Stress relaxation as thermally activated process

CREEP AND STRESS RELAXATION AS THERMALLY ACTIVATED PROCESSES 231 10.2.4 Comparison of single-integral models... 

Creep and Stress Relaxation as Thermally Activated Processes... 

An hysteresis phenomenon is observed when the plateau pressure determined for an absorption isotherm is higher than the plateau pressure measured at the same temperature for the desorption process. Hysteresis is caused by the large stresses associated with the metal to hydride transformation which give rise to internal defects such as dislocations and stacking faults. Hysteresis decreases with increasing temperature as thermally activated stress relaxation processes set in. It is in general important to eliminate or at least minimize hysteresis for most applications. 

With regard to a real particle-matrix system exhibiting plastic deformation, the initial temperature, Ti < 0.5 X Tmq, represents the homologous temperature below which the stress relaxation does not occur as a consequence of thermal-activated processes [20, 21], where Tmq is the melting temperature of the particle ( = / ) or the matrix q = m), and if either Tmp < Tmm or Tmp > T m then either T p or Tmm is considered, respectively. Slowly cooling the real particle-matrix system at temperature T > 7], the thermal stresses are completely released by plastic deformations [20, 21]. 

Annealing in metals can first lead to stress relaxation in which stored internal strain energy due to plastic deformation is relieved by thermally activated dislocation motion (see Figure 5.18). Because there is enhanced atomic mobility at elevated temperatures, dislocation density can decrease during the recovery process. At still higher temperatures, a process known as recrystallization is possible, in which a new set of... 

At elevated temperatures, the behaviour of polymers is much more complex because thermally activated rearrangements and movements within and between the chains can occur, which are frequently reversible. These processes are mainly responsible for the physical and mechanical properties of polymers. They are called relaxation processes and are the topic of this section. Their name is due to the fact that they may cause a relaxation i.e., a reduction of applied stresses, as we will see later. 

If the stressed polymer has deformed viscoelastically by relaxation, the deformed configuration has a higher energy than the initial one. Upon unloading, the molecules return to their initial positions. This process again requires thermal activation and is therefore time-dependent as well. 

If a constant strain is imposed on a metal or alloy, the stress relaxes with time as the system reduces its free energy. Dislocations are annihilated and the remaining dislocations move to lower-energy configurations. This is the nature of the recovery process. At higher temperatures, diffusional processes equivalent to creep occur. This phenomenon is very important for solder joints in electronic devices because the device spends much time at the strain extremes when it is turned on and off or put into a sleep mode and returned to active duty. Stress at constant strain vs. time curves for Sn-3.5Ag solder at 25 °C and 80 ° C, and at 0.3 % strain maximum are given in Fig. 6(a,b). The stress as a result of the coefficient of thermal expansion mismatches initially decreases very rapidly with time to a more or less steady state value. At 25 °C, the steady state value is about 15 MPa and is rather independent of the initial stress value however, at 80°C, the stress relaxes to zero. Thus when an electronic device is turned on, the thermal stress will relax to a low value possibly zero during use. 

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