Life's Devices by Steven Vogel;

Life's Devices by Steven Vogel;

Author:Steven Vogel; [Неизв.]
Language: eng
Format: epub
Publisher: Princeton University Press
Published: 2020-03-21T21:00:00+00:00


FIGURE 9.2. Two materials of the same strength and extensibility but with very different values of work of extension. While the values of strength and extensibility have been set equal to emphasize the contrast, the shapes of the curves realistically represent tendon and arterial wall.

Table 9.4 gives this “strain energy storage,” or “work of extension,” (or in some accounts “toughness” or “resilience,” names used here for other variables) for a variety of materials under, unfortunately, not exactly comparable test conditions. The main uncertainty is, of course, just how far to extend a material in a test—to the elastic limit (the maximum distance from which it will snap back), to some safe working limit before fracture, or to the breaking point itself.

The inclusion of work normalized to mass as well as volume in the table is an attempt to find a more reasonable basis for comparing natural and technological materials. Thus, collagen is only a little better than steel on a volumetric basis but far surpasses it relative to mass. Natural rubber is a peculiar case—it’s very extensible but not at all strong; the area under its stress-strain curve, though, gives some idea why it is useful for powering model airplanes! Spider silk is out of sight, about which more later.

(5) And a final property from these stress-strain graphs. Say one stretches a material and then allows it to recover elastically its original length while measuring its stress throughout the test. A real material will not recover along quite the same stress-strain curve that described the stretch; instead it will trace a curve somewhat below that along which it ascended (Figure 9.3). Therefore, the area under the curve representing the stretch will be greater than that representing recovery. What this means is that not all the work put into stretch comes out again during relaxation in mechanical form.

TABLE 9.4. STRAIN ENERGY STORAGE, OR WORK OF EXTENSION, AT PARTICULAR STRAIN LEVELS

Work/Vol.

(MJ·m3) Work/Mass

(J·kg–1) Strain

(%) Stress

(MPa)

Yew wood 0.5 900 0.9 120

Spring steel 1.0 130 0.3 700

Keratin (horn) 1.8 1500 4.0 90

Collagen (tendon) 2.8 2500 8.0 70

Bone 3.0 1500 2.0 150

Rubber (“natural”) 10.0 8000 300.0 7

Spider silk 200 160,000 30 1400

(Data from Gordon 1978; Currey 1984; Wainwright et al. 1976)



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