International Baccalaureate: Design & Technology
Topic 8/Option B: Microstructures and Macrostructures
The microstructures topic is included to provide students with the concepts used to explain the properties of materials. These concepts also allow a designer to choose, based on a design context, the type of microstructure needed. This allows the designer to specify a type of material and a specific treatment, such as annealing, quenching or case-hardening.
8.1.1
State that all matter is composed of particles.
8.1.2
Define atom, molecule and ion.
Atoms are the smallest parts into which elements can be chemically divided. All atoms have the same basic structure.
8.1.3
Outline a bond as a force of attraction between particles.
8.1.4
Draw an equilibrium position of a particle in a bond using a general potential energy–separation curve.
Link to website giving an explanation
8.1.5
Define an element, compound, pure substance, mixture, alloy and composite. Composites are formed when two or more materials (from any material group) are combined to obtain different properties to those available from the original materials.
8.2.1
Draw and describe an ionic bond, a covalent bond and a metallic bond.
• In an ionic bond the opposing charges of the ions hold the crystal (eg NaCl) together in a lattice. The ions can often be separated easily in water but the electrons stay attached to their respective ions inside the crystal.
• In a covalent bond the outer electrons of some atoms can come close enough to overlap and be shared between the nuclei, thereby forming a covalent bond. Each pair of electrons shared is called a covalent bond. Mention of sigma, pi, double or triple bonds is not required.
• Metallic bonding involves outer electrons but these are freer and they can flow through the crystalline structure. The bonding is caused by attraction between the positively charged metal atom nuclei and the negatively charged cloud of free electrons, and is spread throughout the lattice—“Positively charged nuclei in a sea of electrons”. Specific arrangements of metal atoms in crystals are not required.
8.2.2
State that the three bonds in 8.2.1 are called primary bonds and their relative strengths are ionic > metallic > covalent. No quantitative details are required.
8.2.3
Outline secondary bonds as weak forces of attraction between molecules.
8.2.4
Explain what is meant by a network covalent (giant) structure, with reference to diamond and sand (SiO2). In diamond each carbon is covalently bonded to four other carbon atoms, tetrahedrally arranged. The carbons at the edges are attached to hydrogen atoms. In sand (silica, SiO2) the arrangement is also tetrahedral. Both sand and diamond are very hard.
8.2.5
Describe a crystal as a regular arrangement of particles (atoms, ions or molecules). Details of types of crystal are not required.
8.2.6
Define an amorphous material and a fibre structure.
• Amorphous materials do not have regular structures or crystal patterns. Short range order may occur as far as next neighbour atoms. The general appearance of amorphous materials is glossy and they can occur in ceramics, polymers and metals.
• Fibres have a length-to-thickness ratio of at least 80 but are generally much longer. Textile fibres and food are made up of polymers.
8.2.7
Explain melting in terms of the behaviour of particles and the bonding.
8.2.8
Explain boiling in terms of the behaviour of particles and the bonding.
8.2.9
Discuss the significance of pure substances melting at a fixed temperature, and mixtures softening over a range of temperatures before melting. The working of materials in a plastic condition is possible because of the range of temperatures over which a mixture softens.
8.3 The IB Properties/Bonding Matrix
8.3.1
Explain how the properties specified in the IB properties/materials matrix (see 3.3) can be organized into a properties/bonding matrix.
8.3.2
Describe the relative values of the materials in the IB properties/bonding matrix in terms of their bonding.
8.3.3
Analyse data related to the IB properties/bonding matrix.
8.3.4
Evaluate the importance of the IB properties/bonding matrix in a given design context.
8.4 The Properties of Metals and Alloys
8.4.1
State that metals (pure or alloyed) exist as crystals. Details of crystal types are not required.
8.4.2
Draw and describe what is meant by grain size.
