Advances in ceramic composites reinforced by continuous fibers Brian N Cox* and Frank W Zokt Ceramic matrix composites reinforced with continuous fibers loads. The serious difficulties of ensuring durability are on the verge of insertion into hot engineering structures cratures are being confronted; oxidation Yet current research is only beginning to attack some of pesting of SiC fibers at intermediate temperatures, fiber the most critical problems. Key developments in the last eep at higher temperatures, and the chemical stability 24 months include the formulation of constitutive laws for of interfaces arc all hot topics. Textile reinforcement, continuum mechanics analyses: the discovery of stable weak especially with 3D architecture, has appeared as the xide-oxide interface systems: the analysis of how fiber creep solution to the unavoidable vulnerability of brittle matrix s life at high temperatures; confrontation of the problem composites to delamination. And even the central axiom of oxidation pesting at intermediate temperatures in Sic that CMCs cannot be tough unless the fiber/matrix based systems: re-examination of the maxim that interfaces interfaces are weak is now being challenged must be weak; and the advent of textile reinforcement as the olution to delaminat Modeling the inelastic regime Major progress has been made in the last year or two in devcloping design and reliability codes suitable for Addresses Rockwell Science Center, 1049 Camino Dos Rios PO Box 1085 field use from the wealth of micromechanical models in Thousand Oaks, CA 91358, USA the CMC literature. Effort has focused on generating Materials Department,University of California at Santa Barbara, CA constitutive laws for insertion into finite element models, ith the goal of reducing Current Opinion in Solid State Materials Science 1996 CMCs to standard continuum mechanics Current Chemistry Ltd ISSN 1359-0286 Nonlinearity in CMCs at room temperature involves matrix cracking, stochastic fiber fracture, damage local Abbreviation CMCs ceramic matrix composites ization, and fiber pullout. Two groups have presented exhaustive studies for unidirectional composites of the relation between micromechanical properties (including the interfacial friction stress, residual stresses, constituent Introduction elastic moduli. fiber radius, and fiber volume fraction) The period covered by this review (1995 and the and the macroscopic stress-strain response under aligned beginning of 1996, with selected inclusion of papers from loads prior to damage localization and ultimate failur 1994)marks a major epoch in the history of research [3-9]. Prior and well established models of matrix cracks into continuous fiber reinforced ceramic matrix composites bridged by sliding fibers are used as the physical basis (CMCs). From the carly 1980s, when CMC research for modeling. Micromechanical properties are deduced first enjoyed large scale funding and the attention of directly from experimental hysteresis loops, obviating any significant groups all over the world, effort has been detailed tests of interface conditions, for cxample fibcr concentrated on a simple paradigm of the ideal CMC. It pullout or pushout tests. One group has couched its work must have a weak fiber/matrix interface to allow energy in the language of micromechanics more familiar ro the absorption during fracture by the deflection of cracks, in CMC community [3, 451; the other in the language of the complete absence of any dislocation based toughening. continuum damage mechanics, but with a thermodynat Freed of stress concentration when the matrix cracked, potential function derived from the same micromechanics strong fibers would continue to bear high loads. This 16, 7, 8, 9]. They offer equivalent treatments of nonlinear approach to protecting CMCs from intrinsic faws, notches, ity up to localization, with some variations in the poi and damage was pursued almost entirely in the context of of view and in the level of micromechanical detail used unidirectionally reinforced CMCs, with aligned loads; and in fitting data. Both sets of work are essential readi mostly in terms of room temperature phenomena. It is now More empirical (probably unnecessarily so)treatments of very well understood. (See [1, 2] for recent articles covering nonlinearity in unidirectional CMCs have also appeared many aspects of work up to 1995.) Structural applications almost never involve uniaxial Recent extensions of the continuum damage approach stresses; and the long sought pay-off for CMCs will also deal with predicting the onset of damage localization, certainly come at high temperatures. Now we see at which is required to model ultimate failure and the notch ast the reduction of micromechanical models and our sensitivity of strength [11]. Localization and subsequent dctailcd understanding of matrix cracking and statistical fiber pullout involve distributions of Aaw strengths and fiber failure to constitutive laws suitable for use in stress redistribution effects which are complex and not finite element calculations of structures under complex generally well known in a particular material.There666 Advances in ceramic by continuous fibers Brian N