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A yield strength or yield point is the material property defined as the stress at which a material begins to deform plastically. Prior to the yield point the material will deform elastically and will return to its original shape when the applied stress is removed. Once the yield point is passed, some fraction of the deformation will be permanent and non-reversible. In the three-dimensional space of the principal stresses (), an infinite number of yield points form together a yield surface. The yield point determines the limits of performance for mechanical components, since it represents the upper limit to forces that can be applied without deformation. In structural engineering, this is a soft failure mode which does not normally cause catastrophic failure or ultimate failure unless it accelerates buckling. Yield strength is the critical material property exploited by many fundamental techniques of material-working: to reshape material with pressure (such as forging, rolling, or pressing), to separate material by cutting (such as machining) or shearing, and to join components rigidly with fasteners. == Definition == It is often difficult to precisely define yielding due to the wide variety of stress–strain curves exhibited by real materials. In addition, there are several possible ways to define yielding:〔G. Dieter, ''Mechanical Metallurgy'', McGraw-Hill, 1986〕 ; True elastic limit : The lowest stress at which dislocations move. This definition is rarely used, since dislocations move at very low stresses, and detecting such movement is very difficult. ; Proportionality limit : Up to this amount of stress, stress is proportional to strain (Hooke's law), so the stress-strain graph is a straight line, and the gradient will be equal to the elastic modulus of the material. ; Elastic limit (yield strength) : Beyond the elastic limit, permanent deformation will occur. The elastic limit is therefore the lowest stress at which permanent deformation can be measured. This requires a manual load-unload procedure, and the accuracy is critically dependent on the equipment used and operator skill. For elastomers, such as rubber, the elastic limit is much larger than the proportionality limit. Also, precise strain measurements have shown that plastic strain begins at low stresses. ; Yield point : The point in the stress-strain curve at which the curve levels off and plastic deformation begins to occur.〔.〕 ; Offset yield point () : When a yield point is not easily defined based on the shape of the stress-strain curve an ''offset yield point'' is arbitrarily defined. The value for this is commonly set at 0.1 or 0.2% plastic strain.〔.〕 The offset value is given as a subscript, e.g., Rp0.2=310 MPa.〔ISO 6892-1:2009〕 High strength steel and aluminum alloys do not exhibit a yield point, so this offset yield point is used on these materials.〔 ; Upper and lower yield points : Some metals, such as mild steel, reach an upper yield point before dropping rapidly to a lower yield point. The material response is linear up until the upper yield point, but the lower yield point is used in structural engineering as a conservative value. If a metal is only stressed to the upper yield point, and beyond, Lüders bands can develop.〔Degarmo, p. 377.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Yield (engineering)」の詳細全文を読む スポンサード リンク
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