Evans_Toughening_Rev - journal J Am Ceram Soc 73(21 187-205(1990 Perspective on the Development of High-Toughness Ceramics Anthony G Evans M

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journal J. Am. Ceram Soc., 73 (21 187-205 (1990) Perspective on the Development of High-Toughness Ceramics Anthony G. Evans* Materials Department, College of Engineering, University of California, Santa Barbara, California 931 06 1. Major Developments (1) Synopsis THE science governing the strength and fracture of structural ceramics has devel- oped from a mostly empirical topic in 1965 into a mature discipline that now sets the standards in the field of mechanical be- havior. The intent of this review is to pro- vide a perspective regarding this evolution, followed by succinct descrip- tions of current understanding. The rapid developments in the field are considered to have commenced upon the first con- certed attempt to apply fracture mechan- ics concepts to ceramics, beginning in the middle 1960s (Fig. 1). This allowed a dis- tinction between the separate contribu- tions to strength from the flaws in the material and from the microstructure, as manifest in the fracture toughness. Anoth- er contribution that accelerated the learn- ing process was the development of indentation techniques, which allowed trends in the damage resistance of new ceramics to be assessed on a routine ba- sis. However, the most important develop- ment, which originated at about the same time, was the discovery of toughened zir- conia alloys. The ensuing research on these alloys established two vital prece- dents. Firstly, the introduction of the materials-by-design approach, which es- tablished the benefits that accrue from the strong collaboration between processing, characterization, testing, and modeling A. H. Heuer-contributing editor Manuscript No 197905 Received November 20, 1989, approved December 13, 1989 Presented in part at the 91st Annual Meeting of the American Ceramic Society. Indianapolis, IN, April 24, 1989 (Orton Memorial Lecture) [Key words toughness, fracture microcracking, zir- conia. R curve ] 'Member, American Ceramic Society (Fig. 2). Secondly, the topic attracted the attention of outstanding scientists who have since continued to provide invalua- ble contributions to progress in the field. Still another important development was the appreciation that the mechanical properties of ceramics could be apprecia- bly enhanced by the incorporation of ce- ramic fibers and whiskers. Finally, a recent discovery that may have a similar substan- tive impact on the ceramics field concerns metal-toughened ceramics and compos- ites formed by directed metal oxidation. (2) Introduction of Fracture Mechanics The mathematical framework and the experimental techniques of linear elastic fracture mechanics that had developed from the early 1950s and had been exten- sively applied to metals were first recog- nized to have importance to the ceramic field by Davidge and by Wiederhorn. These authors examined a variety of ex- perimental approaches and began the process whereby the fracture toughness could be established as a material param- eter and eventually be related to micro- structure. At the same time, initial attempts were made to separate the contributions to strength, S, from the flaw size, a, and
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This note was uploaded on 06/10/2011 for the course EMA 6715 taught by Professor Mecholsky during the Fall '08 term at University of Florida.

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Evans_Toughening_Rev - journal J Am Ceram Soc 73(21 187-205(1990 Perspective on the Development of High-Toughness Ceramics Anthony G Evans M

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