Lesson Notes By Weeks and Term v4 - SHS 3

Lesson Video

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Performance objectives

• Define key terms related to tensile testing.
• Understand how a tensile test is performed.
• Calculate stress, strain, and modulus of elasticity.
• Plot and interpret a stress-strain graph.
• Recognize real-world applications of tensile properties.

Lesson summary

This lesson explores one of the most important tests in engineering: the tensile test. In Ghana, we see steel used everywhere – from the "iron rods" (reinforcement bars) in the foundations of new buildings in Accra and Kumasi, to the chassis of Kantanka vehicles, to the metal gates and burglar-proofing made by local welders. How do engineers and manufacturers know that this steel is strong enough for the job and won't suddenly fail, causing a disaster? The answer is by testing its properties. The tensile test allows us to pull a sample of a material like mild steel until it breaks, carefully measuring how it stretches and resists the pulling force.

Lesson notes

This section breaks down the essential knowledge needed to understand the tensile test. A. What is a Tensile Test?

A tensile test is a fundamental materials science test in which a sample is subjected to a controlled pulling force (tension) until it fails. The test measures the material's response to this stretching force, providing valuable information about its mechanical properties.

The sample, often a "dog-bone" shape, is clamped into a machine called a Tensometer or a Universal Testing Machine (UTM). The machine pulls the sample at a constant, slow rate while simultaneously measuring two things: The force (Load) being applied. The elongation (Extension) of the sample. B. Key Definitions and Calculations

The raw data (Force and Extension) from the test are not very useful for comparing different materials because they depend on the size of the sample. To standardise the results, we convert them into Stress and Strain. Stress (σ) Definition: Stress is the measure of the internal force acting within a material per unit of its cross-sectional area. It's essentially "force intensity". Formula: ``` Stress (σ) = Force (F) / Original Cross-sectional Area (A₀) ``` Units: The standard unit is the Pascal (Pa), which is 1 Newton per square meter (N/m²). In engineering, forces are large, so we often use Megapascals (MPa), where 1 MPa = 1,000,000 Pa. Example Calculation: A mild steel rod with a diameter of 10 mm is pulled with a force of 15,000 N. Calculate the stress in the rod. Step 1: Find the cross-sectional area (A₀). The rod is circular, so Area = π * r². Diameter = 10 mm, so Radius (r) = 5 mm = 0.005 m. A₀ = π * (0.005 m)² A₀ ≈ 0.00007854 m² Step 2: Calculate the stress (σ). Force (F) = 15,000 N σ = F / A₀ = 15,000 N / 0.00007854 m² σ ≈ 190,985,931 Pa σ ≈ 191 MPa (rounded to three significant figures) Strain (ε) Definition: Strain is the measure of the deformation or "stretch" of a material in response to stress. It's the change in length divided by the original length. Formula: ``` Strain (ε) = Change in Length (ΔL) / Original Length (L₀) ``` Where ΔL = Final Length - Original Length. Units: Since strain is a ratio of length to length (e.g., mm/mm or m/m), it has no units. It is a dimensionless quantity, often expressed as a decimal or a percentage. Example Calculation: A 50 mm long section of the steel rod from the previous example stretches to 50.05 mm under the 15,000 N load. Calculate the strain. Step 1: Find the change in length (ΔL). Original Length (L₀) = 50 mm Final Length = 50.05 mm ΔL = 50.05 mm - 50 mm = 0.05 mm Step 2: Calculate the strain (ε). ε = ΔL / L₀ = 0.05 mm / 50 mm ε = 0.001 The Stress-Strain Graph for Mild Steel

Evaluation guide

• Measure the tensile properties of steel using the tensile test.
• Experiential Learning: Let learners in mixed -ability groups perform a tensile test on mild steel to
• determine its tensile properties such as stress, strain, modulus of elasticity and elastic deformation.
• Collaborative Learning: Let learn ers in their mixed -ability groups plot t he stress -strain graph