Notch Sensitivity and Stress Redistribution in Three Ceramic-Matrix Composites

Fiber-reinforced ceramic-matrix composites (CMCs) depend upon inelastic mechanisms to diffuse stress concentrations associated with holes, notches, and cracks. These mechanisms consist of fiber debonding and pullout, multiple matrix cracking, and shear band formation. In order to understand these effects, experiments have been conducted on several double-edge-notched CMCs that exhibit different stress redistribution mechanisms. Stresses have been measured and mechanisms identified by using a combination of methods including X-ray imaging, edge replication, and thermoelastic analysis. Multiple matrix cracking was found to be the most effective stress redistribution mechanism.     

Frictional Stress Evaluation along the Fiber-Matrix Interface in Ceramic Matrix Composites

A finite element algorithm, developed for frictional comact problems, has been used to evaluate the shear stresses along the fiber-matrix interface in a ceramic composite and the load point fiber displacements dining fiber compression. The induced peak shear stress and the shear stress gradient were found to increase with increasing coefficients of friction. Calculated fiber displacements asymptotically decayed to the perfectly bonded condition as the coefficient of friction was increased. The computed average interfacial shear stress showed remarkable agreement with recent experimental findings hi the SiC-LAS system.     

Influence of Interfacial Shear Stress on First-Matrix Cracking Stress in Ceramic-Matrix Composites

The first-matrix cracking stress and fiber-matrix interfacial shear stress were measured in zircon-matrix composites uniaxially reinforced with either uncoated or BN-coated silicon carbide filaments to study the role of intentional changes in interfacial shear stress on first-matrix cracking stress. The first-matrix cracking stress was measured by mechanical tests performed in either tension or flexure, and the filament-matrix interfacial shear stress was measured by a fiber pushout test. The first-matrix cracking stress was independent of the measured interfacial shear stress and did not conform to the predictions of a number of energy-based micromechanics models. In contrast, the first-matrix cracking stress showed a good correlation with the first-matrix cracking strain, which is hypothesized to be a more realistic criterion for first-matrix cracking in this class of filament-reinforced ceramic-matrix composites.     

Evaluation of Damage Evolution in Ceramic-Matrix Composites Using Thermoelastic Stress Analysis

Thermoelastic stress analysis (TSA) has been used to monitor damage evolution in several composite systems. The method is used to measure full-field hydrostatic stress maps across the entire visible surface of a sample, to quantify the stress redistribution that is caused by damage and to image the existing damage state in composites. Stress maps and damage images are constructed by measuring the thermoelastic and dissipational thermal signatures during cyclic loading. To explore the general utility of the method, test samples of several ceramic-matrix and cement-matrix composites have been fabricated and tested according to a prescribed damage schedule. The model materials have been chosen to illustrate the effect of each of three damage mechanisms: a single crack that is bridged by fibers, multiple matrix cracking, and shear bands. It is shown that the TSA method can be used to quantify the effect of damage and identify the operative damage mechanism. Each mechanism is identified by a characteristic thermal signature, and each is shown to be effective at redistributing stress and diffusing stress concentrations. The proposed experimental method presents a new way to measure the current damage state of a composite material.     

Matrix Cracking in Ceramic-Matrix Composites

Matrix cracking in ceramic-matrix composites with unbonded frictional interface has been studied using fracture mechanics theory. The critical stress for extension of a fiber-bridged crack has been analyzed using the stress-intensity approach. The analysis uses a new shear-lag formulation of the crack-closure traction applied by the bridging fibers based on the assumption of a constant sliding friction stress over the sliding length of the fiber-matrix interface. The new formulation satisfies two required limiting conditions: (a) when the stress in the bridging fiber approaches the far-field applied stress, the crack-opening displacement approaches a steady-state upper limit that is in agreement with the previous formulations; and (b) in the limit of zero crack opening, the stress in the bridging fiber approaches the far-field fiber stress. This lower limit of the bridging stress is distinctly different from the previous formulations. For all other conditions, the closure traction is a function of the far-field applied stress in addition to the local crack-opening displacement, the interfacial sliding friction stress, and the material properties. Numerical calculations using the stress-intensity approach indicate that the critical stress for crack extension decreases with increasing crack length and approaches a constant steady-state value for large cracks. The steady-state matrix-cracking stress agrees with a steady-state energy balance analysis applied to the continuum model, but it is slightly less than the matrix-cracking stress predicted by such theories of steady-state cracking as that of Aveston, Cooper, and Kelly. The origin of this difference and a method for reconciliation of the two theoretical approaches are discussed.     

