Materials and Mechanical Engineering Sections 1. Introduction、2. Atomic Bonds、3. Material Properties、4. Selection of Materials Objectives After learning this chapter, you should be able to do the following : Understand how material properties are used to qualify materials for engineering design. . . Understand how traditional and composite materials are used in engineering design. 1.Introduction Material science encompasses the study of the structure and properties of any material, as well as using this body of knowledge to create new types of materials, and to tailor the properties of a material for specific uses. ? Materials are the foundation and fabric of manufactured products. ? Mechanical design is dependent on and limited by materials. ? New materials and processes enable other new technologies to be commercially successful. .Properties of materials are usually the deciding factor in choosing which materials should be used for a particular application. . Material properties depend on the material microstructure, which in turn results from its composition and processing. 2. Atomic Bonds Metallic Bonds In a metal, the outer electrons are shared among all the atoms in the solid. Each atom gives up its outer electrons and becomes slightly positively charged. The negatively charged electrons hold the metal atoms together. . Since the electrons are free to move, they lead to good thermal and electrical conductivity. Ionic Bonds Atoms like to have a filled outer shell of electrons. Sometimes, by transferring electrons from one atom to another, electron shells are filled. The donor atom will take a positive charge, and the acceptor will have a negative charge. The charged atoms or ions will be attracted to each other, and form bonds. Covalent Bonds Some atoms like to share electrons to complete their outer shells. Each pair of shared atoms is called a covalent bond. Hydrogen Bonds Hydrogen bonds are common in covalently bonded molecules which contain hydrogen, such as water (H2O). Notes There are two types of bonds: 1.Primary bonds ? ( Metallic Bonds, Covalent Bonds, Ionic Bonds ) 2.Secondary bonds (Hydrogen Bonds, etc.) Primary bonds are the strongest bonds which hold atoms together. Secondary bonds are much weaker than primary bonds. They often provide a "weak link" for deformation or fracture. 3. Material Properties The principal properties of materials which are of importance to the engineer in selecting materials can be broadly divided into : Mechanical Properties ( concerned mainly with strength ) Physical Properties ( such as melting temperature,density, etc ) 3.1 Tensile Test The mechanical properties used in engineering are determined by performing a tensile test. . Typical test machines may test the specimen in different ways including tension and compression. Stress-Strain Curve In a static tensile test, the stress-strain curve is produced. Characteristics of the curve include a linear region and a region of rapid elongation known as the plastic region. The point at which the linear region ends is called the proportional limit. The slope of the curve in the linear region is called the Young’s modulus. The proportional limit defines the point where a small increase in the stress yields a large deformation. This phenomenon is called yielding. Most engineering designs tend to avoid the plastic region. If the goal of a design is to design within the linear region, then all stresses in the structure or component must be below the yield stress. A material that can undergo large plastic deformation before fracture is called a ductile material. A material that exhibits little or no plastic deformation at failure is called a brittle material. 3.2 Young’s Modulus Young's modulus measures the resistance of a material to elastic (recoverable) deformation under load. . A stiff material has a high Young's modulus and changes its shape only slightly under elastic loads. A flexible material has a low Young's modulus and changes its shape considerably. . A stiff material requires high loads to elastically deform it - not to be confused with a strong material, which requires high loads to permanently deform (or break) it. Measurement (Tensile Test) 3.3 Strength The strength of a material is its resistance to failure by permanent deformation (usually by yielding). . A strong material requires high loads to permanently deform (or break) it - not to be confused with a stiff material, which requires high loads to elastically deform it. Measurement (Tensile Test) 3.4 Toughness （韧性） Toughness is the resistance of a material to being broken in two, by a crack running across it - this is called "fracture" and absorbs energy. . A tough material requires a lot of energy to break it, usually because the fracture process causes a lot of plastic deformation. . A brittle material may be strong but once a crack has started the material fractures easily because little energy is absorbed (e.g. glass). Measurement (Compact Tension Test) 3.5 Elongation （延展性）Measurement (Tensile Test) 3.6 Density Density is a measure of how heavy an object is for a given size, i.e. the mass of material per unit volume. . The weight of a product is a very common factor in design. In transport applications, lightweight design is very important - for example, to reduce the environmental impact of cars, or to increase the payload of aircraft. 3.7 Max. Service Temperature The strength of a material tends to fall quickly when a certain temperature is reached. This temperature limits the maximum operating temperature for which the material is useful. 3.8 Resistivity Resistivity is a measure of the resistance to electrical conduction for a given size of material. Resistivity is affected by temperature - for most materials the resistivity increases with temperature. An exception is semiconductors (e.g. silicon) in which the resistivity decreases with temperature. Measurement: The resistivity can be calculated quite easily be measuring the resistance of a piece of wire of constant cross-section and known length. 4. Selection of Materials The main criteria to influence the selection of materials for any particular engineering product can be summarized as the following: Property requirements、Processing requirements、Economic requirements Ultimately our final choice will involve a compromise. There is rarely, if ever, an ideal solution. 