Читать книгу Engineering Physics of High-Temperature Materials - Nirmal K. Sinha - Страница 77

At low homologous temperatures, “elastic” response dominates the deformation in most crystalline solids. Very little inelastic strain occurs during load application time until the stress reaches a limiting level called yield strength or after a reasonably long time of sustained load. If a polycrystalline specimen is uniformly loaded to a uniaxial tension or compression, the specimen deforms elastically to a limiting strain, known as elastic limit. Simplest assumption is that all the grains also suffer the same strain as that of the specimen as a whole. Most solids, with very low porosities, exhibit linear elastic behavior. Unless specifically mentioned, linear elastic response is assumed to be the norm for all solids of engineering importance. Customarily, the porosity is assumed to be very low. However, the compaction of materials with high porosity subjected to load results in nonlinear response. Porous materials, such as firebricks and ceramic foams (used as insulations and protective covers), exhibit stress‐wise nonlinear elastic response, but we will not address such materials in this book.

With increase in temperature, two other mechanisms start to contribute to the deformation: “delayed elastic” and “viscous.” The delayed elastic response may also be called “anelastic,” but we will refrain from using it as per the suggestion of British Standard Institution (1975). Mechanical response of materials at high temperatures can be described as “Elasto – Delayed‐Elastic – Viscous” or simply response (see Chapter 5). Irrespective of loading conditions, monotonic or cyclic, total strain can be described generally as:

(2.3)

Equation (2.3) is a descripted representation of the EDEV model. EDEV response can be divided into three distinct types, depending on stress dependence of delayed elastic and viscous characteristics. Solid materials can be classified broadly into three distinct groups based on phenomenological deformation mechanism. This trinity of classification can be described in a simple manner only if the elastic response is assumed to be independent of stress. Most engineering materials fall into this category. However, porous materials exhibit nonlinear elastic behavior as mentioned earlier. Nonetheless, the room is open for further subdivision to take into account, if necessary, materials with high porosity exhibiting nonlinear elastic response.