Boron Nitride Interphase in Ceramic-Matrix Composites

A BN interphase has been deposited, by isothermal/isobaric chemical vapor infiltration (ICVI) from BF₃─NH₃, within a preform made from ex-polycarbosilane (ex-PCS) fibers, at about 1000°C. In a second step, the BN-treated preform was densified with SiC deposited from CH₃SiCl₃─H₂ at about the same temperature. From a thermodynamic standpoint, ex-PCS fibers could be regarded as unreactive vs the BF₃─NH₃ gas phase assuming they are coated with a thin layer of carbon or/and silica. The as-deposited interphase consists of turbostratic BN (N/B < 1) containing oxygen. The SiC infiltration acts as an annealing treatment: (i) the BN interphase becomes almost stoichiometric and free of oxygen; (ii) the fibers undergo a decomposition process yielding a SiO₂/C layer at the BN/fiber interface. The weaker link in the interfacial sequence seems to be the BN/SiO₂ interface. Deflection of microcracks arising from the failure of the matrix takes place at (or nearby) that particular interface.     

Failure of Crossply Ceramic-Matrix Composites

The fast-fracture and stress-rupture of a crossply ceramic-matrix composite with a matrix through-crack are examined numerically to assess the importance of fiber architecture and the associated stress concentrations at the 0/90 ply interface on failure. Fiber bridging in the cracked 0 ply is modeled using a line-spring bridging model that incorporates stochastic and time-dependent fiber fracture. A finite-element model is used to determine the stresses throughout the crossply in the presence of the bridged crack. For both SiC/SiC and a typical oxide/oxide, the fast-fracture simulations show that as global failure is approached, a significant fraction of fibers near the 0/90 interface are broken, greatly reducing the stress concentration. For fibers with low Weibull moduli ( m < 10), the tensile strength is thus nearly identical to that of a unidirectional composite scaled by the appropriate fiber volume fraction, while for fibers with larger Weibull moduli ( m ≥ 10), there are modest (10−17%) reductions in tensile strength. Stress-rupture simulations show that initially high stress concentrations are relieved as fibers fail with evolving time near the 0/90 interface and shed load away from the interface. For a wide range of fiber properties, efficient load redistribution occurs such that the crossply rupture lifetime is generally within an order of magnitude of the unidirectional lifetime, when the applied stress is normalized by the relevant fast-fracture strength. Overall, stress concentrations at the 0/90 interface are largely relieved with increasing load or time due to the nonlinear bridging response and preferential fiber failure near the interface, resulting in crossplies that respond very similarly to unidirectional composites.     

Application of Weakest-Link Fracture Statistics to Fiber-Reinforced Ceramic-Matrix Composites

The strength and reliability of fiber-reinforced ceramic-matrix composites (CMCs) are dependent on whether conditions of local or global load sharing prevail. Global load sharing is promoted by a low interfacial sliding stress and is manifested in a zero-tangent modulus at the point of tensile failure along with random fiber failures and extensive fiber pullout. In this paper, it is demonstrated that conditions of global load sharing are not present in two commonly studied CMCs, despite the fibrous appearance of their fracture surfaces. This behavior is manifested in a volume-dependent strength, as evidenced by strength differences measured in tension and flexure (accounting for the nonlinear stress distribution in flexure). Methods of weakest-link statistics are used to relate the strengths measured in the two test configurations. Estimates for the Weibull moduli of the two systems are obtained from the experiments and compared with values obtained through Monte Carlo simulations based on a three-dimensional-lattice Greens function method. The implications of these results on the strength of large components and of small regions of high stress concentration are discussed briefly.     

Oxidation Embrittlement Probe for Ceramic-Matrix Composites

A test method for probing oxidation embrittlement in ceramic-matrix composites has been devised and demonstrated for three composites: SiC/magnesium aluminosilite, SiC/SiC, and SiC/Al₂O₃. The method identifies a "pest" temperature T _p. The growth of an embrittled region from the surface, at T _P, has been illustrated. The embrittlement mechanism involves oxygen ingress through matrix cracks and silicate formation at the fibers, causing fiber failure.     

Failure Mechanisms in Ceramic-Fiber/Ceramic-Matrix Composites

Mechanisms of failure in a unidirectional Sic-fibe/glass-ceramic composite are investigated using in situ observations during tensile and flexural loading. These experiments show that failure in tension occurs in several stages (similar to certain other brittle fiber composites): multiple matrix cracking, followed by fiber fracture and pullout. In flexural loading the failure process is more complex. Consequently, the flexural test cannot be used for measurement of tensile strength (although it can be used for measurement of the stress for matrix cracking). The application of conventional fracture mechanics to describe tensile failure is discussed. The in situ observations provide direct indication of the importance of frictional bonding between the matrix and fibers. Some novel methods for measuring the frictional forces and residual stresses are investigated, and the influence of surface damage on strength is assessed.     

Strength versus Gauge Length in Ceramic-Matrix Composites

The strength of ceramic-matrix composites as a function of sample gauge length is derived as a function of the composite constituent material properties. Within the context of a global load-sharing assumption for how load is transferred from broken to unbroken fibers, the analysis shows that, for samples shorter than 0.8δ_c (where δ_c is the characteristic-slip length determining composite pullout), the composite ultimate strength increases with decreasing gauge length. For samples longer than 0.8δ_c, the strength is independent of gauge length. Implications of these results on the performance of composites with small-scale stress concentrators is briefly discussed.     

