Although the tensile test previously discussed gives a good idea of the shape of the stress-strain curve, it is not very accurate for strain readings. The graph above shows the two measurements of strain on a duralumin specimen. The purple set of measurements come from the test machine via the displacement of the cross head. This is assumed to be the extension of the specimen, and then converted to a nominal strain by dividing the extension by the initial length. The grey set of measurements were recorded by a strain gauge, pictured below
The graph shows the difference beteween the two sets of measurements. The strain gauge measures the actual strain, but only works up to relatively small strains. The cross head displacement records the correct shape of the curve, up to large strains as needed, but is not particularly accurate in giving actual strain due to the method of measuring. As can be seen above, the machine readings do not have a very straight line initially, and this is due to slack within the machine being taken up gradually as the clamps tighten onto the specimen.
As the displacement transducer measures the total displacement of the machine and the specimen, the apparent displacement is much larger than expected. This is because as a stress is applied, many other parts of the machine stretch slightly, and so increase the displacement. As a result when the Young's modulus is caluculated, the displayed strain results in a value of around 20GPa (on the steepest part of the graph), while the strain gauge gives a value of 80GPa. The real value for aluminium is 70GPa, and it can be seen that the strain gauge gives a more accurate result.
For the test used here, the interest was in the plastic deformation of the specimen, and so it was appropriate to use the displacement measurement. However, it should always be remembered that if accurate strain readings are required, they must be acquired using a strain gauge.
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