National Physical Laboratory

Materials

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Introduction

Early materials used by human societies, for building shelters and food production, were simply gathered. These materials, such as stone, wood and bone, were processed in a simple mechanical way that imposed a low degree of measurement requirement (ie length and mass estimates). However with increases in population it became important to choose the correct material for the job and also to consider the appropriate resource costs of production. The only practical way that this can be done, when issues such as processability, characterisation, in-service performance and disposal have to be considered, is to have reliable methods of measuring the required materials properties.

Properties of Materials

Everyone can see that metal is different to glass and that plastic is different to rubber, but why? It is all down to the materials properties. These determine everything from melting point to hardness. Some of these properties are explained below although there are many more including adhesive tack, glass transition temperature, elasticity, toughness, fatigue resistance, creep resistance, magnetic properties, refractive index, luminescence, surface tension, flow stress, residual stress, impact resistance, permittivity, electric flux density, polarisation, piezoelectricity and semiconductor properties to name just a few.

Press
Press

Mechanical

Hardness

The hardness of a material is its resistance to being permanently deformed or bent, measured by seeing how far an indenter will penetrate into the material. Metals are often 'worked' or heat treated to increase their hardness.

Stiffness

A stiff material is one for which a large stress (force applied per unit of cross-sectional area) is required to produce a small strain (fractional change in length). The ratio of stress to strain is known as the Young's modulus of the material. This can be measured by recording the change in distance between two points on a test piece as it is subjected to a tensile force.

Strength

A strong material is one able to withstand large stresses before either breaking or deforming such that strain is no longer proportional to stress. Some materials have different strengths according to the nature of the stress applied. Concrete, for example, is strong under compression but has relatively poor tensile strength.

Density

Density is the mass of a material per unit volume. Low density materials with high strength are used in the manufacture of aircraft, for example.

Flow

Viscosity

Viscosity is the resistance of a material to flow. Many manufacturing processes rely on the flow of materials, from molten chocolate to molten metal, so a knowledge of viscosity is important in process design. Viscosity can be measured in many ways, for example by timing a ball as it falls through a liquid, by seeing how much drag is experienced by a submerged rotating bob or by observing the flow of a liquid down a slope.

Turbine
Turbine blades

Thermal

Melting Point

The melting point of a material is the temperature at which liquid begins to form as it is heated. Domestic pipes are often damaged in winter when the water they carry falls below its melting point and begins to expand due to the formation of ice. Water is unusual in that it is less dense in the solid state (ice) than in the liquid state.

Specific Heat Capacity

The specific heat capacity of a material is the energy required to raise the temperature of 1 kg of that material by 1 ┬░C. It is normally measured by comparing the temperature rises experienced by test and reference materials when both are heated at a constant rate. Heat capacity has a direct bearing on the energy costs of any process involving the heating of materials, from making steel to frying chips.

Thermal Conductivity

Thermal conductivity is the rate at which heat flows through a cross-sectional area of material. It is an important property in polymer processing. The cooling time of injection moulded hub caps can be reduced from 69 seconds to less than 40 seconds by an increase of just 20% in the thermal conductivity of the polymer used.

Thermal Expansivity

Thermal expansivity is the ability of a material to change its volume in response to changes in temperature. The height of the arch of the Sydney harbour bridge can change by up to 18 cm as it is heated by the sun.

Sping Corrosion
Spring corrosion

Performance

Wear Resistance

Wear caused by materials rubbing together is an important factor in determining the lifetime of components such as prosthetic joints. The susceptibility of materials to wear can be measured by pressing a pin onto the surface of a rotating test disc and seeing how much is worn away over a period of time.

Corrosion Resistance

Corrosion is the gradual deterioration of a material due to its environment. Iron, for example, will become 'rusty' in the presence of water and oxygen from the air due to the formation of iron oxide. Plastics may become prone to cracking when accidentally exposed to fluids such as organic liquids. Coatings like paint are often used to protect materials from hostile environments and so from corrosion.

Chemical

Composition

The formation of certain atomic structures or 'phases' within a material can lead to problems in its use. The 'sigma' phase, for example, is known to cause embrittlement in steels. Computer software can be used to predict composition ranges where unwanted phases do not form, so aiding materials design.

Electrical

Electrical Conductivity

Electrical conductivity is a measure of how easily a material allows electrical current to flow through it. Metals like copper, aluminium and iron have much higher electrical conductivities than ceramics, plastics, glass and rubber.

Engine Block
Engine block

Measuring Materials Properties

Because of the vast number of properties, the measurement of materials requires a wide range of experimental techniques. These include tensile testing, hardness testing, impact testing, thermal analysis, X-ray diffraction, microscopy and chemical analysis.

Mathematical Modelling of Materials

Ultimately the properties of materials are governed by the nature of the atoms they contain and how these interact with each other. A knowledge of these fundamentals may eventually allow many materials properties to be reliably predicted. Most current materials models start at scales larger than atoms but are often very successful in representing observed materials property values and hence aid our understanding.

Some of the Materials that have Changed the World

4000 BC: Iron Tensile strength, magnetic
100 BC: Concrete Compressive strength, mouldability durability
Glass 50 BC: Glass Transparency, refractive properties, compressive strength
Rubber 1840s: Rubber Elasticity, water repellence, electrical resistivity
Steel 1850s: Steel Tensile strength, hardness, processability
Aluminimum 1880s: Aluminium Strength: weight ratio, corrosion resistance
Polyethylene 1930s: Polyethylene Processability, light, thermal and electrical insulation, chemical resistance
Silicon 1950s: Silicon Semiconductor


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