Continuous Fiber Ceramic Matrix Composites Research Articles

FEM predictions of damage in continous fiber ceramic matrix composites under transverse tension using the crack band method

This paper develops a numerical micromechanics fracture model based on the crack band methodology for the prediction of damage in continuous fiber ceramic matrix composites (CMCs). The energy based crack band formulation offers mesh objective predictions in finite element simulations. Detailed finite element models of CMCs are created based on the actual microstructure and compared with randomly created representative volume elements (RVE). The influence of geometrical inhomogeneities, such as fiber clustering and variations of the fiber coating thickness, on the stress-strain response and the crack development are investigated. Furthermore, FEM models of multi-layer cross-ply laminates are created including thousands of explicitly modeled fibers and fiber coatings. Crack density predictions are compared with well established analytical models by Laws and Dvorak [1] and Nairn [2]. An eight layer FEM model is used to predict damage propagation within each layer. The Numerical prediction of the stress-strain response is compared with experimental results of smooth bar tensile coupons.

Multiscale damage characterization in continuous fiber ceramic matrix composites using digital image correlation

Damage in continuous fiber CMCs with weakly bonded fiber–matrix interfaces evolves in several stages that span multiple length scales. Comprehensive damage characterization necessitates identifying how damage initiates as well as how it accumulates (through final failure), which is not feasible with information gathered from a single length scale. Using digital image correlation to measure full-field surface deformations, damage evolutions in continuous fiber SiC/SiC laminates were analyzed at three distinct length scales: constituent, lamina, and laminate. Constituent scale analyses indicated that fine matrix cracks initiated in localized regions of transverse fiber coatings at low stresses. Investigations at the larger lamina scale revealed that some, but not all, of the coating cracks evolved into matrix cracks, the propagation of which was dependent on the state of stress near the crack tip and the local microstructure. Many of these matrix cracks morphed into cracks large enough to be detected at the (largest) laminate scale. The density of the large matrix cracks increased with load, reaching saturation prior to failure. While the constituent scale identifies when (with respect to stress state) and where (with respect to local microstructure) damage initiates, the lamina scale elucidates damage progression between neighboring constituents. Only laminate scale fields of view are large enough to capture the accumulation of the large matrix cracks that ultimately lead to final fracture. However, as spatial resolution is reduced at this scale, finer cracks (which may provide pathways for environmental ingress) go undetected. As each length scale provides a unique perspective of damage evolution in CMCs, multiscale analysis is essential for comprehensive damage characterization.

In Situ Nondestructive Evaluation of the Accumulative Damage in Continuous Ceramic Fiber-Ceramic Matrix Composites (CFCCs) using Submillimeter Range Electromagnetic Wave

Monolithic ceramic materials usually show catastrophic failure behavior and this property limits application as structural components because the failure occurs easily from a small flaw that usually exists in the materials. To overcome this, incorporation of a fine ceramic fiber into a brittle ceramic has been examined and the obtained material is named as continuous fiber-ceramic matrix composites (CFCCs).

Effect of Microstructure on the Mechanical Behavior of Continuous‐Fiber‐Reinforced Ceramic‐Matrix Composites

The mechanical behavior of a unidirectional continuous‐fiber ceramic‐matrix composite (CFCMC) was correlated with matrix‐rich channels in the microstructure. A large population of CFCMCs was prepared via alkoxide infiltration, which incorporated either uncoated Nicalon fibers (64 samples) or BN‐coated fibers (118 samples). No structure/property correlation was observed for the uncoated composites, because of the uniformity of the microstructure. For the BN‐coated composites, both the flexure strength and work of fracture (WOF) were correlated with oriented matrix‐rich channels. The channels were <90 μm wide and were distributed throughout the cross section. The BN‐coated CFCMCs exhibited laminate‐like behavior: the strength was statistically higher when the channels were aligned parallel with the applied load and the WOF was statistically higher when the channels were perpendicular to the load. Grouping the specimens on the basis of channel orientation, relative to applied load, reduced the variance in strength and WOF. This categorization also resulted in consistent, predictable failure behavior. This observation implies that prior CFCMC data that do not consider microstructure orientation may show wider scatter in mechanical behavior than is warranted.

