramic Matrix Composites

Using a simple model it is shown that, analogous to crack formation in single phase ceramics, delaminations can significantly decrease the effective transverse thermal conductivity, coefficient of thermal expansion, and, depending on the external constraints, the magnitude of maximum thermal stress and/or thermal deformation in laminated fiber-reinforced ceramic matrix composites subjected to transverse heat flow.

Observations on the Role of Cracks in the Non-Linear Deformation and Fracture Behavior of Polycrystalline Ceramics

A review is presented of the authors’ research on the role of cracks in the non-linear deformation and fracture behavior of polycrystalline structural ceramics. The presence of cracks can have two principal effects on the creep behavior, namely “crack-enhanced creep,” which increases the rate of creep and “elastic creep by crack growth,” due to the accompanying decrease in Young’s modulus. These two mechanisms serve to explain observations on the anomalous creep behavior reported in the literature such as nonlinear creep at low stress levels, increasing creep rate with increasing grain size, anomalously high activation energies and diffusion coefficients and large differences in the creep rates in tension and compression.