Question

in a tensile test after which point is necking observed

Answer 1

Necking in a ductile material is observed after the body crosses the point of ultimate strength on the stress-strain curve.

Necking starts when the plot of nominal stress against nominal strain reaches a maximum (plateau). This is not always easy to identify in terms of the strain at which it occurs, since it is commonly a rather flat plateau, but the stress level is usually fairly well defined.