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The relationship between the stress and strain that a particular material displays are 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 a variety of loadings (stress). These curves reveal many of the properties of a material (including data to establish the Modulus of Elasticity, E). Generally speaking, curves representing the relationship between stress and strain in any form of deformation can be regarded as stress-strain curves. The stress and strain can be normal, shear, or mixture, also can be uniaxial, biaxial, or multiaxial, even change with time. The form of deformation can be compression, stretching, torsion, rotation, and so on. If not mentioned otherwise, stress–strain curve refers to the relationship between axial normal stress and axial normal strain of materials measured in a tension test.The Walker Brothers – The Sun Ain’t Gonna Shine AnymoreConsider a bar of original cross sectional area A being subjected to equal and opposite forces F pulling at the ends so the bar is under tension. The material is experiencing a stress defined to be the ratio of the force to the cross sectional area of the bar, as well as an axial elongation:σ=FA0{displaystyle mathrm {sigma } ={tfrac {F}{A_{0}}}}ϵ=L −L0L0=Δ LL0{displaystyle mathrm {epsilon } ={tfrac {L-L_{0}}{L_{0}}}={tfrac {Delta L}{L_{0}}}} Subscript 0 denotes the original dimensions of the sample. The SI unit for stress is newton per square meter, or pascal (1 pascal = 1 Pa = 1 N/m2), and for the strain is “1”. Stress-strain curve for this material is plotted by elongating the sample and recording the stress variation with strain until the sample fractures. By convention, the strain is set to the horizontal axis and stress is set to a vertical axis. Note that for engineering purposes we often assume the cross-section area of the material does not change during the whole deformation process. This is not true since the actual area will decrease while deforming due to elastic and plastic deformation. The curve assuming the cross-section area is fixed is called the “engineering stress-strain curve”, while the curve based on the actual cross-section area is the “true stress-strain curve”. If not mentioned otherwise, the relationship between the true stress-strain curve and the engineering stress-strain curve will be discussed later.see more at WikipediaCheck More at http://workout.vydio-x.com/