Machine elements - deflection and stress problem

[Triathlete]

Homework Statement

Homework Equations

δ = Pl/AE
σ = P/A

The Attempt at a Solution

I had calculated the deformation for both the steel and aluminum, and added them together to give me δ = 0.481 mm, which is incorrect. I also thought I could simply plug in the force P = 200kN and the respective cross sectional areas for each material into the stress formula, which also gave me incorrect answers. Not sure where to go from here.

 

[SteamKing]

Homework Statement

View attachment 95263

Homework Equations

δ = Pl/AE
σ = P/A

The Attempt at a Solution

I had calculated the deformation for both the steel and aluminum, and added them together to give me δ = 0.481 mm, which is incorrect.

Not sure why you would do that. The steel and the aluminum pieces are not in series. Each of the pieces must stretch the same amount, since there is a flange welded to one end and a tensile force is then applied to the assembly.

I also thought I could simply plug in the force P = 200kN and the respective cross sectional areas for each material into the stress formula, which also gave me incorrect answers. Not sure where to go from here.View attachment 95263

Instead of two stiff springs in series, why don't you think of this assembly as two stiff springs in parallel? What's the spring constant for two springs of different spring constants in parallel?