Dear Radzi,

It will be helpful for us to reply if you attach a picture of your specimens with your question.

Standard jig design can be used in dissimilar welds

If you review the standards, you can understand the "spirit" of the tensile test. You must guarantee the tension area and not compromise the welding area in the fixing device. You can design or adapt conventional jigs for your test if you take care about this recomendations

We have welded such things in recent past. SS and Cu. Please specify if stresses are your concern. We have used one generic design, most probably as suggested by Sergio and Enrique. And also another improvised one to adjust stresses.