Lesson Notes By Weeks and Term v5 - Grade 10

Lesson Video

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Performance objectives

• Define and explain key mechanical properties of materials.
• Identify appropriate materials for given applications.
• Describe the difference between elastic and plastic deformation.
• Explain the importance of heat treatment for steel.
• Perform basic calculations related to stress and strain.

Lesson summary

This week, we delve deeper into the fascinating world of materials and their properties. Understanding these properties is crucial in Mechanical Technology because it allows us to select the right material for a specific job. Imagine building a bridge across the Orange River – you wouldn't use cardboard, would you? You need to understand which materials are strong enough to withstand the loads, resistant to corrosion from the environment, and durable enough to last for years. Similarly, designing a robust and reliable bakkie chassis requires a thorough understanding of material properties.

Lesson notes

Let's explore the mechanical properties that define how materials behave under different forces and conditions. 2.1 Tensile Strength: Tensile strength is a material's resistance to being pulled apart. It's the maximum stress a material can withstand while being stretched before breaking. Think about the cables supporting a suspension bridge. They need high tensile strength to hold the weight of the bridge and the traffic crossing it. We measure tensile strength in Pascals (Pa) or Megapascals (MPa). Imagine pulling on a steel wire versus a rubber band. The steel wire will withstand a much larger force before breaking because of its higher tensile strength. 2.2 Compressive Strength: Compressive strength is a material's resistance to being crushed or squeezed. It's the maximum stress a material can withstand while being compressed before failing. Think of the concrete pillars supporting a building. They need high compressive strength to bear the weight of the building. Brick, concrete, and stone possess high compressive strength. 2.3 Shear Strength: Shear strength is a material's resistance to forces that cause it to slide or shear along a plane. Imagine trying to cut a piece of paper with scissors. The scissors apply a shear force to the paper. Rivets holding two metal plates together need to have high shear strength. This is also important in the design of tools like bolt cutters and guillotines. 2.4 Ductility: Ductility is a material's ability to be stretched into a wire without breaking. Think of copper wires used in electrical wiring. Copper is highly ductile, allowing it to be drawn into thin wires. Gold is even more ductile than copper! A ductile material will undergo significant plastic deformation before fracture. 2.5 Malleability: Malleability is a material's ability to be hammered or rolled into thin sheets without breaking. Think of aluminium foil used in kitchens. Aluminium is highly malleable. Gold is also very malleable, which is why it is used in jewellery making. Malleable materials readily deform under compression. 2.6 Hardness: Hardness is a material's resistance to indentation or scratching. Diamond is the hardest naturally occurring material. Hardness is often measured using tests like the Brinell or Rockwell hardness tests. Think of a file used to sharpen tools – the file needs to be harder than the tool it's sharpening. 2.7 Toughness: Toughness is a material's ability to absorb energy and plastically deform before fracturing. A tough material can withstand both stress and strain before breaking. Think of the bumper on a car. It needs to be tough enough to absorb the impact of a collision without shattering. 2.8 Elasticity: Elasticity is a material's ability to return to its original shape after a force is applied and then removed. Think of a rubber band. When you stretch it and then release it, it returns to its original length. Elasticity is important in springs and other components that need to return to their original shape after being deformed. 2.9 Elastic vs.

Plastic Deformation: Elastic Deformation: A temporary change in shape that disappears when the force is removed. The material returns to its original dimensions.

Plastic Deformation: A permanent change in shape that remains even after the force is removed. The material does not return to its original dimensions. Bending a metal bar permanently is an example of plastic deformation. 2.10 Heat Treatment of Steel: Heat treatment processes like annealing, hardening, tempering, and normalizing can significantly alter the mechanical properties of steel.

Annealing: Softens steel, improves ductility, and relieves internal stresses.

Hardening: Increases the hardness and strength of steel, but also makes it more brittle.

Tempering: Reduces the brittleness of hardened steel while retaining some of its hardness and strength.

Normalizing: Refines the grain structure of steel, improving its toughness and machinability.

Example 1: Choosing a Material for a Car Chassis A car chassis needs to be strong, stiff, and able to withstand impacts.

Therefore, we need a material with high tensile strength, high yield strength (resistance to permanent deformation), and good toughness. Mild steel is a common choice because it offers a good balance of these properties at a reasonable cost.

However, high-performance vehicles may use high-strength steel alloys or even carbon fiber composites for increased strength and reduced weight.

Example 2: Calculating Stress Stress is defined as force per unit area.

The formula is: Stress (σ) = Force (F) / Area (A) Let's say a steel bar with a cross-sectional area of 0.001 m² is subjected to a tensile force of 10,000

N. What is the stress on the bar?

Solution: σ = F/A σ = 10,000 N / 0.001 m² σ = 10,000,000 Pa = 10 MPa Therefore, the stress on the steel bar is 10 MPa.

Example 3: Calculating Strain Strain is defined as the change in length divided by the original length.