Stress-strain behavior fibers

Stress—Strain Curve. Other than the necessity for adequate tensile strength to allow processibiUty and adequate finished fabric strength, the performance characteristics of many textile items are governed by properties of fibers measured at relatively low strains (up to 5% extension) and by the change ia these properties as a function of varyiag environmental conditions (48). Thus, the whole stress—strain behavior of fibers from 2ero to ultimate extension should be studied, and various parameters should be selected to identify characteristics that can be related to performance. 

Typical patterns of stress—strain behavior and the relationship of molecular motion on stress—strain behavior have been discussed (10,18,19,21,49—51). At times, it becomes desirable to characterize stress—strain behavior numerically so that a large amount of information can be condensed and many fibers exhibiting different behaviors can be compared. Procedures for measurement of stress—strain parameters are described ia ASTMD3822 andD2101 (10). 

The properties of the lamina constituents, the fibers and the matrix, have been only briefly discussed so far. Their stress-strain behavior is typified as one of the four classes depicted in Figure 1-8. Fibers generally exhibit linear elastic behavior, although reinforcing steel bars in concrete are more nearly elastic-pertectly plastic. Aluminum, as well as... 

Robert M. Jones ar Harold S. Morgan, Analysis of Nonlinear Stress-Strain Behavior of Fiber-Reinforced Composite Materials, AIAA Journal, December 1977, pp. 1669-1676. 

Harold S. Morgan and Robert M. Jones, Analysis of Nonlinear Stress-Strain Behavior of Laminated Fiber-Reinforced Composite Materials, Proceedings of the 1978 International Conference on Composite Materials, Bryan R. Noton, Robert A. Signorelli, Kenneth N. Street, and Leslie N. Phillips (Editors), Toronto, Canada, 16-20 April 1978, American Institute of Mining, Metallurgical a Petroleum Engineers, New York, 1978, pp. 337-352. 

The stress-strain behavior of plastics in flexure generally follows from the behavior observed in tension and compression for either unreinforced or reinforced plastics. The flexural modulus of elasticity is nominally the average between the tension and compression moduli. The flexural yield point is generally that which is observed in tension, but this is not easily discerned, because the strain gradient in the flexural RP sample essentially eliminates any abrupt change in the flexural stress-strain relationship when the extreme fibers start to yield. 

Figure 12.10 displays the stress-strain behavior of PET fibers that were prepared from the same spun yam, but drawn to different ratios. The curves represent the elongation and stress in terms of initial fiber area (decitex2). The open circles represent true stress values, where stress values at break are corrected for the decreased area of the fiber after extension on the testing device. 

FIGURE 14.6 Typical stress-strain behavior for fibers, plastics, and elastomers. 

At the end of Section 5.4.2.5, the statement was made that most continuous, unidirectional fiber-reinforced composites are used to produce layers that are subsequently assembled to form laminate composites. In this section, we expound upon this statement by examining the mechanics of laminate composites, first through a generalized description of their mechanics, then with some specific stress-strain behavior. 

In cross-ply laminates, the stress-strain behavior is slightly nonlinear, as illustrated in Figure 5.123. The stress-strain behavior of a unidirectional lamina along the fiber axis is shown in the top curve, while the stress-strain behavior for transverse loading is illustrated in the bottom curve. The stress-strain curve of the cross-ply composite, in the middle, exhibits a knee, indicated by strength ajc, which corresponds to the rupture of the fibers in the 90° ply. The 0° ply then bears the load, until it too ruptures at a composite fracture strength of ct/. 

Figure 16. Comparison of effects of fiber reinforcement vs. high modulus carbon black on stress-strain behavior of rubber...

Figure 17 illustrates the effect of orientation on the stress-strain properties of the rayon composite shown in Figures 10 and 11. The upper curve represents stress-strain behavior for stress applied parallel to the fiber orientation direction. In the lower curve the force is applied perpendicularly. Even a small degree of orientation has a large effect on the anisotropy of the composite. The differences in tensile strength, modulus and elongation at break in the two directions are considerable. 

Generally, when testing materials with a nonlinear stress-strain behavior, the tests should be conducted under uniform stress fields, such that the associated damage evolution is also uniform over the gauge section where the material s response is measured. Because the stress field varies with distance from the neutral axis in bending tests, uniaxial tension or compression tests are preferred when characterizing the strength and failure behavior of fiber-reinforced composites. 

Fig. 6.2 Room temperature stress-strain behavior of a woven 0°/90° Q/SiC composite. Because of processing-related matrix cracking and progressive fracture near the crossover points of fiber bundles, Stage II behavior (non-linear stress-strain response) is observed from the onset of loading. Above a strain of approximately 0.5% the composite exhibits Stage III (linear) behavior.

Stress Distribution and High Temperature Creep Rate of Discontinuous Fiber Reinforced Metals, Acta Metallurgies et Materialia, 38, 1941-1953 (1990). 26. A. G. Evans, J. W. Hutchinson, and R. M. McMeeking, Stress-Strain Behavior of Metal Matrix Composites with Discontinuous Reinforcements, Scripta Metallurgica et Materialia, 25, 3-8 (1991). 

The strength of the fiber-matrix interface is one of the key parameters responsible for the stress-strain behavior and damage tolerance of ceramic composites. Two different types of tests are available to measure the fiber-matrix interfacial properties in fiber-reinforced ceramic composites. The first is based on an indentation technique to either push the individual fiber into or through the matrix. The second test method relies on pulling a single fiber out of a matrix. These methods have been compared59 to one another for a glass matrix material, and yield similar results. 

The Technora fiber made by Teijin shows properties that are a compromise between conventional fibers and rigid-rod fibers. Table 4.3 provides a summary of these. In terms of its stress-strain behavior, it can be said that Technora fiber lies in between Kevlar 49 and Kevlar 29. 

The stress-strain behavior of iodinated fibers in water illustrates further the contribution of disulfide bonds to the stiffness of the fiber (Hoare et al., 1962). When strained into the post-yield region wool fibers containing loosely combined iodine are drastically modified and on removal of iodine with Na2S20s have the physical properties of wool containing few disulfide bonds. It has been suggested that the disulfide bonds become more reactive when stressed (cf. Speakman, 1936a Bindley and Human, 1957). 

Figure 7a and b show typical three-dimensional surface topographic images of the PHB fibers drawn at a draw ratio of 4.0 and 7. The surfaces of the fibers differ considerably. Depending on the draw ratio, spherulitic or fibril-like surface structures were formed. The textile physical properties of the fibers can be explained by these different structures. The fibers, spun at a draw ratio of 4.0, are brittle without a sufficient elongation at break visible in the stress-strain curve (Fig. 5). The fibers spun at a draw ratio of 7 show a completely different stress-strain behavior with a sufficient elongation at break and a sufficient tenacity, as can be seen from the stress-strain curve (Fig. 5).

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