Explain the reason of lowering the value of load after ultimate load during the test
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Engineering stress is defined as the force measured by the load cell divided by the initial cross section of the specimen measured at the beginning of the test.
Plastic deformation occurs at constant volume. This means that the more you plastically lengthen a material in one direction, the more the other directions shrink, effectively resulting a smaller area to withstand the applied load. Moreover, at a certain point an instability known as “necking” occurs. This is the localized reduction in cross section of one part of the specimen.
Even though the material continues to harden, the reduction in area is so great that the force necessary to keep deforming the sample decreases. As a result, the engineering stress goes down (although the true stress in that cross section continues to increase).
Engineering stress is defined as the force measured by the load cell divided by the initial cross section of the specimen measured at the beginning of the test.
Plastic deformation occurs at constant volume. This means that the more you plastically lengthen a material in one direction, the more the other directions shrink, effectively resulting a smaller area to withstand the applied load. Moreover, at a certain point an instability known as “necking” occurs. This is the localized reduction in cross section of one part of the specimen.
Even though the material continues to harden, the reduction in area is so great that the force necessary to keep deforming the sample decreases. As a result, the engineering stress goes down (although the true stress in that cross section continues to increase).
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