Fibers stress relaxation

Another aspect of plasticity is the time dependent progressive deformation under constant load, known as creep. This process occurs when a fiber is loaded above the yield value and continues over several logarithmic decades of time. The extension under fixed load, or creep, is analogous to the relaxation of stress under fixed extension. Stress relaxation is the process whereby the stress that is generated as a result of a deformation is dissipated as a function of time. Both of these time dependent processes are reflections of plastic flow resulting from various molecular motions in the fiber. As a direct consequence of creep and stress relaxation, the shape of a stress—strain curve is in many cases strongly dependent on the rate of deformation, as is illustrated in Figure 6. 

DiCarlo, J. A., Creep Stress Relaxation Modeling of Polycrystalline Ceramic Fibers, NASA, 1994. 

Analogous results have been found for stress relaxation. In fibers, orientation increases the stress relaxation modulus compared to the unoriented polymer (69,247,248,250). Orientation also appears in some cases to decrease the rate, as well as the absolute value, at which the stress relaxes, especially at long times. However, in other cases, the stress relaxes more rapidly in the direction parallel to the chain orientation despite the increase in modulus (247.248,250). It appears that orientation can in some cases increase the ease with which one chain can slip by another. This could result from elimination of some chain entanglements or from more than normal free volume due to the quench-cooling of oriented polymers. 

Olefin fiber production, economic aspects of, 11 242-243 Olefin fibers, 11 224-246 applications of, 11 243-244 creep, stress relaxation, and elastic recovery in, 11 227-228 extrusion of, 11 231-234 hard-elastic, 11 242 high-strength, 11 241-242 manufacture and processing of,... 

Creep, Stress Relaxation, Elastic Recovery. Olefin fibers exhibit creep, or time-dependent deformation under load, and undergo stress relaxation, or the spontaneous relief of internal stress. High molecular weight and high orientation reduce creep. 

Trombitas, K., Wu, Y., and McNabb, M. (2003). Molecular basis of passive stress relaxation in human soleus fibers Assessment of role of immunoglobulin domain unfolding. Biophys. J. 85, 3142-3153. 

In terms of dyeing and finishing, CT is more similar to purely synthetic fibers than CA. It can be permanently pleated. For stress relaxation, articles made of CT, like those made of PES, are heat set (thermofixed) after dyeing [80, pp. 92-100], CT, like PES, can be dyed by the thermosol process (see Section 4.12.1). 

Background At elevated temperatures the rapid application of a sustained creep load to a fiber-reinforced ceramic typically produces an instantaneous elastic strain, followed by time-dependent creep deformation. Because the elastic constants, creep rates and stress-relaxation behavior of the fibers and matrix typically differ, a time-dependent redistribution in stress between the fibers and matrix will occur during creep. Even in the absence of an applied load, stress redistribution can occur if differences in the thermal expansion coefficients of the fibers and matrix generate residual stresses when a component is heated. For temperatures sufficient to cause the creep deformation of either constituent, this mismatch in creep resistance causes a progres-... 

Fig. 5.1 Idealized representation of the transient change in fiber and matrix stress that occurs during the isothermal tensile creep and creep recovery of a fiber-reinforced ceramic (the loading and unloading transients have been exaggerated for clarity). It is assumed that the fibers have a much higher creep resistance than the matrix. The matrix stress reaches a maximum at the end of the initial loading transient. After full application of the creep load, the matrix stress relaxes and the fiber stress increases. Upon specimen unloading, elastic contraction of the composite occurs, followed by a time-dependent decrease in fiber stress and increase in matrix stress. Overall, creep tends to increase the difference in stress between constituents and recovery tends to minimize the difference in stress. After Wu and Holmes.15...

The latter result indicates that the volumetric strains can be relaxed to some extent by matrix creep. This contrasts with the 3-D case where complete compatibility of strains precludes such relaxation. The extent to which the relaxation occurs has not yet been calculated. However, if it is assumed that the relaxation can be complete so that the matrix volumetric strain is zero, then the fiber stress tends towards aal3lf and, therefore, the composite strain approaches... 

The associated phenomenon of stress relaxation (continuous decrease in stress at constant strain) has been demonstrated in animal fibers at all levels of elongation (Rigby, 1955, 1959 Feughelman and Mitchell, 1959). 

On the other hand the nature of the retractive forces in the yield and post-yield regions has been the subject of much controversy. Bull (1945), Woods (1946a,b), Astbury (1947), Elod and Zahn (1949a), and Breuer (1962) have concluded from the effects of temperature on the retractive forces that entropy contributes very little to retractive forces at strains up to 30 %. Meyer and Haselbach (1949) and Meyer et al. (1952), however, consider that the fibers must reach an equilibrium condition before measurements are made and conclude that the forces are entirely entropic. There can be no doubt that after stress relaxation at high temperatures the residual force is largely entropic (Feughelman and Mitchell, 1959), but this force is only a fraction of the initial force. 

Stress-relaxation measurements confirm Burley s interchange hypothesis. Fibers pretreated with iodine or nitrous acid, both of which oxidize disulfide groups and probably also thiol groups (Sookne and Harris, 1937 Cockbum etal., 1948 Crewtherand Dowling, 1961a), were characterized by... 

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