Subodh K. Mital

Micromechanics for Ceramic Matrix Composites via Fiber Substructuring

A generic unit cell model which includes a unique fiber substructuring concept is proposed for the development of micromechanics equations for continuous fiber reinforced ceramic composites. The unit cell consists of three constituents: fiber, matrix and an interphase. In the present approach, the unit cell is further subdivided into several slices and the equations of micromechanics are derived for each slice. These are subsequently integrated to obtain ply level properties. A stand-alone computer code containing the micromechanics model as a module is currently being developed specifically for the analysis of ceramic matrix composites. Towards this development, equivalent ply property results for a SiC (silicon carbide fiber) /Ti-15-3 (titanium matrix) composite with a 0.5 fiber volume ratio are presented and compared with those obtained from customary micromechanics models to illustrate the concept. Also, comparisons with limited experimental data for the ceramic matrix composite, SiC/RBSN (Reaction Bonded Silicon Nitride) with a 0.3 fiber volume ratio are given to validate the concepts.

Multiscale Modeling of Ceramic Matrix Composites

Results of multiscale modeling simulations of the nonlinear response of SiC/SiC ceramic matrix composites are reported, wherein the microstructure of the ceramic matrix is captured. This micro scale architecture, which contains free Si material as well as the SiC ceramic, is responsible for residual stresses that play an important role in the subsequent thermo-mechanical behavior of the SiC/SiC composite. Using the novel Multiscale Generalized Method of Cells recursive micromechanics theory, the microstructure of the matrix, as well as the microstructure of the composite (fiber and matrix) can be captured.

Fiber Pushout Test: A Three-Dimensional Finite Element Computational Simulation

A fiber pushthrough process has been computationally simulated using a three-dimensional (3-D) finite element method. The interphase material is replaced by an anisotropic material with greatly reduced shear modulus, such that the simulation becomes linear up to the fiber pushthrough load. Such a procedure is easily implemented and is computationally very effective. It can be used to predict fiber pushthrough load for a composite system at any temperature. The average interface shear strength obtained from pushthrough load can easily be separated into its two components: one that comes from frictional stresses and the other that comes from chemical adhesion between fiber and the matrix and mechanical interlocking that develops as a result of shrinkage of the composite because of phase change during the processing. Step-by-step procedures are described to perform the computational simulation, to establish bounds on interfacial bond, and to interpret interfacial bond quality.

Metal matrix composites microfracture: Computational simulation

Abstract Fiber/matrix fracture and fiber-matrix interface debonding in a metal matrix composite (MMC) are computationally simulated. These simulations are part of a research activity to develop computational methods for microfracture, microfracture propagation and fracture toughness of metal matrix composites. The three-dimensional finite element model used in the simulation consists of a group of nine unidirectional fibers in a three by three unit cell array of SiC/Ti15 metal matrix composite with a fiber volume ratio of 0.35. This computational procedure is used to predict the direction of crack growth based on strain energy release rate. It is also used to predict stress redistribution to surrounding matrix/ fibers due to initial and progressive fracture of fiber/matrix and due to debonding of the fiber-matrix interface. Microfracture results for various loading cases such as longitudinal, transverse, shear and bending are presented and discussed. Step-by-step procedures are outlined to evaluate composite microfracture for a given composite system.

MICROMECHANICS FOR PARTICULATE-REINFORCED COMPOSITES

A set of micromechanics equations for the analysis of particulate-reinforced composites is developed using the mechanics of materials approach. Simplified equations are used to compute homogenized or equivalent thermal and mechanical properties of particulate-reinforced composites in terms of the properties of the constituent materials. The microstress equations are also presented here to decompose the applied stresses on the overall composite to the microstresses in the constituent materials. The properties of a “generic” particulate composite as well as those of a particle-reinforced metal matrix composite are predicted and compared with other theories as well as some experimental data. The micromechanics predictions ate in excellent agreement with the measured values.

Micro-fracture in high-temperature metal-matrix laminates

Abstract Computational procedures are described to evaluate the behavior of high-temperature metal-matrix composites. Typical results are presented from studies done on compliant layers to reduce the residual thermal micro-stresses and the effect of partial bonding on composite properties. A three-dimensional finite element analysis in conjunction with strain energy release rates is also described to evaluate composite microfracture. Typical results for micro-fracture in composites subjected to thermo-mechanical cyclic loading are discussed and presented. The results show that interfacial debonding follows fiber or matrix micro-fracture.

