Hi Peter
In all the discussion I somehow seemed to miss the most important part.
"The strain at the interface between an adjacent pair of nodes is the same for both copper and aluminum, but because those two materials have different values of Young's Modulus (E) the stress in each element will be different."
Does this mean that the nodes at the interface have the material properties of both aluminium and copper and the deformations based on the matrix evaluation is calculated based on both material properties. Then to satisfy equilibrium, the deformation which corroborates with the equilibrium condition is accepted as the deformation of this node?
And then based on this deformation the strain is calculated and consecutively the stress (which is different for both the material because of the different Young's modulus) is calculated?
Is that what you meant or am I missing something here?

Also, could you please mention the name of the book again? I can not seem to find it in the comment chain.

Best regards Amit