Mechanical creeping

Regenerated cellulose Hydrophilic, low-fouling Sensitive to temperature, pH, CI2, microbial attack, mechanical creep... 

The effect of heat, whether generated externally or by other mechanisms within the polymer itself, will be to raise the temperature. An increase in temperature will accelerate most of the degradation mechanisms listed, such as oxidation, chemical attack or mechanical creep. Temperature alone can cause thermal expansion, while at very high temperatures polymers will decompose, although this may be masked by the effects of oxidation. 

The mechanical properties of Nafion materials have not been of the most critical importance, as in the case of commercial thermoplastics or composite materials that are expected to be load-bearing. Rather, the primary focus has been on transport properties. To be sure, the mechanical integrity of membranes as mounted in cells, and under the perturbation of pressure gradients, swelling-dehydration cycles, mechanical creep, extreme temperatures, and the onset of brittleness and tear resistance, is important and must be taken into consideration. 

Identify the different types of mechanical creep and calculate creep rate. 

The integration of equation (7-8) is carried out using the integrating factor technique, which was fully explained in Chapter 3, equation (3-28). It should be clear that equation (7-9) represents the electrical analogue of the mechanical-creep experiment on a single Voigt element. 

Practical consequences of Eg modification in polymer films include significant changes of dissolution, diffusional and etching characteristics, mechanical creep behavior, and adhesion. Figure 17.30 shows a plot of the effective diffusion coefficient of perfluorooctane sulfonate photoacid as a function of film thickness of partially protected poly(4-t-butyloxycarbonyloxstyrene). The profile shows asymptotic behavior at 600 A, below which diffusion slows down remarkably, probably due to interfacial and confinement effects. Clearly, the interaction of the first few hundred angstroms of the film with the substrate determines its adhesion and can alter its electrical and optical properties as well as its topographical and surface characteristics. ... 

The mechanical properties to maintain spatial containment of the solar cells and Interconnects, and to resist mechanical creep. The level of mechanical properties also must not exceed values that would Impose undue mechanical stresses on the solar cell. 

Similarly, it must exhibit no significant mechanical creep at the upper operating temperature extreme of 90 C in order for the layup, cell positions, etc. to remain Intact when tilted at an angle facing the sun. 

In this mechanism creep occurs by dislocation motion, i.e., glide and climb. For the climb-controlled process the creep rate can be expressed as... 

In this mechanism creep b Burgers vector sion distance). 

Interpretation of mechanical measurements in terms of molecular structure was until fairly recently confined essentially to identification of the temperatures of the major viscoelastic relaxations through extensional or torsional dynamic mechanical studies. Now, however, investigations of the elastic constants and their temperature dependence—allied with dynamic mechanical, creep and both wide and small angle X-ray diffraction— are yielding fairly detailed pictures of the interrelation of the crystalline and less well ordered regions of some oriented solid polymers. 

The deformation by mechanical creep of the claddings, which tends to decrease the gaps and, therefore, the Doppler effect. 

Experimental crack growth rates, da/dN, are modelled using a linear summation of cyclic and creep contributions. A good agreement is obtained using a fracture mechanics creep parameter C s(t), estimated from a reference stress approach, which continously takes into account the effects of primary and secondary creep during the tests. Detailed results will be presented at the SMIRT 14 conference (LYON, August 17-22, 1997). 

As in other fabrication and finishing operations, special considerations must be given to mechanical fastening because of the nature of the plastic material. Care must be taken to avoid overstressing the parts. Mechanical creep can result in loss of preload in poorly designed systems. Reliable mechanically fastened plastic joints require ... 

RFcr = for reduced pore size caused by mechanical creep of material RFjn = for reduced pore space due to adjacent materials intruding into the geotextile... 

Plots of the product permeability versus time on a log-log coordinate system are often linear over relatively long time periods, as shown in Fig. 20.5-2. Similar behavior is observed in asymmetric reverse osmosis membranes. The log-log plotting approach provides a simple and reasonably satisfactory means of predicting the performance change of fibers under long-term operation by exuapolation of short-term data. Mechanical creep and volume recovery in glassy polymers aftw an initi perturbation also ate known to be reasonably represented on such log-log plots. ... 

In the NCTL test (see Sect. 3.2.13) ductile failure occurs at tensile stresses above 50 % of the stress at yield. Under about 30 % of the stress at yield the specimen fails through a brittle fracture. There is a range between 30 % and 50 %, where both mechanisms - creep and plastic deformation of the entire material cross-section as well as stress cracking - determine the fracture behaviour. This intermediate range and the range of ductile failure will be briefly dealt with below. The attention is first directed to stress crack formation that results in an unambiguous brittle failure. 

Mechanical creeping. Under meehanical stress the CP film experiences a viscoelastic irreversible deformation on each cycle. How to recover fliis deformation is a pending point. 

It was the conclusion of the evaluation that the two submitted gasoline storage tank spouts failed via slow crack initiation through a creep rupture mechanism. Creep is the application of static, or near static, stresses to a component over an extended period of time. The exertion of stresses below the yield point of the material results in disentanglement of the polymer chains without substantial permanent deformation. The time required to produce cracking is a complex function of the applied load, temperature, and the condition of the material. 

---