Characterization of the as Manufactured Variability in a CVI SiC/SiC Woven Composite

The microstructure of a 2D woven ceramic matrix composite displays significant variability and irregularity. For example, a chemical vapor infiltrated (CVI) SiC/SiC composite exhibits significant amount of porosity arranged in irregular patterns. Furthermore, the fiber tows within a ply frequently have irregular shape and spacing, and the stacked plies are often misaligned and nested within each other. The goal of an ongoing project at NASA Glenn is to investigate the effects of the complex microstructure and its variability on the properties and the durability of the material. One key requirement for this effort is the development of methods to characterize the distribution in as-fabricated ceramic matrix composite (CMC) microstructures with the objective of correlating microstructural distribution parameters with mechanical performance. An initial task in this effort was to perform quantitative image analysis of polished cross sections of CVI SiC/SiC composite specimens. This analysis provided sample distributions of various microstructural composite features, including: inter-tow pore sizes and shapes, transverse sectioned tow sizes and shapes, and within ply tow spacing. This information can then be used to quantify the effect of extreme values of these features on the local stress state with the goal of determining the likelihood of matrix cracking at a given external load.Copyright

Telescoping composite mechanics for composite behavior simulation

Abstract Telescoping composite mechanics are described and implemented in terms of recursive laminate theory. The initial elemental scale is defined where simple equations are derived. Subsequently these mechanics are applied to homogeneous composites, hybrid composites, smart composites and composite enhanced reinforced concrete structures. Results from those applications are presented in terms of tables/figures to illustrate the versatility and generality of telescoping composite mechanics. Comparisons with methods such as approximate, single cell, and 2D and 3D finite element demonstrate the predictive accuracy and computational effectiveness of composite telescoping mechanics.

Probabilistic Micromechanics/Macromechanics for Ceramic Matrix Composites:

Ceramic matrix composites (CMCs) are known to display a considerable amount of scatter in their properties due to variations involved in fiber/matrix properties, interphase properties, interphase bonding, amount of matrix voids, and many geometric or fabrication process related parameters such as ply thickness and ply orientation. This paper summarizes the preliminary studies related to the incorporation of formal probabilistic descriptions of the material behavior and fabrication related parameters into micromechanics and macromechanics for CMCs. This process involves a synergistic coupling of two existing methodologies: namely ceramic matrix composite micro- and macromechanics analysis, and a fast probability integration (FPI) technique to obtain probabilistic composite behavior/response. Preliminary results in the form of cumulative probability distributions and information on the response probability sensitivities to primitive variables for a unidirectional SiC/RBSN ceramic matrix composite are presented. The cumulative distribution functions are computed for composite moduli, thermal expansion coefficients, thermal conductivities and longitudinal tensile strength at room temperature. Variations in the constituent properties that directly affect the above mentioned composite properties are accounted for via assumed probabilistic distributions. Collectively the results show that the present technique provides valuable information on the composite properties and sensitivity factors which are useful to the design/test engineers. Furthermore, the present methodology is computationally more efficient than a standard Monte-Carlo simulation technique and the agreement between the two is excellent as shown via select examples.

Development of Design Analysis Methods for Carbon Silicon Carbide Composite Structures

The stress—strain behavior at room temperature and at 1100° C (2000°F) is measured for two carbon fiber-reinforced silicon carbide (C/SiC) composite materials: a two dimensional (2D) plain-weave quasi-isotropic laminate and a 3D angle interlock woven composite. Previously developed micromechanics-based material models are calibrated by correlating the predicted material property values with the measured values. Four-point beam-bending subelement specimens are fabricated with these two fiber architectures and four-point bending tests are performed at room temperature and at 1100°C. Displacements and strains are measured at the mid-span of the beam and recorded as a function of load magnitude. The calibrated material models are used in concert with a nonlinear finite-element solution using ABAQUS to simulate the structural response of the two materials in the four-point beam bending tests. The structural response predicted by the nonlinear analysis method compared favorably with the measured response for both materials and both test temperatures. Results show that the material models scale-up fairly well from coupons to subcomponent level.