Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Recall and apply concepts of materials (metals, polymers, composites).
• Analyze and solve problems related to forces, stress, and strain.
• Define and apply workshop safety procedures (OHSA).
• Interpret Occupational Health and Safety Act regulations.
• Differentiate between various joining methods.

Lesson summary

Welcome, Grade 12 Mechanical Technology students, to our first week of exam revision! This week is dedicated to laying a strong foundation for your upcoming final examinations. Mechanical Technology plays a crucial role in South Africa, from the manufacturing industries in Gauteng to the mining operations in Mpumalanga and the agricultural sector in the Western Cape. Understanding these principles is not just about passing exams; it’s about contributing to our country's economic growth and technological advancement. Many of you may pursue careers in engineering, manufacturing, or maintenance, and a solid grasp of these fundamentals is essential.

Lesson notes

This week focuses on revising foundational concepts. We will cover materials, forces, stress/strain, safety, and joining methods.

A. Materials: Ferrous Metals: These are metals containing iron (Fe). Examples include steel (carbon steel, stainless steel, alloy steel), cast iron, and wrought iron. They are known for their strength and durability but are susceptible to corrosion. In South Africa, ferrous metals are critical in construction (steel reinforcing bars), automotive manufacturing (engine blocks, chassis), and mining (excavation equipment).

Non-Ferrous Metals: These are metals that do not contain iron. Examples include aluminum, copper, brass, bronze, and titanium. They are often lighter than ferrous metals, resistant to corrosion, and possess good electrical conductivity. Non-ferrous metals are vital in the electrical industry (copper wiring), aerospace (aluminum alloys), and plumbing (copper pipes).

Polymers: These are large molecules made up of repeating structural units (monomers). Examples include polyethylene (PE), polyvinyl chloride (PVC), polypropylene (PP), and nylon. They are generally lightweight, corrosion-resistant, and can be molded into complex shapes. Polymers are widely used in packaging, piping, automotive components, and textiles. South Africa has a significant plastics industry.

Composites: These are materials made from two or more constituent materials with significantly different physical or chemical properties that, when combined, produce a material with characteristics different from the individual components. Examples include fiberglass (glass fibers in a polymer matrix) and carbon fiber reinforced polymers (CFRP). Composites offer high strength-to-weight ratios and corrosion resistance, making them suitable for aerospace, automotive, and sporting goods applications.

Properties of Materials: Key properties to consider are: Tensile Strength: The maximum stress a material can withstand while being stretched or pulled before breaking.

Yield Strength: The stress at which a material begins to deform permanently.

Hardness: Resistance to indentation or scratching.

Ductility: The ability of a material to be drawn into a wire.

Malleability: The ability of a material to be hammered or rolled into thin sheets.

Brittleness: The tendency of a material to fracture without significant deformation.

Corrosion Resistance: The ability of a material to resist degradation due to chemical reactions with its environment.

Example: Consider the chassis of a bakkie (pickup truck) in South Africa. Why is steel used? Steel provides the necessary tensile strength and yield strength to withstand the stresses of carrying heavy loads and navigating rough terrain.

However, to prevent corrosion in coastal regions, the steel may be galvanized (coated with zinc) or treated with a corrosion-resistant paint.

B. Forces, Stress, and Strain: Force: A push or pull that can cause an object to accelerate or deform. Measured in Newtons (N). Stress (σ): The force acting per unit area within a material. It is a measure of the internal forces that molecules within a continuous material exert on each other. σ = Force (F) / Area (A). Measured in Pascals (Pa) or N/m 2 . Strain (ε): The deformation of a material caused by stress. It is a dimensionless quantity. ε = Change in Length (ΔL) / Original Length (L).

Types of Stress: Tensile stress (pulling), compressive stress (pushing), shear stress (sliding).

Hooke's Law: States that stress is proportional to strain within the elastic limit of a material: σ = Eε, where E is the modulus of elasticity (Young's modulus).

Factor of Safety (FoS): A ratio of the ultimate tensile strength (UTS) or yield strength to the working stress. FoS = UTS/Working Stress or Yield Strength/Working Stress. A higher FoS indicates a safer design.