Several posters have given good info. The elastic (Young's) modulus
does not change with heat treatment, so the 80 ksi steel stretches the
same as the 120 ksi for the same dimensions. The easiest way to solve
the problem is to calculate how much load is needed to stretch each
individual piece a set amount. Then for each increment of displacement,
the total load needed will be the sum of each, since they are attached
and must displace the same amount as load is applied. This is true up
to the yield point of the first piece to reach yield. After that things
get more involved, with plastic deformation, etc.
With different cross sectional areas, they will not stretch the same,
and the yield stress may be less than the sum of the two pieces. Each
pice must be analyzed separately, just as all parts of a structure must
be. Often times I see peolple adding steel straps to beef up aluminum
structure. This is not the best way as the higher modulus of the steel
means that it usually carries most of the load. In order to make a
situation like this work, you need to match the load versus elongation
of each piece to get the most out of it. You just have to do the
calculations!
By the way, since you didn't give the yield strengths, I can't
calculate which one will hit yield first. But to answer your question,
assuming that the two straps are attached by the
same bolts, they must stretch the same under load. This means that
since the 80 ksi steel has a larger area, it takes more load to stretch
it a set amount. But since it has a larger area, its psi is lower so it
may hit yield at the same time as the 120 ksi steel. It does carry 50%
more load than the 120 ksi material. For every 100 lbs load, 60 lbs
goes thru the 80 ksi, and 40 lbs through the 120ksi.


Dick wrote:
okay, got pounds per Square Inch and elongation differences and the failure
sequence in my noodle.

For simplicity, let's use 120,000 and 80,000 psi numbers.

So if the 120,000 # piece is .667 square inches
and the 80,000# piece is 1.0 square inch,,,are the elongations the same???

Thanks, Dick