The objective of this program has been to elucidate the underlying mechanisms by which porous matrices give rise to composite toughness in the absence of interphases between the fibers and the matrix, the role of microstructure and processing in attaining this behavior, and the mechanics associated with the deformation, cracking and failure of the composites. The broader goal has been to develop the knowledge base necessary to exploit the nonlinear phenomena occurring within the matrix. Properties of prime interest include in-plane tensile strength and notch sensitivity, subject to both fiber-dominated and matrix-dominated loadings. Recognizing that changes in the structure of the porous matrix have opposing effects on the properties of interest (enhancing some while compromising others), the research has sought to identify the microstructural parameters that provide a desirable balance of composite properties. The key variables explored were those pertaining to the characteristics of the porosity, notably its content, as well as the nature and extent of the bonds which determine both the strength of the particle network within the matrix and the strength of the fiber-matrix interface.The highlights of the program are summarized below. Additional details are in the appended papers. Mullite/Alumina Mixtures for Use as Porous MatricesWeakly bonded particle mixtures of mullite and alumina have been assessed as candidate matrices for use in porous matrix ceramic composites. Conditions for the deflection of a matrix crack at a fiber-matrix have been used to identify the combinations of modulus and toughness of the fibers and the matrix for which damage tolerant behavior is expected to occur in the composite. Accordingly, an experimental study on modulus and toughness of particle mixtures as well as the changes in these properties following aging at elevatedPublic reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instruct.ons, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. ABSTRACT (Maximum 200 Words)The program has focused on the mechanics of deformation and fracture of a family of porous-matrix all-oxide continuous fiber ceramic composites (CFCCs). The activities have lead to substantive developments in the mechanistic understanding of: (i) mechanical properties and thermal stability of porous mullite/alumina matrices; (ii) strength, fracture resistance and notch sensitivity of porous-matrix composites; and (iii) stability and performance of oxide composites in gas turbine combustion environments. Models to describe the pertinent phenomena have been developed and validated.14. 2 temperature was designed and conducted. Models for these properties based on bonded particle aggregates have been calibrated. Upon combining the experimental and modeling results, predictions are made of the critical aging times at which damage tolerance is lost because of ...
The elastic/plastic response of ceramic microballoon reinforced metal matrix composites subject to uniaxial loading are examined using finite element analysis. The microballoons are assumed to be spherical and their morphology characterized by the ratio of wall thickness, t, to radius, R. The key parmeter investigated are the relative wall thickness, t/R, the modulus ratio (matrix/ceramic) and the yield and hardening characteristics of the matrix. The emphasis of the study is on the overall stress-strain response of the composite, the development of matrix plastidty and the development of stress within the microballoon. Typically, the volume fraction of microballoons is in the range of 50-60%. A micrograph of one such composite is shown in Fig. 1. The composites are being targeted for marine applications requiring high specific strength and stiffness and high damping capacity.The potential advantages of these systems over continuous fiber reinforced materials include superior mechanical isotropy, lower production costs and greater flexibility in component fabrication. Their main advantage over conventional particulate-reinforcedMMCs is their reduced density, a result of the void space within the reinforcements.The present paper examines the elastic/plastic response of microballoon reinforced MMCs subject to uniaxial loading (compressive or tensile). The majority of the results are based on a finite element analysis of axisymmetric unit cells. The key parameters investigated are: (i) the ratio of the matrix modulus, Em to that of the ceramic, Er, (ii) the microballoon morphology, characterized by the ratio of wall thickness, t, to microballoon radius, RX and (iii) the work hardening characteristics of the matrix. The focus of the work is on the overall stress-strain response of the composite, the development of matrix plasticity and the development of stresses within the microballoons. NUMERICAL MODELFinite element calculations were conducted on cylindrical unit cells containing a microballoon at the cell center. The height of the cylinder was taken to be equal to the diameter. The ratio of the microbaUoon wall thickness, t, to the microballoon radius, R, was varied from 0 to 1. In all cases, the total volume fraction of microbaloons was 50%.A typical finite element mesh for t/R -02. is shown in Fig. 2.The boundary conditions were prescribed to model a state of uniform tension or compression, applied parallel to the axis of the cylinder. The top face of the cylinder was required to remain planar, with an average normal traction, a, and zero shear traction. The lateral face was also required to remain planar but with zero average normal and shear tractions acting on it. The model was implemented using the finite element program ABAQUS, with 8-noded 2-dimensional axisymmetric biquadrilateral elements. A typical mesh contained 480 elements and 1565 nodes. The overall strain was increased by increments of Aseo = 0.1 for PAO < 5, •e•o = 0.2 for 5 < e/e1 < 10,and Ae/-o = 0.5 for PAO > 10. The effective (deviatoric) str...
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