Lesson Notes By Weeks and Term v5 - Grade 10
Basic mechanical materials and properties – Week 7 focus
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Basic mechanical materials and properties – Week 7 focus
Download the Lessonotes Mobile South Africa app for faster lesson access on Android and iPhone.
Subject: Mechanical Technology
Class: Grade 10
Term: 1st Term
Week: 7
Theme: General lesson support
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In Mechanical Technology, understanding the properties of materials is absolutely crucial. The materials we use dictate the strength, durability, and functionality of everything from a simple spanner to complex machinery used in South African mines or the automotive industry. Selecting the right material for the job is key to ensuring safety, efficiency, and longevity. This week, we will delve deeper into the mechanical properties of materials, building upon what we learned in previous weeks. Understanding these properties allows you to make informed decisions when designing or repairing mechanical components, ensuring they can withstand the forces and conditions they will encounter.
2. 1. Elasticity Elasticity is the ability of a material to return to its original shape and size after the removal of an applied force or load. Think of a rubber band – when you stretch it and then release it, it returns to its original length. The material undergoes elastic deformation which is temporary and reversible. Within the elastic limit, stress is proportional to strain (Hooke’s Law). Beyond this limit, the material will undergo plastic deformation.
Example: The springs in the suspension of a taxi need to be highly elastic to absorb bumps and provide a comfortable ride for passengers. 2.
2. Plasticity Plasticity is the ability of a material to undergo permanent deformation without fracturing or breaking. When a force is applied and then removed, the material does not return to its original shape; it retains the deformation. This is called plastic deformation and is permanent and irreversible.
Example: The process of bending metal sheets to create the body panels of a car relies on the plasticity of the metal. Once bent, the metal retains its new shape. 2.
3. Ductility Ductility is the ability of a material to be drawn into a wire or elongated under tensile stress without fracturing. A ductile material can be stretched into a thin wire. Ductility is often measured by percent elongation or reduction in area.
Example: Copper is a very ductile material, which is why it's used extensively in electrical wiring. It can be easily drawn into long, thin wires. 2.
4. Malleability Malleability is the ability of a material to be deformed into thin sheets by hammering or rolling without fracturing. A malleable material can be flattened or shaped into sheets.
Example: Gold is a very malleable metal, which is why it can be hammered into thin gold leaf used for decorative purposes. Steel, used to create car panels, also needs to have sufficient malleability. 2.
5. Toughness Toughness is the ability of a material to absorb energy and plastically deform before fracturing. It is a measure of a material's resistance to crack propagation. A tough material can withstand both high stress and high strain. It is NOT the same as hardness. Think of it as the amount of "abuse" a material can take before failing. Toughness is the area under the stress-strain curve.
Example: The steel used in the chassis of a truck needs to be tough to withstand the impact of bumps and potholes on South African roads. 2.
6. Hardness Hardness is the resistance of a material to localized plastic deformation, usually by indentation or scratching. Hardness is often measured using tests like the Vickers hardness test or the Rockwell hardness test. A hard material resists scratching and wear.
Example: The cutting tools used in machining operations need to be very hard to cut through other materials. High-speed steel and cemented carbides are examples of hard materials used for cutting tools. 2.
7. Brittleness Brittleness is the tendency of a material to fracture without significant plastic deformation. Brittle materials break easily with little or no bending or stretching. They have low toughness.
Example: Glass is a brittle material. When dropped, it shatters into many pieces without bending or deforming. Cast iron is also relatively brittle. 2.8 Stress-Strain Relationship When an external force (load) is applied to a material, it experiences stress and strain. Stress (σ): is defined as the force (F) acting per unit area (A). σ = F/
A. Measured in Pascals (Pa) or N/m². Strain (ε): is defined as the change in length (ΔL) divided by the original length (L). ε = ΔL/L. It is a dimensionless quantity. The relationship between stress and strain is unique for each material and is graphically represented by a stress-strain curve. 2.9 Worked
Examples: Example 1: Calculating Stress A steel cable with a cross-sectional area of 0.001 m² is used to lift a container weighing 10,000
N. Calculate the stress in the cable.
Solution: Stress (σ) = Force (F) / Area (A) σ = 10,000 N / 0.001 m² σ = 10,000,000 Pa or 10 MPa
Commentary: This calculation shows how much force is distributed over the cross-sectional area of the cable. The high stress value indicates that the cable needs to be strong enough to withstand this force without breaking.
Example 2: Calculating Strain A 2-meter long aluminium bar is subjected to a tensile force that causes it to elongate by 2 mm. Calculate the strain in the bar.
Solution: Strain (ε) = Change in length (ΔL) / Original length (L) ΔL = 2 mm = 0.002 m L = 2 m ε = 0.002 m / 2 m ε = 0.001 (dimensionless)
Commentary: The strain value is small because the elongation is small compared to the original length. Strain represents the relative deformation of the material.
Example 3: Material Selection A farmer needs to build a fence. Which material properties are most important for the fence posts, and why?