Matrix-Enabled Damage Tolerance in Oxide CFCCs

Abstract

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 ...