= Linear reversibility and yield in tensile tests
<Linearity> and reversibility answer different questions. The <proportional limit> ends the interval in which axial <stress> is proportional to axial <strain>. <Elastic deformation> means that deformation is recovered after unloading; a nonlinear response can still be elastic. Conversely, a short straight portion of a loading curve does not by itself demonstrate recovery.
To check recovery, unload and examine the local <extensometer> <strain> relative to its initial value, allowing for instrument resolution, drift and any time dependence. A residual extension can indicate <plasticity in continuum mechanics>, but grip slip can produce a residual in <crosshead displacement> without the same local residual. Compare the channels before assigning the effect to the specimen. <Yield point> concerns the onset of irreversible deformation; a specified offset <proof stress> is an operational convention rather than <Young's modulus>. For example, a $0.2\%$ offset <proof stress> is the intersection of the loading curve with $\sigma=E(\varepsilon-0.002)$, using the correctly referenced <strain>. Report the fitted <stress>/<strain> window and the unloading evidence alongside the modulus, so that apparatus seating, departure from <linearity> and onset of <plasticity in continuum mechanics> remain distinguishable.
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