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PROPERTIES OF MATERIALS - ABSORBENCY, DENSITY AND FUSABILITY |
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ABSORBENCY |
The ability of a material to absorb another substance, that in itself is capable of being absorbed. Under normal circumstances, this would mean a liquid or gas being ‘absorbed’ by a material. It is the structure of the material that determines its absorbency level. The ability to soak up moisture, such as a sponge. Synthetic sponges are typically made from materials like polyurethane, cellulose, or polyester. These materials are absorbent. Cellulose-Based Polymers have a high level of absorbency and are also used in sponges and paper towel. Paper towels are used to dry hands and to absorb spillages. In agriculture, absorbent polymers ( super-absorbent polymers - SAPs) are mixed with soil, in order to retain moisture. This is particularly useful during a dry period. A good example is Polyacrylamide. Medical dressings often include Polyvinyl Alcohol, which absorbs fluids from wounds and after operations. Absorbent materials tend to have a high level of porosity, allowing liquids to be trapped within the material. A sponge is a typical example. Absorbent materials are able to ‘draw up’ the liquid due to capillary action. The pores within the structure of the material, store the liquid. |
The absorbency of a material (e.g. a sponge) can be tested, by submerging it in water and then determining if it has increased in weight, a few minutes later. This is called a submergence test. |
Absorbency can be determined through a drop test. This involves dropping a known quantity of liquid onto a standard unit of material. The time it takes for the liquid to be absorbed is recorded and can be compared to that of other materials. |
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A capillary rise test is an excellent way to test a materials absorbency. A standardised length of material is placed in a liquid. The height the liquid rises up the material in a set time, is measured. |
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DENSITY |
Density is calculated by dividing a material’s mass (in kilograms) by its volume (in cubic metres). For example, a cubic metre of a sponge is lighter and therefore much less dense, than a cubic metre of stainless steel, which is much heavier. |
Our technician demonstrates density, by lifting a low density standard unit of foam, followed by a high density standard unit of stainless steel. Despite both being the same size / volume, the unit of foam is easy to lift and very light. On the other hand, the stainless steel unit is extremely heavy. Our technician cannot hold it in position and he drops it. |
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SOME LOW DENSITY MATERIALS |
Aerogels, Polystyrene Foam (Styrofoam), Balsa Wood, Aluminum, Titanium, Polyethylene and Cork |
SOME HIGH DENSITY MATERIALS |
Osmium, Iridium, Platinum, Gold, Tungsten, Lead: A dense metal used in batteries and Uranium: |
FUSABILITY |
The ability of a material to be transformed from a solid state to a liquid state, due to the application of heat. Materials with a high level of fusability tend to have lower melting points. This is seen when soldering components to a printed circuit board. |
Our technician demonstrates fusability by welding to metals together, forming a permanent joint. |
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Solder is used to join components to a PCB, when constructing an electronic circuit. Solder has a high level of fusability. It melts at a relatively low temperature, at just above 200 degrees Celsius (lead free solder). |
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BRITTLENESS |
Brittleness is a property that describes the fracturing / breaking of a material, that takes place without prior deformation. Good examples of brittle materials include glass, concrete, ceramics, graphite, silicon and cast iron. The Tay Bridge disaster of 1879, occurred as a result of the brittleness of vital cast iron parts of the structure. The material could not resist the weight of a train passing over it, during a storm. The structure was unable to absorb forces / stresses by flexing (as steel can). |
Brittleness is a property avoided by structural engineers. However, it does have its uses. Safety glass used in windows, is designed to ‘shatter’ into small, less dangerous pieces. |
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STIFFNESS |
The resistance of a material to deformation when under an applied force. The material property called ‘stiffness’, refers to a materials resistance to deformation when an external force / load is applied. Materials with this property resist bending, stretching and flexing. Materials such as steel and even carbon fibre, have high levels of ‘stiffness’ (although they still have some flexibility, which is a useful characteristic that can be exploited in engineering and construction). A material such as rubber has little ‘stiffness’ value and consequences bends and flexes with relative ease. There are certain practical applications for materials that have high levels of ‘stiffness’ Concrete is used by the construction industry for building. It is worth remembering that successful materials such as steel require a high stiffness value and a measure of flexibility. |
COMMERCIAL JET AIRFRAME |
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THERMAL EXPANSION |
The tendency of a material to expand when heated and contract when cooled. Materials with thermal expansion properties change in size, dependent on the temperature. When heated up they expand and when cooled they contract. The value assigned to thermal expansion is called the ‘coefficient of thermal expansion’. This indicates the change in ‘size’ of a material, per degree of temperature. An understanding of thermal expansion is vital in the construction and engineering industries. For example. Steel expands when the temperature rises and contracts when cooled. A knowledge of its thermal expansion, allows a civil engineer to design and build a safe bridge, because he / she knows how much the various components will expand and contract and how they will interact with each other. The main electronic circuit inside a computer (motherboard), warms up when in use and expands. Understanding the degree of expansion and its effect on individual components, allows the computer designer to ensure the computer is sufficiently cooled by internal fans and where they should be positioned. A knowledge and understanding of thermal expansion, allows architects, engineers and designers to manufacture reliable and safe products. |
A practical example is a bimetallic strip, found in thermostats, . These are used to regulate temperature inside many products, such as electric irons. The two metals of the bimetallic strip (steel and copper / brass) expand at different rates, which results in bending. This opens or closes the electrical circuit, ensuring the electric iron does not overheat. |
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