Introducing and Classifying Materials

1. State that materials can be classified into groups according to similarities in 
   their microstructures and properties.

2. Explain that several classifications are recognized but that no single 
   classification is “perfect”.

3. State that materials are classified into groups: 

• timber - natural wood or composite (plywood)

• metals - ferrous (iron or iron alloys) or nonferrous

• ceramics - earthenware, porcelain, and stoneware

• plastics - thermoplastics or thermosets

• textile fibers - natural or synthetic

•  food - vegetable or animal origin

• composites - fiber glass, carbon fiber

Properties of Materials (4h)

Physical Properties

4. Define the physical properties of: 

• density

• electrical resistively

• thermal conductivity

• thermal expansion - important if joining dissimilar materials

• hardness - resistance to penetration or scratching 

6. Define stress and explain how it is different from pressure although both use the same units.

• Stress = (force) / (unit of area) at any given point inside an object
• Pressure = (force) / (unit of area) applied to the outside surface of an object
• Pressure is an external load. Stress is an internal condition resulting from external loads of forces and pressures.

7. Define strain: the ratio of a change in dimension to the original value of that dimension.

8. Mechanical Properties

• tensile strength - the amount of tensile stress a material can withstand before failure

  • yield stress - max stress before permanent deformation

  • ultimate tensile stress - max stress before catastrophic failure

  • rupture stress - max stress at catastrophic failure

• stiffness - modulus of elasticity or Young's modulus

• toughness - resistance to abrasion and cutting. Work required to make a material fail catastrophically. Tough materials are generally not brittle.

• ductility - ability to extrude

• malleability - ability to shape plastically. Note: the IB syllabus makes a special point to make a distinction between ductility and malleability. For all practical purposes. ductile materials are also malleable.

9. Explain a design context where each of the above properties is an important consideration.

Draw and describe a stress/strain graph (see at right) elastic region yield stress plastic flow region ultimate stress (UTS).

11. Explain the relationship of the 2 most common structural members (beams and columns).

• Beams : horizontal
• Columns: vertical

12. Explain how both modulus of elasticity and moment of inertia are related to stiffness.

• Modulus of elasticity or Young’s modulus (a material property) - directly proportional to stiffness
• Area Moment of inertia (a design property) - directly proportional to stiffness

13. Calculate the Young’s modulus of a material.

    (Young’s modulus) = stress / strain

14. Calculate area moment of inertia for a beam with a rectangular cross section.

I ₀ = (bh³) / 12

 

where:

I ₀ = Area Moment of Inertial for a rectangular cross section of a beam

b = width

h = height in the direction of the load

15. Define elastic stability (the tendency of a structure to resist buckling) and state why it is an important consideration in design.
16. Give examples of elastic instability.

• External pressure applied to a plastic soft drink bottle's exterior
• Long thin columns

 

Aesthetic characteristics

17. Outline the characteristics of taste, smell, appearance, texture and color.
18. Explain a design context where each of the above characteristics are an
    important consideration.

 The IB Properties/Materials Matrix

19. Explain how all the groups and sub-groups of materials shown above can be
    organized into a properties/materials matrix
20. Explain the relative values of the properties in the IB properties/materials
    matrix.