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| [[Image:StressStrainWEB.svg|thumb|300px|''Fig.1'' 'Stress–strain curve''' showing typical yield behavior for nonferrous alloys. Stress (<math> \sigma </math>) is shown as a function of strain (<math> \epsilon </math>)<br>
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| 1: Elastic (proportionality) limit<br>
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| 2: Offset yield strength (0.2% proof strength)]]
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| The relationship between the [[stress (mechanics)|stress]] and [[Deformation (mechanics)|strain]] that a particular material displays is known as that particular material's Stress-Strain curve. It is unique for each material and is found by recording the amount of deformation (strain) at distinct intervals of tensile or compressive loading (stress). These curves reveal many of the properties of a material (including data to establish the [[Modulus of Elasticity]], E).<ref>Luebkeman, C., & Peting, D. (2012, 04 28). Stress-strain curves. Retrieved from http://pages.uoregon.edu/struct/courseware/461/461_lectures/461_lecture24/461_lecture24.html.</ref>
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| Stress–strain curves of various materials vary widely, and different tensile tests conducted on the same material yield different results, depending upon the temperature of the specimen and the speed of the loading. It is possible, however, to distinguish some common characteristics among the stress-strain curves of various groups of materials and, on this basis, to divide materials into two broad categories; namely, the ductile materials and the brittle materials.<ref>Beer, F, Johnston, R, Dewolf, J, & Mazurek, D. (2009). Mechanics of materials. New York: McGraw-Hill companies. P 51.</ref>
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| Consider a bar of cross sectional area A being subjected to equal and opposite forces F pulling at the ends. If this were a rope, we would say that it is experiencing a tension force. Taking this concept over, we say that the bar is under tension, and is experiencing a stress that we define to be the ratio of the force to the cross sectional area:
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| Stress = F/A
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| This stress is called the tensile stress because every part of the object is subjected to a tension. The SI unit of stress is the Newton per square meter, which is called the Pascal.
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| 1 Pascal = 1 Pa = 1 N/m<sup>2</sup>
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| Shear Stress and Strain
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| Now consider a force that is applied tangentially to an object
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| The ratio of the shearing force to the area A is called the shear stress
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| If the object is twisted through an angle q, then the strain is
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| Shear Strain = tan q
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| Finally, we can define the shear modulus, MS, as
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| The shear modulus is also known as the torsion modulus
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| ==Ductile materials==
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| [[Image:Stress v strain A36 2.svg|thumb|300px|''Fig 2.'' A stress–strain curve typical of structural [[steel]]<br>
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| 1. Ultimate Strength<br>
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| 2. [[Yield Strength]]<br>
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| 3. Rupture<br>
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| 4. Strain hardening region<br>
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| 5. Necking region.<br />
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| A: Apparent stress (F/A<sub>0</sub>)<br />
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| B: Actual stress (F/A)]]
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| [[Ductile]] materials, which includes structural steel, as well as many alloys of other metals, are characterized by their ability to yield at normal temperatures.<ref>Beer, F, Johnston, R, Dewolf, J, & Mazurek, D. (2009). Mechanics of materials. New York: McGraw-Hill companies. P 58.</ref>
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| Low carbon steel generally exhibits a very linear stress–strain relationship up to a well defined [[yield strength|yield point]] (''Fig.2''). The linear portion of the curve is the elastic region and the slope is the [[modulus of elasticity]] or [[Young's Modulus]]. After the yield point, the curve typically decreases slightly because of [[dislocations]] escaping from [[Cottrell atmosphere]]s. As deformation continues, the stress increases on account of [[strain hardening]] until it reaches the [[ultimate strength]]. Until this point, the cross-sectional area decreases uniformly because of [[Poisson's ratio|Poisson contractions]]. The actual rupture point is in the same vertical line as the visual rupture point.
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| However, beyond this point a ''[[Necking (engineering)|neck]]'' forms where the local cross-sectional area decreases more quickly than the rest of the sample resulting in an increase in the [[Stress (physics)|true stress]]. As shown in ''Fig.2'', On an engineering stress–strain curve this is seen as a decrease in the ''apparent stress''. However if the curve is plotted in terms of ''true stress'' and ''true strain'' the stress will continue to rise until failure. Eventually the neck becomes unstable and the specimen ruptures ([[fractures]]).
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| Less ductile materials such as aluminum and medium to high carbon steels do not have a well-defined yield point.There are generally two types of yield points,upper and lower yield point. For these materials the yield strength is typically determined by the "offset yield method", by which a line is drawn parallel to the linear elastic portion of the curve and intersecting the abscissa at some arbitrary value (generally from 0.1% to 0.2%). The intersection of this line and the stress–strain curve is reported as the yield point. The elastic region is the portion of the curve where the material will return to its original shape if the load is removed. The plastic region is the portion where some permanent deformation will occur, even if the load is removed. Failure point is when the object ruptures.
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| [[Image:Stress v strain brittle 2.png|thumb|300px|''Fig.3'' Stress–strain curve for brittle materials.]]
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| Brittle materials, which includes cast iron, glass, and stone, are characterized by the fact that rupture occurs without any noticeable prior change in the rate of elongation.<ref>Beer, F, Johnston, R, Dewolf, J, & Mazurek, D. (2009). Mechanics of materials. New York: McGraw-Hill companies. P 59.</ref>
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| [[Brittle]] materials such as [[concrete]] or [[carbon fiber]] do not have a yield point, and do not strain-harden. Therefore the ultimate strength and breaking strength are the same. A typical stress-strain curve is shown in ''Fig.3''. Typical brittle materials like [[glass]] do not show any [[plastic deformation]] but fail while the deformation is [[Elasticity (physics)|elastic]]. One of the characteristics of a brittle failure is that the two broken parts can be reassembled to produce the same shape as the original component as there will not be a neck formation like in the case of ductile materials. A typical stress strain curve for a brittle material will be linear. <!-- This phrase means nothing, the part before the comma does not seem to match the part after -->Testing of several identical specimen, [[cast iron]], or [[soil]], tensile strength is negligible compared to the compressive strength and it is assumed zero for many engineering applications. [[Glass fiber]]s have a [[tensile strength]] stronger than steel, but bulk glass usually does not. This is because of the [[stress intensity factor]] associated with defects in the material. As the size of the sample gets larger, the size of defects also grows. In general, the tensile strength of a rope is always less than the sum of the tensile strengths of its individual fibers.
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| ==See also==
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| *[[Elastomers]]
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| *[[Strength of materials]]
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| *[[Tensometer]]
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| *[[Universal testing machine]]
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| *[[Stress-strain index]]
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| *[[Stress–strain analysis]]
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| == References ==
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| {{reflist}}
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| ==External links==
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| *[http://www.bssm.org/ British Society for Strain Measurement]
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| *[http://www.mathalino.com/reviewer/mechanics-and-strength-of-materials/stress-strain-diagram Stress–strain diagram]
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| *[http://www.keytometals.com/page.aspx?ID=CheckArticle&site=kts&NM=43 Engineering stress–strain curve]
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| {{DEFAULTSORT:Stress-Strain Curve}}
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| [[Category:Elasticity (physics)]]
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My name is Evelyne and I am studying Psychology and Philosophy at Tubarao / Brazil.
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