= Yield point
A <yield point> marks the onset of permanent <deformation> according to a stated experimental criterion. In a <tensile testing> experiment, low loads can produce recoverable extension, while higher loads leave a residual extension after unloading. A sharp transition may be visible in the <stress-strain curve>, but many specimens change gradually from mainly elastic to increasingly plastic response.
When no unambiguous sharp point exists, an offset <proof stress> gives an operational definition: draw a line parallel to the initial elastic slope and displaced by a specified <engineering strain>, then identify its intersection with the loading curve. The chosen offset must be reported. The <yield point> is distinct from both the limit of exact proportionality and the <tensile strength> at maximum load. This matters because a small measurable departure from linear response does not by itself establish a particular amount of permanent <plastic strain>.
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