Contraction connections are mechanically and thermally high stressed joints in high current devices. A special type is an aluminium - copper - contraction connection, where the inner and the outer ring are plated. As all electrical joints contraction connections may age due to creep, interdiffusion, chemical corrosion or fretting.
Because of the geometry and the used materials the initial joint resistance and especially changes in joint resistance during the lifetime of contraction connections are In the range of nano-ohms and therefore not easy to measure. Hence a method was developed, to calculate the joint resistance by means of the Finite Element Method.
To do so the ageing behaviour of plated aluminium - plated copper - joints was studied in long-term experiments. No change in joint resistance and so ageing due to interdiffusion, fretting or chemical corrosion was observed in joints with interlayers between the aluminium current bar and the plating.
The main ageing mechanism in a contraction connection is therefore creep. The strain - stress - curves and creep parameters of aluminium and copper as well as the resistance of the plating depending on the contact pressure were determined experimentally. Using the Finite Element Method the influence of the oversize for a given geometry on the initial tangential stresses, contact pressure and the initial joint resistance is shown. The higher the oversize is, the higher are the mechanical stresses and the lower is the initial joint resistance. The Influence on the long-term behaviour is negligible, since higher mechanical stresses after assembling mean higher creep deformation. Within short time the mechanical stresses are independent of the oversize and so is the joint resistance.
The joint resistance of the contraction connection under consideration will increase to maximum 1.8-times the initial resistance if the connection is operated at its limiting temperature all the time. In reality the joint temperature of contraction connections will change between ambient and limiting temperature due to changes in the operating current. That may lead to loss of mechanical and electrical contact between the two rings.
With the developed method It is possible to calculate the joint resistance of contraction connections depending on geometry, oversize, used materials, temperature and time.