4.1 Mild Steel Steels are the most important engineering materials, and cover a wide range of alloys based on iron and carbon. . Mild steel contains 0.1-0.2 %C. They are cheap, strong steels used for construction, transport and packaging. . All steels have a high density and a high Young's modulus. The strength of mild steel is improved by cold working. It is inherently very tough. . Mild steel rusts easily, and must be protected by painting, galvanizing or other coatings. Design strengths: ? High strength-to-weight ratio ? High stiffness-to-weight ratio ? Good strength with high toughness ? Very cheap ? Easy to shape ? Easy to weld ? Easy to recycle Design weaknesses: ? High density ? Poor electrical and thermal conductivity 4.2 Alloy Steel Alloy steels are mostly fairly cheap, covering a range of carbon contents (0.1-1.0%). The high carbon content steels respond well to heat treatment to give very high strength and good toughness for gears, drive shafts, pressure vessels, tools. . Alloy steels containing other elements as well as carbon are classified into low alloy and high alloy, depending on the amount of additional alloying elements. Heat-treated high alloy steels give very high strengths, but are more expensive. . Alloy carbon steels rust easily, and must be protected by painting or other coatings. Design strengths: ? High strength with good toughness ? High stiffness ? Mostly very cheap ? Quite easy to shape ? Easy to weld ? Easy to recycle Design weaknesses: ? High density ? Poor electrical and thermal conductivity 4.3 Stainless Steel Stainless steels are more expensive steels containing typically 25% of Chromium and Nickel, which gives excellent corrosion resistance and also high strength and toughness (used for chemical plant and surgical instruments). Design strengths: ? High strength with good toughness ? High stiffness ? Mostly very cheap ? Quite easy to shape ? Quite easy to weld ? Easy to recycle Design weaknesses: ? High density ? Poor electrical and thermal conductivity 4.4 Aluminum Alloy Aluminum is a lightweight, reasonably cheap metal widely used for packaging and transport. It has only been widely available and used for the last 60 years. . Raw aluminum has low strength and high ductility (ideal for foil). Strength is increased by alloying and heat treatment. Some alloys are cast, others are used for wrought products. . Aluminum is quite reactive, but protects itself very effectively with a thin oxide layer to resist corrosion. Design strengths: ? High strength-to-weight ratio ? High stiffness-to-weight ratio ? High electrical and thermal conductivity ? Easy to shape ? Easy to recycle Design weaknesses: ? Difficult to arc weld 4.5 Titanium Alloys Titanium alloys are quite low density, stiff, strong alloys and are expensive. They are used most in sports products (e.g. golf clubs and bicycles) and in aircraft (e.g. engine fan blades). . Pure titanium has moderate strength, but the standard titanium alloy contains 6% aluminum and 4% vanadium, which gives the high strength needed in a jet engine. . Titanium is a reactive metal when hot, but has good corrosion resistance at room temperature. Design strengths: ? High strength, even at high temperatures ? High stiffness ? Corrosion resistant, even resistant to salt water Design weaknesses: ? High cost ? Chemically very reactive when hot ? Quite difficult to shape - usually cast 4.6 Silicon Silicon is doped with very low levels of other elements to give it the particular "semiconducting" electrical properties needed for transistors and microchips. . To supply the huge demand for computer chips, processes have developed so that it can be produced as very large high purity crystals. . Silicon is the base material used for the manufacture of computer chips, and is therefore one of the most important materials. Design strengths: ? Semiconducting properties Typical products: ? Transistors ? Computer chips 4.7 Diamond Diamond is covalently bonded pure carbon, and has the highest Young's modulus and hardness of all materials. . Diamond is naturally occurring but can also be manufactured. It is increasingly used for its very high hardness in cutting tools. Design strengths: ? Excellent corrosion resistance ? Low density ? High electrical resistance. ? High hardness Design weaknesses: ? Low tensile strength ? Low toughness ? Difficult to shape 4.8 Composite Composites are formed from two or more types of materials. Examples include polymer/ceramics and metal/ceramics composites. . Composites are used because overall properties of the composites are superior to those of the individual components. . For example: polymer/ceramics composites have a greater modulus than the polymer component, but are not as brittle as ceramics. There are two types of composites: Fiber Reinforced Composites Particle Reinforced Composites 4.9 Example 1 (Automobile) The new Lincoln LS represents a current example of the use of light weight materials on a high volume production vehicle Aluminum, plastics and magnesium are selected to achieve weight reduction. (Totally more than 20% of vehicle weight) The Ford P2000, is a good example of what the mix might be for vehicles by the end of the next two decades The P2000 meets the goal of 50% weight reduction in the body and chassis. To do this light weight materials are used in every application where feasible. 4.10 Example 2 (Aircraft) Structural materials mass distribution on the Boeing 747 and 777 : Aluminum alloys constitute by far the biggest proportion of structural mass of most modern aircraft, with steels, titanium alloys and structural composites all accounting for approximately 10%. A new series of aluminum alloys have recently been developed which contain the element lithium. These alloys are lighter and stiffer than existing alloys, and are now finding use on the latest aircraft designs. Notes Titanium has a density approximately twice that of aluminum, but when alloyed with other elements, can exhibit very high mechanical properties. The reason titanium alloys are not used more extensively on airframes is due to cost. Titanium alloys cost up to 10 times more than aluminum alloys Structural composite materials are finding increasing use on modern aircraft (especially military aircraft) because of their very attractive low density and high mechanical properties.