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Properties Of Materials

Physical properties

4.2.1 Define density, electrical resistivity, thermal conductivity, thermal expansion and hardness.

Density
The mass per unit volume of a material
Electrical resistivity
This is a measure of a materialís ability to conduct electricity.
Thermal conductivity
A measure of how fast heat is conducted through a slab of material with a given temperature difference across the slab.
Thermal expansion (expansivity)
A measure of the degree of increase in dimensions when an object is heated. This can be measured by an increase in length, area or volume. The expansivity can be measured as the fractional increase in dimension per kelvin increase in temperature.
Hardness
The resistance a material offers to penetration or scratching.
Density Thermal expansion
Hardness Electric conductivity

4.2.2 Explain a design context where each of the properties in 4.2.1 is an important consideration.

  • 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 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.

Activity#1
Find a product or design context for each physical property which is crucial to the design and explain why this is so.

Mechanical properties

4.2.3 Define tensile strength, stiffness, toughness and ductility.

Tensile strength
The ability of a material to withstand pulling forces.
Stiffness
The resistance of an elastic body to deflection by an applied force.
Toughness
The ability of a material to resist the propagation of cracks
Ductility
The ability of a material to be drawn or extruded into a wire or other extended shape.
Toughness Stiffness Ductility

4.2.4 Explain a design context where each of the properties in 4.2.3 is an important consideration.

  • 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).

Activity#2
Find a product or design context for each mechanical property which is crucial to the design and explain why.

Aesthetic characteristics

4.2.5 Outline the characteristics of taste, smell, appearance, texture and color.

  • Such characteristics, are very important for the design/choice of material of the product.

Smell: the ability to perceive odours such as sweet, acrid or fragrant

Texture: it is how something feels or looks, it can be rough or smooth

Color: can be warm (eg browns) or cool (eg blues) can have psychological affects (eg greens are claming)

Appearance: the design of appearance in a product must be aestheticly pleasing to attract a customer. unless it is for a certain market.

4.2.6 Explain a design context where each of the characteristics in 4.2.5 is an important consideration.

Some of these properties are only relevant to food, while others can be applied to more than one material group. Although these properties activate peopleís senses, responses to them vary from one individual to another, and they are difficult to quantify scientifically, unlike the other properties


Activity#3
Find a product or design context for each aesthetic property which is crucial to the design and explain why.

PBS Building Big Lab

References

Bulleted list and italicised paragraphs are excerpted from Design Technology: guide. Cardiff Wales, UK: International Baccalaureate Organization, 2007.

Images are clickable links to its location.

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Page last modified on September 10, 2014, at 11:13 PM