8.4.3
Explain how grain size can be controlled and modified by the rate of cooling of the molten metal, or by heat treatment after solidification. Reheating a solid metal or alloy allows material to diffuse between neighbouring grains and the grain structure to change. Slow cooling allows larger grains to form; rapid cooling produces smaller grains. Directional properties in the structure may be achieved by selectively cooling one area of the solid.
8.4.4
8.4.5
Explain in words and diagrams how metals work harden after being plastically deformed. Beyond the yield stress metals and alloys harden when plastically deformed. See 8.7.4 for details.
8.4.6
Describe in words and diagrams how the tensile strength of a metal is increased by alloying.
8.4.7
Explain in words and diagrams the effect of alloying on malleability and ductility. 8.4.6–8.4.7 The increased strength and hardness, and reduced malleability and ductility, of alloys compared to pure metals is due to the presence of “foreign” atoms which interfere with the movements of atoms in the crystals during plastic deformation.
8.4.8
Explain, using words and a diagram, how the movement of free electrons makes metals very good electrical and thermal conductors. The explanation should involve the mobility of the “sea” of electrons.
8.4.9
Evaluate the importance of bonding and structure to the properties of metals and alloys in a given design context.
8.5 The Properties of Thermoplastics and Thermosets
8.5.1
Draw and describe the structure and bonding of a thermoplastic. Thermoplastics are linear chain molecules, sometimes with side bonding of the molecules, but with weak, secondary bonding between the chains.
8.5.2
Explain the effect of load on a thermoplastic, with reference to orientation of chains. Deformation occurs in two ways: elastic in which initially coiled chains are stretched and plastic at higher loads, where secondary bonds weaken and allow the molecular chains to slide over each other. Creep and flow are important. No quantitative details required.
8.5.3
Explain the reversible effect of temperature on a thermoplastic, with reference to orientation of chains. Increase in temperature causes plastic deformation, see 8.5.2.
8.5.4
Draw and describe the structure and bonding of a thermoset. Thermosets are formed by making primary (covalent) bonds which form strong, primary cross-links between adjacent polymer chains. This gives the thermosets a rigid three-dimensional structure.
8.5.5
Explain the non-reversible effect of temperature on a thermoset. Heating increases the number of permanent cross links and so hardens the plastic.
8.5.6
Discuss the properties and uses of two thermoplastics including polypropene.
8.5.7
Discuss the properties and uses of two thermosets including polyurethane.
8.5.8
Discuss the importance of the properties of plastics on the recycling of plastics.
8.5.9
Evaluate the importance of the properties of thermoplastics and thermosets on product design.
8.6 The Properties of Composite Materials
8.6.1
State that composites are a combination of two or more materials which are bonded together to improve their mechanical, physical, chemical or electrical properties. Recall of specific composites (other than those mentioned in 8.6.2–8.6.6) and details of bonding are not required.
8.6.2
Describe the structure of wood as a natural composite material. Cellulose fibres in a lignin matrix—the tensile strength is greater along the grain (fibre) than across the grain (matrix).
8.6.3
Discuss in outline the evolution of synthetic composite materials with reference to wattle-and-daub, mortar, papier mâché, reinforced concrete, glass reinforced plastic (GRP), carbon reinforced plastic (CRP) and high temperature superconductors. Emphasize the historical aspects. The details of bonding or proportions of materials are not required.
8.6.4
Outline the structure of KevlarTM (aramid) fibre. Aramid fibre comprises linear chains of hydrocarbon rings. Aramid chains behave like rigid rods and they are aligned along the length of the fibre during the manufacturing process. They have a very high tensile structure.
8.6.5
Outline two examples where KevlarTM fibres twisted into ropes and woven into sheets (mats) are used.
8.6.6
Explain why KevlarTM is suited to these applications with reference to its tensile strength, elasticity and water resistance. KevlarTM fibres do not absorb water, have a high tensile strength and are non-stretch.
Rope—light, non-stretch ropes used in sailing boats.
Sheets—sails that hold their shape under great stress in high-performance yachts; composite structures with resin eg motor racing cars.
