| Tensile Strength |
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| materials science | |
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Tensile strength measures the force required to pull something such as rope, wire, or a structural beam to the point where it breaks. Specifically, the tensile strength of a material is the maximum amount of Tensile Stress that it can be subjected to before failure. The definition of failure can vary according to material type and design methodology. This is an important concept in Engineering , especially in the fields of Material Science , Mechanical Engineering and Structural Engineering . There are three typical definitions of tensile strength:
CONCEPT The various definitions of tensile strength are shown in the following stress-strain graph for low-carbon Steel : 1. Ultimate Strength 2. Yield Strength 3. Rupture 4. Strain Hardening region 5. Necking region.]] Steel has a very linear stress-strain relationship up to a sharply defined Yield Point , as shown in the figure. For stresses below this yield strength all deformation is recoverable, and the material will relax into its initial shape when the load is removed. For stresses above the yield point, a portion of the deformation is not recoverable, and the material will not relax into its initial shape. This unrecoverable deformation is known as Plastic Deformation . For many applications plastic deformation is unacceptable, and the yield strength is used as the design limitation. After the yield point, steel and many other Ductile Metals will undergo a period of Strain Hardening , in which the stress increases again with increasing strain up to the ''ultimate strength''. If the material is unloaded at this point, the stress-strain curve will be parallel to that portion of the curve between the origin and the yield point. If it is re-loaded it will follow the unloading curve up again to the ultimate strength, which has become the new yield strength. After steel has been loaded to its ultimate strength it begins to "neck" as the cross-sectional area of the specimen decreases due to plastic flow. Necking is accompanied by a region of decreasing stress with increasing strain on the stress-strain curve. After a period of necking, the material will rupture and the stored elastic energy is released as noise and heat. The stress on the material at the time of rupture is known as the ''breaking stress''. Note that if the graph is plotted in terms of ''true stress'' and ''true strain'' necking will not be observed on the curve as true stress is corrected for the decrease in cross-sectional area. Necking is also not observed for materials loaded in compression. Ductile metals other than steel typically do not have a well defined yield point. For these materials the yield strength is typically defined by the "0.2% offset strain". The yield strength at 0.2% offset is determined by finding the intersection of the stress-strain curve with a line parallel to the initial slope of the curve and which intercepts the abscissa at 0.002. A stress-strain curve typical of aluminum along with the 0.2% offset line is shown in the figure below. | ||
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