Fibers creep

This review is intended to focus on ceramic matrix composite materials. However, the creep models which exist and which will be discussed are generic in the sense that they can apply to materials with polymer, metal or ceramic matrices. Only a case-by-case distinction between linear and nonlinear behavior separates the materials into classes of response. The temperature-dependent issue of whether the fibers creep or do not creep permits further classification. Therefore, in the review of the models, it is more attractive to use a classification scheme which accords with the nature of the material response rather than one which identifies the materials per se. Thus, this review could apply to polymer, metal or ceramic matrix materials equally well. 

Another case is when the fibers creep linearly and the matrix creeps with an index of 2. Then the matrix tends towards a stress... 

Creep of fibers also occurs much more rapidly than happens in bulk materials, in part because of the much smaller grain sizes present in the fibers. Creep and creep rupture test techniques for composites are described elsewhere in this volume (Chapter 4 Weiderhom and Fuller) but performing such tests on small diameter fibers requires somewhat different apparatus.57 To date, only a small number of fiber compositions have been characterized.58... 

Researchers have examined the creep and creep recovery of textile fibers extensively (13-21). For example, Hunt and Darlington (16, 17) studied the effects of temperature, humidity, and previous thermal history on the creep properties of Nylon 6,6. They were able to explain the shift in creep curves with changes in temperature and humidity. Lead-erman (19) studied the time dependence of creep at different temperatures and humidities. Shifts in creep curves due to changes in temperature and humidity were explained with simple equations and convenient shift factors. Morton and Hearle (21) also examined the dependence of fiber creep on temperature and humidity. Meredith (20) studied many mechanical properties, including creep of several generic fiber types. Phenomenological theory of linear viscoelasticity of semicrystalline polymers has been tested with creep measurements performed on textile fibers (18). From these works one can readily appreciate that creep behavior is affected by many factors on both practical and theoretical levels. 

Adsorption of transition metal species during processing, cellulose, 388 Advisory panels for standards for archival materials, potential benefits, 311 Age of cellulosic textiles determination from single-fiber creep measurements, 19-38 effect on crystallinity, 417 effect on rate of degradation, 416-17 Aged paper... 

Recommendation 3. Studies should be directed toward improving oxide fiber creep resistance and understanding the underlying scientific mechanisms. 

To gain a mechanistic understanding of SiC fiber creep, it is of interest to examine the property of fiber creep strength. This property is defined as the applied stress required to... 

TABLE 3. 1000-hr upper use-temperatures for SiC ceramic fibers as estimated from single fiber creep-rupture results in air (and argon)... 

It is difficult to arrive at a coherent picture of the distribution of strains in a cold-drawing fiber. Creep is always taking place in a cold-drawing specimen, yet theorists have concentrated heavily on travelling wave solutions. These cannot account for continuing creep in a drawn region far away from the boundaries of a neck. Perhaps travelling waves can be related to sufficiently... 

Figure 13.28 Plot to give creep rate deceleration factor (CROP). (Reproduced from Cawood, M.J., Channell, A.D. and Capaccio, C. (1993) Crack initiation and fiber creep in polyethylene. Polymer, 34, 423. Copyright (1993) Elsevier Ltd.)...

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