繊維強化複合材料の新展開 連続繊維強化セラミックスの破壊と力学特性

繊維強化複合材料の新展開 連続繊維強化セラミックスの破壊と力学特性

Nondestructive Characterization of Ceramic Composites Used as Combustor Liners in Advanced Gas Turbines

Nondestructive characterization (NDC) methods, which can provide full-field information about components prior to and during use, are critical to the reliable application of continuous fiber ceramic matrix composites in high-firing-temperature (>1350°C) gas turbines. [For combustor liners, although they are nonmechanical load-bearing components, both thermal characteristics and mechanical integrity are vitally important.] NDC methods being developed to provide necessary information include x-ray computed tomography (mainly for through-wall density and delamination detection), infrared-based thermal diffusivity imaging, and single-wall through-transmission x-ray imaging (mainly for fiber content and alignment detection). Correlation of the data obtained from NDC methods with subscale combustor liner tests have shown positive results at thermal cycling temperatures from 700°C to 1177°C.

Fabrication, Processing, and Characterization of Braided, Continuous SiC Fiber-Reinforced/CVI SiC Matrix Ceramic Composites

Abstract Three-dimensionally reinforced continuous-fiber ceramic matrix composites (CFCCs) were fabricated from preforms of braided SiC fiber (Nicalon(™)) tows that had been coated with 0.4 μm thick layer of pyrolytic graphite. A hybrid infiltration process of chemical vapor infiltration and polymer impregnation pyrolysis was used to form the matrix. The as-fabricated CFCCs were composed of ∼34 vol% fiber and ∼36 vol%matrix with ∼29 vol% residual porosity. Appropriate specimens were tested at 20° and 1000°C along the longitudinal braiding direction in uniaxial tension, compression and three and four-point flexure. Linear stress-strain responses to well-defined proportional limits (∼75 MPa) occurred for tension and flexure at both temperatures. Nonlinear stress-strain behaviour occurred beyond the proportional limit up to fracture at ultimate strengths in tension on the order of 175-200 MPa for 20°C and 100 MPa for 1000°C. SEM fractography revealed fiber pullout for the 20°C tests and evidence of brittle fracture due to environmental degradation for the 1000°C tests.

Processing of damage-tolerant, oxidation-resistant ceramic matrix composites by a precursor infiltration and pyrolysis method

Damage-tolerant, continuous fiber ceramic matrix composites have been produced by an inexpensive method. According to this method, the space between the fibers is filled with a powder. The powder particles are heat treated to form a porous framework without shrinkage, which is then strengthened with an inorganic synthesized from a precursor. High particle packing densities can be achieved within the fiber preform provided that the particle-to-fiber diameter ratio is small. Filling the interstices with a powder increased the composite density and also limits the size of the crack-like voids within the matrix. In this review we describe the mechanical properties of partially dense materials produced from powders to show that a porous matrix can be strong. We demonstrate that the packing density of particles around fibers is highest when the particle-to-fiber diameter ratio is small. The kinetics and mechanical behavior of composite systems is summarized to demonstrate the requirements of damage-tolerant properties. An all-oxide ceramic matrix composite produced by this method is discussed.

High-speed photographic studies of laser drilling of ceramics and ceramic composites

High-speed photographic techniques were used to study plumes generated above a material during drilling of ceramics and ceramic composites with a carbon dioxide laser. The prinicipal objectives were to identify the mechanism of material removal (spattering, particulate and fiber debris, liquid droplets) and plume phenomena (plume shapes and sizes) for ceramics and ceramic composites. High-speed photographic (1000 frames per second) visualization of laser drilling was undertaken for two monolithic ceramics, (sintered α-silicon carbide (α-SiC) and hot-pressed silicon nitride (Si3N4)) and two continuous fiber—ceramic matrix composites, (carbon fibers in a silicon carbide matrix (C–SiC) and silicon carbide fibers in a silicon carbide matrix (SiC–SiC)). The results of this study indicate that each of these ceramic materials decomposes differently during laser processing and that the material removal mechanism for a given material is possibly different at different stages of the process. In all cases, material removal is initially through high-temperature decomposition and/or vaporization. As the hole depth increases, the walls of the hole become steeper resulting in a substantial reduction of the laser flux incident on the surface; this in turn appears to modify the mechanism of material removal depending upon the composition and porosity of the material. The photographic evidence presented here clearly shows ablative material removal for sintered α-silicon carbide (α-SiC), periodic ejection of liquid silicon during laser drilling of silicon nitride and ejection of particulate/fiber debris during laser processing of the composite materials SiC–SiC and C–SiC.