BOOK REVIEWS A. Peras and V. Dauknis

METHODS OF STRENGTH TESTING FOR REFRACTORY CERAMICS Reviewed by G. Gogotsi

Modern refractory technology has recently been combined with the use of metals in engineering structures subject to considerable thermal loads and there has been a tendency for operating conditions to become more severe, which has led to increased research on the strength of ceramic materials. There have therefore recently been many publications on this topic, but all the surveys devote the main attention to researches on consolidated singlephase polycrystalline materials or single crystals, i.e., materials that are not particularly promising for extensive practical use. A distinctive feature of this book is that the main interest centers on porous and other heterogeneous materials that are produced commercially and used in high-temperature systems. The monograph surveys analytical and experimental studies on the strength of refractory ceramics over a wide range of temperatures. A considerable fraction of the space is given to data obtained in the Institute of Power-Engineering Technical Physics Problems, Academy of Sciences of the Lithuanian SSR, under the direction and with the direct participation of the authors, who are leading specialists in the field of refractory strength. The advantage of this monograph is that it deals extensively with the methods required to support experimental studies. The volume consists of five chapters and a bibliography; the information is highly upto-date and presented in a very readable form. The first chapter is essentially an introduction and discusses the current state of research on the strength of ceramics; it is shown that the specific features of this class of material mean that techniques derived from strength testing for metals cannot be transferred to this class of material without resulting in considerable error. Published information on the physicomechanical characteristics of ceramics in relation to composition and structure is used to show that there can be major differences between materials of this type, and therefore a differential approach is required in researching the strength of each kind. The second chapter deals with the basic strength laws for refractory ceramics uniaxial loading, and the statistical theories of strength are examined in detail, as Weibull's strength theory. Considerable interest is attached to the suggestion statistical aspects should be incorporated in the formulation of any experiment on sicomechanical properties of such materials.

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There is also a detailed discussion, perhaps somewhat idealized, of the effects of porosity on the strength characteristics; this chapter also includes data on the effects of temperature on the behavior of ceramic materials under load. The effects of structural and other factors on the measurements are examined, particularly those from tests under various heating conditions. The third chapter deals with methods used in determining the physicomechanical characteristics of ceramics. There is a reasonably detailed description of the pros and cons of each of the test methods for use in tension, bending, axial or diametrical compression, and so on. Estimates are made of the errors that can occur in tension and bending tests. There is a novel approach to accuracy evaluation in strength determination, which shows in particular that eccentricity in load application to an inhomogeneous material in tension is not so important as it is in tests on homogeneous materials. Descriptions are given of the types * Mukslas, Vilnius, 1977, 183 pp. Translated from Problemy Prochnosti, No. i0, pp. 124-125, October, 1978.

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9 1979 Plenum Publishing Corporation

of specimen required for various types of measurement, and details are given of the design of clamps and heaters for use in strength researches. The fourth chapter deals with various pieces of equipment built over a period of many years at the Institute of Power-Engineering Physicotechnical Problems, Academy of Sciences of the Lithuanian SSR, particularly for researching short-term strength of refractories over wide temperature ranges. The authors discuss the design principles of such systems, particularly the design of some major units and the choice of materials for the parts to work at the highest temperatures; experience accumulated over a long period is summarized. This information is of considerable value, since the equipments have some novel design features that may prove of use in other areas. Further, the description of the design is accompanied by an analysis of the results obtained under various loading conditions. We may note particularly a system for tension testing in which the strain diagram is recorded; this type of system is recommended for the determination of strength characteristics, deformation, elasticity, and brittleness of ceramic materials. The fifth chapter deals with some aspects of creep in refractory ceramics; much of the evidence of interest to the experimental worker is collected here. Creep mechanisms are examined in general terms for ceramics at high temperatures, and a generalized deformation diagram is given, as well as a diagram for creep rates under various forms of loading etc. The chapter also describes instruments built by the authors for creep testing, together with details of the modes of operation. Particular interest is attached to creep in pure bending, on account of the inequality of the creep rates under compression and tension. The authors discuss the stress state in a bent beam and derive formulas that incorporate the different forms of behavior of the material in tension and in compression, particularly the possible displacement of the neutral axis. Calculations on these formulas correspond much better with measurements than to those obtained by ordinary methods. Finally, the authors draw the very important conclusion that primary importance attached to the definition of unified standard techniques for strength testing of ceramics, particularly the vital need for a standardized set of mechanical characteristics to be supplied when any ceramic material is recommended for practical use. These positive features of the book are accompanied by the following flaws. It is hardly correct to say that tension testing is the most widely usable method, for in fact such tests are the most widely used as being irreplaceable at high temperatures (at temperatures more than 0.7-0.8 of the melting point of the material), i.e., when the ceramic becomes plastic. However, this method of loading has often to be abandoned on account of insuperable difficulties in eliminating various sources of error. Also, insufficient attention is given to the energy aspects of failure; effects related to the energy aspect are very important for heterogeneous materials and may be more important than the effects considered in the statistical theories of strength. There seems little doubt that neglect of this feature is responsible for the lack of discussion of the rigidity in the test systems. Further, some of the ideas put forward are debatable and require confirmation. Unfortunately, in certain instances major conclusions are drawn from data derived from tests on atypical ceramic materials. It is recommended that the book should be supplemented with results from fractographic studies if it is to be reprinted, since these might illustrate some of the conclusions and demonstrate the basis for Suggestions made in discussing the mechanisms of deformation and failure. In conclusion, it must be emphasized that this monograph is concerned primarily with materials not intended to bear major loads, but the conclusions are the value in research on any ceramic. Taken as a whole, the deficiencies and debatable points do not detract substantially from. the generally considerable value of the book, which will undoubtedly be of great assistance to those researching the strength of ceramics, particularly if they have recently entered this area. 1243