After a certain amount of yield, the load has to be increased to increase the elongation. Under stress, the specimen elongates; at the same time, its cross section decreases evenly. At one point, however, the test specimen cross section begins to decrease visibily. In technical terms, it starts to neck. Because of the reduced cross section at the necking, elongation continues to increase, even at a reduced load.
Finally, the specimen will break. The maximum load divided by the original cross section gives the maximum tensile strength of the material. Comparing the stress/strain diagrams of different materials (see Figure 2), shows that the low-strength, high-elongation material (Material 1 on the graph) will have a different diagram than a higher-strength, less-elastic material (Material 2). A high-strength, low-elongation material will again provide a different stress/strain graph (Material 3).
Figure 2 also shows that Materials 2 and 3, assuming that both are steels, have the same slope up to the yield point. This means that the elongation created by a given load is the same for both the low-strength and high-strength steels. Of course, the high-strength material will accept a larger load than will the low-strength material.
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| Figure 2: When comparing the stress/strain diagrams of different materials, it is obvious that the low-strength, high-elongation material (Material 1) will have a different diagram than a higher-strength, less-elastic material (Material 2). A high-strength, low-elongation material (Material 3) will provide a different stress/strain graph. |
