Mr. Rogers' IB Design Technology Objectives
Syllabus 1st Quarter 2nd Quarter 3rd Quarter 4th Quarter
Design Project Materials Structures  

Topic 9 Structures

Unit Plan Practice Test Study Guide


Introducing and Classifying Materials

  1. 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.
  1. Define strain: the ratio of a change in dimension to the original value of that dimension.

  2. Calculate a tensile or compressive stress given a force and area.

  3. Calculate a tensile or compressive strain given an original dimension and the change in the dimension.

  4. 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.

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

  1. Draw and describe a stress/strain graph (see at right)

  • elastic region

  • yield stress

  • plastic flow region

  • ultimate stress (UTS).

  1. Explain the relationship of the 2 most common structural members (beams and columns).
  • Beams : horizontal
  • Columns: vertical
  1. 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
  1. Calculate the Young’s modulus of a material.

    (Young’s modulus) = stress / strain

  1. Calculate area moment of inertia for a beam with a rectangular cross section.
I 0 = (bh3) / 12
I 0 = Area Moment of Inertial for a rectangular cross section of a beam
b = width
h = height in the direction of the load
  1. Define elastic stability (the tendency of a structure to resist buckling) and state why it is an important consideration in design.
  2. Give examples of elastic instability.
  • External pressure applied to a plastic soft drink bottle's exterior
  • Long thin columns


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