tailieunhanh - Introduction to Elasticity Part 13

Tham khảo tài liệu 'introduction to elasticity part 13', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Figure 14 The body-centered tetragonal structure of martensite. Kinetics of creep in crystalline materials Creep is the term used to describe the tendency of many materials to exhibit continuing deformation even though the stress is held constant. Viscoelastic polymers exhibit creep as was discussed in Module 19. However creep also occurs in polycrystalline metallic and ceramic systems most importantly when the the temperature is higher than approximately half their absolute melting temperature. This high-temperature creep can occur at stresses less than the yield stress but is related to this module s discussion of dislocation-controlled yield since dislocation motion often underlies the creep process as well. High-temperature creep is of concern in such applications as jet engines or nuclear reactors. This form of creep often consists of three distinct regimes as seen in Fig. 15 primary creep in which the material appears to harden so the creep rate diminishes with time secondary or steady state creep in which hardening and softening mechanisms appear to balance to produce a constant creep rate én and tertiary creep in which the material softens until creep rupture occurs. The entire creep curve reflects a competition between hardening mechanisms such as dislocation pileup and mechanisms such as dislocation climb and cross-slip which are termed recovery and which augment dislocation mobility. Figure 15 The three stages of creep. In most applications the secondary regime consumes most of the time to failure so much of 12 the modeling effort has been directed to this stage. The secondary creep rate èii can often be described by a general nonlinear expression of the form ẼII Aơm exp -X 11 p RT 8 where A and m are adjustable constants EC is an apparent activation energy for creep Ơ is the stress R is the Gas Constant to be replaced by Boltzman s constant if a molar basis is not used and T is the absolute temperature. This is known as the Weertman-Dorn equation. .