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DOE Fundamentals Handbook, Volume 1 and 2. This will result in a decrease in the mobility of the dislocations and a strengthening of the material. The more dislocations within a material, the more they will interact and become pinned or tangled. The dislocation density in a metal increases with deformation or cold work because of dislocation multiplication or the formation of new dislocations. Dislocations can move if the atoms from one of the surrounding planes break their bonds and rebond with the atoms at the terminating edge. When a metal is plastically deformed, dislocations move and additional dislocations are generated. It is a process of making a metal harder and stronger through plastic deformation. It is called cold-working because the plastic deformation must occurs at a temperature low enough that atoms cannot rearrange themselves. Strain hardening is also called work-hardening or cold-working. In this region, the stress mainly increases as material elongates, except that there is a nearly flat region at the beginning. This region starts as the strain goes beyond yield point, and ends at the ultimate strength point, which is the maximal stress shown in the stress-strain curve. One of stages in the stress-strain curve is the strain hardening region.
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Ultimate tensile strengths vary from 50 MPa for an aluminum to as high as 3000 MPa for very high-strength steels. However, it is dependent on other factors, such as the preparation of the specimen, the presence or otherwise of surface defects, and the temperature of the test environment and material. It is an intensive property therefore its value does not depend on the size of the test specimen. Even though deformations can continue to increase, the stress usually decreases after the ultimate strength has been achieved. The stress-strain curve contains no higher stress than the ultimate strength. When a ductile material reaches its ultimate strength, it experiences necking where the cross-sectional area reduces locally. Often, this value is significantly more than the yield stress (as much as 50 to 60 percent more than the yield for some types of metals). Ultimate tensile strength is often shortened to “tensile strength” or even to “the ultimate.” If this stress is applied and maintained, fracture will result.
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This corresponds to the maximum stress that can be sustained by a structure in tension. The ultimate tensile strength is the maximum on the engineering stress-strain curve. The following points describe the different regions of the stress-strain curve and the importance of several specific locations. In this case we have to distinguish between stress-strain characteristics of ductile and brittle materials. To clarify, materials can miss one or more stages shown in the figure, or have totally different stages. There are several stages showing different behaviors, which suggests different mechanical properties. A schematic diagram for the stress-strain curve of low carbon steel at room temperature is shown in the figure.