8.6.7
Evaluate the importance of the properties of composite materials in a design context.
8.7 Young’s Modulus—Stress and Strain
Young's Modulus Graph Explained
8.7.1
The load on a structural member divided by its cross-sectional area is called the “stress in the member”. State that stress (load) is force per unit area acting on a body or system.
8.7.2
State that strain is the ratio of a change in dimension to the original value of that dimension.
The strain in a material is a measure of the relative change of shape it undergoes when subjected to a load. It is independent of the size of the structural member.
8.7.3
8.7.4
Draw and describe a stress/strain graph and identify the elastic region, yield stress, plastic flow region and ultimate stress (UTS).For most materials the elastic region is a straight line which changes to a curved line (plastic region). Quantitative details of specific materials are not required.
8.7.5
Explain the relevance of the stress/strain graph identified in 8.7.4 in design contexts.
8.7.6
Outline the importance of yield stress. This is the stress at the yield point. Many materials undergo plastic as well as elastic deformation.
8.7.7
Describe the difference between elastic and plastic strains.In the straight portion of the stress/strain graph the material behaves elastically, ie if the stress on the material is released before it breaks, the extension (strain) relaxes and the material returns to its original length.
8.7.8
The stiffness of a material is called the “elastic modulus”. In relation to tensile and compressive loads the elastic modulus is called “Young’s modulus”.
8.7.9
Calculate the Young’s modulus of a material.
Young’s modulus = stress
strain
8.7.10
Use an example where stiffness of a material is important. Explain how knowledge of the Young’s modulus of a material affects the selection of materials for particular applications.
8.7.11
Evaluate the importance of stress and strain in a design context
8.8.1
Outline what is meant by an external load acting as a structure. This involves loads where physical contact is made.
8.8.2
Outline what is meant by body load. This is a load without physical contact, eg a structure’s own weight.
8.8.3
Describe the difference between weight and mass.
8.8.4
State the units of weight and mass.
8.8.5
Outline the relationships of external loads to internal forces and the concept of the balance of equilibrium of forces within a structure. Describe how a structure “works” by interpreting how external loads give rise to internal forces within the structural members. A static structure is in equilibrium, otherwise it would move, ie the forces acting upon it are equal in size and opposite in direction.
8.8.6
Outline the differences between tensile and compressive forces and how they affect equilibrium within a structure. Tensile loads tend to extend or stretch a structural member. Compressive loads tend to compress or shorten a structural member. Tensile and compressive forces must balance if the structure is to maintain equilibrium. Only forces that are parallel or perpendicular need to be considered. Knowledge of trigonometry or quantitative resolution of vectors into components is not required.
8.8.7
Calculate a tensile or compressive strain given values of force and area.
Stress =force
area
8.8.8
Calculate a tensile or compressive strain given values of the original dimension and the change in dimension.
Strain = change of length
original length
8.8.9
Evaluate the importance of forces in a design context.
8.9 The Strength and Stiffness of Structures
8.9.1
Describe the relationship between deflection and stiffness in structures. If an external load is applied to some part of a structure, that part will be deflected to some extent, depending on the size of the load and the stiffness of the structure.
8.9.2
Calculate the stiffness of a structure.
Stiffness = load
deflection
8.9.3
Outline what is meant by “bending moment” in relation to structures.
8.9.4
Outline what is meant by “moment arm”. The load x distance from the pivot is called the “moment” about the pivot. The distance between the load and the pivot is called the “moment arm”.
8.9.5
Explain the need for a factor of safety in structural design. Structures are designed to take higher loads than those they are normally expected to support.
8.9.6
Calculate the factor of safety for a structure.
Factor of safety = design load
normal maximum load
8.9.7
Apply the concept of factor of safety to other areas of design. A factor of safety is simply a ratio of the quantitative value of a design (factor) divided by the normal maximum expected value, eg stiffness, fuel tank volumes, electrical resistance, engine acceleration.
8.9.8
Evaluate the importance of strength and stiffness in a design context.