MBSims, don't get upset if I make a couple of corrections on your statement.
In the SI metric system, Pascal is the unit of pressure, not force. The unit of force is the Newton (simplified N). One Pascal is one Newton per square meter. As it's a very small amount of pressure, it's usual to use kilo or megaPascals (kPa or MPa).
Also, the point at which the specimen under tensile stress starts to neck down is not the yield point. In carbon and low alloy steel, the yield point comes right after the elastic limit, i.e., the point at which the specimen looses its elastic properties and elongations are permanent, as Malcolm correctly says.
In the yield point, carbon and low alloy steels continue to elongate at an approximately constant tensile strength. In the tensile test this point lasts a few seconds, then the specimen enters the plastic region, in which the elongations are permanent. It is in the highest point of this region that the specimen starts to neck down.
In the tensile test diagram the yield point is recognized because it consists of a short "sawteeth" horizontal line located right after the elastic region and before the plastic one.
Unfortunately for Radiation, ASTM standards seldom specify a minimum shear strength for the materials they cover. They ususally specify minimum tensile strengths. In this case, Radiation can go after Malcolm's and MBSlims' reccomendations, in that the shear stress is approximately 60% (or 58%, if you prefer) of the ultimate tensile.
Giovanni S. Crisi
Sao Paulo - Brazil