Generalized strain ratio

The modulus term in this equation can be obtained in the same way as in the previous example. However, the difference in this case is the term V. For elastic materials this is called Poissons Ratio and is the ratio of the transverse strain to the axial strain (See Appendix C). For any particular metal this is a constant, generally in the range 0.28 to 0.35. For plastics V is not a constant. It is dependent on time, temperature, stress, etc and so it is often given the alternative names of Creep Contraction Ratio or Lateral Strain Ratio. There is very little published information on the creep contraction ratio for plastics but generally it varies from about 0.33 for hard plastics (such as acrylic) to almost 0.5 for elastomers. Some typical values are given in Table 2.1 but do remember that these may change in specific loading situations. 

The physical meaning of the relaxation modulus C(t) is apparent in terms of this simple experiment. In general, the ratio of a stress to the corresponding strain is called a modulus, and for a perfectly elastic solid the equilibrium shear modulus G is defined as a/y, G(t) is its time-dependent analog as measured in an experiment with this particular time pattern. This experiment can be performed on both viscoelastic liquids and viscoelastic solids. 

The ratio of stress to strain in the initial linear portion of the stress—strain curve indicates the abiUty of a material to resist deformation and return to its original form. This modulus of elasticity, or Young s modulus, is related to many of the mechanical performance characteristics of textile products. The modulus of elasticity can be affected by drawing, ie, elongating the fiber environment, ie, wet or dry, temperature or other procedures. Values for commercial acetate and triacetate fibers are generally in the 2.2—4.0 N/tex (25—45 gf/den) range. 

A number of substituted triazines are used as herbicides, and their biodegradation has been discussed in Chapter 10, Part 1. Treatment of soil contaminated with atrazine (2-chloro-4-(ethylamino)-6-isopropylamino-l,3,5-triazine) illustrated a number of significant features. Although the soil that was used had the potential for degradation, a laboratory experiment with Pseudomonas sp. strain ADP that had an established potential for atrazine degradation revealed important limitations. There was a substantial decline in the numbers of Pseudomonas sp. strain ADP and only limited mineralization. Supplementation with citrate or succinate increased the survival of the strain, and successful mineralization was dependent on the preservation of a carbon/nitrogen ratio >10 (Silva et al. 2004). The last would apply generally to substrates with a low C/N ratio such as triazines. 

In this equation e is the longitudinal strain and er is the strain in the width (transverse) direction or the direction perpendicular to the applied force It can be shown that when Poisson s ratio is 0.50, the volume of the specimen remains constant while being stretched. This condition of constant volume holds for liquids and ideal rubbers. In general, there is an increase in volume, which is given by... 

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