Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 128

Ceramics (including glasses and glass‐ceramics) are brittle and, thus, they undergo only elastic deformation prior to fracture (Figure 4.6). The majority of ceramics used as biomaterials such as alumina, zirconia‐toughened alumina (ZTA), yttria‐stabilized zirconia (YSZ), and silicon nitride typically have a higher Young’s modulus than most metals such as titanium and its alloys, stainless steel and cobalt–chromium alloys (Table 4.1). The strength of these ceramics, often determined in flexural testing, is also typically higher than the tensile strength of many metals and alloys. Consequently, the stress–strain curve consists of straight line and the stress at failure is often larger than the yield stress of many metals. Glasses typically have a lower Young’s modulus and flexural strength than most ceramics used in biomedical and engineering applications. The key parameters obtained from the stress–strain curves for brittle materials are their Young’s modulus, strength and strain to failure in flexural loading. In comparison, metals show a stress–strain behavior characterized by an elastic deformation followed by plastic deformation, and the key parameters are their Young’s modulus, yield strength, elastic limit, and elongation to failure.

Figure 4.6 Schematic stress–strain curves to illustrate the characteristic response of brittle ceramics and ductile metals.

At room temperature, polymers can show a brittle, ductile, or elastic response depending on their composition and structure (Figure 4.7). Some amorphous polymers such as polystyrene, for example, show a brittle response under some appropriate rate of loading, fracturing at tensile strains of less than approximately 0.5%. In comparison, semicrystalline polymers typically show a ductile response. Polyethylene, for example, shows a ductile response in which the strain to failure can be up to a few hundred percent. This difference in mechanical response between polystyrene and polyethylene is attributed to differences in their structure (see Section 8.2.6). Amorphous polymers with a crosslinked structure such as natural rubber show an elastic response over a very large strain (a few hundred percent for natural rubber) followed by failure, but the stress–strain curve is nonlinear. The strength and elastic modulus of polymers are far lower than those for ceramics and metals (Figure 1.5).

Figure 4.7 Schematic stress–strain curves to illustrate the characteristic response shown by different polymers near room temperature. A given polymer can show all three types of response depending on the rate of loading (or strain rate) and the temperature of the test.

As viscoelastic materials, polymers show a mechanical response that is highly sensitive to the rate of loading (or the time over which the load is applied) and to temperature. A given polymer, for example, can show all three types of response illustrated in Figure 4.7 depending on the rate of loading and the temperature. High loading rate or low temperature typically leads to a brittle response whereas a low loading rate and higher temperature lead to an elastic response comparable to a rubber. At intermediate loading rate or temperature, a ductile response is commonly observed. Many natural materials, such as collagen, bone, cartilage, tendons, and ligaments show a viscoelastic response similar to that of synthetic polymers whereas elastin, a major component of skin, shows an elastic response similar to that of rubber (Chapter 10).