Topic 4: Materials (17
hours)
4.1 Introducing and
classifying materials
1 hour
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Assessment statement |
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Notes |
References |
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4.1.1 |
Define atom, molecule, alloy and composite. |
1 |
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4.1.2 |
Describe a bond as a force of attraction between atoms. |
2 |
Consider
and differentiate between the three main types of bond: ionic, covalent and
metallic. |
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4.1.3 |
Describe how materials are
classified into groups according to similarities in their microstructures and
properties. |
2 |
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4.1.4 |
Explain that several
classifications are recognized but that no single classification is
“perfect”. |
3 |
It is convenient to be able to classify materials into
categories (albeit crude in nature) that have characteristic combinations of
properties. |
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4.1.5 |
Describe that, for this course, materials are classified into groups:
timber, metals, plastics, ceramics, food and composites; and that some of
these groups have subdivisions. |
2 |
In each group there can
be subdivisions, for example, for timber (natural wood and man-made), metals
(ferrous and non-ferrous), plastics (thermoplastics, thermosets),
ceramics (earthenware, porcelain, stoneware, glass),
textile fibres (natural or synthetic), food
(vegetable or animal origin) and composites (difficult to classify due to
variability and continual development of new composite materials). Food is
included here for completeness, although it is dealt with in detail as an
option. |
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4.2 Properties of materials 3 hours |
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Physical properties 4.2.1 |
Define density, electrical
resistivity, thermal conductivity, thermal
expansion and hardness. |
1 |
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Resistant Materials Technologypg 20 and 21.
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4.2.2 |
Explain a design
context where each of the properties in 4.2.1 is an important consideration. |
3 |
Density is important in relation to product weight and
size (for example, for portability). Prepackaged food is sold by weight or
volume, and a particular consistency is required. Electrical resistivity is particularly important in selecting
materials as conductors or insulators. Thermal conductivity is important for
objects that will be heated or must conduct or insulate against heat. Thermal
expansion (expansivity) is important where two
dissimilar materials are joined. These may then experience large temperature
changes while staying joined. Hardness is important where resistance to
penetration or scratching is required. Ceramic floor tiles are extremely hard
and resistant to scratching. |
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Mechanical properties 4.2.3 |
Define tensile
strength, stiffness, toughness and ductility. |
1 |
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4.2.4 |
Explain a design context where each of the properties in 4.2.3
is an important consideration. |
3 |
Tensile strength is
important in selecting materials for ropes and cables, for example, for an elevator.
Stiffness is important when maintaining shape is crucial to performance, for
example, an aircraft wing. Toughness is important where abrasion and cutting
may take place. Ductility is important when metals are extruded (not to be
confused with malleability, the ability to be shaped plastically). |
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Aesthetic characteristics 4.2.5 |
Outline the
characteristics of taste, smell, appearance, texture and colour. |
2 |
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4.2.6 |
Explain a design context where each of the characteristics in 4.2.5
is an important consideration. |
3 |
Some of these properties are only relevant to food, while
others can be applied to more than one material group. Although these
properties activate people’s senses, responses to them vary from one
individual to another, and they are difficult to quantify scientifically,
unlike the other properties. |
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4.3 Timber 3 hours |
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4.3.1 |
Describe the structure
of natural timber. |
2 |
Natural timber is a natural composite material comprising
cellulose fibres in a lignin matrix. The tensile
strength of timber is greater along the grain (fibre)
than across the grain (matrix). |
Design and Technology (Caborn, Mould & Cave)pg 309-311 Resistant Materials Technologypg 72 - 75. |
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4.3.2 |
Outline that timber can be classified according to the
conditions needed for tree growth. |
2 |
Consider temperate and tropical conditions. A general knowledge
of the geographical distribution of world timber resources is required. |
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4.3.3 |
Outline that conifer trees are referred to as softwoods and that
these grow only in temperate regions. |
2 |
Recognize the
characteristics of softwood trees. |
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4.3.4 |
Outline that deciduous trees are referred to as hardwoods and
that these grow in both temperate and tropical regions. |
2 |
Recognize the
characteristics of hardwood trees. |
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4.3.5 |
Discuss the issues relating to the consideration of timber as a
renewable resource. |
3 |
Consider time to reach maturity, soil erosion, greenhouse
effect and extinction of species. The issues should be placed in local,
national and international contexts. |
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4.3.6 |
List two examples of composite timbers. |
1 |
Consider particle board (chipboard) and plywood. |
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4.3.7 |
Compare the characteristics of particle board, laminated woods (for
example, plywood), pine wood (a softwood) and mahogany (a hardwood). |
3 |
Consider composition, hardness, tensile
strength, resistance to damp environments, longevity and the aesthetic
properties of grain, colour and texture. The
ability to produce sketches showing cross-sectional views of the structure of
the materials is expected. |
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4.3.8 |
Outline criteria for the selection of timber for different
structural and aesthetic design contexts. |
2 |
Consider timber for buildings, furniture and children’s
toys. |
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4.3.9 |
Describe the reasons for treating or finishing wood. |
2 |
Consider reducing attack by organisms and chemicals,
enhancing aesthetic properties and modifying other properties. |
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4.3.10 |
Explain three differences in the selection of timbers for
flooring if it were made of a hardwood, a softwood
or a composite material. |
3 |
Consider durability,
ease of maintenance and aesthetics. |
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4.4 Metals 3 hours |
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4.4.1 |
Draw and describe a metallic bond. |
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Metals are often
described as positively charged nuclei in a sea of electrons. The outer
electrons of the metal atom nuclei are free and can flow through the crystalline
structure. The bonding is caused by attraction between the positively charged
metallic atom nuclei and the negatively charged cloud of free electrons.
Specific arrangements of metal atoms are not required. |
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4.4.2 |
Explain how the movement of free electrons makes metals very
good electrical and thermal conductors. |
3 |
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4.4.3 |
State that metals (pure or alloyed) exist as crystals. |
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4.4.4 |
Draw and describe what is meant by grain size. |
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4.4.5 |
Explain how grain size can be controlled and modified by the rate
of cooling of the molten metal, or by heat treatment after solidification. |
3 |
Reheating a solid metal or alloy allows material to diffuse
between neighbouring grains and the grain structure
to change. Slow cooling allows larger grains to form; rapid cooling produces
smaller grains. Directional properties in the structure may be achieved by
selectively cooling one area of the solid. |
Resistant Materials Technology pg 66, 67, 80 &
81. |
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4.4.6 |
Define plastic deformation. |
1 |
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4.4.7 |
Explain how metals work-harden after being plastically deformed. |
3 |
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4.4.8 |
Describe how the tensile strength of a metal is increased by
alloying. |
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4.4.9 |
Explain the effect of alloying on malleability and ductility. |
3 |
The presence of “foreign” atoms in the crystalline structure of the metal interferes with the movement of atoms in the
structure during plastic deformation. |
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4.4.10 |
Describe a superalloy. |
2 |
The strength of most
metals decreases as the temperature is increased. Superalloys
are metallic alloys that can be used at high temperatures, often in excess of
0.7 of their absolute melting temperature. |
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4.4.11 |
List two design criteria for superalloys. |
1 |
Consider creep and
oxidation resistance. |
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4.4.12 |
Identify applications for superalloys. |
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