In-Plane Mechanical Properties of Several Ceramic-Matrix Composites

An attempt has been made to assess the generalized in-plane inelastic deformation and rupture properties of typical laminated or woven ceramic-matrix composites. The assessment is made by first identifying two principal classes of behavior. These classes are distinguished by the ratio of the elastic properties of the fibers to those of the matrix, which determines the mechanisms of deformation and rupture. These mechanisms, in turn, control the magnitude and orientation sensitivity of the stress/strain curves. Assessment of the inelastic deformations is achieved by first establishing the evolution of matrix cracks and their influence on the elastic moduli. Subsequent evaluation is made by using constituent properties, particularly the interface debonding and sliding resistances in the presence of matrix cracks. This is achieved by analyses of hysteresis loops, using a matrix cracking model. This model provides a representation of the influence of load direction on the interface responses and the inelastic strains. The ultimate strength is controlled by two mechanisms. It is fiber-controlled in 0/90 tension but becomes matrix controlled in ±45° tension. A model characterizing this mechanism change has yet to be devised.     

Pillared Smectite Clay Coatings for Ceramic-Matrix Composites

This paper describes a novel route for the low-temperature formation of mullite, from pillared smectite clay precursors, for use as fiber coatings in ceramic-matrix composites. In particular, alumina-pillared bentonite converts in part to mullite at the unusually low temperature of about 800°C. The clay precursors display excellent film-forming capability and have been coated onto silicon carbide fibers. Mechanical tests on composites of the coated fibers and a borosilicate glass demonstrate their success as debond coatings, suggesting that this approach is a viable and simple route to oxide coatings for fibers.     

Room-Temperature Mechanical Behavior of Fiber-Reinforced Ceramic-Matrix Composites

Ceramic-matrix composites reinforced with SiC fibers were tested at room temperature both in, flexural and tensile configurations. The stress-strain behavior for composites tested in tension was correlated with progressive microcracking and failure processes. Significant differences between failure modes in tension and flexure were observed.     

Multilayered Oxide Interphase Concept for Ceramic-Matrix Composites

Hi-Nicalon/SiC minicomposite specimens containing three oxide interphase layers (amorphous SiO₂, monoclinic ZrO₂, and amorphous SiO₂) were prepared by chemical vapor deposition. The minicomposites exhibited graceful composite failure behavior with reasonable load-carrying capability in room-temperature tensile tests. Much of the composite behavior and load-carrying capability was retained even after matrix precracking and subsequent oxidation in air at 960°C for 10 h. In both the as-prepared and oxidized specimens, crack deflection and fiber pull-out occurred preferentially within the multilayered interphase region. The potential merits and uncertainties associated with this multilayered oxide interphase approach were discussed in the context of designing environmentally durable interfaces for ceramic-matrix composites.     

Synergism of Toughening Mechanisms in Whisker-Reinforced Ceramic-Matrix Composites

The improved fracture resistance of whisker-reinforced ceramic-matrix composites involves more than one energy-absorbing mechanism. The possible mechanisms are reviewed and a micromechanical model evaluating the relative contributions to the overall toughness is presented. The mechanisms involve microcracking, load transfer, bridging, and crack deflection. The synergism of these mechanisms is examined using an energy release rate balance equation. The basic assumption of the proposed model is that the load transfer between the matrix and the whiskers is due to Coulomb friction. The model has been applied to an Al₂O₃/SiC whisker composite and shows reasonable agreement with reported experimental results. The role of the thermal residual stresses is also examined in light of the frictional load transfer assumption.     

Steady-State Mechanics of Delamination Cracking in Laminated Ceramic-Matrix Composites

A fracture mechanics delamination cracking model has been developed for brittle-matrix composite laminates. The near-tip mechanics is discussed in the context of material orthotropy and composite material inhomogeneities. A fracture mechanics framework based on the near-tip energy release rate and the associated phase angle Ψ has been adopted. In the case of steady-state delamination cracking in a prenotched cross-ply symmetric laminated beam, analytical expressions for the steady-state energy release rate, _ss, have been obtained for the combined applied loading of an axial force and a bending moment. Parameter studies assessing the effects on _ss of load coupling, crack location, and lamination morphology which includes the total number of layers, layer thickness, and material properties are presented. Thus, composite homogenization criteria with respect to the total number of layers placed along the beam height can be obtained for a wide range of material selection. The associated phase angle Ψ at the delamination crack tip is discussed in the context of existing solutions. The analysis has been developed based on a theory for structural laminates. The delamination model can be used in conjunction with experimental data obtained from model geometries to extract the mixed-mode transverse composite fracture toughness. Thus, conditions for stable delamination crack growth can be established and design criteria based on toughness for composite laminates and composite fasteners can be obtained.