Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 113

Biomaterials often have to withstand a complex combination of loads that may operate in more than one direction. An implant designed to heal large defects in the long bones of the limbs, for example, should be capable of withstanding appropriate physiological loads not only in compression but also in bending (flexure) and twisting (torsion). The implant should also be capable of withstanding loads due to walking, running, and jumping which are composed predominantly of repeated cyclic loads and not just a constant load. Thus, an understanding of the mechanical response of biomaterials when subjected to these different loading modes is essential.

Different classes of materials, such as metals, ceramics, and polymers, show different characteristic responses when subjected to mechanical forces, such as the ductility of metals and the brittleness of ceramics. Unlike metals and ceramics, synthetic polymers and many natural materials show a property called viscoelasticity in which the mechanical response depends not just on the magnitude of the load but also on the time over which the load is applied. These different mechanical responses strongly influence the selection and design of biomaterials.

Our discussion of mechanical properties is confined to materials composed of a single type of solid. These properties can be modified or improved by combining two or more different solids to form composites, a topic considered in Chapter 12. While the general aspects of mechanical testing procedures are discussed when appropriate, these procedures are not covered in detail because standard procedures are well described by organizations such as the American Society for Testing and Materials (ASTM) and the International Organization for Standardization (ISO).