Why is the measure of percent elongation prior to necking be considered a more reliable measure of ductility?

Answer 1

When a material specimen, like plastic, is undergoing a tension test, eventually the tension exceeds the strength of the material to retain its shape. The specimen stretches, but since it must conserve volume, it gets narrower too, which is pretty intuitive. That's called necking, especially when it happens at one place in the specimen. Brittle materials don't deform like that - they just snap before becoming elongated. Ductile materials deform significantly before breaking (by necking). That's what Necking is... now... what other measure of ductility would you use? Brittle materials have a pretty linear stress-strain relationship that ends abruptly with fracture. Ductile materials start to strain more, producing a stress-strain curve before failing. Isn't that just another way of measuring the same effect, deformation prior to failure?

Answer 2

Necking would be consistent with permanent deformation and the yield stress of the material. In many instances, once a material has passed its yield point, it is considered to have failed. The design is usually intended to stay in the elastic region and this is were design properties (strength, ductility, etc) are crucial for success.

Also, theoretically, yielding induces strain hardening and alters the properties of the material. There is a lot of phD work done on this area but there are a wide variety of things that can change measurements and lab results once you get past the yield point. Unless you have a great understanding, it may be difficult to duplicate exact results making any result suspect and therefore unreliable.