stress stiffness factor

That sounds like a term that would apply to structural support members. Pipe is generally tested to specific pressure.

Slope of the Stress strain curve(Tensile Test) of an elastic material.

Try Wiki:

http://en.wikipedia.org/wiki/Young's_modulus

Milo

I believe that you may be interested in the "Flexibility Factor" used during the stress analysis of piping systems.

Flexibiity factors are important, particularly for piping elbows, because they represent the additional flexibility in a system subject to expansion and bending. Piping elbows tend to ovalize when moments are imposed on adjacent piping legs

In the USA, piping codes such as ASME B31.1 and B31.3 contain infomation to calculate flex factors.

Generally speaking, thinner walled elbows "flex" more than thicker walls. Large diameter "pipe bends" are stiffer than the 1.5 D elbows commonly used.

Hope that this helps....

GOOGLE piping "flexibility factors" and analysis for more info...

MAkes sense to me. Stiffness is well, what stiffness is.

Your factors would seem to make more sense.

GA from me.

Milo