Tensile Test – Purpose , Graph , Results , Specimen Detail
The main principle of the tensile test is denotes the resistance of a material to a tensile load applied axially to a specimen. It is very important to the tensile test to be considered is the standard dimensions and profiles are adhered to. The typical progress of tensile test can be seen in figure.
Let’s now look at another figure. In this figure, the gauge length (L0) is the length over which the elongation of the specimen is measured. The minimum parallel length (Lc) is the minimum length over which the specimen must maintain a constant cross- sectional area before the test load is applied. The lengths L0, Lc. Li and the cross- sectional area (A) are all specified in BS 18.
The elongation obtained for a given force depends upon the length and area of the cross-section of the specimen or component, since-
Elongation = Applied Force × L/E × A
where, L = Length, A = Cross-sectional area E = Elastic modulus.
Therefore if the ratio [L/A] is kept constant (as it is in a proportional test piece), and E remains constant for a given material, then comparisons can be made between elongation and applied force for specimens of different sizes.
Tensile Test Results:
The load applied to the specimen and the corresponding extension can be plotted in the form of a graph, as shown in figure.
From A to B the extension is proportional to the applied load. Also, if the load is removed the specimen returns to its original length. Under these relatively lightly loaded conditions the material is showing elastic properties.
From B to C it can be seen from the graph that the metal suddenly extends with no increase in load. If the load is removed at this point the metal will not spring back to its original length and it is said to have taken a permanent set. Therefore, B is called “limit of proportionality”, and if the force is increased beyond this point a stage is reached where a sudden extension takes place with no increase in force. This is known as the “yield point” C.
The yield stress is the stress at the yield point; that is, the load at B divided by the original cross-section area of the specimen. Usually, a performer works at 50 percent of this figure to allow for a ‘factor of safety’.
From C to D extension is no longer proportional to the load, and if the load is removed little or no spring back will occur. Due to this relatively greater loads the material is showing plastic properties.
The point D is referred to as the ‘ultimate tensile strength’ when referred extension graphs or the ‘ultimate tensile stress’ (UTS) when referred to stress-strain graphs. The ultimate tensile stress is calculated by dividing the load at D by the original cross-sectional area of the specimen. Although a useful figure for comparing the relative strengths of materials, it has little practical value since engineering equipment is not usually operated so near to the breaking point.
From D to E the specimen appears to be stretching under reduced load conditions. In fact the specimen is thinning out (necking) so that the ‘load per unit area’ or stress is actually increasing. The specimen finally work hardens to such an extent that it breaks at E.
In general, values of load and extension are of limited use since they apply to one particular size of specimen and it is more usual to plot the stress-stain curve.
Stress and strain are calculated as follows:
Therefore ductility is usually expressed, for practical purposes, as the percentage; Elongation in gauge length of a standard test piece at the point of fracture when subjected to a tensile test to destruction.
Stress = ( Load / Cross section area )
Strain = ( Extension / Original Length )
Percentage elongation = ( Increase in Length /Original Length) x 100
The increase in length is determined by fitting the pieces of the fractured specimen together carefully and measuring the length at failure.
Increase in length (elongation) = Length at failure – Original length
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