Creep stages

Depending on the type of waving product used, there may be several variations to the procedure outlined above. Thus, instead of wrapping with lotion, the hair is wound wet and the lotion appHed to curled hair. Some instmctions also suggest a creep stage for better tightness and durabiUty. This is simply a 30 min wait between rinsing off the lotion and appHcation of the neutralizer. 

Fig. 4.1 Tensile creep curves for siliconized silicon carbide (Carborundum KX01). Over most of the data range, these data can be represented by a constant creep rate there is a short primary creep stage, and almost no tertiary creep. The rupture strain decreases with increasing creep rate. The strain to failure, =1.5%, indicates brittle behavior even at low rates of creep detormation. Figure from Ref. 28.

Figures 2 and 3 also present the expenmental results of the examination of steady-state creep. They are similar to those for primary creep. Therefore, the kinetics of deformation in the secondary creep stage may be expressed by an Arrhenius type equation analogous to that for the primary stage [24]...

Since the tertiary creep stage, accelerating creep, embraces a short time region, compared with the previous stages, it is difficult to investigate the kinetics of deformation on this stage. The next Section will be devoted to the final phenomenon of creep evolution, the fracture. 

Several tensile creep tests were carried out on SCS-6[o]/MoSiz-50Si3N4 composite specimens between 1273 and 1473 K in vacuum. Test durations of-1000 hr were achieved and some idea of long term durability was obtained. Specimens tested at these temperatures exhibited a short primary creep stage and an extended secondary stage. The minimum creep rates ranged from 1.0 x 10 to 2.0 x 10 at 70 MPa between 1373 and 1473 K. 

Primary (transient) creep can be considered as a consolidation process during which the structure of the material adjusts itself to the following steady-state creep stage. In some instances, like in cross-Unked elastomers at low stresses, the steady state is absent, with the creep rate decreasing to zero, and the total creep strain remaining constant. In this case, primary creep is a delayed response of the material to the applied stress. At higher stress levels, chain scission, oxidation effects etc. may influence this simple behavior. 

FIGURE 24.11. A schematic of a creep curve. A = instantaneous initial deformation which may contain a piastic component B = primary, C = secondary and D = tertiary creep stage. 

We observe that the recovery curve is almost a mirror image of the primary creep stage. 

Under the continuous earthquake activities, deformation was ongoing and the hypergene fissures developed more quickly. When the tensile fractures in the scarp deepened later on to a certain depth, the stress accumulation around the locked patches brought this part of the rock into an accelerated creep stage. After a strong earthquake, the still intact patches were broken along the sliding plane, which led to the development of the landslide. 

Cold rolling led also to an increase in the creep strain and secondary creep strain-rate. The creep activation energy was found to increase with increasing rolling reduction. Within the secondary creep stage, the creep process in polypropylene is mainly due to the a-relaxation process and most of the creep strain was recoverable. 

Equation 4.10 is the power law that can be used to model creep as a time-dependent function. However, it is expected that the initial slope corresponding to the first creep stage is different from the longer term slope. As the first creep stage takes place in less than an hour, this equation can be corrected for a better fit by subtracting a constant b equivalent to the instantaneous strain. The corrected equation can be written as ... 

The presence or even predominance of one of the three creep stages depends upon the following factors (a) the material properties and microstructure (b) the temperature and (c) the applied stress. In the case of Sn-Ag-Cu lead-free solders exposed to the accelerated aging conditions of - 55 and 125 °C (- 67 and 257 °F) and hold times of 15 min., time-dependent deformation will be largely primary creep. On the other hand, service conditions that expose the Sn-Ag-Cu solder to several hours at temperatures exceeding 125 °C, or 257 °F (e.g., automotive underhood applications) will potentially place (depending on the applied stress) the material into the steady-state regime. At this time, there is no data that correlate the final failure of a solder interconnection to laboratory tertiary creep data. This lack of correlation is likely due... 

It is apparent that quantitative analyses of creep behavior nearly always emphasize the steady-state regime. Steady-state creep represents a dynamic equilibrium condition. That equilibrium can be a balance between defect creation and annihilation rates or can result from a constant defect velocity. Irrespective of the mechanistic details, the rate kinetics of the minimum creep rate, dddtj, can provide valuable insight into the processes responsible for creep deformation, including those active in the primary creep stage. 

The creep of the ternary B2 phase (Ni,Fe)Al exhibits a power-law behavior at high temperatures—e.g. 60% of the melting temperature or higher—with creep rates between about 10 and 10 s (Rudy and Sauthoff, 1985 Rudy, 1986 Jung et al., 1987), i.e. the observed secondary creep rates, which refer to the steady-state creep stage, are described by the familiar Dorn equation for dislocation creep (Mukherjee et al., 1969) ... 

In the case of NijAl, the initial transient creep stage exhibiting normal primary creep, i.e. hardening with decelerating creep rate, is succeeded by a second transient stage with anomalous primary creep, i.e. softening with accelerating creep rate, which is known... 

A set of creep experiments at different load levels can be used to build creep-strength time curves indicating at which time- and load-related limits a failure under static load condition is to be expected for a certain adhesive-substrate combination. The construction of a so called creep rupture envelope by connecting the points of the onset of the tertiary creep stage is shown in Fig. 34.12. 

A typical creep curve has three stages primary, secondary, and tertiary. The test specimen has an instant extension as soon as the load is applied. It is marked as the initial strain, in Figure 4.14. The deformation rate will gradually slow down in the primary creep stage, and reaches a constant creep rate in the secondary creep stage. This constant creep rate is also the minimum creep rate and is usually referred to as the steady-state creep rate, or simply the creep rate. The slope of the curve can be calculated using Equation 4.19, where e is the creep rate and e and t are creep deformation and time, respectively. 

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