tailieunhanh - Rules of Thumb for Mechanical Engineers Episode 10

Tham khảo tài liệu 'rules of thumb for mechanical engineers episode 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 260 Rules of Thumb for Mechanical Engineers In this chapter material properties a few definitions and some typical applications will be presented as guidelines for material selection. The most important rule for material selection is that the operating conditions must be well defined. These conditions include temperature environment impurities stress strain cost and any limitations for life cycle such as creep or fatigue. CLASSES OF MATERIALS Materials can be broken down into three major classes each having specific characteristics. Metals ceramics and polymers constitute these classes and some typical properties and applications will be described. In addition fabrication methods for the materials will be described. Metals are characterized by metallic bonding in which the electrons are shared in a veritable sea of electtons. Each nucleus is surrounded by electrons but the electrons are not specifically attached to any particular nucleus. Metals are further characterized by theft surface appearance and generally have a metallic sheen and can be polished to a mirror finish. Metals are also capable of sustaining large loads they are ductile and they have reversible elastic properties to a point. They can be alloyed to alter their physical and chemical properties. Metals and alloys are defined by the major alloying element present. Common engineering alloys consist of iron nickel cobalt aluminum magnesium titanium and copper. These alloys will be discussed in some detail after some definitions are presented. Polymers are long chains of carbon and hydrogen arranged in specific bonding orientations. The bonds are generally covalent bonds. Noncrystalline or amorphous polymers have a random arrangement of polymer chains. Crystalline polymers have specific polymer chain arrangements. The unit cells smallest repeating arrangement of atoms to make the full structure are large 10 run relative to metal unit cells nm . Polymer properties can be elastic viscous or .