Читать книгу Engineering Physics of High-Temperature Materials - Nirmal K. Sinha - Страница 29

The deformation in a crystalline material consists of two major parts – an elastic part associated with the recoverable distortions of its atomic lattice structure, and an inelastic part. The term “inelastic” is associated with the part of the deformation that is “really a deviation” from the time‐independent (isothermal) elastic response. In classical materials science literature and for ordinary temperatures (less than 0.2 T _m, where T _m is the melting point in kelvin), the inelastic deformation is assumed (by default) to be time independent and is commonly referred to as “plastic.” However, experimental evidence tends to indicate that the classically described inelastic part actually includes a time‐dependent recoverable portion, called anelastic or delayed elastic strain. This portion is more observable as temperature increases.

Experimental and theoretical concepts of time‐independent plastic deformation and the large volume of associated literature involving various yield functions, yield surfaces, envelopes, etc. have become the backbone for many engineering practices. These developments occurred before our knowledge of crystalline defects, particularly lattice vacancies (point defects) and dislocations (line defects) and their dynamics, broadened our understanding of microstructure‐sensitive micromechanical processes of failure in engineering materials. For historical reasons, the literature is still not clear about the exact definition of the term “plastic.” There are confusions between the classical “rate‐insensitive” or “time‐independent” plasticity (which is physically impossible because crystalline defects take time to move) and time‐dependent, rate‐sensitive flow, called “creep,” often treated as independent mechanisms. Moreover, the term “plastic” usually lumps together the permanent and the time‐dependent and hence rate‐sensitive recoverable anelastic or delayed elastic component of deformation readily observed at temperatures >0.3 T _m.

In this book, we try to avoid the term “plastic” and associated terms, like “viscoplasticity,” and others. We will use the term “viscous” for any permanent deformation. Viscous deformation provides a measure of irreversible changes in the structure of the material. This name is consistent with the term “viscoelastic,” irrespective of stress‐wise “linear” or “nonlinear” viscoelasticity (known as power‐law creep). Unfortunately, the use of the term “viscoelasticity,” because of historical usage in association with classical plasticity, may lead one to think of only linear stress dependency. We realize that the use of this term may therefore cause some uneasiness among the investigators accustomed to the characteristics often associated with this terminology in classical theories of plasticity. Of course, metallurgists liberally use plastic deformation for creep strain in metals. We will return to this topic in Section 1.7.