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Testing Materials (Describing
Materials (Metals (Metals have a wide range…
Testing Materials
Describing
Materials
Metals
Metals have a wide range of mechanical properties. Pure metals tend to be soft. Lead can be bent into shape by hand - gold and copper can easily be hammered into shape. Metals that can be shaped easily are called malleable, whilst those that can be drawn into wires are called ductile. Metal alloys, such as steel, are usually harder than the pure metals used to make them.
Polymers
Polymers include familiar synthetic materials such as polythene and Perspex, as well as natural materials such as leather and cotton. Glassy polymers have properties similar to glass and often replace glass in spectacle lenses. These materials are brittle. Semi-crystalline polymers are tough - they can undergo considerable deformation and can absorb more energy before breaking (compared to more brittle polymers).
Ceramics
Ceramics are hard, meaning that they are difficult to scratch. They are brittle, meaning that they will shatter into jagged pieces - think of a broken plate. Ceramics are also stiff - they are difficult to stretch or bend. Examples of ceramics include china and other pottery, as well as more modern 'engineering ceramics' such as alumina and silicon carbide.
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Stress, Strain, and
the Young Modulus
Strain
A long wire will stretch more than a shorter wire of the same cross-sectional area if the same load is hung from both. Therefore, to make a fair comparison, we can calculate the strain. Strain is the fractional increase in length. Unlike extension, strain does not depend on the original length of the specimen.
Strain ε = extension/original length = x/L
Strain is a ratio of two lengths and is often given as a percentage, for example, a strain of 5%.
Young's Modulus
The Young's Modulus E gives a measure of the stiffness of a material rather than the stiffness of a particular specimen.
E = stress/strain = σ/ε = (F/A)/(x/L) = FL/xA
The units for Young's Modulus are Nm^-2 or Pa. These are the same as the units of stress. This is because strain is a ratio of two lengths and so has no units - strain is dimensionless. Since F and L are large values, divided by the product of the much smaller x and A, Young's Modulus is often a very large number.
Stress
When you compare the strength of a girder to that of a spider thread you are comparing specimens, not materials. It will take a bigger force to break or fracture the girder than the thread. But the girder has a far greater cross-sectional area than the thread. To make a fair comparison we use the concept of stress, which is force per unit area.
Fracture Stress is the stress at which a material breaks. Stress is found by determining the cross-sectional area of the specimen under investigation, measuring the force on the specimen and dividing the force by the cross-sectional area.
It is also important to know the yield stress of materials. This is the stress at which a material begins to deform plastically and become permanently deformed. Imagine you are designing a steel bridge - the breaking stress is an important factor but so is the yield stress as this is the value at which the materials will begin to bend or buckle.
Choosing
Materials
The choice of materials to use in a product depends to a large extent on the mechanical properties of the material, but other factors such as cost and the look of the finished product are also considered.