Читать книгу Engineering Physics of High-Temperature Materials - Nirmal K. Sinha - Страница 35

The history of cultures and nations, including various economic and political aspects of the inhabitants, provides opportunities to look back and make judgments that can, eventually, influence and improve our understanding of the global society. Looking back is always healthy as long as the approach is rational and forward looking. This approach has been the key to success for the development of science and technology and building bonds between diverse societies and linguistic groups of the world using a more‐or‐less common multidisciplinary scientific and technical language and jargon. A thorough and critical, but unbiased (hopefully), review of literature is therefore essential for embarking on any scientific work. It is said, “Hindsight 20‐20.” Why not apply this approach with a fresh outlook to high‐temperature materials science? But then, what would be that approach?

Materials exhibit elastic and inelastic deformation on application of a load. Inelastic deformation is commonly known as plastic. The paradigm of plasticity theories was developed on the basis of engineering experience with materials at low homologous temperatures. Plastic deformation is thought to occur when stress exceeds a specific range. The thoughts of practitioners in several engineering disciplines are molded by theories of plasticity proven to be very successful in explaining failures. Plastic deformation is traditionally assumed to be independent of time and hence independent of strain rate or stress rate. As a consequence, failure processes of geological materials have continued to be presented/discussed in terms of yield functions, yield surfaces, yield diagrams, envelops, etc. As the operational temperature rises, complex issues related to time–temperature effects complicate matters.

Inadvertently, plasticity theories have created confusion for many aspects of engineering materials science in general at elevated temperatures. Yield strength is, for example, very subjective and depends on the user of the information and materials. For example, “yield strength” in mechanical metallurgy and materials engineering, in general, is used to mean the stress corresponding to a specific strain, such as 0.2% offset strain on stress‐strain diagrams. Strength of engineering materials, especially at elevated temperatures, is known to be rate dependent, and inelastic deformation leading to permanent changes in a solid depends on time, among other parameters. Consequently, a small shift in paradigm occurred. Yield strengths had to be defined with respect to a specific range of loading rates. Nowadays, measurement of 0.2% yield is undertaken through uniaxial constant strain rate or more often constant crosshead or displacement rates with respect to “tensile tests” or “compressive tests” (mainly for ceramics, rocks, and ice) at some specific strain rate between 5 × 10⁻⁵ s⁻¹ and 1.2 × 10⁻⁴ s⁻¹ (ASTM 1998). These considerations led us to the use of the word “viscous” for any permanent deformation, irrespective of the micromechanisms (dislocation or diffusion) involved in inducing the changes in the shape of a body. By looking back at the history of the theoretical front and engineering practices, we will try (as mentioned earlier in several places) to avoid the use of the term “plastic strain” in this book. However, we recognize that the terms like plastic deformation and plastic strain continue to be used strongly for describing inelastic strain even for high temperatures, such as creep strain. We recognize that paradigm shifts take time. For this reason, we will often remind the reader about the equivalency of the two terms: viscous and plastic.