NATIONAL DIPLOMA IN ENGINEERING
UNIT 10 – PROPERTIES AND APPLICATIONS OF ENGINEERING MATERIALS ASSIGNMENT 1: Structures and Properties of Engineering Materials.
Special attention should be paid to the Grading Criteria, as this will influence the way you answer each of the tasks. For each question all tasks need to be answered correctly to achieve the relevant criteria.
Question 1: (P1)
With the aid of clear, annotated diagrams, describe the structure associated with the following engineering materials: - a)
Iron at room temperature.
To meet the criteria you will need to describe the following: - •
The type of atomic lattice structure it has.
The way the grain structure develops as the metal cools from a molten state.
BODY CENTERED CUBIC STUCTURE
Iron has a BCC crystal lattice structure at room temperature. The open-packed planes of atoms make the structure rigid, and hard. When external forces are applied the atoms don’t easily move over each other.
When iron is heated to around 800 º C its crystal lattice structure changes to FCC. The closely packed atoms move more readily over each other when external forces are applied.
FACED CENTERED CUBIC
When the molten iron solidifies, latent heat is given off and crystals or grains start to form at different points. These embryo grains are known as dendrites, which multiply and grow until the material is solidified. The size of the grains depends on the cooling rate.
METALLIC DENDRITE GROWTH
FORMATION OF GRAINS
To meet the criteria you will need to consider the following: - •
Whether the material is a thermoplastic or thermosetting polymer. •
The atomic structure associated with this class of polymer, NOT the chemical constituents.
Polycarbonate is a thermoplastic.
Thermoplastics have a linear or branched molecular structure which determines their strength, solid at room temperature. The polymer chain is held together by relatively weak van der Waals forces; because the molecules are long the forces are considerable. The molecules become tangled. When heated the van der Waals forces are weakened, the molecules are able to slide over one another, the material becomes weaker and less rigid, so it can easily be moulded.
Repeating chemical structure unit of
Polycarbonate made from Bisphenol A
To meet the criteria you will need to describe how the structure develops as it solidifies from the molten state. The amorphous structure of glass makes it brittle because glass doesn't contain planes of atoms that can slip past each other, there is no way to relieve stress; the amorphous structure is formed by cooling. When cooled quickly from its molten state the resulting solid is amorphous and transparent. Amorphous structure of glass is more loosely packed and random.
Glass-fibre reinforced plastic.
To meet the criteria you will need to describe: -
The matrix material
The reinforcement material
How they are blended to make the final composite material
Glass-fibre reinforced plastic (GRP) is a composite material made by combining two materials where one of the materials is a reinforcement (glass fibre) and the other material is a matrix (resin). Epoxy and polyester resins are the usual matrix materials. The combination of the glass fibre and matrix provide characteristics superior to either of the materials alone. The most widely used reinforcement material is fibreglass in polyester resin, which is commonly referred to as just fibreglass. Fibreglass is lightweight, corrosion resistant, easily processed, and has good mechanical properties. The glass fibres woven in different directions are impregnated with the resin while in its liquid state, the result, after curing, is a cohesive completely integrated matrix of resin and fibres. The matrix can have a surprising range of properties. In general,...
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