Frederick A. Leckie

Strength variability in alumina fiber-reinforced aluminum matrix composites

Abstract The strength variability of an Al2% Cu alloy matrix reinforced with 65 vol.% Nextel -610 Al 2 O 3 fibers has been investigated, with the aim of identifying and separating the contributions associated with the variabilities in both the fiber bundle strength and the fiber volume fraction. Strength distributions have been measured using three test geometries, including three- and four-point flexure and uniaxial tension. The measured distributions are rationalized on the basis of a fiber strength distribution that follows Weibull statistics and a fiber volume fraction distribution characterized by a Gaussian. The fiber bundle strength distribution is found to be extremely narrow, with a Weibull modulus in the range of ∼ 50–60. In addition, the coefficient of variation in the fiber volume fraction distribution is inferred to be ∼ 6%; by comparison, measurements made on relatively large specimens yield a coefficient of variation of ∼ 3%. The differences in these values are attributed to local volume fraction variations which are not detectable by the global measurements. The measured strengths are compared with the predicted values based on the theoretical work of Curtin and co-workers, incorporating the effects of local load sharing between broken fibers and their neighbors. Good correlations are obtained between the experimental data and the model predictions.

Tensile and flexural ultimate strength of fiber-reinforced ceramic-matrix composites

Abstract A constitutive equation has been derived for fiber-reinforced ceramic-matrix composites, based on fiber breakage and distributed fiber pull-out. Length-dependent and length-independent regimes, governed by the size of the specimen, are differentiated. The constitutive equation is used to predict the ultimate strength of fiber-reinforced ceramic-matrix composites subjected to tensile and flexural loadings.

Modeling of anisotropic behavior of weakly bonded fiber reinforced MMC's

The anisotropic mechanical behavior of a continuous fiber reinforced Ti alloy matrix composite which possesses a weak fiber matrix interface is modeled numerically. Effects of interface properties and residual stresses incurred during the fabrication are addressed in detail. The computational modeling is guided by comparison with experimental data. The study provides an understanding which will be used to model the multiaxial behavior of weakly bonded composites and to provide a tool for predicting the failure of composite structures.

Matrix cracking and debonding of ceramic-matrix composites

The effects of matrix cracking and debonding which occur in ceramic-matrix composites are described by a micromechanical model. The cracking and debonding processes induce loss of stiffness, inelastic strains, hysteresis loops and crack closure. These features are analysed within the framework of Continuum Mechanics by the introduction of internal variables identified in the micromechanical analysis. The evolution laws of the internal variables can be determined by combining the experimental data with micromechanical modeling. The influences of residual stress fields due to processing are also included. Comparisons are made between theoretical predictions and the results of experiments performed on layered materials.

The effect of fiber architecture on the mechanical properties of carbon/carbon fiber composites

The mechanical performance of carbon-fiber matrix composites with different fiber architectures is investigated for various loading modes. All the composites were fabricated from nominally equal constituents and identical consolidation processes, leaving as the only variables, the variations caused by the different fiber weave structures. The fiber architecture drastically affects both composite strength and deformation characteristics. Some systems are almost linear up to a final brittle failure while others exhibit a pronounced non-linearity prior to failure. It is found that the composite tensile strength is dictated by both fiber volume fraction and weave architecture. The weaving can have a beneficial effect in spite of introducing new fiber flaws and stress concentrations, because it causes the composite to be less flaw sensitive. These features are addressed analytically by considering the statistical aspects of the fiber strength and the formation of critical defects.

Component Design-Based Model for Deformation and Rupture of Tough Fibre-Reinforced Ceramic Matrix Composites

A model is developed for the deformation and fracture of tough fibre-reinforced ceramic matrix composites for use in component design. The model is based on a single continuum damage state parameter given by the ratio of the number of failed to unfailed fibres. The composite is assumed to be instantaneously elastic and the stress-strain response is determined from the stress-damage history. The model includes the effects of debonding and fibre pull-out, through weakest fibre Weibull statistics, and estimates of composite ductility have been made. The model quantitatively describes the dominant mechanism of deformation and fracture, and in the following paper, it is shown how the model may be used in component design.

On the mechanical behaviour under cyclic loading of ceramic matrix composites

Abstract In this paper, a constitutive law is presented to model the mechanical behaviour of ceramic matrix composites. It allows matrix-cracking, interfacial debonding, sliding and wear to be accounted for in the framework of continuum mechanics. Based upon micromechanical studies, a 1D and 2D model was derived. An application was performed on a [0,90] SiC/SiC composite.

Localization due to damage in fiber-reinforced composites

Abstract Fiber pull-out is one of the fracture features of fiber-reinforced ceramic matrix composites. The onset of this mechanism is predicted by using Continuum Damage Mechanics, and corresponds to a localization of the deformations. Alter deriving two damage models from a uniaxial bundle approach, different configurations are analysed through analytical and numerical (F.E. calculations) methods. For one model some very simple criteria can be derived, whereas for the second one none of these criteria can be derived and the general criterion of localization has to be used.

Optimization of Coating Layers in the Design of Ceramic Fiber Reinforced Metal Matrix Composites

The potential of using coating layers to reduce thermal stresses in the matrix of composites with a mismatch in coefficients of thermal expansion (CTE) of fiber and matrix is investigated. Two thermoelastic solutions based on a 3-cylinder model are developed and a comprehensive sensitivity study is conducted. It is shown that the performance of the layer can be defined by the product of its CTE and thickness, and that a compensating layer with a sufficiently high CTE can reduce the thermal stresses in the matrix significantly. A practical procedure offering a window of coating layer candidates is proposed. Easy-to-use contours of matrix stress reduction factors are produced.

The mechanical behavior of an alumina carbon/epoxy laminate

An experimental study has been made of a laminate consisting of monolithic thin alumina plates alternating with unidirectional carbon/epoxy (C/E) prepreg tapes. The main advantages of this system over the traditional means of reinforcing ceramics, are the avoidance of large flaws due to processing, which occur in fiber reinforced brittle matrix composites, and the nearly isotropic behavior under biaxial loading. In addition, the multiple fracture mechanism occurring in the system gives rise to pseudo ductile behavior and enhanced strain energy dissipation. The mechanical behavior of the laminate is explored. The effects of the number of layers, volume fraction and transverse properties are also investigated. The loss of stiffness with increase of the applied strain is estimated using a simple shear lag theory, which includes the plastic behavior of the interface.