Lesson Notes By Weeks and Term v4 - SHS 3
Properties of Materials
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Properties of Materials
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Subject: Manufacturing Engineering
Class: SHS 3
Term: 2nd Term
Week: 12
Grade code: 3.1.2.LI.2
Strand code: 1
Sub-strand code: 2
Content standard code: 3.1.2.CS.2
Indicator code: 3.1.2.LI.2
Theme: Manufacturing Materials and Technologies
Subtheme: Properties of Materials
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Welcome, future engineers! Today, we will explore a critical topic: why materials fail. In Ghana, we see materials all around us—from the steel in our buildings and bridges to the plastics in our chairs and the metal in our cars and tro-tros. Sometimes, these materials break, and it's not always because of a single, large force. They can fail slowly over time or after being used again and again. Understanding *how* and *why* they fail is essential for designing safe, reliable, and long-lasting products. This knowledge helps us prevent accidents, save money, and build a stronger nation. We will investigate three key ways materials fail: creep, fatigue, and fracture.
Material failure is the point at which a material or component can no longer perform its intended function safely. Fracture is the physical separation of the material, but creep and fatigue are the *processes* that often lead to that final fracture. A. Creep: The Slow Stretch
Definition: Creep is the slow, continuous, and permanent deformation of a material under a constant stress over a long period. It is often more significant at high temperatures.
Analogy: Imagine placing a very heavy stack of books in the middle of a wooden shelf. When you first place them, the shelf might bend a little. If you leave the books there for several years, you might notice the shelf has sagged permanently. This slow, gradual sagging under a constant weight is creep.
Key Factors: Load: The load is constant or static (not changing). Time: Creep happens over a long duration (months, years). It is a time-dependent failure. Temperature: Creep is much more likely to occur at high temperatures. For metals, it becomes significant at temperatures above 0.4 times their melting point (in Kelvin).