Cox* and Frank W Zokt Ceramic matrix composites reinforced with continuous fibers are on the verge of insertion into hot engineering structures. Yet current research is only beginning to attack some of the most critical problems. Key developments in the last 24 months include the formulation of constitutive laws for continuum mechanics analyses; the discovery of stable weak oxide-oxide interface systems; the analysis of how fiber creep limits life at high temperatures; confrontation of the problem of oxidation pesting at intermediate temperatures in Sic based systems; re-examination of the maxim that interfaces must be weak; and the advent of textile reinforcement as the solution to delamination problems. Addresses *Rockwell Science Center, 1049 Camino DOS Rios, PO Box 1065, Thousand Oaks, CA 91356, USA +Materials Department, University of California at Santa Barbara, CA 93106, USA Current Opinion in Solid State & Materials Science 1996, 1:666-673 0 Current Chemistry Ltd ISSN 1359-0266 Abbreviation CMCs ceramic matrix composites Introduction The period covered by this review (1995 and the beginning of 1996, with selected inclusion of papers from 1994) marks a major epoch in the history of research into continuous fiber reinforced ceramic matrix composites (ChlCs). From the early 198Os, when CMC research first enjoyed large scale funding and the attention of significant groups all over the world, effort has been concentrated on a simple paradigm of the ideal ChlC. It must have a weak fiber/matrix interface to allow energy absorption during fracture by the deflection of cracks, in the complete absence of any dislocation based toughening. Freed of stress concentration when the matrix cracked, strong fibers would continue to bear high loads. This approach to protecting CMCs from intrinsic flaws, notches, and damage was pursued almost entirely in the context of unidirectionally reinforced ChlCs, with aligned loads; and mostly in terms of room temperature phenomena. It is now very well understood. (See [1,‘2] for recent articles covering many aspects of work up to 1995.) Structural applications almost never involve uniaxial stresses; and the long sought pay-off for ChlCs will certainly come at high temperatures. Now we see at last the reduction of micromechanical models and our detailed understanding of matrix cracking and statistical fiber failure to constitutive laws suitable for use in finite element calculations of structures under complex loads. The serious difficulties of ensuring durability at high temperatures are being confronted; oxidation pesting of SIC fibers at intermediate temperatures, fiber creep at higher temperatures, and the chemical stability of interfaces are all hot topics. Textile reinforcement, especially with 3D architecture, has appeared as the solution to the unavoidable vulnerability of brittle matrix composites to delamination. And even the central axiom that CMCs cannot be tough unless the fiber/matrix interfaces are weak is now being challenged. Modeling the inelastic regime Major progress has been made in the last year or two in developing design and reliability codes suitable for field use from the wealth of micromechanical models in the Ch,iC literature. Effort has focused on generating constitutive laws for insertion into finite element models, with the goal of reducing the treatment of nonlinearity in CIVICS to standard continuum mechanics. Nonlinearity in CMCs at room temperature involves matrix cracking, stochastic fiber fracture, damage local￾ization, and fiber pullout. Two groups have presented exhaustive studies for unidirectional composites of the relation between micromechanical properties (including the interfacial friction stress, residual stresses, constituent elastic moduli, fiber radius, and fiber volume fraction) and the macroscopic stress-strain response under aligned loads prior to damage localization and ultimate failure [3-91. Prior and well established models of matrix cracks bridged by sliding fibers are used as the physical basis for modeling. Micromechanical properties are deduced directly from experimental hysteresis loops, obviating any detailed tests of interface conditions, for example fiber pullout or pushout tests. One group has couched its work in the language of micromechanics more familiar to the ChlC community [3*,4’,5]; the other in the language of continuum damage mechanics, but with a thermodynamic potential function derived from the same micromechanics [6,7,8”,9]. They offer equivalent treatments of nonlinear￾ity up to localization, with some variations in the point of view and in the level of micromechanical detail used in fitting data. Both sets of work are essential reading. hlore empirical (probably unnecessarily so) treatments of nonlinearity in unidirectional CMCs have also appeared 1101. Recent extensions of the continuum damage approach also deal with predicting the onset of damage localization, which is required to model ultimate failure and the notch sensitivity of strength [ 111. Localization and subsequent fiber pullout involve distributions of flaw strengths and stress redistribution effects which are complex and not generally well known in a particular material. There